The origin of birds and their flight has been heavily debated in the field of evolutionary biology since the late nineteenth century. Birds are the only living descendants of dinosaurs and, for paleontologists, the famous Archaeopteryx has played a pivotal role in this discussion. Living during the Jurassic period about 150 million years ago in what is now southern Germany, Archaeopteryx is generally accepted as the oldest known flying bird. Yet, with the discovery of other bird-like dinosaurs from the same period, a question has arisen as to whether Archaeopteryx is the only flying bird from the Jurassic.

To answer this question, Rauhut et al. carefully examined a fossil of an isolated wing skeleton that was recently discovered in the same region of Germany where Archaeopteryx was found. The new specimen shows several characteristics that are otherwise only found in modern birds and not seen in Archaeopteryx. As such, this fossil represents a new species and the most bird-like bird discovered from the Jurassic. Rauhut et al. named the species Alcmonavis poeschli, after the ancient name for a river that flows near the discovery site, the Greek word for bird, and Roland Pöschl – the collector who found the specimen.

The wing of Alcmonavis also shows several features related to the attachment of flight muscles that suggest it was better adapted for flapping flight than Archaeopteryx. Together these findings are mostly consistent with the hypothesis that birds first started flying by flapping their wings rather than starting from a gliding stage. However, more detailed studies of the anatomy of primitive birds and their close relatives are needed to further test this hypothesis.

The so-called ‘Solnhofen limestones’ of southern Germany have long been known for their exceptionally preserved fossils (see Arratia et al., 2015). Historically, most fossils reported from these rocks come from the Altmühltal Formation (sensu Niebuhr and Pürner, 2014; see that publication for a clarification of the confusing history of naming the Late Jurassic limestone units of the Franconian Alb), as these were subject to intensive quarrying for several commercial purposes, from construction to lithography, probably since Roman times (Neumeyer, 2015). However, fossils have been known from the underlying Torleite and the overlying Mörnsheim formations for a long time (see e.g. Tischlinger, 2001), and more recent work in these units has shown that they are, at least at some localities, actually more fossiliferous than the Altmühltal Formation (see e.g. Viohl and Zapp, 2007; Heyng et al., 2011; Heyng et al., 2015; Albersdörfer and Häckel, 2015; Viohl, 2015).

Arguably the historically most important fossil taxon from the ‘Solnhofen limestones’ is the famous 'urvogel' Archaeopteryx, which has played a crucial role in the debate about the origin of birds (e.g. Huxley, 1868; Heilmann, 1926; Ostrom, 1976a; Wellnhofer, 2008; Wellnhofer, 2009). This taxon is so far only known from the lower Tithonian of Bavaria, Germany, with the vast majority of specimens (nine out of eleven) being derived from the Altmühltal Formation (Rauhut et al., 2018). Only one specimen (the 12^th specimen; see comments on numbering of specimens below) comes from the Kimmeridgian/Tithonian boundary in the lowermost part of the Painten Formation (Rauhut et al., 2018), and a single, fragmentary skeleton was reported from the Mörnsheim Formation (Figure 1), which overlies the Altmühltal Formation (Mäuser, 1997; Tischlinger, 2009), and was recently made the type of a new species of this genus, Archaeopteryx albersdoerferi Kundrát et al., 2019). In total, the genus Archaeopteryx seems to span some 700,000 to one million years, and notable variation between the different specimens might indicate evolutionary changes over this time (Rauhut et al., 2018).

Figure 1

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For more than 150 years, Archaeopteryx was the only Jurassic representative known of the Paraves, the clade of theropod dinosaurs that includes birds (Avialae; see Gauthier, 1986) and their closest relatives, dromaeosaurids and troodontids, which are united in a monophyletic clade, Deinonychosauria, in most phylogenetic analyses. However, the discovery of a diverse fauna of paravian theropods from slightly older rocks in China in the last decades (e.g. Godefroit et al., 2013a; Godefroit et al., 2013b; Sullivan et al., 2014; Xu et al., 2015; Lefèvre et al., 2017), and the identification of the ‘Haarlem specimen of Archaeopteryx’ as a separate taxon, Ostromia crassipes (von Meyer, 1857), representing a different clade of paravians, the Anchiornithidae (Foth and Rauhut, 2017), highlight the complexity of paravian evolution, diversity and distribution in the Late Jurassic, and make a careful re-evaluation of all maniraptoran specimens from the Late Jurassic plattenkalks of southern Germany necessary.

Here we report on a new paravian specimen from the Lower Tithonian Mörnsheim Formation, representing the second theropod specimen from this unit, which overlies the Altmühltal Formation. The new specimen represents the largest avialan theropod yet recorded from the Jurassic and provides further evidence on the forelimb anatomy and the origin of flapping flight in basal avialans.

Description

Preservation

The new specimen, SNSB-BSPG 2017 I 133, consists of the partially disarticulated, but associated skeleton of the right wing of an avialan theropod (Figure 2). The humerus is rotated and slightly displaced from the remains of the arm, and radius and ulna are disarticulated, but preserved in close proximity. The manus is disarticulated from the radius and ulna and the metacarpus overlies the distal ulna. The digits of the hand are mainly preserved in articulation, with only phalanx I-1 forming an unnatural angle with its respective metacarpal, and the ungual of digit I being displaced, lying at the distal end of the ulna.

Figure 2

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The head of the humerus is not preserved, as it lay in a mud-filled crack within the rock (U. Leonhardt, pers. com. to OR, 10.2017) and the proximal shaft of the radius is partially reconstructed. The bone preservation is generally rather poor, with all longbones being compressed, collapsing the shafts, and fractured, and the entepicondyle of the humerus is largely lost, as is the distal end of metacarpal I. However, the phosphatized remains of the original keratinous sheath of the unguals are rather nicely preserved, as it is often the case in theropod specimens from the Late Jurassic plattenkalks of southern Germany.

Humerus

The right humerus is exposed in anteromedial view (Figure 3). As noted above, the bone is missing its proximal end, so its exact length cannot be established (the apparent outline of the bone in the sediment is an artefact of preparation, as the outline has been sculpted in the matrix used to fill the crack). However, the medial edge preserves the distal end of the internal tuberosity, indicating that only a small portion of the proximal end is missing, so that the complete length can be estimated to be approximately 90 mm (±2.5 mm; length as preserved: 81.4 mm). As in all maniraptoran theropods (Rauhut, 2003), the internal tuberosity was obviously proximodistally elongate and rectangular in outline, with approximately its distal third being preserved. Distally, it fades into the relatively sharp medial edge of the shaft in an oblique step. On the anterolateral side of the humerus, the distal half of the large, rectangular deltopectoral crest is preserved. Whereas the anterolateral edge of the proximal part of the crest seems to be rather sharp-edged, the distalmost c. 11 mm show an elongate oval facet for the insertion of the m. pectoralis (Figure 3). This facet is inclined anteromedially and reaches a maximal transverse width of 2.2 mm. A similar facet is also developed in Sapeornis (Provini et al., 2009; pers. obs. by CF on specimen JZT-DB 0047), Jeholornis (Lefèvre et al., 2014), Jixiangornis (Chiappe and Meng, 2016), Confuciusornis (e.g. JME 1997/1; Chiappe et al., 1999), but it is absent in Archaeopteryx, anchiornithids and non-avian theropods (see discussion). Distal to this insertion, the deltopectoral crest fades into the anterolateral side of the shaft in an oblique, slightly concave step. As in all specimens of Archaeopteryx, other basal avialans like Confuciusornis (Chiappe et al., 1999), and, to a lesser degree, in non-avialan maniraptorans (e.g. Ostrom, 1969; Clark et al., 1999; Pei et al., 2014), the proximal part of the humerus that houses the deltopectoral crest is angled posteriorly in respect to the central shaft, with its posteromedial edge (excluding the internal tuberosity) forming an angle of approximately 38° towards the long axis of the shaft. This angle is larger than in non-avialan Paraves and Anchiornis (e.g., Ostrom, 1969; Burnham, 2004; Pei et al., 2014; Pei et al., 2017). In Archaeopteryx, the angle varies between 30° (Daiting specimen) and 33° (Maxberg and Ottman and Steil specimens), while it is similarly increased in more derived avialans (see discussion).

