Bone microstructure and diagenesis of saurischian dinosaurs from the Upper Cretaceous (Neuquén Group), Argentina

Abstract

ABSTRACT. The Neuquén Basin in northwestern Patagonia, Argentina, holds the most important record of Cretaceous dinosaurs in South America.The Neuquén Group (Upper Cretaceous) is the richest dinosaur-bearing unit of the basin. It comprises the Río Limay, the Río Neuquén and the Río Colorado subgroups. In this study, dinosaur remains from the Río Neuquén and the Río Colorado subgroups outcropping in Mendoza are examined. In this group, isolated, disarticulated or partially articulated sauropods and theropods are abundant. However, little is known about the diagenetic history of fossil assemblages. In southern Mendoza, three fossiliferous sites were found in the areas of Paso de las Bardas (Quebrada Norte) and Cerro Guillermo (CG1, CG2). This study aims to add to the knowledge of diagenetic processes involving dinosaur remains from the Neuquén Group, as well as their relation to the depositional environment. Histologic features and diagenetic processes of dinosaur bones were analyzed through thin sections in order to interpret the degree of taphonomic alteration. The fossil-diagenetic processes inferred include substitution, fracturing, plastic deformation and different permineralization events. Combined analyses through X-ray diffractometry (XRD) and petrographic studies reveal the substitution of hydroxyapatite by francolite. The presence of fluorine -in one of the cases-suggests a link between the elemental composition and depositional environments: floodplain and fluvial channel. Permineralization stages include infilling of vascular canals, trabeculae and fractures with iron oxides and iron carbonate minerals during the burial history. This contribution represents an integral approach to the study of Cretaceous dinosaurs for assessing the diagenetic changes in the bone microstructure and the differential preservation of fossil remains in fluvial environments.

Taphonomic analysis of saurischian dinosaurs from the Plottier Formation (Upper Cretaceous), Mendoza, Argentina

“…According to this study, after the burial, the main diagenetic processes that affected the dinosaur bones were compaction, plastic deformation, permineralization and substitution. The ionic substitution of the hydroxyapatite by francolite -in the bone microstructure-was confirmed, in the first moment through the XRD analysis (see Previtera, 2017), and also by the SEM-EDS method performed here. This is a typical mineral replacement of fossilized bone (Lucas and Prévôt, 1991;Kolodny et al, 1996;Elorza et al, 1999) and it is common in many other dinosaur sites (e.g., Hubert et al, 1996;Pereda-Suberbiola et al, 2000;Paik et al, 2001;Luque et al, 2009;Rogers et al, 2010;Piga et al, 2011;among others).…”

“…Furthermore, diage netic processes, mineralization and compaction, which occurred in the burial environments (floodplain and fluvial channel) has been recognized in a previous study (Previtera, 2017). According to this study, after the burial, the main diagenetic processes that affected the dinosaur bones were compaction, plastic deformation, permineralization and substitution.…”

“…Especially, the existence of "open fractures" in saurischian bones indicates post-fossilization weathering processes -subaerial exposure-flaking and fracturing occurred during exhumation events (telodiagenesis). These processes are the result of the differences in burial depth, temperature and geostatic pressure suffered by the fossils in each burial environment (Previtera, 2017). Similar fossil-diagenetic features have been identified in other cretaceous tetrapod assemblages of Patagonia (González Riga and Astini, 2007;González Riga et al, 2009;Casal et al, 2013;Previtera, 2011Previtera, , 2013.…”

“…8A). These cracks were subsequently filled by minerals during permineralization events occurring within the sediment -diagenetic stages-as described by the author (see Previtera, 2017). Appendicular bones, dorsal ribs and the scapula show transverse and oblique fractures with smooth edges (Fig.…”

“…This record includes abundant sauropods and theropods of considerable significance from both systematic and phylogenetic viewpoints (e.g., Bonaparte, 1991;Novas, 1996Novas, , 1997Novas and Puerta, 1997;Salgado et al, 1997;Powell, 2003;González Riga, 2003;Wilson, 2002;Calvo and González Riga, 2003;Calvo et al, 2004;Curry Rogers, 2005;Coria and Currie, 2006;Calvo et al, 2007;González Riga et al, 2008, 2009González Riga and Ortiz David, 2014;Casal et al, 2014;González Riga et al, 2016, among others). Unlike the numerous systematic studies published, sedimentological and taphonomic analyses have been poorly documented (e.g., Garrido et al, 2001;Chiappe et al, 2004;González Riga and Astini, 2007;Previtera, 2011Previtera, , 2017 and they are lagging behind taxonomic and palaeobiogeographic researches. The paucity of taphonomic studies diminishes not only the option of accurate comparisons among different vertebrate assemblages but their stratigraphic provenance as well.…”