Bone histology reveals an unusual growth strategy in Lessemsaurus sauropoides, a basal sauropod dinosaur from the Late Triassic of Argentina

APALDETTI, CECILIA; CERDA, IGNACIO ALEJANDRO; MARTINEZ, RICARDO NESTOR; POL, DIEGO

Santa Clara del Mar

Jornada; XXXI Jornadas Argentinas de Paleontología de Vertebrados; 2017

Sauropod dinosaurs are considered to have uninterrupted rapid rates of growth (inferred from the lacks of growth marks in their long bones during most of its ontogeny), which differs from their more basal relatives (non-sauropod sauropodomorphs), which have a slower cyclical growth dynamics (evidenced by the cyclical deposition of growth marks during all the ontogeny). In order to evaluate the early evolution of the sauropod growth strategies, the growth pattern of the basal sauropod Lessemsaurus sauropoides Bonaparte 1999 (Late Triassic, mid-Norian; Los Colorados Formation) is deduced from its long bone histology. Two femora (PVL 4822/64 and PVL 6580) and two scapulae (PVL 4822/5 and PVSJ 1079) belonging to two different individuals of similar sizes (femora mid-shaft circumferences equal 37 and 36 cm) were sampled. Lessemsaurus exhibits well defined growth marks throughout the cortical bone, revealing a cyclical growth strategy, as occur in non-sauropod sauropodomorphs. The presence of thick zones of highly vascualrized reticular fibrolamellar bone tissue between the growth marks indicates, however, high rates of bone deposition during the phases of active growth. Hence, although Lessemsaurus exhibits the primitive (i.e. cyclical) growth strategy typical among non-sauropod sauropodomorph dinosaurs, its growth rate during active growth appears to be highly accelerated. Our data reveals a different and novel growth strategy, in which both basal and derived features arose at the beginning of the Sauropodomorpha evolution. Such strategy could be associated with the very large body size reached by Lessemsaurus, suggesting an early and different way toward sauropodomorph gigantism.