Figure 3

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The length of the humerus distal to the deltopectoral crest is 64.8 mm as measured from the distalmost edge of the apex of the crest and c. 60 mm from the point where the distal end of the crest fades into the shaft. The humeral shaft distal to the deltopectoral crest is very slightly sigmoidal, with the proximal c. 30 mm being slightly convex anteriorly and concave posteriorly, whereas the distal end is slightly flexed anteriorly. Although the compaction of the shaft makes secure interpretation difficult, it seems that it was wider transversely than anteroposteriorly over its entire length. The minimal width of the shaft is c. 6.5 mm (all measurements should be seen with caution due to the compaction of the bones). The distal end gradually expands transversely to approximately 200% of the minimal shaft width (as far as this can be established; 13 mm as preserved) and the distal condyles expand anteriorly. A large, deep, triangular groove is present proximal to and between the condyles on the anterior side of the humerus (Figure 3). Although this groove might be somewhat exaggerated by the compaction of the bone, it nonetheless seems to be a genuine character of the bone. The medial rim of the groove is formed by a broad longitudinal ridge that is slightly offset laterally from the medial margin of the distal humerus and fades into the shaft some 15 mm proximal to the distal end. This groove and ridge have not been described for Archaeopteryx so far (due to the fact that the anterior side of the distal humerus is not exposed in any of the specimens referred to this genus), but they are also present in the humeri of the dromaeosaurids Deinonychus (Ostrom, 1969), and Bambiraptor (Burnham, 2004), in Balaur (Brusatte et al., 2013) and the basal avialans Confuciusornis (Chiappe et al., 1999) and Sapeornis (Provini et al., 2009). In modern birds, a depression in a similar position serves as the attachment of the brachialis muscle, the fossa musculi brachialis (Baumel and Witmer, 1993), and this might be a likely identification in these paravian theropods as well. The entepicondyle (ulnar condyle or condyles dorsalis) is largely missing, but its impression in the rock indicates that it was anteriorly expanded and posteromedially rounded. The ectepicondyle (radial condyle or condyles ventralis) is considerably expanded anteriorly and has a well-developed, anterodistally facing, transversely concave roller joint, similar to the condition in Deinonychus (Ostrom, 1969).

Ulna

The ulna is a slender element (Figure 4) and, with a length of 82 mm, slightly shorter than the estimated length of the humerus, as in Archaeopteryx (Wellnhofer, 2008; Wellnhofer, 2009; Foth et al., 2014; Rauhut et al., 2018), non-avialan maniraptorans, Confuciusornis (e.g. Chiappe et al., 1999), and Apsaravis (Clarke and Norell, 2002), whereas both elements are of subequal length in Sapeornis (e.g. Zhou and Zhang, 2003a; Provini et al., 2009), and the ulna is longer than the humerus in many more derived avialans (e.g. Hu et al., 2014; Wang et al., 2016). It is exposed in anteromedial view. As in many other maniraptoran theropods (Gauthier, 1986), the proximal half of the ulna is slightly bowed, being convex posteriorly. The proximal end of the ulna is only slightly expanded from the shaft, and the olecranon process is poorly developed (Figure 5), as in most maniraptorans, including birds (Rauhut, 2003). The proximal articular surface shows a small, almost round, proximally and very slightly anteriorly facing concavity anteriorly, and a smaller convexity posteriorly. Thus, in contrast to Deinonychus (Ostrom, 1969), other dromaeosaurids (Norell and Makovicky, 1999; Hwang et al., 2002; Burnham, 2004), and Balaur (Brusatte et al., 2013), but similar to Jeholornis (Lefèvre et al., 2014; Chiappe and Meng, 2016) the proximal end is oval rather than triangular in outline in proximal view.

Figure 4

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Figure 5

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Below the proximal articular surface, a small, but sharply defined longitudinal ridge is present on the anteromedial side of the ulna (Figure 5). This ridge extends from the anteromedial edge of the proximal articular surface, is slightly offset from the medial side of the bone and fades into the shaft some 9 mm below the articular surface. A very similar ridge is present in the 12th specimen of Archaeopteryx (Rauhut et al., 2018: fig. 22), Confuciusornis (JM 1997/1), Ichthyornis (Clarke, 2004: fig. 52C) and many modern birds, where it marks the margin of the impressio brachialis (Baumel and Witmer, 1993), but such a ridge is absent in non-avialan theropods. Laterally, a small lateral tubercle is present proximally, defining the posterior border of the radial fossa and rapidly disappearing into the ulnar shaft distally. The tubercle is offset from the ulnar articular surface by a ridge, but its proximal surface is convex and does not show an additional articular surface, as it is the case in the cotyla dorsalis of modern birds. A pronounced tubercle for the insertion of the biceps muscle cannot be identified, but the bone is strongly crushed in the area where this tubercle would be expected.

The shaft is slender and narrows gradually, but only slightly from approximately 7 mm at the proximal end to approximately 5 mm at its mid-length. From there, it expands again towards the distal end, which seems to be anteroposteriorly flattened and expanded transversely (Figure 6), as in for example the dromaeosaurid Bambiraptor (Burnham, 2004), the enantiornithine Rapaxavis (O'Connor et al., 2011) or the ornithuromorphs Archaeorhynchus (Zhou et al., 2013) or Gansus (Wang et al., 2016). Whereas the lateral expansion is more gradual, the medial side has an abrupt, rounded expansion that starts some 6 mm proximal to the distal end forming a dorsal ulnar condyle (in Rapaxavis and Archaeorhynchus the distal end of the ulna has a gradual medial expansion and an abrupt lateral expansion; O'Connor et al., 2011; Zhou et al., 2013). In contrast, the distal ulna of Archaeopteryx seems to be less abruptly and more symmetrically expanded. The proximal end of the condylus dorsalis forms a small, anteromedially directed ridge, whereas the expansion flexes gradually into the distal side distally. The general shape of the distal ulna resembles that of the Late Cretaceous ornithurine Ichthyornis (Clarke, 2004: Figure 52C) and the posterior side of the ulna in modern birds, but it differs from the latter in the absence of a tuberculum carpale (Baumel and Witmer, 1993). The maximal distal expansion is 9 mm as preserved. The distal articular surface is gently rounded transversely, flexing higher proximally on the medial than on the lateral side (Figure 6). However, it is rather poorly preserved, so nothing can be said about its details.

Figure 6

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Radius

As in all theropods, the radius is more slender than the ulna (Figure 4), its minimal shaft width being 3 mm. The bone is straight, and its length cannot be established precisely, as the distal end is overlain by the metacarpus, but it is approximately 80–82 mm. The radius seems to be mainly exposed in medial view.

Although the shaft generally widens gently proximally, the proximal end is rather rapidly expanded to an anteroposterior width of c. 6 mm, with this abrupt expansion starting some 4 mm below the proximal end (Figure 7A). In contrast to the situation in Bambiraptor (Burnham, 2004), Anchiornis (Pei et al., 2017), Balaur (Brusatte et al., 2013), and Confuciusornis (Chiappe et al., 1999), but similar to Deinonychus (Ostrom, 1969), the anterior and posterior sides are almost equally expanded. The proximal articular end extends slightly more proximally posteriorly, where the articular surface is slightly convex and gradually curves into the posterior side of the shaft, whereas the anterior two thirds of the articular surface are gently concave.

Figure 7

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An unusual feature of the proximal radius is a raised, medially directed crest on the anteromedial side of the shaft Figure 7A,B). This crest expands abruptly medially some 3.5 mm distal to the proximal articular surface and extends for at least 9 mm distally, before it obliquely disappears into the shaft (although the crest is damaged in parts, enough is preserved to establish its approximately shape and extent; Figure 7B). At its highest section, the crest extends at least 1.5 mm from the shaft medially. In position and general appearance, this crest corresponds to the tuberculum bicipitale radii of birds (Baumel and Witmer, 1993), and we thus interpret it as the insertion of the M. biceps brachii. In basal paravian theropods, a similar crest on the proximal radius has so far only been described in Bambiraptor (Burnham, 2004), but it is absent in Deinonychus (Ostrom, 1969), Pyroraptor (Allain and Taquet, 2000), Anchiornis (Pei et al., 2017), and Balaur (Brusatte et al., 2013). In Confuciusornis (Chiappe et al., 1999) and at least some specimens of Archaeopteryx (pers. obs.), a small tuberculum bicipitale radii is present, but it is triangular in shape and less pronounced than in SNSB- BSPG 2017 I 133 (see below).

The midsection of the radial shaft is more or less intact, showing a longitudinal groove running along the medial side of the shaft (Figure 7C). Such a groove is absent in Archaeopteryx, but has been described for some species of Enantiornithes (e.g., Chiappe and Walker, 2002; Sanz et al., 2002; Hu et al., 2015). A similar groove was also described for Jeholornis, but interpreted as the result of crushing (Lefèvre et al., 2014). However, as such groove is also present in other specimens of Jeholornis, including the holotype (Chiappe and Meng, 2016): 32, 36), this structure might be authentic. The shaft has its minimal width at about mid-length, from where it again gradually expands distally to a width of 4 mm at the point where the radius is overlain by the metacarpus. Nothing can be said about the morphology of the distal end of the bone, as it suffered severe damage.

Carpus

The carpus is represented by the semilunate carpal that is preserved in articulation with the metacarpus (Figure 4), two small bones preserved below the distal end of the ulna (Figures 6 and 8), and a very small element preserved in articulation with the proximal end of metacarpal III (Figures 8 and 9). In agreement with Botelho et al., 2014, the small element articulated with metacarpal III is interpreted as distal carpal 3. The other two carpals are poorly preserved, but probably represent the radiale, preserved a small distance away below the lateral side of the distal ulna, and the pisiforme (=ulnare; see Botelho et al., 2014), which is preserved directly below the medial side of the ulna and is partially covered by the first manual ungual.

