DEPOSITIONAL SETTING AND PALEOECOLOGICAL SIGNIFICANCE OF A NEW SAUROPOD BONEBED IN THE JAVELINA FORMATION (CRETACEOUS) OF BIG BEND NATIONAL PARK, TEXAS

A new Alamosaurus locality has been discovered in the Javelina Formation of Big Bend National Park. This site is significant because: it is the first bonebed containing the remains of Alamosaurus; the sauropod remains are all from juveniles; and, the depositional setting at this site is that of a shallow lake.

Alamosaurus is the only taxon recorded from this site. Based on the number of femora, there are at least three individuals represented. Comparison of limb element length of specimens from this site with those of adults shows these juveniles are approximately 50%-60% adult size, or approximately 10 to 13 meters long. Outcrop pattern is poor in this area but the current stratigraphic approximation is that this site is 60 meters above the contact with the underlying Aguja Formation.

The site of deposition is an approximately 67 million -year-old lake bed. Abundant charophyte fossils indicate deposition in shallow, relatively clear, alkaline waters. The lake bottom was fine mud. Bones are dispersed through a two meter interval but are concentrated along the base of the unit. Many limb bones have high angle plunges that in some extreme instances approach vertical. This bone orientation pattern, the contorted nature of the entombing sediments, and the suggestion of large sauropod footprints at the upper contact of the bone-bearing unit suggests that this site experienced bioturbation (dinoturbation) probably by adults sauropods.