TAPHONOMY OF A PARTIAL HADROSAURID SKELETON POSSESSING SUBADULT TYRANNOSAUR FEEDING TRACES

In 2007 a partial hadrosaurid skeleton was collected from the Hell Creek Formation (latest Maastrichtian) of southeastern Montana. The skeleton consists of heavily weathered pelvic elements and two anterior caudal vertebrae. The bones were encased in a six-meter thick massive fine-grained sandstone lens with well-rounded rip-up clasts at the base, and overlying a 30 cm siderite bed. Smaller unidentifiable bone fragments were also recovered, and were bisected and sheered at the sandstone-siderite contact. Based on the lithology and pre-burial condition of the skeleton, the site is interpreted as the base of a point-bar sequence where the skeleton was exposed and eroded for a considerable period of time prior to burial by continued fluvial processes. In addition to fluvial erosion, one of the anterior caudal vertebrae possesses three V-shaped indentations on the ventral surface of the centrum consistent with bite marks to a large theropod dinosaur. The indentations show no signs of healing, and are inferred to have been created peri- or post-mortem. To test the origin of the bite marks, a series of Tyrannosaurus rex maxillae of varying ontogenetic stages were digitized using a desktop 3D scanner and photogrammetric techniques and the resulting indentations were measured for cross-sectional morphology and tooth spacing. These results were compared with the depressions present on the hadrosaur vertebra. Maxillae of adult specimens of T. rex were found to be much too large and widely spaced to have produced the indentations in question. However, the maxilla of a subadult Tyrannosaurus histologically dated to be approximately 11 years old at the time of death produced similarly shaped and spaced indentations, suggesting the bite was caused by a subadult Tyrannosaurus. While feeding traces attributable to adult tyrannosaurids is well-documented, the identification of subadult Tyrannosaurus feeding traces adds new insight into the role of subadult theropods in paleoecological interactions and biostratinomic processes.