TITANOSAUR OSTEODERM ONTOGENY, ANATOMY, AND FUNCTION: NEW DATA FROM RAPETOSAURUS KRAUSEI (MAEVARANO FORMATION, MADAGASCAR)

Osteoderms have been attributed to ten genera of titanosaur sauropods, but they are typically isolated and their anatomy and functional/phylogenetic significance remains unresolved. Here we report on two osteoderms that occur with associated/articulated skeletons of Rapetosaurus krausei, a titanosaur from the Upper Cretaceous Maevarano Formation of Madagascar. These specimens are significant because they allow referral of two osteoderm morphotypes to R. krausei, and provide insight on the ontogeny, anatomy, and potential functions of titanosaur osteoderms.

Osteoderms were first associated with titanosaurs more than a century ago with the assignment of a large, cylindrical dermal bone to “Titanosaurus madagascariensis.” Recent work in the Maevarano Formation has yielded three distinctive bonebeds at site MAD 93-18 (BB1, BB2, BB3) that preserve the remains of R. krausei. BB1 preserves a single skeleton of an adult (femur length = 146 cm) that includes the most massive sauropod osteoderm yet discovered (FMNH PR 2342). FMNH PR 2342 was closely associated with pelvic elements, is ellipsoid, and exhibits a characteristic bulb and root morphology. It measures 57 x 26.7 x 19.2 cm, and tapers to a height of 3.6 cm and width of 12.5 cm near the thinnest part of the element. The surface exhibits a number of distinctive textures including a cross-hatched internal surface, a ridged texture at the intersection of the bulb and root, and a rugose, articular-cartilaginous-type texture on the broader of two lateral surfaces.

BB3 yielded a subadult skeleton of R. krausei that exhibits no duplication of elements and some instances of articulation (e.g., foot, ribs, hindlimb, and forelimb in anatomical position). A cylindrical osteoderm recovered in close association with a series of four mid-caudal vertebrae from this specimen measures 14 x 9 x 8 cm, exhibits an external midpoint convexity, and tapers to ~2 cm near the thinnest part of the element. The osteoderm surface is characterized by thin, disorganized bony spicules and deep internal vascular canals.

Taphonomic data suggest that Rapetosaurus possessed only a few osteoderms in its skin, that osteoderm morphology is related either to anatomical location or ontogeny, and that large osteoderms may result from the fusion of multiple, smaller dermal elements.