Figure 8

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Figure 9

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Both proximal carpals have a rather poorly defined, granular bone surface, probably indicating incomplete ossification (Figure 6). The radiale is a small element, maximally 4.3 mm wide and 2.8 mm deep. It is roughly trapezoidal in outline, has rounded edges, is slightly waisted, and tapers towards its medial(?) side. The exposed (distal?) surface is subdivided by an oblique ridge into two slightly angled facets, a larger, concave lateral(?) facet, presumably for the articulation with the trochlea of the semilunate carpal, and a smaller, triangular lateral facet towards the tapering edge, which might represent the attachment of a propatagial tendon (see Wang et al., 2017, for the presence of propatagia in maniraptoran theropods).

The pisiforme is slightly larger, being approximately 4.8 mm wide transversely (Figure 6). It is poorly preserved and partially overlapped by the distal ulna and the manual ungual I, so nothing can be said about its exact shape. However, the bone seems to have a triangular, posteroproximally tapering process at its posteroproximal corner, possibly for the attachment of the m. flexor carpi ulnaris (see Vazquez, 1994).

The semilunate carpal is considerably larger (Figures 8 and 9), being 6.4 mm wide transversely, and maximally 3.7 mm deep proximodistally on the palmar side. It is preserved in articulation with metacarpal II, but is separated from the latter by a clear sutural line and even slightly detached from metacarpal I on the palmar side, and was thus not fused into a carpometacarpus, similar to the situation found in non-avian Pennaraptora (Botelho et al., 2014), and other basal avialans like Archaeopteryx (Wellnhofer, 2009), Sapeornis (Zhou and Zhang, 2003a; Botelho et al., 2014) and Confuciusornis (Chiappe et al., 1999). The proximal articular surface is strongly convex transversely, with the medial side being more strongly convex than the lateral, so that the proximalmost point of the carpal is slightly placed medial to its mid-width (Figure 9). The distal side of the carpal is subdivided into a larger, gently concave lateral facet for the contact with metacarpal II and a much smaller, more strongly concave facet for metacarpal I. The plantar side of the semilunate carpal is exposed below the medial side of metacarpal I. It extends further medially than the palmar side, bordering the entire plantar medial expansion of the proximal articular surface of metacarpal I, as in Archaeopteryx and more basal paravians. Assuming a similar lateral expansion as the palmar side of the carpal, the plantar side is approximately 9.5 mm wide, but only 2.3 mm deep proximodistally at the point where the exposed section disappears below the palmar trochlea. The palmar ridge of the articular trochlea of the semilunate carpal is thus shorter and more strongly convex than the plantar ridge of the trochlea (Figure 9), as in other paravian theropods, such as Deinonychus (Ostrom, 1969), Bambiraptor (Burnham, 2004) and other taxa (see Xu et al., 2014). Plantar and palmar trochlear ridges seem to be separated by a deep transverse groove on the proximal articular surface. The lateral edge of the semilunate carpal overlaps half of the area of the proximal end of distal carpal three and metacarpal III, although the latter are offset distally from the carpus.

Distal carpal three is a very small, round nubbin of bone some 1 mm wide transversely and slightly less deep proximodistally, which is attached to the proximal end of metacarpal III.

Metacarpus

As in all maniraptorans, the metacarpus consists of three elements (Figure 8), which are here identified as metacarpals I-III, in agreement with the vast majority of works on theropods (but see Xu et al., 2009, for an alternative interpretation). The metacarpus is preserved in articulation and exposed in palmar view. Metacarpal II is notably more robust than either metacarpal I or III (Figure 8), with the width of its proximal shaft (c. 5 mm) being approximately twice that of metacarpal III (2.5 mm) and more than 185% of the width of the proximal articular surface of metacarpal I (2.7 mm). This differs from the more equal widths of metacarpal I and II in Archaeopteryx (e.g. Wellnhofer, 1974; Wellnhofer, 2008; Mayr et al., 2007), but also the less marked difference in other basal avialans, such as Sapeornis (Zhou and Zhang, 2003a) and Confuciusornis (Chiappe et al., 1999). In contrast, these proportions resemble the condition found in Jeholornis (Zhou and Zhang, 2002; Lefèvre et al., 2014).

Metacarpal I is unfortunately incompletely preserved, so that its exact length and maximum width cannot be established (Figures 8 and 9). However, the position of the proximal end of phalanx I-1 might indicate the distal end of this metacarpal, in which case it would have been little more than 7 mm long, or about 17–17.5% of the length of metacarpal II and thus relatively much shorter than in Archaeopteryx (25–31%). The proximal articular surface of metacarpal I is 3 mm wide transversely and semilunate in outline, with a plantar-palmarly expanded lateral side and a transversely expanded plantar side (Figure 9). As the plantar side is embedded in the sediment, it cannot be said whether a plantar process is present laterally, as it is the case in the dromaeosaurids Deinonychus (Ostrom, 1969) and Bambiraptor (Burnham, 2004). The mediopalmar side of the proximal end shows a notable depression, distal to which the medial margin seems to have been somewhat expanded, but is broken off. The lateral side of metacarpal I is closely appressed to metacarpal II over its entire preserved length, and the proximal part of the palmar side forms a small lateral flange that overlaps the mediopalmar edge of metacarpal II (Figure 9). The distal condyles are broken off, so nothing can be said about the structure of the distal articulation.

Metacarpal II is massive, apparently subrectangular in cross-section and largely straight, with only the distal articular end being very slightly flexed (Figure 8). The exact width of the proximal end is difficult to measure, as its palmar side is partially overlapped by flanges of metacarpals I and III, but it is approximately 5.8 mm. From there, the bone gradually tapers over its proximal two thirds to a minimal width of 3.2 mm some 25 mm from the proximal end, distal to which it expands again slightly. The distal end is slightly flexed medially and exposed somewhat obliquely, and has a maximal width of approximately 4.7 mm. The palmar surface of the distal end is collapsed, but the distal articulation was obviously strongly ginglymoidal, with a lateral condyle that is slightly wider and more distally expanded lateral than the medial condyle, which are separated by a marked, broad groove (Figure 10A,B). A shallow collateral extensor depression is present on the obliquely exposed lateral side of the distal end.

Figure 10

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Metacarpal III is slender and slightly shorter than metacarpal II. The bone is straight and closely appressed to metacarpal II over its entire length (Figure 8), as in most basal paravian theropods, but unlike the flexed third metacarpal with a well-developed spatium intermetacarpale in most modern birds and some basal avialans, such as Jeholornis (Zhou and Zhang, 2002; Zhou and Zhang, 2003b; Lefèvre et al., 2014). The proximal end is displaced distally by slightly more than 1 mm from the level of the proximal end of metacarpal I and II, as in oviraptorids (Longrich et al., 2010; Balanoff and Norell, 2012), dromaeosaurids (Lü and Brusatte, 2015), Archaeopteryx (Elzanowski, 2002; Mayr et al., 2007), confuciusornithids (e.g. Zhang et al., 2008), enantiornithines (e.g. Zhang et al., 2013), and more derived birds. The proximal end has a medially flared, rounded flange that overlaps the lateropalmar edge of metacarpal II and thus reaches a maximal transverse width of 4 mm (Figure 9). The flange disappears into the shaft some 4 mm from the proximal end, distal to which the shaft is 2.7 mm wide. The shaft remains of subequal width over most of its length, tapering only very slightly and reaching a minimal width of 2.4 mm some 5 mm from the distal end. Distally, the bone expands slightly again, mainly due to a rounded mediopalmar flange, to a maximal distal width of 2.9 mm. The distal articular surface is not ginglymoidal, but convex transversely, with a palmar depression between the slightly more massive lateral side of the articular surface and the mediopalmar flange (Figure 10A,B).

Digits

The hand of the new specimen shows the typical theropodan phalangeal formula of 2-3-4 (Figure 8), with digit II being by far the longest of the three digits, followed by the subequal digit I (65% of the length of digit II) and digit III (64% of the length of digit II). Considering isolated phalanges, phalanx I-1 is the longest manual phalanx, being only very slightly longer (28.5 mm) than phalanx II-2 (28 mm).

Phalanx I-1 is long, slender, and slightly bowed, being convex plantarly (Figure 10C). The phalanx seems to be higher than wide (although this might be exaggerated by compression), and its length is approximately 6.5 times its proximal height. The proximal articular end is poorly preserved, but it seems to have been narrow and weakly ginglymoidal. On the palmar side of the proximal end a notable round tubercle probably marks the insertion of a flexor tendon. Directly distal to the proximal articular end, a short, but stout lateropalmar flange seems to have been present (Figure 10C). At mid-shaft, the phalanx seems to have a shallow longitudinal lateral groove, similar to, but less developed than in Ostromia (Foth and Rauhut, 2017); however, as this groove is only apparent in the mid-section of the shaft, and the bone has generally suffered from strong compression, collapsing both the proximal and distal part, some uncertainty remains if this groove represents an original feature or an artefact of preservation. Interestingly, however, such a groove is not present in any of the other manual phalanges, but a similar structure is present in the also apparently uncrushed mid-shaft of the radius, and we thus tentatively regard this feature as a true character of this phalanx. The distal articular end of phalanx I- one is ginglymoidal, with the palmar side of the articular facet extending considerably further proximally than the plantar side. A well-developed collateral ligament pit is present and displaced plantarly from the mid-height of the distal end.

