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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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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