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

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 supports
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.