Abstract

Titanosauria is a clade of macronarian sauropod dinosaurs that includes species ranging from the largest-known terrestrial vertebrates to insular dwarfs no bigger than elephants. Though most sauropod dinosaurs exhibit highly vascularized fibrolamellar bone signaling rapid growth rates comparable to those of extant mammals, diminutive titanosaurs apparently exhibit histological traits consistent with reduced primary growth rates and/or truncated active growth to reach small adult body sizes. A better understanding of the evolution of titanosaur body size requires additional sampling of the 40+ known titanosaur species. The best-preserved and most complete titanosaur yet discovered is Rapetosaurus krausei from the Maastrichtian Maevarano Formation in Madagascar. Skeletal material from many individuals spanning a wide range of ontogeny, from neonates to large-bodied adults, makes Rapetosaurus an ideal candidate for an analysis of growth. We analyzed Rapetosaurus histology using a growth series of twenty-five forelimb, hindlimb, and girdle elements representing different ontogenetic stages, including samples from the largest Rapetosaurus known (femur length = 143 cm). Primary bone histology in Rapetosaurus is highly vascularized fibrolamellar tissue as found in other sauropods and most sampled titanosaurs. Secondary remodeling begins early, as noted in other titanosaur taxa, and by mid-ontogeny is pervasive in most sampled elements. Some of the smallest juveniles in our sample exhibit an unusual pattern of bone erosion and replacement that occur in the context of peripheral lines of arrested growth that signal a temporary pause in bone apposition. We hypothesize that these unusual signals may relate to the drought-stressed ecosystem inhabited by Rapetosaurus.