The ungual of the first digit (Figure 10D) is slightly smaller than the ungual of digit II, as in Anchiornis (Hu et al., 2009) and Archaeopteryx (Wellnhofer, 2008; Wellnhofer, 2009), whereas ungual I is larger than that of the second digit in more basal tetanurans (e.g. Allosaurus: Gilmore, 1920; oviraptorosaurs: Clark et al., 1999; Balanoff and Norell, 2012; Deinonychus: Ostrom, 1969), but also in Sapeornis (Zhou and Zhang, 2003a) and Confuciusornis (Chiappe et al., 1999). The ungual of the first digit is strongly curved, with the tip of the bony claw being placed approximately 5.6 mm below the proximal articular facet, when the latter is oriented perpendicular. The proximal end has a maximal height of 7.2 mm, of which 3.9 mm are accounted for by the proximal articular facet, whereas the rest forms the very strongly developed flexor tubercle. The latter expands downwards from the articular surface to form a rounded palmar tubercle, the distopalmar extremity of which is expanded transversely into a small, triangular lateral tubercle. Distally, the flexor tubercle fades into the palmar margin of the ungual. When the proximal articular end is oriented perpendicular, the plantar margin of the ungual arches slightly upwards in its proximal part, as in most coelurosaurs, but in contrast to more basal theropods. A plantar lip at the proximal end of the ungual is absent. The ungual has a single, more or less centrally placed claw groove, palmar to which the bone is considerably wider transversely than on the plantar side. The palmar margin of the bony ungual is notably flattened, with the claw showing its greatest transverse width where the lateral side flexes into the palmar side, and tapering slightly towards the claw groove. The keratinous sheath of the claw is well preserved and largely hides the distal end of the bony ungual. It extends the length of the claw for approximately one third of the length of the bony element (measuring the maximum straight length of the ungual) and follows the curvature of the latter, so that the distal end of the sheath is placed approximately 9.5 mm below the proximal articular facet when the latter is oriented vertically.

In the second digit, the first phalanx is shorter (c. 81%) than the second phalanx, as in all theropods. However, the first phalanx of the second digit, which is exposed in palmar view, is strikingly robust, being by far the most robust phalanx of the manus (Figures 8 and 10A,B), as it is the case in Sapeornis (Provini et al., 2009; Gao et al., 2012), Confuciusornis (Chiappe et al., 1999) and more derived avialans, whereas this phalanx is only slightly more robust than phalanx II-2 in Archaeopteryx (Mayr et al., 2007; Wellnhofer, 2008; Wellnhofer, 2009) and more basal theropods. Thus, with a maximal transverse width of 5.5 mm, the proximal end of phalanx II-1 is more robust than the distal end of its respective metacarpal, and the bone narrows only slightly to a minimal width of 4.4 mm just distal to its mid-length. Although the phalanx is compressed, the preserved width seems to reflect the true width of the element, as indicated by the fitting articulation with metacarpal II. In contrast to the condition in more derived birds, but as in Sapeornis (Provini et al., 2009) and Confuciusornis (Chiappe et al., 1999), the phalanx is not flattened but seems to have been rather robust, although it is largely collapsed. The proximal end of the phalanx has two well-developed, concave articular facets separated by a median ridge, fitting with the ginglymoidal distal articular end of the second metacarpal (Figure 10A,B). In the shaft, the medial side of the bone is gently concave over its entire length, whereas the lateral side is slightly bulbously convex over its proximal third and becomes only slightly concave distally. The distal articular end of this phalanx is ginglymoidal, being somewhat twisted so that the palmar condyles face slightly lateropalmarly (Figure 10A,B), which is furthermore slightly exaggerated by compression. The articular end thus consists of two well-rounded condyles separated by a notable groove. A shallow collateral ligament depression is present on the medial side of the end, being slightly displaced palmarly from the mid-height of the bone.

Phalanx II-2 is long, slender, and slightly flexed, being convex on the plantar side; it is exposed in medial view. The proximal end is 4.4 mm high plantar-palmarly and seems to have been narrow transversely, although this is certainly exaggerated by compression. The proximal articular end is concave. The height of the bone diminishes rapidly directly distal to the articular end, and then more gradually to a minimal value of 2.7 mm approximately two-thirds of the length of the bone from the proximal end. From here, the bone expands slightly again towards the strongly ginglymoidal distal articular end. The latter is very similar to the distal articular end of phalanx I-1 in extending further proximally on the palmar side and having a well-developed, plantarly displaced collateral ligament pit (Figure 10E).

The ungual of the second digit (Figure 10E) is slightly longer proximodistally than the ungula of digit I, but less strongly curved, so that the distal tip of the bony ungual is placed 5.5 mm below the articular facet. The flexor tubercle is slightly more offset distally from the proximal end than in the first ungual and accounts for c. 3 mm of the maximal proximal height of 6.6. mm. Furthermore, in contrast to ungual I this element shows a well-developed proximal lip on the plantar surface. Otherwise the ungual is similar to that of digit I, in having a transverse expansion of the palmar extremity of the flexor tubercle and a flattened palmar margin. The dorsal margin arches upwards from the articular end before the claw flexes downward again, as in the first ungual. The ungual sheath is also well preserved and extends to c. 13 mm below the proximal articular surface.

In digit III, the proximal two phalanges are shorter, taken together, than phalanx III-3 (Figure 8), as in maniraptoriforms generally (Rauhut, 2003). Of the two proximal phalanges, phalanx II-1 is considerably longer (c. 150%) than phalanx II-2, but both are poorly preserved (Figure 10A,B). Both phalanges are strongly compressed and collapsed, but the transverse width of phalanx III-1 can be estimated to be approximately 1.5 mm, or only about 25–30% that of the maximal width of phalanx II-1. Both phalanges seem to have been ginglymoidal, but nothing can be said about any morphological details. Phalanx III-3 is long and slender but straight, not flexed as it is the case in phalanges I-1 and II-2. The proximal end is poorly preserved but was approximately 2.5 mm high plantar-palmarly. From this end, the phalanx gradually tapers to a minimal height of 0.9 mm just proximal to the expansion of the ginglymoidal distal articular end. The latter is again generally similar to the distal ends of phalanges I-1 and II-2, being more narrow on the plantar than on the palmar side (Figure 10E).

The ungual of digit III (Figure 10F) is by far the smallest of the manual unguals, as in most theropods. It is also less markedly curved than the other unguals, with the tip of the bone being placed approximately 2.4 mm below the articular facet. The flexor tubercle is proximally placed and similarly pronounced as in the first ungual and accounts for c. 2.2 mm of the maximal proximal height of 4.6 mm of the bone. In other characters, such as the upward arching proximal flexure of the claw, the singly claw groove, and the flattened palmar margin, the ungual is similar to the other manual unguals. This is also true for a transverse expansion of the distopalmar extremity of the flexor tubercle, further indicating that this represents the original condition and does not stem from compression or deformation. As in the other unguals, the sheath extends the ungual for slightly more than one third of the length of the bony element, and its tip is placed c. 5.2 mm below the proximal articular facet.

All data generated or analysed during this study are included in the manuscript and supporting files.

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Thank you for submitting your article "A non-archaeopterygid bird (Dinosauria; Theropoda; Avialiae) from the Late Jurassic of southern Germany" for consideration by eLife. Your article has been reviewed by four peer reviewers, including Trevor Worthy as Guest Editor and Reviewer #2, and the evaluation has been overseen by Diethard Tautz as the Senior Editor. The following individuals involved in review of your submission have agreed to reveal their identity: Jingmai Oconnor (Reviewer #4).

The reviewers have discussed the reviews with one another and the Reviewing Editor has drafted this decision to help you prepare a revised submission.

Summary:

This manuscript describes a new fossil, the thirteenth urvogel from the Solnhofen archipelago (where Archaeopteryx derives from), showing that it represents a second avialan taxon that is slightly more derived than Archaeopteryx. The conclusion that follows is that some flight adaptations and the transition towards Aves evolved earlier than previously thought. This is a major discovery - if substantiated - and thus one that requires extraordinary justification. You provide an excellent morphological description of an important new specimen, in a well written manuscript supplied with wonderful photographs in both normal and UV light. However, while it reveals some new anatomical details, the specimen is fragmentary, with only wing bones, and some of the key conclusions of the manuscript are rather poorly supported by the actual data as presented. In addition, some speculative ideas are presented as too definitive in the abstract and conclusion. Added comparisons and attention to the points raised below may possibly overcome these obstacles; hence a revision is invited.

Essential revisions:

1) Adequate Diagnosis from Archaeopteryx

Two reviewers have strong reservations as to whether the fossil is adequately diagnosed as distinct from Archaeopteryx and more ancestral avialan. There is also concern how well the erection of a new genus name for the fossil is substantiated. In the diagnosis, the authors differentiate the fossil from "all other theropods", but what would be needed is an explicit differentiation from Archaeopteryx and the newly erected "Ostromia".

