Middle Jurassic fossils document an early stage in salamander evolution

Abstract

Salamanders are an important group of living amphibians and model organisms for understanding locomotion, development, regeneration, feeding, and toxicity in tetrapods. However, their origin and early radiation remain poorly understood, with early fossil stem-salamanders so far represented by larval or incompletely known taxa. This poor record also limits understanding of the origin of Lissamphibia (i.e., frogs, salamanders, and caecilians). We report fossils from the Middle Jurassic of Scotland representing almost the entire skeleton of the enigmatic stem-salamander Marmorerpeton . We use computed tomography to visualize high-resolution three-dimensional anatomy, describing morphologies that were poorly characterized in early salamanders, including the braincase, scapulocoracoid, and lower jaw. We use these data in the context of a phylogenetic analysis intended to resolve the relationships of early and stem-salamanders, including representation of important outgroups alongside data from high-resolution imaging of extant species. Marmorerpeton is united with Karaurus , Kokartus , and others from the Middle Jurassic–Lower Cretaceous of Asia, providing evidence for an early radiation of robustly built neotenous stem-salamanders. These taxa display morphological specializations similar to the extant cryptobranchid “giant” salamanders. Our analysis also demonstrates stem-group affinities for a larger sample of Jurassic species than previously recognized, highlighting an unappreciated diversity of stem-salamanders and cautioning against the use of single species (e.g., Karaurus) as exemplars for stem-salamander anatomy. These phylogenetic findings, combined with knowledge of the near-complete skeletal anatomy of Mamorerpeton, advance our understanding of evolutionary changes on the salamander stem-lineage and provide important data on early salamanders and the origins of Batrachia and Lissamphibia.

Biogeographic history of Palearctic caudates revealed by a critical appraisal of their fossil record quality and spatio-temporal distribution

“…Based on these considerations, we agree with Jones et al . [ 30 ] that Valdotriton belongs to the stem of Salamandroidea and we suggest that Kiyatriton and Apricosiren might also occupy a similar phylogenetic position.…”

“…Several genera and species are known from the Middle/Late Jurassic boundary of China, including the stem-urodeles Chunerpeton tianyiensis [ 30 , 37 , 38 ] and Neimengtriton daohugouensis [ 30 , 39 , 40 ]. Beiyanerpeton jianpingensis [ 41 ] and Qinglongtriton gangouensis [ 42 ] were recovered in an early-branching position within Salamandroidea by Jia & Gao [ 43 ], whereas Linglongtriton daxishanensis [ 43 ] and Pangerpeton sinensis [ 44 ] were placed on the stem-hynobiid lineage [ 43 ].…”

“…However, all these taxa were identified as stem-urodeles in the recent analysis of Jones et al . [ 30 ]. In general, Jurassic taxa from China are mostly known from slabs, often as skeletal impressions, making it difficult to evaluate their relationships with other fossil species that are only known from isolated, three-dimensional remains (but see [ 44 ] and [ 43 ] for detailed studies of their possible phylogenetic relationships).…”

“…For example, most phylogenetic analyses that incorporate fossil taxa only include species known from essentially complete skeletons (e.g. [ 30 , 115 , 187 ]). Furthermore, taphonomic biases influence the preservation of disarticulated remains, with the largest and most robust elements more often represented than other parts of the skeleton in palaeontological collections [ 95 , 153 ].…”

“…Stem salamandroids are rare in the Palearctic fossil record. The only taxon supported as a stem salamandroid by phylogenetic analyses [ 30 ] is Valdotriton from the Early Cretaceous Spain, and a possible stem salamandroid position is herein suggested for the contemporaneous UK genus Apricosiren , as well as Kiyatriton from the Middle Jurassic–Early Cretaceous of Russia, although these taxa are only known from disarticulated elements. As such, the scarce record prevents any possible consideration on the Palearctic biogeographic history of the clade Salamandroidea.…”