Location of the studied regions and lithological columns. (a–b) Permian-Triassic paleogeographic map of North China and the studied successions (stars) and outcrops (points). Base map of a is modified from Sun et al. (2012). (c) Depositional facies and detailed distribution of trace fossils in three main successions.

Ichnofossils from the Heshanggou Formation of North China. (a) Shallow tiers Kouphichnium and Helminthoidichnites (He) are crosscut by immediate tier Palaeophycus (Pa). (b) Skolithos cf. serratus with faint oblique striations. (c) Y-shaped Psilonichnus isp. (d) Downward unbranched and tapered rhizocretion. (e) Shallow tiers Beaconites coronus, arrows show tightly stacked arcuate meniscus. (f) Camborygma isp. shows enlarged terminal chamber and possible transverse scratches (arrows). (g) Gordia isp. with darker and finer infills. (h) High density Palaeophycus tubularis preserved on the sole of thick sandstone. (i) Inclined Planolites beverleyensis within siltstone. (j) Taenidium barretti (arrows) pass through the rippled surface. (k) Horizontal Camborygma, the outer surface is intertwined with tinny root traces (arrows). (l) Biserial Diplichnites gouldi. (m-n) Internode cross-section of Neocalamites stem and micro-CT structure, showing the clear ribs and grooves. (o-p) Large ?Beaconites on top of rippled sandstone, rectangle in (p) shows meniscus-like portions comprising of gritty infillings. Scale bar of (d, f, h, o-p) are 40 mm, the rest are all 20 mm.

Vertebrates from the Heshanggou Formation of North China. (a–b) Skull elements of Xilousuchus sapingensis and drawings. (c) Fugusuchus hejiapanensis. (d) Eumetabolodon bathycephalus. (e–f) Shaanbeikannemeyeria xilouensis and drawing. (g) Pentaedrusaurus ordosianus. (h) Hazhenia concava, (i) Dashed line shows the boundary between the Liujiagou and Heshanggou formations. Arrow displays fossil horizon of the inserted picture at the base of the Heshanggou Formation. (b, c, f) are from Li et al. (2008). Scale bars are 40 mm.

Ichnofossil criteria in North China and global terrestrial ecosystem changes from latest Permian to earliest Middle Triassic. Geochronological timescale is based on the latest version of the International Chronostratigraphic Chart (https://stratigraphy.org/). Plant richness from Shu et al. (2022). Numbers in Burrow size column represent the mean trace fossil sizes from the investigated interval. Drainage conditions of paleosols are inferred from preservation of rhizoliths and their relative depths. Ranges of microbially induced sedimentary structures (MISS) are from Chu et al. (2017). Atmospheric CO2 curves are modified from Joachimski et al. (2022). Abbreviations: Ind. = Induan; G. = Griesbachian; D. = Dienerian.

Lithology, sedimentology and ichnology of the studied sections.

Early Triassic geochronological scale in North China.

The Geo-magnetic polarity timescale (GPTS) is from Hounslow and Muttoni (2010) and Hounslow and Balabanov (2018). Magnetostratigraphy and the CA-ID-TIMS age of 1 are from Guo et al. (2022). Detrital zircon U-Pb ages are based on: 2 = tuffaceous claystones sampled from the uppermost part of the Sunjiagou Formation (Lu et al., 2022), 2 = around middle part of the Liujiagou Formation (Zhu et al., 2019), 3 = sandstone from the base of the Heshanggou Formation and tuffs from the bottom Ermaying Formation (EMY; Zhu et al., 2022). G. = Griesbachian, D. = Dienerian.

Brief description of lithofacies, facies associations (FA) and trace fossils at the studied sections.

Ichnofauna from the Liujiagou Formation at Liulin section.