Does the phrase "all theropods" in the diagnosis include birds as well? If it does, some of the features, such as a "large deltopectoral crest", are erroneously listed, as these do occur in avians (or "avialans" sensu the authors). One reviewer was not convinced by the diagnosis distinguishing it from other theropods, noting that the phalanges seem very mobile (whereas the digits are typically preserved straight indicating more rigidity (which would be necessary for flight) in Archaeopteryx; also the morphology of the semilunate is more non-avian than avian, and the same seems possible with regards to the distal humerus, and the proportions of the claws relative to their digits (not including horny sheath). We think a stronger diagnosis is therefore required. Throughout the description, anywhere comparison is only made with avians, non-avians should be added (and vice versa).

2) Comments on specific morphologies

There are reservations concerning the interpretation of the humerus that need addressing.

Some of the alleged differences between the new fossil and previously described specimens are controversial. For example, one of the key features listed by the authors, the alleged presence of an impression for musculus pectoralis, cannot be established with certainty in some other Archaeopteryx fossils, such as the London specimen, in which the deltopectoral crest appears to be abraded in the corresponding region (see Figure 12 of the manuscript). This facet in the new fossil does not appear to be substantially different from that found in Mei long (Figure 14B), and certainly it is much less developed than the corresponding muscular impression in more crownward birds. Reviewer Oconnor noted that the attachment for the pectoralis on the distal end of the deltopectoral crest has not been widely described, at least not in non-ornithothoracine Early Cretaceous birds (it is mentioned in Martinavis; Walker et al., 2007), so taxa that have this feature should be listed together with citations (be careful, although it may appear present from a figure, if it wasn't described, its appearance in a photograph is possibly an artefact of preservation/photography, e.g., Sapeornis IVPP V13396 – Provini et al., 2008, or the holotype of Jeholornis. It is not mentioned in any of the papers cited, and in Oconnor's experience with basal Jehol birds is not common and so must have been independently evolved in Alcomavis, or perhaps only appears in the most ontogenetically mature specimens of Archaeopteryx (and potentially other Mesozoic birds).

It is not accurate to estimate the length of the humerus as it is incomplete and to do so based on assumptions about the size of the internal tuberosity. Better to just state its minimal length (and not use this element for ratios and comparison). Which other taxa show elongate oval facet for the insertion of the m. pectoralis (make sure to check non-avians)?

With regards to the angle of the proximal portion of the humerus relative to the shaft the authors say to a lesser degree in non-avians but this seems a bit misleading – it is also strongly angled in a number of non-avian paravians, especially it seems in dromaeosaurids (Ji et al., 2000 – Sinornithosaurus; Microraptor, any number of references) but also present in Anchiornis (this is another reason why line drawing reconstructions of the forelimb in 4-5 key taxa would be really helpful). Reviewer Worthy suggested that the shaft angle (Subsection “Humerus”, etc) is perhaps increased by inclusion of the bicipital crest. The bicipital crest can be seen to join the shaft roughly co-level with the distal end of the deltopectoral crest, and thus the bicipital crest adds width to the ventral (here posteromedial) side of the shaft. Exclusion of the bicipital crest would lead to a lesser angle of the proximal section of the shaft relative to that more distally. Thus, using the ventral shaft margin as a proxy for shaft angle is not homologous between taxa that have a definite and enlarged bicipital crest and those that do not. A revision of definition of this metric would avoid this issue. Applies to the Discussion section as well.

Ulna – since the humerus is not fully preserved, it is not accurate to compare an estimated length and then derive comparisons. Compare proximal end of ulna to Anchiornis. Although caution is mentioned often, the distal ulna seems pretty crushed flat but still described heavily. The features described are only present in Archaeorhynchus or also present in other stem birds? Label the anteromedially directed ridge in Figure 6.

Radius – compare longitudinal groove to that in enantiornithines.

Carpals – MC I seems to be broken and it was unlikely it was as short as described. MC II is thicker than I and III in Microraptor as well; also, Mc III is slightly offset proximally.

Digits – Digit I ungual largest in Anchiornis (see Pei et al., 2017); digit II phalanx 1 is certainly robust but it appears strongly exaggerated as preserved, as two surfaces are exposed and flattened so it appears to be a single exposed surface of a hyper-robust phalanx. If this is taken into account, then the proximal articular surface of the phalanx is slightly narrower than the matching articular surface on metacarpal II. I do not see this described convexity on the plantar surface of phalanx II-2. The flexed morphology described in the penultimate phalanges of digits I and II is very subtle, at best, and should be described as so.

Unguals – it is stated (Discussion section) that the unguals are larger (proportionately) in Archaeopteryx but if the ungual (not including horny sheaths which may or may not be complete or in situ) is compared to the length of the penultimate phalanx, they are proportionately larger in Alcomavis. A table of ratios would be helpful to further help illustrate differences between this specimen and other urvogels and closely related taxa. Confuciusornis does not show a reduction in manual claws from I to III (II is smallest). Since this specimen is larger than other Archaeopteryx specimens, the pronounced muscle attachments could be explained by ontogeny (as suggested by the authors themselves somewhat), especially given that the difference between these taxa is pretty minor if the robustness of phalanx II-1 is taken out.

3) Advisability of creating a new nomen

Even if the new specimen can be shown to differ from other Archaeopteryx fossils in some features, it is contentious, and some would say not advisable to assign a name to these fragmentary remains. The past years have seen an inflation in new and rather poorly diagnosed Archaeopteryx-like avians from the Solnhofen limestone, and in light of the very fragmentary preservation of the fossil, the manuscript would be scientifically more rigorous if the authors opted against naming a new taxon.

4) Phylogenetic analysis

The phylogenetic analysis is presented in rather a cursory way. It is unacceptable not to have support values in the presented phylogenetic trees. For example, it is misrepresentative to say the strict consensus is well resolved (subsection “Phylogenetic position of Alcmonavis”) yet not show the evidence that there is in fact very weak support for such resolution. At the very least Bremner support values should be given so that readers can see which of these clades has any semblance of support – one assumes that as no values are given and given the statement, that clade support is very low indeed – but readers deserve to be shown this, not have these data occluded. Ideally bootstrap values over 50% should be shown because anything less shows the clades are highly unstable and probably are not real in terms of evolutionary history. As presented, the placement of Alcmonavis is discussed as an afterthought in this section. Surely the focus should be on the new taxon and then use known taxa secondly to validate the tree.

Given it is well established that missing data drive a taxon to the base of the clade in which it would otherwise reside (e.g., Sansom, 2015), some comment on the placement of the new taxon is warranted; why, what character evidence precludes that, for example, the new taxon really lies within Jelholornithidae or another slightly more derived position, than the node it appears in the tree? Character evidence for the critical nodes should be discussed, identifying the derived characters that place the new fossil in a more crownward position than Archaeopteryx.

Sansom, 2015.

The phylogenetic analyses should state which multistate characters were ordered (if any).

Major clade names, including all Family names used in text, should be shown on the trees in the Supplementary file.

5) Improvement of figures

While the currently presented figures are excellent, some more are needed. Key to demonstrating the distinction from Archaeopteryx, are photos and or line drawings of the important elements of the new fossil in direct comparison with those of "Archaeopteryx" that explicitly show the differences in morphology and proportions. It is also desirable to add some figures, at least in Supplementary file, of key features of the new fossil directly compared with a meaningful selection of stem birds and non-avian maniraptorans to aid with comparison.

6) Tabular presentation of key data to facilitate comparison

Use of two more tables might facilitate comparisons better. For example, the section on Comments on the numbering and naming of avialan specimens from the Solnhofen Archipelago is useful, but quite long. Perhaps include the data in a table listing all 13 specimens, their current taxonomy, their numbers and correlative 'names' and institution, and include just a couple sentences to justify the new names for the eleventh to thirteenth specimens. If length is an issue then it could be in Supplementary file, but it is of more use to the audience in the main text, i.e. not lost/invisible in the Supplementary file.

Similarly, a table with comparative key measurements for the 12 Archaeopteryx specimens including ratios should also be provided, so demonstrating how similar this specimen is to other urvogel specimens.

7) Discussion on possible flight mode of the fossil

This section is entirely speculative and not well substantiated. Various authors have detailed that flapping flight capabilities are unlikely for Archaeopteryx. The different morphology of the supracoracoideus muscle is just one argument contra flapping flight capabilities of Archaeopteryx, and the anatomy of Archaeopteryx does not show any evidence that this animal was capable of take-off through flapping flight (comparisons with extant pigeons certainly are not appropriate, as the latter have a completely different skeletal morphology than any Jurassic avians). Even though Voeten et al. recently proposed flapping flight capabilities of Archaeopteryx, their arguments based on the thickness of the bone cortex are far from compelling. Likewise, it is not clear why the presence of a tubercle for musculus biceps brachii on the radius would indicate flapping flight capabilities (so much the more as the presence/absence of this feature in other Archaeopteryx specimens is difficult to assess). Given the speculative nature of this interpretation, to include in the abstract a sentence like "Several modifications, especially in muscle attachments related to the downstroke of the wing, indicate a rapid adaptation of the forelimb for active flapping flight in the early evolution of birds" is not justified by the data (so much the more as the discussion is far less definitive).

