Tafonomía de mamíferos pleistocenos colectados por Darwin en Punta Alta (Buenos Aires, Argentina): análisis de la colección digitalizada del Museo de historia natural de Londres

Abstract

Charles Darwin visited the southwest of the Buenos Aires province during his trip aboard the H.M.S. Beagle in 1832 and 1833, especially the area of Bahía Blanca and its surroundings, making geological and paleontological observations. One of the highlights was the discovery of fossil mammals in Punta Alta. In this degree thesis, the remains of a fossil assemblage from Punta Alta, composed of Equus (Amerhippus) neogeus, Megatherium americanum, Mylodon darwini, Scelidotherium leptocephalum, Toxodon platensis and Toxodon sp., and some fragments of Mammalia ident., were analyzed. The remains, stored and digitized by the staff of the Natural History Museum of London, are available in free access virtual repositories. The purpose of this thesis was to identify various taphonomic attributes using 2D and 3D images in order to verify if the remains were deposited in the context of a fluvial system. The recovered assemblage of mammals presents an NMI = 9 and corresponds to the typical Pleistocene megafauna of the Pampean region of Argentina. In particular, the presence of the lithoptern Macrauchenia patachonica and the equid Equus (Amerhippus) neogeus, indicate a late Pleistocene age for the assemblage. The most represented elements are the skull and the ribs and they are the most affected by weathering processes. The least represented bone elements include vertebrae and astragalus and exhibit the highest degree of abrasion. Most of the remains were disarticulated and incomplete, with the exception of the holotype of S. leptocephalum. In addition, skeletal elements bioeroded by root growth, bacterial activity, and traces of Polychaeta were identified. Some of the remains were impregnated, an attribute that caught Darwin's attention. The identified taphonomic attributes reflect that the bone remains were affected by different processes, both before and after burial. The results obtained are consistent with the interpretation of a fluvial environment. It is considered that some materials were quickly accumulated and buried, while others remained exposed on the surface for a longer period of time and were resedimented. Marine trace fossils suggest that some of the remains were affected by reelaboration. In this context, the association is considered to be composed of both autochthonous (represented only by a nearly complete skeleton of S. leptocephalum) and para-autochthonous specimens. Most of the attributes included in this analysis were observed by Darwin, even though taphonomy had not been defined as a science, which is why his facet as a taphonomist is considered.