A tiny new Middle Triassic stem-lepidosauromorph from Germany: implications for the early evolution of lepidosauromorphs and the Vellberg fauna

www.nature.com/scientificreports OPEN A tiny new Middle Triassic stemlepidosauromorph from Germany: implications for the early evolution of lepidosauromorphs and the Vellberg fauna Gabriela Sobral1*, Tiago R. Simões2 & Rainer R. Schoch1 The Middle Triassic was a time of major changes in tetrapod faunas worldwide, but the fossil record for this interval is largely obscure for terrestrial faunas. This poses a severe limitation to our understanding on the earliest stages of diversification of lineages representing some of the most diverse faunas in the world today, such as lepidosauromorphs (e.g., lizards and tuataras). Here, we report a tiny new lepidosauromorph from the Middle Triassic from Vellberg (Germany), which combines a mosaic of features from both early evolving squamates and rhynchocephalians, such as the simultaneous occurrence of a splenial bone and partial development of acrodonty. Phylogenetic analyses applying different optimality criteria, and combined morphological and molecular data, consistently recover the new taxon as a stem-lepidosauromorph, implying stem-lepidosauromorph species coinhabited areas comprising today’s central Europe at the same time as the earliest known rhynchocephalians and squamates. It further demonstrates a more complex evolutionary scenario for dental evolution in early lepidosauromorphs, with independent acquisitions of acrodonty early in their evolutionary history. The small size of most terrestrial vertebrates from Vellberg is conspicuous, contrasting to younger Triassic deposits worldwide, but comparable to Early Triassic faunas, suggesting a potential long-lasting Lilliput effect in this fauna. The Middle Triassic was a time of major changes in terrestrial tetrapod faunas on a global scale. Despite the most recent divergence time estimates based on molecular and morphological data indicating the origin of most diapsid lineages during the Permian1,2, several lineages of diapsids are recognized in the fossil record for the first time only in the Middle Triassic, such as squamates, rhynchocephalians, tanystropheids, and drepanosaurs2–4. Other lineages that first appeared in the fossil record in the Early Triassic or Late Permian considerably increase in abundance and taxonomic representation during the Middle Triassic, such as ichthyosauromorphs, sauropterygians, and turtles5–7. Additionally, there is a generally poor vertebrate fossil record for the Early Triassic, partially owing to the relatively long period of recovery from the Permian-Triassic mass extinction8,9. All of those factors combined make the Middle Triassic of fundamental importance to understand the recovery of global faunas after the greatest mass extinction of the Phanerozoic, and the initial diversification of characteristic components of the modern vertebrae biota, such as archosauromorph and lepidosauromorph reptiles. Among the tetrapod lineages that began diversifying in the fossil record at least by the Middle Triassic, lepidosauromorphs are one of the most diverse, representing one of the largest lineages of diapsid reptiles today (alongside birds), with ca. 10,500 described species10,11. The earliest putative stem-lepidosauromorphs first appear in the fossil record in the Late Permian—e.g. Palaeagama12,13—whereas crown lepidosauromorphs are first recognized in the Middle Triassic, represented by the oldest known squamate2 and rhynchocephalians14,15. However, the Triassic record of lepidosauromorphs is, in general, still extremely poor when compared to other lineages of diapsid reptiles4–7. Currently, only a few Triassic localities provide diagnostic lepidosauromorph taxa, including: 1 Staatliches Museum für Naturkunde Stuttgart, Rosenstein 1, D-70191, Stuttgart, Germany. 2Department of Organismic and Evolutionary Biology, Museum of Comparative Zoology, Harvard University, Cambridge, MA, 02138, USA. *email: Scientific Reports | (2020) 10:2273 | https://doi.org/10.1038/s41598-020-58883-x 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. Picture (a) and line drawing (b) of the holotype material of Vellbergia bartholomaei. Scale bars approximately 5 mm. Abbreviations: af articulation facet, de dentary, ds dental shelf, fr frontal, mx maxilla, na nasal, de dentary, ju jugal, pa parietal, pm premaxilla, po postorbital, pof postfrontal, prf prefrontal, q quadrate, qj quadratojugal, sp splenial. the Czatkowice quarry in Poland (Early Triassic); the Vellberg locality in Germany (Middle Triassic); the Dont Formation in the Italian dolomites (Middle Triassic); the Lossiemouth Sandstone Formation in Northeast Scotland (Middle-Late Triassic); the Santa Maria Formation (Linha Bernardino locality) and Caturrita Formation (Linha São Luiz locality), in Southern Brazil (Late Triassic); and the Tytherington and Cromhall quarries in South West Britain (Late Triassic)14,16–21. Most of those localities bear only one or two valid lepidosauromorph species, with the most speciose of those currently represented by the collection of quarries in Southwest Britain, which include remains from six rhynchocephalian species: Diphydontosaurus avonis, Planocephalosaurus robinsonae, Clevosaurus hudsoni, Clevosaurus minor, Clevosaurus cambrica, Clevosaurus sectumsemper22–26. Therefore, most of the early fossil record of lepidosauromorphs remains largely unknown. Here, we report a new partially articulated fossil lepidosauromorph from the Middle Triassic deposits of Vellberg in Southern Germany. The new species described here falls into the smallest size cluster so far collected from the site, and likely represents the first juvenile individual from that locality. This new taxon depicts a mosaic of features that are generally observed in both early evolving rhynchocephalians and squamates, suggesting stem-lepidosauromorphs may have survived up to the Middle Triassic. It further demonstrates a more complex scenario on dental evolutionary patterns among early lepidosaurs. Finally, this and other findings from Vellberg indicate this is one of the richest sites in the world to understand early lepidosauromorph evolution. Together Scientific Reports | (2020) 10:2273 | https://doi.org/10.1038/s41598-020-58883-x 2 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 2. CT images of Vellbergia. Cross section trough (a) the left maxilla and (b) right dentary with grey miniatures below showing position of the slices. 3D renderings of the (c) postorbital (with CT slice) and (d) squamosal. Scale bars: (a and b) 0.7 mm, (c) 1.5 mm and 0.7 mm, (d) 0.95 mm. Abbreviations: d dentary, ds dental shelf, fo foramen, or orbital rim, pp posterior process, rd ridge, sp splenial, t tooth. with other taxa from this locality, Vellberg may also hold fundamental clues to understand the evolution of body size subsequent to the Permian-Triassic mass extinction. Results Systematic palaeontology Lepidosauromorpha Gauthier, 1984 Vellbergia n. g. Vell (...truncated)