Given the great similarities of the general skeletal architecture of this fragmentary wing to the known Archaeopteryx remains (three unfused metacarpals with large ungual phalanges, similar phalangeal proportions etc.), it is highly unlikely that substantial differences in flight mode to Archaeopteryx existed. The speculative nature of this discussion and interpretation should be addressed.

8) Biogeographic interpretations.

These speculations towards the end of the manuscript should be deleted altogether.

Essential revisions:

1) Adequate Diagnosis from Archaeopteryx

Two reviewers have strong reservations as to whether the fossil is adequately diagnosed as distinct from Archaeopteryx and more ancestral avialan. There is also concern how well the erection of a new genus name for the fossil is substantiated. In the diagnosis, the authors differentiate the fossil from "all other theropods", but what would be needed is an explicit differentiation from Archaeopteryx and the newly erected "Ostromia".

The explicit differentiation of Alcmonavis to Archaeopteryx and Ostromia was already provided in the first part of the Discussion section of the previous manuscript. To avoid confusion, we rearranged the manuscript and put the diagnosis after the description and discussion of the differences so that the reader already has this information before the new taxon is defined.

We extended the comparison between Alcmonavis to Archaeopteryx and Ostromia by including statistical analyses on various limb proportions.

Does the phrase "all theropods" in the diagnosis include birds as well? If it does, some of the features, such as a "large deltopectoral crest", are erroneously listed, as these do occur in avians (or "avialans" sensu the authors). One reviewer was not convinced by the diagnosis distinguishing it from other theropods, noting that the phalanges seem very mobile (whereas the digits are typically preserved straight indicating more rigidity (which would be necessary for flight) in Archaeopteryx; also the morphology of the semilunate is more non-avian than avian, and the same seems possible with regards to the distal humerus, and the proportions of the claws relative to their digits (not including horny sheath). We think a stronger diagnosis is therefore required. Throughout the description, anywhere comparison is only made with avians, non-avians should be added (and vice versa).

As stated in the text, Alcomonavis is diagnosed based on a combination of characters, i.e., single characters listed in the diagnosis do not have to be unique. Of course, large deltopectoral crests are typical for many basal birds, but in its extension the deltopectoral crest of Alcmonavis is larger than in Archaeopteryx, anchiornithids and other non-avian theropod dinosaurs.

The reviewer is correct that in many pygostylian birds, the digits are rigid, especially in Ornithothoraces. This also seems to be the case in most Archaopteryx specimens, but also in Anchiornis or Microraptor. However, we believe that the preservation of the phalanges in Alcmonavis is not an indicator of missing rigidity, but a taphonomic artefact caused by an advanced stage of decay; indeed, fossils of the Mörnsheim Formation tend to be more strongly disarticulated and disrupted than those of the underlying Altmühltal Formation, which has yielded the vast majority of specimens of Archaeopteryx. Moreover, some specimens of Archaeopteryx, including the "chicken wings" and the twelfth specimen, also show strongly flexed phalanges. Indeed, similar “mobile” preservations can be also found in Sapeornis (IVPP V13396), Pengornis (IVPP V15336), Gansus (BMNHC-PH1392) and many specimens of Confuciusornis (DNHM-D2454, IVPP V10928, IVPP V12352, IVPP V13156 or JME 1997-1) (see e.g., Falk et al., 2016; Wang et al., 2019 or Chiappe and Meng, 2016), so we consider this simply to be an artefact of preservation.

We already stated that the morphology of the semilunate carpal of Alcomonavis resembles that of non-avian theropods like Deinonychus, being not fused to the metacarpus. However, the latter situation is also not present in other basal birds, including Jeholornis, Sapeornis or Confuciusornis. In fact, a fusion of carpal and metacarpal bones happens first time within Ornithothoraces, like in Archaeorhynchus (see e.g., Botehlo et al., 2014). However, even this partial fusion in Archaeorhynchus seem to be occur very late in ontogeny (Wang and Zhou, 2017). Despite the fact that we did not include any diagnostic character from the carpal region, we added a comparison to the description.

All measurements of the unguals based on the bony element. The sheath was not taken into account. Our statistical analyses shows that UI is significant smaller when compared to the ulna length and UII when compared to the length of manual phalanx II-1.

Concerning the diagnosis we don't see how the "strength" of the diagnosis depends on how bird-like or not the new taxon is – although no clear autapomorphic characters can currently be identified, the combination of characters seen in this wing skeleton is unique within both non-avialan and avialan theropods.

We have added comparisons with derived maniraptoran theropod dinosaurs.

2) Comments on specific morphologies

There are reservations concerning the interpretation of the humerus that need addressing.

Some of the alleged differences between the new fossil and previously described specimens are controversial. For example, one of the key features listed by the authors, the alleged presence of an impression for musculus pectoralis, cannot be established with certainty in some other Archaeopteryx fossils, such as the London specimen, in which the deltopectoral crest appears to be abraded in the corresponding region (see Figure 12 of the manuscript). This facet in the new fossil does not appear to be substantially different from that found in Mei long (Figure 14B), and certainly it is much less developed than the corresponding muscular impression in more crownward birds. Reviewer Oconnor noted that the attachment for the pectoralis on the distal end of the deltopectoral crest has not been widely described, at least not in non-ornithothoracine Early Cretaceous birds (it is mentioned in Martinavis; Walker et al., 2007), so taxa that have this feature should be listed together with citations (be careful, although it may appear present from a figure, if it wasn't described, its appearance in a photograph is possibly an artefact of preservation/photography, e.g., Sapeornis IVPP V13396 – Provini et al., 2008, or the holotype of Jeholornis. It is not mentioned in any of the papers cited, and in Oconnor's experience with basal Jehol birds is not common and so must have been independently evolved in Alcomavis, or perhaps only appears in the most ontogenetically mature specimens of Archaeopteryx (and potentially other Mesozoic birds).

The reviewer is right that the margin of the deltopectoral crest in London specimens is slightly abraded, increasing towards proximal. More distal the margin of the crest is still intact so that the presence of a facet can be ruled out (pers. obs. by OR). Furthermore, the crest is partially preserved also in the Thermopolis and Maxberg specimen (cast of the latter at the BSPG), none of which shows any sign of such a facet. To strengthen our case, we added a photo of the right humerus of the Thermopolis specimen, which shows the same situation to Figure 12. Furthermore, we added another photo showing the same region in Alcmonavis for comparison.

Based on the figure we provided in the previous version, it appears that Mei long possesses such a facet, but actually the photo shows the margin of the crest, which faces anterior (in many theropods the deltopectoral crest is not flat but slightly curved). Like in Ornitholestes (discussed in the manuscript) the margin is slightly expanded transversally. We changed the relevant sections in the manuscript accordingly.

We do not follow the argumentation that the appearance of undescribed structures in photographs are artefacts of preservation/photography. We believe that the illustration of anatomical details is part of a description, and it becomes even more essential, when the written part is rather brief (which is often the case, unfortunately). In this context, the documentation of preservational artefacts is also necessary to avoid misinterpretations. Nevertheless, the presence of the facet in Sapeornis is confirmed by pers. obs. of CF, and in Confuciusornis by pers. obs. of OR. The structure of the respective specimen is now illustrated in Figure 14. As it is not properly figured, we did not cite the holotype of Jeholornis, but a referred specimen, which was described by Lefèvre et al., (2014).

Although muscle attachments often become more conspicuous during ontogeny, the London specimen, which, as shown, does not have such as facet, is certainly not a young individual, and is not so much smaller than the new specimen. Thus, this specimen should show some sort of pronounced facet, which is not the case.

It is not accurate to estimate the length of the humerus as it is incomplete and to do so based on assumptions about the size of the internal tuberosity. Better to just state its minimal length (and not use this element for ratios and comparison). Which other taxa show elongate oval facet for the insertion of the m. pectoralis (make sure to check non-avians)?

We now added the actual length of the humerus as preserved. However, as fossils often suffered damage during the preservation or excavation process it is common in palaeontology to provide estimated length for respective elements. The estimation of the humerus length is based on ratios of complete elements known from other paravians. Nevertheless, as the correct length of the element is not known, the humerus length was not considered for statistical comparisons.

In the previous version, taxa with an oval facet were listed in the Discussion section. For comparison, we added taxa to the description and made a reference to the discussion.

With regards to the angle of the proximal portion of the humerus relative to the shaft the authors say to a lesser degree in non-avians but this seems a bit misleading – it is also strongly angled in a number of non-avian paravians, especially it seems in dromaeosaurids (Ji et al., 2000 – Sinornithosaurus; Microraptor, any number of references) but also present in Anchiornis (this is another reason why line drawing reconstructions of the forelimb in 4-5 key taxa would be really helpful). Reviewer Worthy suggested that the shaft angle (Subsection “Humerus”, etc) is perhaps increased by inclusion of the bicipital crest. The bicipital crest can be seen to join the shaft roughly co-level with the distal end of the deltopectoral crest, and thus the bicipital crest adds width to the ventral (here posteromedial) side of the shaft. Exclusion of the bicipital crest would lead to a lesser angle of the proximal section of the shaft relative to that more distally. Thus, using the ventral shaft margin as a proxy for shaft angle is not homologous between taxa that have a definite and enlarged bicipital crest and those that do not. A revision of definition of this metric would avoid this issue. Applies to the Discussion section as well.

