Osteohistology of the titanosaur Mendozasaurus (Dinosauria, Sauropoda): preliminary interpretations

GONZÁLEZ RIGA, BERNARDO J.; CURRY ROGERS, KRISTI

Córdoba, Argentina

Congreso; 9ª Congreso Argentino de Paleontología y Bioestratigrafía; 2006

Academia Nacional de Ciencias- Universidad Nacional de Córdoba

Mendozasaurus neguyelap is a titanosaur recovered from a single locality in the late Turonian–Coniacian strata of the Neuquén Basin of central-western Argentina (González Riga, 2003; 2005). At least four juvenile, subadult, and adult specimens of Mendozasaurus offer new anatomical and histological data that detail titanosaur evolution and growth strategy. Here we describe and compare the osteohistology of this ontogenetic series, with a focus on femora, metacarpals, and dorsal ribs.  We followed the methods of other workers (e.g., Curry 1999; Salgado 2003), and studied 20 mm thin sections taken at standard locations in each of the bones in our sample. The most complete sub-adult Mendozasaurus in our sample (femur length ~150 cm; 80 % of adult size) allowed us to compare histological variability among skeletal elements. The femur documents: a) lamellar tissue with some lines of arrested growth” (LAGs) in the periosteal cortex, b) fibro-lamellar bone with well-defined secondary osteons in the mid-cortex and, c) an extensive spongiosa tissue formed by numerous trabeculae that extend into the deep medullary region. The spongiosa structure suggests numerous episodes of erosion and endosteal redeposition.  Metacarpals and dorsal ribs of this specimen exhibit a different structure: a) the absence of LAG in the cortex; b) highly vascular and disorganized primary tissue that form a woven bone matrix throughout the cortex, and, c) secondary osteons only located in the deep cortex. From ontogenetic viewpoint, the rib of a adult specimen (dorsal rib, width: 79 mm) shows a large medullary region in comparison with a sub-adult specimen (dorsal rib, width: 46 mm) and indicates significant remodeling of bones at these ontogenetic stages. Moreover, the adult bones exhibit longitudinal vascularity of fibro-lamellar tissue in the outer cortex, rather than an interweaving pattern of vascular canals. The microstructure of the femur described herein is similar to the well-documented  genus Apatosaurus from North America (Curry, 1999) that exhibits adults and sub-adults specimens with a dense Haversian tissue extensively developed and a periosteal bone marked with LAGs in the typical pattern of the “external fundamental system”. In particular, the LAGs in the femur of Mendozasaurus indicate pauses in the growth late in the sub-adult growth stage. In contrast, metacarpals and dorsal ribs suggest a rapid rate of osteogenesis lacking cyclicity until adult stage. This type of histological variability commonly exists, and highlights the need for standardized sampling of multiple bones per individual.   Study supported by the Project CONICET PIP-5222 and the IANIGLA-CRICYT.     Bibliography  Curry, K.A. 1999. Ontogenetic histology of Apatosaurus (Dinosauria:sauropoda): new insights on growth rates and longevity. Journal of Vertebrate Paleontology 19 (4): 654-665. González Riga,  B.J. 2003. A new titanosaur (Dinosauria, Sauropoda) from the Upper Cretaceous of Mendoza, Argentina. Amehginiana 40: 155-172. González Riga, B.J. 2005. Nuevos restos fósiles de Mendozasaurus neguyelap (Sauropoda: Titanosauridae) del Cretácico Tardío de Mendoza, Argentina. Ameghiniana 42: 535-538. Salgado, L. 2003. Considerations on the bony plates assigned to titanosaurs (Dinosauria, Sauropoda). Ameghiniana 40: 441-456.