DESCRIPTION AND PRELIMINARY TAPHONOMY OF THE PARTIAL SKELETAL REMAINS OF A JUVENILE DIPLODOCID SAUROPOD FROM THE JURASSIC MORRISON FORMATION OF UTAH

A small scapula and ischium from a diplodocid sauropod excavated during May 2001 (with permission from the federal Bureau of Land Management, Utah) from a fine to medium grained, well indurated fluvial sandstone of the Upper Jurassic Brushy Basin Member of the Morrison Formation in Emery County, Utah has been tentatively identified to the genus Diplodocus. The bones of the juvenile are associated with the scattered remains of a single adult diplodocid sauropod which has also been identified as Diplodocus. The quarry is located on the western flank of the San Rafael Swell and the bones are stratigraphically 124 meters (407 ft.) below the base of the Lower Cretaceous Buckhorn Conglomerate Member of the Cedar Mountain Formation. Primary criteria for the preliminary identification of the juvenile sauropod as Diplodocus are based on the scapula. The scapula is 65 cm in length and 25 cm in maximum width. The general shape of the small scapula and a mature Diplodocus scapula are very similar. The angle between the acromion (muscle ridge on plate) and the blade of the scapula is acute, while the proximal plate of the scapula is greatly expanded and the distal (upper) end of the blade is only mildly expanded. These criteria, and others, are indicative of the genus Diplodocus. The bones in the quarry appear to be randomly oriented with respect to the anatomy of the adult and juvenile sauropods, but the long bones seem to have a northerly-southerly alignment (probably as a result of current action). The fluvial sandstone is cross-bedded and the lower portion has rip-up clasts of gray mudstone. We speculate that a mother Diplodocus and her offspring may have perished in a stream channel or the adjacent floodplain (perhaps of dehydration during the dry season). No carnivore teeth were found nor are there any tooth marks on the bones recovered to date. During a flooding event the bones were disarticulated and transported for a short distance in the stream channel.