The titanosaur (Dinosauria: Sauropoda) osteoderm record: review and first definitive specimen from India

Abstract

ABSTRACT Titanosaurs are the only sauropods that possessed osteoderms, although the phylogenetic distribution of this feature within Titanosauria is not yet resolved. Whereas the majority of titanosaur osteoderms are from South America, several have been found in Cretaceous rocks in Africa, Madagascar, and Europe. Here we describe a titanosaur osteoderm from the Maastrichtian of India that extends the geographic range of these armored sauropods. The known spatiotemporal distribution of titanosaurs is broader than the distribution of titanosaur osteoderms. This discrepancy can be explained by one or more of several factors: (1) few titanosaur genera had osteoderms; (2) titanosaurs were not heavily armored; (3) there are strong collection and/or (4) taphonomic biases against these elements. Almost 90 individual titanosaur osteoderms have been reported and can be assigned to ten of the 40+ currently recognized titanosaur genera. Although they are not divisible into discrete size classes, titanosaur osteoderms are here shown to fall into four morphotypes: “ellipsoid,” “keeled,” “cylindrical,” and “mosaic.” No morphotype is unique to any one taxonomic group, geographic area, or time period. Despite the relative scarcity of osteoderms, it appears that armored titanosaurs are relatively diminutive in body size, with average femoral and humeral lengths of about 60 and 70% those of unarmored taxa, respectively. An apparent exception to this relationship is the titanosaur osteoderm from India, which likely pertains to one of the two relatively large-bodied titanosaur species that are known from the Lameta Formation.