An exceptionally preserved Sphenodon-like sphenodontian reveals deep time conservation of the tuatara skeleton and ontogeny

Tiago R. Simões,Stephanie E. Pierce,Grace Kinney-Broderick

doi:10.1038/s42003-022-03144-y

Tiago R. Simões, Stephanie E. Pierce + Show 1 more

Open Access

https://doi.org/10.1038/s42003-022-03144-y

Copy DOI

Abstract

Sphenodontian reptiles are an extremely old evolutionary lineage forming the closest relatives to squamates (lizards and snakes) and were globally distributed and more diverse than squamates during the first half of their evolutionary history. However, the majority of their fossils are highly fragmentary, especially within sphenodontines—the group including its single surviving species, Sphenodon punctatus (the tuatara of New Zealand)—thus severely hampering our understanding on the origins of the tuatara. Here, we present a new sphenodontian species from the Early Jurassic of North America (Arizona, USA) represented by a nearly complete articulated skeleton and dozens of upper and lower jaws forming the most complete ontogenetic series in the sphenodontian fossil record. CT-scanning provides plentitude of data that unambiguously place this new taxon as one of the earliest evolving and oldest known sphenodontines. Comparisons with Sphenodon reveal that fundamental patterns of mandibular ontogeny and skeletal architecture in Sphenodon may have originated at least ~190Mya. In combination with recent findings, our results suggest strong morphological stability and an ancient origin of the modern tuatara morphotype.

Highlights

Sphenodontian reptiles are an extremely old evolutionary lineage forming the closest relatives to squamates and were globally distributed and more diverse than squamates during the first half of their evolutionary history
The higher taxonomic diversity of sphenodontians compared to squamates during the first half of their evolutionary history is contrasted by the quality of their fossil record
A considerable portion of sphenodontian fossil diversity is represented only by fragmentary specimens of little systematic value, such as isolated jaws and teeth—e.g., refs. 1,11–18. This pattern has long posed a severe limitation to construct a robust hypothesis of sphenodontian phylogenetic relationships and evolutionary patterns— see discussions in refs. 3,19—such as reconstructing the morphological changes leading to the considerable diversity of sphenodontian cranial morphotypes in the fossil record and the origin of the modern tuatara skull[20]

Summary

Introduction

Sphenodontian reptiles are an extremely old evolutionary lineage forming the closest relatives to squamates (lizards and snakes) and were globally distributed and more diverse than squamates during the first half of their evolutionary history. Sphenodontians sustained a higher taxonomic diversity compared to squamates during the Triassic and Jurassic, being surpassed by the latter as the most species-rich group of lepidosaur only in the Cretaceous[4] This discrepancy in species richness between squamates and sphenodontians only increased during the Late Cretaceous and Cenozoic[9], culminating in the current 11,000+ species of extant squamates and only one sphenodontian species, Sphenodon punctatus (the tuatara of New Zealand)[10]. 21)—the branch of the sphenodontian tree including Sphenodon, and which is estimated to have originated during the Late Triassic at ~206 Mya[19] During this 200+ Myr evolutionary history, all fossils robustly assigned to sphenodontines were comprised of jaw elements and isolated vertebrae, including all material referable to taxa such as Cynosphenodon, Kawasphenodon, Sphenovipera, among other unnamed Mesozoic specimens[12,14,15,22]. Using high-resolution micro-CT scanning and distinct phylogenetic methods, we find overwhelming support for the new species as one of the earliest evolving and oldest members of the tuatara lineage, revealing insights into the ontogeny and deep time evolutionary origins of the tuatara skull

Methods

Results

Conclusion