The reviewer is correct that the proximal ends of the humeri of Anchiornis, Sinornithosaurus and Microraptor are angled, but less than 30 percent. We already provided these values for Dromaeosauridae and Anchiornis in the Discussion section of the previous version. We added this comparison also to the description.

The bicipital crest (internal tuberosity in non-avialan theropods) is usually included in this measurement, so the values should be comparable.

Ulna – since the humerus is not fully preserved, it is not accurate to compare an estimated length and then derive comparisons. Compare proximal end of ulna to Anchiornis. Although caution is mentioned often, the distal ulna seems pretty crushed flat but still described heavily. The features described are only present in Archaeorhynchus or also present in other stem birds? Label the anteromedially directed ridge in Figure 6.

The most detailed description of Anchiornis (including photos) was provided by Pei et al., 2017. Unfortunately, they describe only a minor expansion and a reduced olecranon process, which is a common morphology in Maniraptora. Nothing is stated concerning the cotylae. Neither the figures in Pei et al., 2017 nor personal photos of various Anchiornis specimens taken by CF can help to add more comparison.

The distal expansion of the ulna is more common in Avialae, so we added more comparisons. It is also present in Archaeopteryx, but is less pronounced and more symmetrically shaped.

We have only described features of the distal ulna that can be established despite the crushing of the bone.

The ridge was labelled in the figure.

Radius – compare longitudinal groove to that in enantiornithines.

We added a comparison into the description section and also discussed the presence of this character in the Discussion section.

Carpals – MC I seems to be broken and it was unlikely it was as short as described. MC II is thicker than I and III in Microraptor as well; also, Mc III is slightly offset proximally.

We noted in the description that Mc I is incomplete. How short this element was is everybody's guess; we only say that IF the position of the first phalanx indicates the distal end of this metacarpal (which is the case in several other similarly preserved theropod specimens), this element was very short. However, we did not use the possible length of this metacarpal for any formal analysis.

Unfortunately, we cannot evaluate this based on the scientific literature. Personal observations of several specimens by CF shows that the proximal end of MCI is strongly expanded in Microraptor (see also measurements provided by Hwang et al., 2002), while it becomes thinner distally. However, this is not the exact situation we found in Alcmonavis and Jeholornis. Generally, Mc II seems to be less robust in Microraptor than in Alcmonavis.

Because the metacarpus of Alcmonavis shows no signs of disarticulation, the offset position of Mc III most likely represents the actual situation. As already stated in the text, this morphology is present in many other Avialae, but also in several oviraptorids and dromaeosaurids (these comparisons were added to the text).

Digits – Digit I ungual largest in Anchiornis (see Pei et al., 2017);

The measurements provided for Anchiornis by Hu et al., 2009; Pei et al., 2017 and Guo et al., 2018 all show that the ungual of digit II is slightly longer than that of digit I. Personal observation of the holotype by CF confirms this.

digit II phalanx 1 is certainly robust but it appears strongly exaggerated as preserved, as two surfaces are exposed and flattened so it appears to be a single exposed surface of a hyper-robust phalanx. If this is taken into account, then the proximal articular surface of the phalanx is slightly narrower than the matching articular surface on metacarpal II. I do not see this described convexity on the plantar surface of phalanx II-2. The flexed morphology described in the penultimate phalanges of digits I and II is very subtle, at best, and should be described as so.

We gave this character a lot of thought and scrutiny of the actual fossil. Although the phalanx might give the impression of two surfaces being exposed, this does not seem to be the case; especially at the proximal end we are confident that this is the true width of the phalanx, whereas the distal end seems to show some internal twist in the phalanx, giving the impression that both palmar and lateral surface are exposed. Again, however, detailed investigation of the specimen indicate that this is not due to compression, but largely reflects the true morphology (although some compression is evident, as stated in the description), with the distal articular surface being slightly twisted. We added a sentence on the preservation in the description.

Concerning the flexed morphology of the penultimate phalanges, we noted in the description that these elements are "slightly bowed" or "slightly flexed". Furthermore, their morphology can be seen in the figure.

Unguals – it is stated (Discussion section) that the unguals are larger (proportionately) in Archaeopteryx but if the ungual (not including horny sheaths which may or may not be complete or in situ) is compared to the length of the penultimate phalanx, they are proportionately larger in Alcomavis. A table of ratios would be helpful to further help illustrate differences between this specimen and other urvogels and closely related taxa. Confuciusornis does not show a reduction in manual claws from I to III (II is smallest). Since this specimen is larger than other Archaeopteryx specimens, the pronounced muscle attachments could be explained by ontogeny (as suggested by the authors themselves somewhat), especially given that the difference between these taxa is pretty minor if the robustness of phalanx II-1 is taken out.

Statistical comparisons with Archaeopteryx reveal that UI of the Altmühl specimen is relatively smaller compared to the Ulna, while UII is relatively smaller compared to the preungular phalanx, both on a significant level. In contrast, UI is not relatively smaller compared to PI-1, because the latter element is relatively shortened, too.

We corrected the observation concerning the claw reduction in Confuciusornis.

As morphology and relative size of the relevant muscle attachments sites on humerus and radius do not really vary throughout the different specimens of Archaeopteryx, ontogenetic variation would imply drastic morphological changes in the final phase of ontogeny. However, as growth curves of tetrapods usually have a logarithmic, inverted exponential or logistic shape (Karkach, 2006), significant shape changes are rather expected in the phase of early ontogeny. Therefore, we consider the differences as interspecific.

3) Advisability of creating a new nomen

Even if the new specimen can be shown to differ from other Archaeopteryx fossils in some features, it is contentious, and some would say not advisable to assign a name to these fragmentary remains. The past years have seen an inflation in new and rather poorly diagnosed Archaeopteryx-like avians from the Solnhofen limestone, and in light of the very fragmentary preservation of the fossil, the manuscript would be scientifically more rigorous if the authors opted against naming a new taxon.

We disagree with the reviewer that there is "an inflation in new and rather poorly diagnosed Archaeopteryx-like avians from the Solnhofen limestone" – on the contrary, there rather is a problem with the assumption that all avialian theropods from the Solnhofen Archipelago (please see the manuscript for why we prefer not to talk about the "Solnhofen limestone") should be Archaeopteryx, as we now know that other, bird-like theropods were around in the Late Jurassic, as outlined by Rauhut et al., (2018) and in the current manuscript. Thus, in an attempt to better diagnose the genus Archaeopteryx, we realized that some specimens cannot be referred to this genus with certainty, and that one specimen, in particular (the Haarlem specimen), showed significant differences. As a species name was already available for this specimen, we thus proposed a new genus name, Ostromia (Foth and Rauhut 2017). However, this is the only non-archaeopterygid paravian theropod named from the Solnhofen Archipelago (and, as outlined, we only provided a new generic name for an existing species that can be shown to not fit in the genus Archaeopteryx), so this is hardly "an inflation in new and rather poorly diagnosed Archaeopteryx-like avians".

Regarding the new specimen, we approached its study with the same premise: First we tried to clarify if the specimen can be referred to Archaeopteryx with any certainty. Apart from the general proportions – which, as outlined in the manuscript, do also not differ significantly from several other paravians and basal avialans – there is no reason to refer this specimen to the genus Archaeopteryx. As our detailed study of the specimen revealed several differences to all other specimens of Archaeopteryx, the character combination shown by this specimen is unique in theropods in general, and a phylogenetic analysis found this specimen in an intermediate position between Archaeopteryx and later avialans, we opted to create a new taxon, after considering alternative options for almost a year.

Fragmentary preservation is not in itself an argument against naming new taxa, as demonstrated by literally thousands of fossil vertebrate species (especially in palaeoornithology it is not unusual that new species are based even on single bones) – what matters is if a new taxon can be adequately diagnosed. As the character combination demonstrated by the new specimen is unique within theropods (as also supported by the phylogenetic analysis), we argue that this is the case for Alcmonavis.

4) Phylogenetic analysis

The phylogenetic analysis is presented in rather a cursory way. It is unacceptable not to have support values in the presented phylogenetic trees. For example, it is misrepresentative to say the strict consensus is well resolved (subsection “Phylogenetic position of Alcmonavis”) yet not show the evidence that there is in fact very weak support for such resolution. At the very least Bremner support values should be given so that readers can see which of these clades has any semblance of support – one assumes that as no values are given and given the statement, that clade support is very low indeed – but readers deserve to be shown this, not have these data occluded. Ideally bootstrap values over 50% should be shown because anything less shows the clades are highly unstable and probably are not real in terms of evolutionary history. As presented, the placement of Alcmonavis is discussed as an afterthought in this section. Surely the focus should be on the new taxon and then use known taxa secondly to validate the tree.

