title: Taphonomy of Late Jurassic (Tithonian) Morrison Formation Apatosaurus sp. Vertebrae Found Associated with Teeth from Allosaurus sp. and Ceratosaurus sp., and Body Size Extrapolation from the Associated Theropod Teeth.
Creators: name: Agyan, Greg C.
Publisher: name: Utah State University
Year: 2021

description: An Apatosaurus sp. locality from Dinosaur National Monument designated DNM-15 was excavated in 1985, and associated with two Allosaurus teeth and one Ceratosaurus tooth that were near one of the caudal vertebrae. The Ceratosaurus tooth was buried between an overlying rib and that same caudal vertebra. The caudal vertebrae of the DNM-15 Apatosaurus were intact and articulated, but the anterior skeleton was mostly absent, with a row of articulated sacral vertebrae in close association with a femur. Two other Allosaurus teeth were reported near the preserved ilium of the Apatosaurus, but they could not be located in the collections.
Field observations of the locality reveal the site to be on a similar stratigraphic level to the Carnegie Quarry. Sandstone and conglomerate deposits at the locality imply that the area was part of a braided channel system similar to the one observed in the Carnegie Quarry, albeit a fine to coarse-grained alluvial deposit rather than a coarser-grained riverbed deposit. Thin section data of the locality matrix of a sample collected from the site reveals the rock to be litharenite, with the high quantities of quartz, chert, and lithics providing evidence of excess clastic sediment input from a regional uplifted topography, likely to the west associated with the initiation of the Sevier Orogeny. Taphonomy and paleohydrology data from the Carnegie Quarry shows that the fluvial process would have had a high enough flow velocity during flooding events to transport both bones and teeth, although neither would have travelled especially far. Most individual sauropods at Dinosaur National Monument are at least partially articulated, including the one from DNM-15. These factors, combined with the many carnivorous dinosaur teeth buried between the Apatosaurus bones at site DNM-15, imply that the association between the vertebrae and teeth is likely due to active scavenging on the carcass before burial rather than allogenic fluvial transport.
Skull and body length data from four allosauroids were used to set up a series of regression plots whose equations were used to extrapolate estimated skull and body lengths from the recovered teeth from the site. The calculations of the DNM-15 Allosaurus teeth yielded an average skull length of just fewer than 80 cm and a body length of about 7 meters for both teeth, implying that both Allosaurus teeth from DNM-15 were shed from the same individual or that they were each shed separately by two similarly sized individuals. The Ceratosaurus size estimation process used a similar data analysis method as the one used for Allosaurus, though four Ceratosaurus specimens were used for this regression data set instead. The calculations for the DNM-15 Ceratosaurus tooth show skull length results averaging out to about a meter long, while the average of the body length values was shown to be approximately 8.7 meters long. The Allosaurus from DNM-15 was of average size, but the calculations for the DNM-15 Ceratosaurus suggest that it could potentially be the largest member of this taxon on record.