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

Abstract

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 making of calibration sausage exemplified by recalibrating the transcriptomic timetree of jawed vertebrates

“…5, 6; also, after a few changes to the dataset, Garberoglio et al, 2019: fig. S2; Sobral et al, 2020: fig. S9, S10), but not in their parsimony analyses of morphological data without or with implied weights (ext.…”

“…2019: fig. S3, and Sobral et al, 2020: fig. S7, S8), where it came out as a stem-sauropsid; the question was unresolved in their Bayesian tip-dating or tip-and-node dating analyses of combined data (ext.…”

“…The minimum age of this calibration, given as 238 Ma, has to be slightly revised to 244 Ma (both in the Middle Triassic) based on Megachirella , the oldest known unambiguous stem-pan-squamate (Renesto and Bernardi, 2013; Simões et al, 2018: table S2, 2020; Garberoglio et al, 2019; Sobral et al, 2020), which is older than the oldest known rhynchocephalian (238–240 Ma: Jones et al, 2013).…”

“…The Early Triassic Sophineta , a large collection of isolated bones, may be a stem-pan-squamate or a stem-pan-lepidosaur (Evans and Borsuk-Białynicka, 2009a; Simões et al, 2018, 2020; Garberoglio et al, 2019; Sobral et al, 2020). The text of Sobral et al (2020) makes clear that Vellbergia , another such animal, is younger than Megachirella , despite being shown as older in Sobral et al (2020: fig. 4).…”

Bite force data suggests relationship between acrodont tooth implantation and strong bite force

“…The transition from pleurodont to acrodont tooth implantation occurred independently several times within Lepidosauria (Acrodonta, Trogonophidae, Rhynchocephalia) and it is even seen in stem lepidosauromorphs (Sobral, Simões & Schoch, 2020), but only in Acrodonta and Rhynchocephalia is the tooth-bone boundary difficult to detect upon initial inspection. Stem Acrodonta do not possess the extensive bone deposition that accompanies ankylosis, nor do they possess the apical tooth implantation that is seen in the crown group, although the roots of the teeth are much shorter than most other iguanian lizards and possess a relatively increased degree of ankylosis (Simões et al, 2015).…”