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

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.

Evaluating the accuracy of methods for detecting correlated rates of molecular and morphological evolution

“…For instance, the tuatara ( Sphenodon punctatus ) diverged from all other squamates ∼220 million years ago (Ma) and is the only extant member of the order Rhynchocephalia (Herrera-Flores et al 2017). Despite the inference that the long lineage leading to the tuatara has experienced little morphological evolution (Herrera-Flores et al 2017; Simões et al 2022b), the mitochondrial control region of the tuatara has been estimated to evolve at a remarkably high rate (Hay et al 2008; Subramanian et al 2009). Earlier work on morphologically conserved horseshoe crabs showed only modest reductions in the rate of mitochondrial evolution compared with scorpions and brine shrimp (Avise et al 1994).…”

First rhynchocephalian (Reptilia, Lepidosauria) from the Cretaceous–Paleogene of India

“…Additionally, GSI/SR/PAL-NR-0101 appears to be from an immature individual, based on its small size, the minor attritional wear of its teeth, and the thin or absent secondary bone on the dentary and around the bases of its teeth (Robinson, 1976; Fraser, 1988). Furthermore, because the hatchling series typically wears down with age and, in some cases, is completely lost in mature individuals (e.g., Robinson, 1976; Fraser, 1988; Simões et al, 2022), comparisons of these teeth are mostly restricted to immature individuals. Lastly, relative to the successional and additional teeth in most rhynchocephalians, the hatchling teeth are simple structures (e.g., labiolingually compressed cones that alternate in size) and provide few distinguishing characters for comparative anatomy and taxonomy.…”

“…Other putative Tiki records include undescribed and unfigured sphenodontian teeth (Datta et al, 2004). Unambiguous rhynchocephalian specimens from the Lower–Middle Jurassic Kota Formation ( Rebbanosaurus jaini and Godavarisaurus latee fi; Evans et al, 2001) are more abundant, better preserved, and complete enough to incorporate into phylogenetic analyses, where they are commonly recovered outside Eusphenodontia: the clade that includes clevosaurs, pleurosaurs, saphaeosaurs, eilenodontines, and sphenodontines (e.g., Apesteguía et al, 2012; Herrera-Flores et al, 2018; Romo de Vivar et al, 2020; Simões et al, 2020, 2022; DeMar et al, in press). A possible third rhynchocephalian from the Kota Formation, Bharatagama rebbanensis , was originally considered an acrodontan iguanian (Evans et al, 2002), but it was recently hypothesized to be a rhynchocephalian (Conrad, 2018).…”

Revealing the rise of a living fossil menagerie

“…Another attribute, the association of persistent morphological stasis with defensive mechanisms against predators and pathogens (Wieland, 1924;Guan et al, 2016), has contributed to recent explosive growth in the study of Ginkgo's protective secondary metabolites and their genetic foundations (Jacobs and Browner, 2000;Singh et al, 2019;. Additional living fossils and their attributes have been prominent for problems entailing rates of evolution and conflicting morphological and genetic signals, for instance in tadpole shrimps (Luchetti et al, 2021), fishes (Clarke et al, 2016, Giles et al, 2017, and Sphenodon, the tuatara (Subramanian et al, 2009;Simões et al, 2022). Lastly, we point to examples in which living fossils have contributed to the early understanding of embryology (Hopwood, 2005) and modern evo-devo (Braasch et al, 2015;Bayramov et al, 2018;Watkins, 2021).…”