(a-b) Ovate to elongated almond-shaped Lockeia siliquaria roughly arranged along a similar direction, fossils were preserved as convex hypichia in sandstone with desiccation cracks (arrow in a). (c-dd) Simple unbranched surficial burrows with sharp turns (arrows in d), self-overcrossing occasionally seen, traces are identified as Gordia indianaensis and preserved in rippled sandstone (arrow in c). (e) Vertical Skolithos linearis in sandstone. (f) Vertical root traces, showing inconsistent burrow sizes and tiny bifurcations (arrows). (g) Kouphichnium didactylum with simple foot imprints and a pusher imprint.

Ichnofauna from the Heshanggou Formation.

(a) Y-shaped Psilonichnus cf. upsilon, the branch portion displays similar size with the main tube. FA3 costal mudplain facies at Shichuanhe section. (b) Diplichnites gouldi, evenly spaced imprints are comma or elongated and incline to the midline. Fossils preserved in FA3 at Shichuanhe section. (c) High-density simple vertical Skolithos linearis at Dayunlin section. (d) Actively filled Planolites beverleyensis (Pl) pass through surficial Helminthoidichnites tenuis (He), which in turn, crosscut by Skolithos (Sk). Dayunlin section. (e) Horizontal Taenidium serpentinum, distance between menisci (arrow) is roughly equal to the burrow width. Dayunlin section. (f) Crowded Palaeophycus tubularis preserved on the sole of thick sandstone. Liulin section. (g) Horizontal to inclined, straight to curve, lined Beaconites coronus with moderately arcuate menisci. Liulin section.

Large burrows from the Heshanggou Formation.

(a-b) Horizontal but inclined unlined burrow with elliptical cross sections, branching and terminal chamber were no found. Possible lateral scratches could be seen on the side (Arrows in b). trace fossil is informally assigned to undesigned burrow. Liuin section. (c-e) LargeBeaconites and other relatively small traces (e.g., Palaeophycus) on the rippled siltstone (ripple marks can be seen at the top-right of c). Burrows display meniscus-like portions, comprising of gritty infillings (Arrow in e), akin to Beaconites reported from breccia facies in the Devonian (Brück, 1987). Mafang outcrops. (f-h) Vertical to subhorizontal burrows with circular cross sections, filled with calcareous sand. Trace fossils are similar to crayfish burrows (Camborygma), although branches, terminal chamber or scratch marks were no found. Hongyatou outcrop. (i-j) In situ preserved Neocalamites stems in the lower part of the Heshanggou Formation at Shichuanhe section and Hongyatou outcrop, respectively. Typically, all the stems are associated with Camborygma and other burrows and trackways.

Root traces from the Heshanggou Formation in studied sections and outcrops.

(a-h) Vertical branched rhizolith with green or pale purple haloes, showing irregular burrow sizes and lateral small rootlets (d, f), Yiyang section. (i) Herringbone-like branched rhizotubules, Yiyang section. (j-k) Vertical tapered unbranched or bifurcated rhiozocretions, Yiyang section. (l) Simple vertical branched roots, Tuncun outcrops. (m) Horizontal small rootlets, Yiyang section. Classification of root traces are based on Klappa (1980) and Kraus and Hasiotis (2006). All scale bars are 20 mm.

Size variation of all ichnogenera from there sections.

EMY = Ermaying Formation.

Size variation of sing ichnogenus from Liulin and Dayulin sections during the Spathian.

EMY = Ermaying Formation.

Reconstruction of the Spathian (Heshanggou Formation) coastal mudplain to alluvial ecosystem in North China.

Plant communities in the coastal mudplain and alluvial facies are depauperated, dominated by Neocalamites and Pleuromeia, and only diversified at the top of the Heshanggou Formation (late Spathian; Shu et al., 2022). Reoccurrence of plants and tetrapods, coupled with diverse invertebrate, including limuloid, crayfish, spinicaudatan, and insect, etc., reveal reorganization of a relatively complex ecosystem in riverain regions during the Spathian.

List of ichnogenera from the Shichuanhe section, Ichnodisparity designations and Tiering level.

List of ichnogenera from the Heshanggou Formation of the Dayulin section, Ichnodisparity designations and tiering level.

List of ichnogenera from the Liulin section, Ichnodisparity designations and tiering level.

Possible trace producers on land.