NORTH AMERICA’S OLDEST HERBIVOROUS DINOSAURS

Recent microvertebrate collecting in the Upper Triassic Chinle Group of Texas, New Mexico and Arizona demonstrates that herbivorous dinosaurs, particularly ornithischians, were considerably more common during the early Late Triassic than previously believed, whereas prosauropods remain limited to a single occurrence. Furthermore, Late Triassic ornithischian dinosaurs are both derived and diverse, with a variety of dental adaptations. At the outset of this study, demonstrable early Late Triassic (Carnian) ornithischian teeth were known from a handful of specimens collected at three or four localities in the Chinle Group. Now, we recognize at least eight ornithischian-bearing localities ranging in age from Otischalkian (late early to early late Carnian) to Adamanian (latest Carnian) age in the Chinle. These teeth represent at least five morphotypes: (1) a primitive, recurved, coarsely denticled, possibly carnivorous ornithischian, (2) teeth of Tecovasaurus murryi; (3) complex teeth with coarse denticles that are themselves finely denticled; (4) the first teeth of Pekinosaurus olseni discovered outside of the type locality in the Newark Supergroup of the eastern USA; (5) fragmentary teeth that may represent the first occurrence of Galtonia outside of its type locality in the Newark Supergroup; (6) tall, asymmetrically denticled, recurved teeth that may represent the premaxillary teeth of Tecovasaurus murryi.

These teeth have numerous biostratigraphic and evolutionary implications. The occurrence of Tecovasaurus in no fewer than four localities of Adamanian age indicates that Tecovasaurus is a potential index fossil of Adamanian time. Records of Pekinosaurus and, possibly, Galtonia in the late Carnian portion of both the Chinle and the Newark Supergroup strengthen existing correlations of these strata. The putative carnivorous ornithischian is known from two localities, both of Otischalkian age, and thus also appears to have some biostratigraphic utility. From an evolutionary perspective, the diversity and abundance of these teeth indicate that ornithischians probably originated during the early Carnian and diversified rapidly during the late Carnian.