We estimated bootstrap and Bremer support values for each node. Bootstrap values are generally low. For the equal-weight parsimony analysis, the phylogenetic position of Alcmonavis is supported by a Bremer support of 2, which is higher than for most other clades, including the phylogenetic position of Archaeopteryx (implied-weight parsimony leads to similar results). The support values for each node are shown for the strict consensus trees in the Supplementary file. The updated version of Figure 15. shows now the Bremer support values for the Avialae.

Given it is well established that missing data drive a taxon to the base of the clade in which it would otherwise reside (e.g., Sansom, 2015), some comment on the placement of the new taxon is warranted; why, what character evidence precludes that, for example, the new taxon really lies within Jelholornithidae or another slightly more derived position, than the node it appears in the tree? Character evidence for the critical nodes should be discussed, identifying the derived characters that place the new fossil in a more crownward position than Archaeopteryx.

In the Discussion section of the phylogeny, we listed the characters that support the more crownward position than Archaeopteryx.

We agree that it could be possible that the missing data places Alcmonavis closer to the base of Avialae. As the autapomorphies for the more derived position of Jeholornis, Sapeornis and co. are based mainly on forelimb characters, we believe that the phylogenetic position of Alcmonavis is robust.

Sansom, 2015.

The phylogenetic analyses should state which multistate characters were ordered (if any).

We provided a character list in the previous version. Ordered characters were labelled as Ordered.

Major clade names, including all Family names used in text, should be shown on the trees in the Supplementary file.

For the two reduced consensus trees we added the most important taxa names that are also mentioned in the text at the relevant nodes.

5) Improvement of figures

While the currently presented figures are excellent, some more are needed. Key to demonstrating the distinction from Archaeopteryx, are photos and or line drawings of the important elements of the new fossil in direct comparison with those of "Archaeopteryx" that explicitly show the differences in morphology and proportions. It is also desirable to add some figures, at least in Supplementary file, of key features of the new fossil directly compared with a meaningful selection of stem birds and non-avian maniraptorans to aid with comparison.

We added a figure comparing the outlines of humeri of several avialan theropods for comparison, plus a figure showing an actual specimen of Sapeornis in the taxonomic Discussion section. We furthermore added several basal paravian and basal avialan mani in the figure showing the manus of Archaeopteryx and changed part of the figure comparing the attachment area of the pectoralis muscle.

6) Tabular presentation of key data to facilitate comparison

Use of two more tables might facilitate comparisons better. For example, the section on Comments on the numbering and naming of avialan specimens from the Solnhofen Archipelago is useful, but quite long. Perhaps include the data in a table listing all 13 specimens, their current taxonomy, their numbers and correlative 'names' and institution, and include just a couple sentences to justify the new names for the eleventh to thirteenth specimens. If length is an issue then it could be in Supplementary file, but it is of more use to the audience in the main text, i.e. not lost/invisible in the Supplementary file.

As one of the peculiarities of Archaeopteryx is the use of the numbers of finds and colloquial names rather than the specimens numbers, even in the scientific literature, we think it to be important to clarify both the numbering and the colloquial names used. Tables with lists of specimens (up to the twelfth specimen) can be found in other publications (e.g. Rauhut et al., 2018), so simply adding three names to these lists seems rather redundant. It is furthermore important to point out that especially the numbering of specimens should be fixed, as current developments of taxonomic identifications or uncertainty make the numbering of Archaeopteryx specimens somewhat ambiguous (e.g. the twelfth specimen has been called eleventh specimen in the popular press in some cases, following the identification of the Haarlem specimen as a separate taxon).

Similarly, a table with comparative key measurements for the 12 Archaeopteryx specimens including ratios should also be provided, so demonstrating how similar this specimen is to other urvogel specimens.

Tables, showing the measurements of various are Archaeopteryx specimens are already available in Mayr et al., 2007 or Wellnhofer, 2009. However, we agree that for comparison it is useful to show those ratios, where Alcmonavis differs from Archaeopteryx, including T and p values. These values are shown in Table 2.

7) Discussion on possible flight mode of the fossil

This section is entirely speculative and not well substantiated. Various authors have detailed that flapping flight capabilities are unlikely for Archaeopteryx. The different morphology of the supracoracoideus muscle is just one argument contra flapping flight capabilities of Archaeopteryx, and the anatomy of Archaeopteryx does not show any evidence that this animal was capable of take-off through flapping flight (comparisons with extant pigeons certainly are not appropriate, as the latter have a completely different skeletal morphology than any Jurassic avians). Even though Voeten et al. recently proposed flapping flight capabilities of Archaeopteryx, their arguments based on the thickness of the bone cortex are far from compelling. Likewise, it is not clear why the presence of a tubercle for musculus biceps brachii on the radius would indicate flapping flight capabilities (so much the more as the presence/absence of this feature in other Archaeopteryx specimens is difficult to assess). Given the speculative nature of this interpretation, to include in the abstract a sentence like "Several modifications, especially in muscle attachments related to the downstroke of the wing, indicate a rapid adaptation of the forelimb for active flapping flight in the early evolution of birds" is not justified by the data (so much the more as the discussion is far less definitive).

We are aware that the flight mode of Archaeopteryx is still highly debated and many scientists favour a gliding flight as it costs less energy. Being familiar with the environment of the Solnhofen Archipelago, we would like to point out that the habitat of Archaeopteryx and Co. was an arid bush land with only a few small trees. Consequently, we find an arboreal life style with passive gliding flight for Archaeopteryx as often suggested rather unlikely.

The mentioned studies on the musculature of the pectoral girdle just show that the configuration was different in Archaeopteryx compared to modern volant birds (it was probably more similar to that of ostriches). If these differences really prevent flapping movements in Archaeopteryx is speculative (ostriches are still able to flap their forelimbs). 3D reconstructions of the shoulder girdle and forelimb of Archaeopteryx, based on a synchrotron scan of the well-preserved Thermopolis specimen, indicate that Archaeopteryx was actually able to perform a wing stroke (Carney, 2016; Heers and Carney, 2017). Due to different muscle configurations the flight stroke was certainly not as well “optimized” than in modern volant birds, but already possible. In the revised version, we cite a couple of recent studies supporting our case.

Showing the presence of the tuberculum bicipitale radii in three specimens of Archaeopteryx, we do not know why this character is controversial for Archaeopteryx. In other specimens the structure is simply not visible due to the orientation of the radius or overlap of matrix or other bones.

To avoid confusion, we never compared the flight performance of Archaeopteryx or Alcmonavis with that of pigeons. We cited the relevant study, because it shows, which muscles are active during different phases of a wing stroke in modern volant birds. Using this as an analogue, we hypothesize that the presence of tuberculum bicipitale radii indicates a stronger m. biceps brachii. We believe that this is related to active movements during flight, because for gliding the wing just need to act as passive airfoil and does not require such modifications in the elbow.

Given the great similarities of the general skeletal architecture of this fragmentary wing to the known Archaeopteryx remains (three unfused metacarpals with large ungual phalanges, similar phalangeal proportions etc.), it is highly unlikely that substantial differences in flight mode to Archaeopteryx existed. The speculative nature of this discussion and interpretation should be addressed.

We already made a very detailed comparison to Archaeopteryx regarding the enlarged muscle attachment sites present in Alcmonavis. Assuming that these areas are associated with stronger muscles, an improvement of the flight ability is likely. Please note, that we never claimed that the flight mode of Alcmonavis is substantially different from Archaeopteryx (we apologize if the term “rapid” may has caused this impression).

Applying the logic of the argument, no improvement for the flight ability of Confuciusornis is to be expected either, as it has three unfused metacarpals with large ungual phalanges, and more or less similar phalangeal proportions.

8) Biogeographic interpretations.

These speculations towards the end of the manuscript should be deleted altogether.

We deleted the Conclusions section.

Author details

1. Oliver WM Rauhut

   1. Staatliche naturwissenschaftliche Sammlungen Bayerns (SNSB), Bayerische Staatssammlung für Paläontologie und Geologie, München, Germany
   2. Department for Earth and Environmental Sciences, Palaeontology and Geobiology, Ludwig-Maximilians-Universität, München, Germany
   3. GeoBioCenter, Ludwig-Maximilians-Universität, München, Germany

   Contribution

   Conceptualization, Data curation, Software, Funding acquisition, Validation, Investigation, Visualization, Methodology, Writing—original draft, Writing—review and editing

   Contributed equally with

   Christian Foth

   For correspondence

   o.rauhut@lrz.uni-muenchen.de

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-3958-603X

2. Helmut Tischlinger

   Tannenweg 16, Stammham, Germany

   Contribution

   Resources, Visualization, Methodology, Writing—review and editing

   Competing interests

   No competing interests declared

3. Christian Foth

   Department of Geosciences, Université de Fribourg, Fribourg, Switzerland

   Contribution

   Formal analysis, Funding acquisition, Validation, Investigation, Methodology, Writing—original draft, Writing—review and editing

   Contributed equally with

   Oliver WM Rauhut

   Competing interests

   No competing interests declared

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