Evidence for gender-specific reproductive tissue in a basal sauropodomorph dinosaur from the Late Triassic of Argentina.

IGNACIO A. CERDA; DIEGO POL

La Rioja.

Jornada; XXVII Jornadas Argentinas de Paleontología de Vertebrados?; 2013

Unambiguous indicators of gender in dinosaurs are usually lost during fossilization, along with other aspects of soft tissue anatomy. One exception is medullary bone (MB), which is an ephemeral bony tissue that forms before ovulation in the medullary cavities of birds as a calcium source for eggshelling. MB has been reported in non avian dinosaurs, including theropods and ornithischians. Here we report the presence of endosteally derived bone tissues lining the interior marrow cavities of an appendicular bone Mussaurus patagonicus Bonaparte and Vince 1979 and we hypothesize that these tissues are homologous to MB. The sampled bone is a right femur (MLP 61-III-20-22) collected from the Laguna Colorada Formation (Laguna Colorada locality, Santa Cruz Province, Late Triassic). The element corresponds to a large individual (=femur length=). The thin sections were obtained from the mid-shaft, below the fourth trochanter. The cortical bone is composed by fibro-lamellar bone tissue with abundant growth marks (lines of arrested growth [LAGs]) in the outer cortex. Distinctive, unusual endosteally formed radial bony spicules project into the medullary cavity. The spicules are composed by woven fibered bone tissue. The location (long bone), distribution (marrow cavity) and histological structure (woven fibered) of the endosteal tissue in Mussaurus are consistent with avian MB. This is the first evidence for a gender-specific tissue in a sauropodomorph dinosaur. As other previously supposed exclusive features of birds (e.g., skeletal postcranial pneumaticity), the presence of MB could be a character developed in a more inclusive group (e.g., Dinosauria).