COMPARATIVE OSTEOHISTOLOGY OF RAPETOSAURUS KRAUSEI (SAUROPODA: TITANOSAURIA) FROM THE LATE CRETACEOUS OF MADAGASCAR

Bone histology yields abundant data on dinosaur life history. Among dinosaurs, sauropods are among the best-sampled taxa with significant sampling gaps only in basal forms and derived titanosaurs. A consensus view has emerged that indicates that sauropods possess highly vascularized fibrolamellar bone indicative of growth rates comparable to those of extant mammals and birds. Growth rates and final size are taxon specific, with several small-to-medium sized sauropods thought to gain these more diminuitive adult body sizes either through reduction of growth rates relative to other neosauropods and/or by shortening the active phase of growth rapid growth throughout ontogeny. In this study, we present an ontogenetic, histologic analysis of the Late Cretaceous Malagasy titanosaur Rapetosaurus krausei that tests the hypothesis that Rapetosaurus growth rates were on par with those of other large-bodied sauropods.

Our sample consisted of ten forelimb, hindlimb, and girdle elements of Rapetosaurus krausei at different ontogenetic stages, including samples from the largest Rapetosaurus yet known (femur length = 143.4 cm). Maximum bone length allowed us to group our sample into five relative age classes (Early Juvenile – Adult). We compared bones in particular age classes from our Rapetosaurus sample to twelve forelimb and girdle elements from a well-known ontogenetic series of Apatosaurus. Primary bone tissue in Rapetosaurus is highly vascularized fibrolamellar bone as in other sauropods. However, even the smallest juveniles in our sample exhibit nearly complete replacement of the primary cortex by secondary Haversian bone. Expressed in Histologic Ontogenetic Stages (HOS), even the smallest Rapetosaurus specimens show a histology otherwise only observed in adult sauropods (HOS 12-14), and Magyarosaurus, a titanosaur thought to exhibit an extremely reduced growth rate and small adult body size. An explanaton of insular dwarfism proposed for Magyarosaurus cannot be argued for Rapetosaurus krausei because large-bodied adults are known. Instead, we hypothesize that the intense remodeling of bone tissue in the Rapetosaurus sample may be related to resource limitations and ecological stress in the Maevarano Formation paleoenvironment.