Daniel Chure

On the Colorado Plateau, numerous Mesozoic stratigraphic units, including the Navajo and Entrada formations, preserve abundant tracks that contrast with their extremely poor body fossil records. This is also true of the Moenkopi Formation, which also preserves abundant tracks attributed to reptiles. Here we report the first occurrence of tetrapod tracks from the Moenkopi Formation in Dinosaur National Monument in northeastern Utah. The Moenkopi Formation in this area can be divided into four informal “members,” with all of the currently known tracksites in Dinosaur National Monument occurring low within the third member. Tracks consist of both footprints and swim tracks and are preserved in the “racetrack” exposures in the western portion of the monument. These traces show morphological characteristics suggesting they were produced under a variety of conditions ranging from subaqueous to subaerial. Footprints are chirotheriid in overall morphology with some resemblance to Protochirotherium and Synaptichnium. Due to the relative paucity of skeletal remains from the Moenkopi Formation these and other tracks from this unit provide important information about the paleoecology of Early-Middle Triassic reptiles.

Sauropod dinosaur bones are common in Mesozoic terrestrial sediments, but sauropod skulls are exceedingly rare—cranial materials are known for less than one third of sauropod genera and even fewer are known from complete skulls. Here we describe the first complete sauropod skull from the Cretaceous of the Americas, Abydosaurus mcintoshi, n. gen., n. sp., known from 104.46 ± 0.95 Ma (megannum) sediments from Dinosaur National Monument, USA. Abydosaurus shares close ancestry with Brachiosaurus, which appeared in the fossil record ca. 45 million years earlier and had substantially broader teeth. A survey of tooth shape in sauropodomorphs demonstrates that sauropods evolved broad crowns during the Early Jurassic but did not evolve narrow crowns until the Late Jurassic, when they occupied their greatest range of crown breadths. During the Cretaceous, brachiosaurids and other lineages independently underwent a marked diminution in tooth breadth, and before the latest Cretaceous broad-crowned sauropods were extinct on all continental landmasses. Differential survival and diversification of narrow-crowned sauropods in the Late Cretaceous appears to be a directed trend that was not correlated with changes in plant diversity or abundance, but may signal a shift towards elevated tooth replacement rates and high-wear dentition. Sauropods lacked many of the complex herbivorous adaptations present within contemporaneous ornithischian herbivores, such as beaks, cheeks, kinesis, and heterodonty. The spartan design of sauropod skulls may be related to their remarkably small size—sauropod skulls account for only 1/200th of total body volume compared to 1/30th body volume in ornithopod dinosaurs.