Taphonomy of sauropod and theropod dinosaurs from the Upper Cretaceous of Mendoza, Argentina

ELENA PREVITERA

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Universidad Nacional y Museo de Ciencias Naturales de la provincia de San Juan

The Neuquén Basin, located in northwestern Patagonia of Argentina, has the most important record of Cretaceous dinosaurs from South America. The Neuquén Group (Cenomanian-early Campanian) is the richest dinosaur-bearing unit of the basin. Taphonomic studies of these vertebrates are scarce since most papers are focused on systematic aspects. Disarticulated, isolated or partially articulated sauropods and theropods are abundant in this group. In this context, the present contribution focuses on three main aspects: biostratinomic and fossil diagenetic processes of dinosaurs and diagenetic features of the sedimentary facies. In Mendoza province, four dinosaur sites were found from Paso de las Bardas (Quebrada Norte site) and Cerro Guillermo (CG1, CG2, CG3 sites). These discoveries led to the recognition of four taphonomic preservational modes (MT): MT1. Articulated bones of titanosaur indet. specimen in well-drained floodplain facies. In this case the bones are preserved articulated in facies with abundant paleosols and intense bioturbation and they do not show evidence of weathering or orientation due to sporadic currents. MT 2. Disarticulated but associated bones of sauropod in poorly drained floodplain facies. This is the case of Malarguesaurus florenciae (González Riga et al., 2009) and two specimens of titanosaur indet. The bones are preserved in poorly drained floodplain facies with poorly developed paleosols. They show a complete disarticulation processes, subaerial biodegradation, weathering and limited hydraulic winnowing. MT 3. Disarticulated bones of theropod indet. in crevasse splays. The bone remains exhibit abrasion and sorting due to intense hydraulic transport related to crevasse splays facies, and MT 4. Isolated bones of sauropod and theropod indet. in channel facies. In this case, the bones are fragmentary remains that show total disarticulation, abrasion and hydraulic sorting processes due to transport within fluvial channel-lag deposits. The fossil diagenetic stages observed in theropods (MT4) and sauropods (MT2) are reconstructed using microscopic studies and compositional analysis using X-ray diffractometry. In these stages, the bones suffered intense plastic deformation, permineralization with iron oxides and other diverse minerals in vascular canals and fractures. These processes are the result of the different burial depths, temperature regimes, and geostatic pressures experienced by sauropod and theropod bones preserved in floodplains and channel facies, respectively. Bone preservation differences observed in both depositional environmental are associated with the rock-fluid system of each sedimentary facies. In summary, according to the biostratinomic and fossil diagenetic studies, the dinosaurs preservation in fluvial systems is strongly controlled by intrinsic factors of the organisms such as the morphology, skeletal relative proportions and taxonomic group, and extrinsic factors such as the depositional environmental and its mineralogical and petrographical conditions.