Early growth of the basal sauropodomorph dinosaur Mussaurus patagonicus Bonaparte and Vince from the Late Triassic of Argentina inferred from its bone histology

IGNACIO A. CERDA; DIEGO POL

Jornada; XXVI Jornadas Argentinas de Paleontología de Vertebrados” Mayo de 2012, Buenos Aires; 2012

Here we describe the bony histology of juvenile specimens of the basal sauropodomorph Mussaurus patagonicus Bonaparte and Vince and interpret what this tells us about the early growth dynamics of this taxon. Thin sections from at least three juvenile specimens MPM 1813 (femur length: 11-12 cm) of M. patagonicus were analyzed. The sampled bones consist of several postcranial elements collected from the Laguna Colorada Formation (Laguna Colorada locality, Santa Cruz Province, Late Triassic). The cortical bone is composed by woven-fibered and fibro-lamellar bone tissue. Vascularization is commonly laminar or plexiform. Growth marks (lines of arrested growth [LAGs]) are absent in all the examined samples. The “epiphyses” of long bones are all formed by a well-developed coating of hypertrophied calcified cartilage. The predominance of woven-fibred bone tissue in cortical bones indicates a markedly fast growth rate in the individuals examined. Moreover, if we assume that M. patagonicus deposited LAGs through its ontogeny (as has been reported in other basal sauropodomorphs), and the first LAG was formed at the first year of life, the absence of growth marks in the bones suggest that the specimens died before reaching their first year of life. Compared with the African taxon Massospondylus carinatus Owen (other basal sauropodomorph for which the bone histology has been previously studied), the bone microstructure of M. patagonicus reveals a faster growth rate for the Patagonian form. In this sense, a minimum age of two years was estimated for juvenile specimens of M. carinatus of comparable body size (femur length: 12,7 cm). Mussaurus patagonicus Bonaparte and Vince and interpret what this tells us about the early growth dynamics of this taxon. Thin sections from at least three juvenile specimens MPM 1813 (femur length: 11-12 cm) of M. patagonicus were analyzed. The sampled bones consist of several postcranial elements collected from the Laguna Colorada Formation (Laguna Colorada locality, Santa Cruz Province, Late Triassic). The cortical bone is composed by woven-fibered and fibro-lamellar bone tissue. Vascularization is commonly laminar or plexiform. Growth marks (lines of arrested growth [LAGs]) are absent in all the examined samples. The “epiphyses” of long bones are all formed by a well-developed coating of hypertrophied calcified cartilage. The predominance of woven-fibred bone tissue in cortical bones indicates a markedly fast growth rate in the individuals examined. Moreover, if we assume that M. patagonicus deposited LAGs through its ontogeny (as has been reported in other basal sauropodomorphs), and the first LAG was formed at the first year of life, the absence of growth marks in the bones suggest that the specimens died before reaching their first year of life. Compared with the African taxon Massospondylus carinatus Owen (other basal sauropodomorph for which the bone histology has been previously studied), the bone microstructure of M. patagonicus reveals a faster growth rate for the Patagonian form. In this sense, a minimum age of two years was estimated for juvenile specimens of M. carinatus of comparable body size (femur length: 12,7 cm).