BONE HISTOLOGY AND MICROANATOMY OF ALAMOSAURUS SANJUANENSIS (SAUROPODA: TITANOSAURIA) FROM THE MAASTRICHTIAN OF BIG BEND NATIONAL PARK, TEXAS

Bones of Alamosaurus sanjuanensis were collected from the Upper Cretaceous Javelina and Black Peaks formations in Big Bend National Park, Texas, for use in the first extensive histological study of a titanosaurian sauropod dinosaur. Samples include cervical, dorsal, and caudal vertebrae ribs; a scapula; humeri; an ulna; a metacarpal; an ischium femora; a tibia: fibulae and a metatarsal, representing a minimum of twenty two individuals between 35% and 100% maximum length. In elements of less than 60% maximum length, cortical tissue is primarily reticular or plexiform fibro-lamellar, often with modulations. Resorption in some specimens appears to have proceeded as outwardly expanding hands. In larger specimens (70-79% maximum length), primary tissue is only visible interstitially as either fibro-lamellar or parallel-fibered with low vascularity and annuli. Secondary osteons are in many places linearly arranged. Haversian systems are so pervasive in many of the largest specimens (80-100% maximum length) that virtually no primary tissue remains. However, one adult sized femur possesses laminar fibro-lamellar tissue in its outer cortex. suggesting that the adult size for Alamosaurus may have been underestimated. Camellae typical of titanosaur dorsal vertebrae ire present in Alamosaurus but absent in caudal vertebrae. Although pneumatic dorsal ribs characterize Titanosauriformes, sections of ribs analyzed in this study show the cavities in Alamosaurus 21 re much more extensive than most previously described. Overall. Alamosaurus bone histology combines characteristics found in less derived sauropods, Such as all early onset of secondary reconstruction, with novel characters such as extensively pneumatic ribs and longer retention of juvenile tissue.