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| Marmorerpeton wakei Jones, Benson, Skutschas, Hill, Panciroli, Schmitt, Walsh & Evans, 2022 Artwork: Brennan Stokkermans |
Significance:
Little is known about stem-lineage salamanders, limiting understanding of their early evolution and of the origins of modern amphibian diversity. We report new, three-dimensionally preserved skeletons of the stem-salamander Marmorerpeton, from 166 million-year-old rocks in Scotland, documenting many phylogenetically informative anatomical traits. High resolution computed tomography (CT) scans reveal unprecedented three-dimensional anatomical detail, illuminating anatomical changes during early salamander evolution. Phylogenetic analysis provides evidence for an anatomically diverse radiation of early stem salamanders distributed across Eurasia during the Middle Jurassic and Early Cretaceous. Our findings highlight the morphological variety of stem-salamanders, undermining the use of single exemplars (e.g., Karaurus; the “Archaeopteryx” of salamanders) to represent early evolutionary transitions.
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.
Marmorerpeton wakei
Marc E. H. Jones, Roger B. J. Benson, Pavel Skutschas, Lucy Hill, Elsa Panciroli, Armin D. Schmitt, Stig A. Walsh and Susan E. Evans. 2022. Middle Jurassic Fossils document An early Stage in Salamander Evolution. PNAS. 119 (30) e2114100119. DOI: 10.1073/pnas.2114100119
Marmorerpeton wakei (mar-more-ER-pet-on WAY-kee),
