New sphenodontian (Reptilia: Lepidosauria) from a novel Late Triassic paleobiota in western North America sheds light on the earliest radiation of herbivorous lepidosaurs

Abstract

Herbivory is a common ecological function among extant lepidosaurs, but little is known about the origin of this feeding strategy within Lepidosauria. Here we describe a sphenodontian (Lepidosauria) from the Late Triassic of western North America, Trullidens purgatorii n. gen. n. sp., that reveals new aspects of the earliest radiation of herbivorous lepidosaurs. This taxon is represented by an isolated lower jaw with robust structure bearing transversely widened dentition and extensive wear facets, suggesting a masticatory apparatus specialized for herbivory. An unusual ‘incisor-like’ tooth is present at the anterior end of the jaw; a unique feature among lepidosaurs, this tooth is convergent with the incisors of extant rodents and lagomorphs. Phylogenetic analyses support the placement of this taxon within opisthodontian sphenodontians, a group sharing derived cranio-dental morphologies specialized for herbivory. The new taxon was recovered in a recently discovered and unnamed series of Upper Triassic strata in southeastern Colorado, USA, exposed in Canyons incised by the Purgatoire River and its tributaries. These strata comprise a dominantly red-bed sequence of conglomerates, sandstones, and siltstones deposited in a fluvio-lacustrine setting, preserving a Late Triassic biota of invertebrate and vertebrate ichnofossils, plant macrofossils, bony fish, temnospondyl amphibians, and reptiles. We use aetosaur osteoderms as biostratigraphic links to the nearby Chinle Formation of Arizona, USA, establishing a middle Norian age for these strata. The presence of an opisthodontian from western equatorial Pangaea in the Norian Stage reveals a near-global radiation of this clade across the Pangaean supercontinent during the Late Triassic. UUID: http://zoobank.org/A737c03f-863a-488e-a860-5cc914548774.

A new Middle Triassic (Ladinian) trilophosaurid stem-archosaur from Germany increases diversity and temporal range of this clade

“…For the assessment of the phylogenetic position of the new taxon, we scored its features into the character-taxon matrix published by Chambli-Trowell et al [20], which is a modified version of that used by Kligman et al [10], itself an iteration of the matrix compiled by Pritchard et al [35]. The modified matrix used here (see electronic supplementary material) consists of 36 taxa and 231 parsimony-informative characters.…”

“…[ 20 ], which is a modified version of that used by Kligman et al . [ 10 ], itself an iteration of the matrix compiled by Pritchard et al . [ 35 ].…”

“…The two cusps are subequal in size in T. phasmalophos [ 31 ]. Teeth of Trilophosaurus have three (in T. buettneri , T. dornorum , and T. jacobsi ) or two (in T. phasmalophos ) large, labiolingually aligned cusps, which are linked by transverse occlusal ridges [ 10 , 27 , 31 ]. Tricuspisaurus thomasi , Variodens inopinatus , and an indeterminate Middle Triassic trilophosaurid from Nova Scotia each have teeth with three labiolingually positioned cusps [ 20 , 33 ].…”

“…In the late Middle and Late Triassic, dicynodonts were represented only by large-bodied kannemeyeriiforms [ 3 ], and hyperodapedontine rhynchosaurs and traversodontid cynodonts became the most common herbivores in Gondwana [ 8 ]. During or toward the end of the Late Triassic, most of these groups became extinct, and some rhynchocephalians [ 9 , 10 ] as well as small- to medium-sized synapsids including tritylodontid cynodonts [ 11 ] and haramiyidan mammaliaforms [ 12 ] evolved craniodental features for herbivory.…”

“…Since the work in the 1940s, many additional specimens of Trilophosaurus have been recovered from the Dockum Group of Texas and the Chinle Formation of Arizona and New Mexico [ 10 , 14 , 15 ]. Two additional species of this genus were recognized on the basis of diagnostic dental features: T. jacobsi , from the Placerias Quarry in the Norian-age lower Chinle Formation near St Johns in Arizona [ 14 , 26 ], and T. dornorum , from the stratigraphically younger Norian-age Sonsela Member of the Chinle Formation of Petrified Forest National Park in Arizona [ 27 ].…”