A DESCRIPTION OF APATOSAURINE REMAINS FROM ARCHES NATIONAL PARK, UTAH USA (UPPER JURASSIC, MORRISON FORMATION)

Arches National Park in southeastern Utah has Mesozoic deposits spanning the Triassic to early Late Cretaceous Periods. The Morrison Formation exposed in the park has yielded vertebrate fossils and petrified wood; however, few of these fossils have been described. Here we redescribe fossil remains that we assign to Apatosaurinae indet. (UMNH VP 3157= ARCH 3151) from Arches National Park (Utah, USA). These ten bones (nine caudal vertebrae and a right pubis) come from the Arches Apatosaur locality.

The matrix was a partially indurated green-gray mudstone possibly indicating reduced iron, standing water, and organic material. The quarry is high in the Brushy Basin Member of the Morrison Formation, a few meters below the contact with the Cedar Mountain Formation.

Most of the caudal vertebrae were exposed on the surface with some loose and others partially embedded in the matrix. These vertebrae were roughly associated with each other. The pubis was almost entirely buried and found in close association with the vertebrae. As these bones were found near each other, with no duplicate material, they likely represent a single individual. No other associated fauna were found at the site.

The partial pubis is 814 mm long. The caudal vertebrae range in length from 131-371 mm and may represent caudals 2, 3, 5, 7, 9, 10, 11, 14, and 20. No taphonomic marks were observed on the heavily weathered bones, most of which are missing their neural spines and caudal ribs. We identify this animal as an apatosaurine based on heart-shaped caudal centra with limited pneumatization and posterior articular surfaces that are slightly convex ventrally and slightly concave dorsally. The robust pubis is also consistent with an apatosaurine identification.

These remains indicate the presence of a geologically young apatosaurine from one of the stratigraphically highest sites in the Morrison Formation. Despite this high stratigraphic placement, this specimen is average in size compared to apatosaurine material found in the Oklahoma Panhandle or C. supremus from the Cope’s Nipple locality. This apatosaurine is important to our understanding of sauropod remains from southern Utah and from high in the Morrison, as well as exemplifying the importance of the vertebrate fossil heritage preserved and protected within the national park system.