Sedimentology and taphonomy of a juvenile Alamosaurus site in the Javelina Formation (Upper Cretaceous), Big Bend National Park, Texas

A partial skeleton of a juvenile sauropod dinosaur was recovered fi"om an outcrop of the Javelina Formation in the Grapevine Hills region of Big Bend National Park, Texas. Paleocene mammals and reptiles were collected a few meters above the sauropod, making this section the most tightly constrained Cretaceous-Tertiary boundary section in the Big Bend region. Freshwater gastropods, fi-eshwater fish, and charophyte algal oogonia suggest the deposits represent a lake.

The sauropod is assigned to Alamosaurus sanjuanensis, based on similarities in the humerus, and the unique morphology of the ischium. Many skeletal elements collected were previously unknown for this species. These include six cervical centra and six cervical neural arches. Other skeletal elements recovered and not previously reported in the literature are one dorsal centrum, four dorsal neural arches, two distal tibiae, one complete fibula, and one partial fibula.

An ontogenetic series of six humeri assigned to Alamosaurus sanjuanensis were measured to investigate changes with growth in this element. Proximal width, distal width, shaft circumference, and shaft diameter were each compared against length. The allometric coefficients are all greater than one, indicating that all these measures increased relatively faster than length, making the humerus more robust with growth.

Taphonomic data suggests that this animal died on its left side, and the skeleton was disarticulated prior to burial. Scavenging may have been responsible for partial disarticulation, with the smaller and easily transported elements being winnowed out by currents that affected the orientation of the remaining bones.

Carbonate nodules were collected along a 192 m section spanning the K/T boundary at this site. Stable isotope analysis revealed a pattem similar to that obtained by Ferguson et al. (1991) from nearby exposures. There is a strong correlation between values, and the trend resembles an evaporative pattem. A large negative excursion in both the carbon and oxygen isotope values occurs at or just above the K/T boundary. This excursion is thought to represent climatic change during the early Paleocene to cooler and wetter conditions in the Big Bend region.