Estudio de estructuras bioerosivas asociadas a restos óseos de un plesiosaurio del cretácico superior de antártida.

Abstract

The study of bioerosive structures recorded in bone remains of marine reptiles constitute a contribution to the paleoecology of sclerobiont assemblages or communities (i.e., organisms that lived associated with this type of material). In addition, to determine the components of these communities from their trace fossils, stages of paleoecological successions can be recognized and thus contribute to the knowledge of the taphonomic history of the bone remains in study. The main objective of this work is to study and recognize macroscopic and microscopic bioerosive structures preserved in the skeletal remains of a plesiosaur (MLP16-I-21-6) recovered from the López de Bertodano Formation (late Maastrichian), Marambio Island, Antarctica. A total of 113 postcranial bone elements belonging to the same individual were identified: 25 vertebral centres, 50 rib fragments, 7 flat bones, 2 pelvic girdle elements, 7 phalanges, and 8 long bones; while some fragments could not be classified. They were observed under a stereoscopic microscope and 3 elements were selected to make histological sections and then they were observed using a polarized light microscope. The material is in good to moderate preservation, with a high degree of fragmentation, and the remains were found in situ, disarticulated but associated. At the macroscopic level, three bioerosive structures were recognized, among which only Trypanites Mägdefrau could be identified. The rest of the traces were classified as Morphotype 1, corresponding to perforations with circular entrances on the surface of the bones with common characteristics between Entobia Bronn and Osedacoides Karl, Brauckmann, and Groening, and, as Morphotype 2 to indeterminate traces, those with certain similarities to those originated by scavengers, although its characterization is not yet conclusive. At the microscopic level, traces with mostly straight tunnels, sometimes branched similar to those caused by endolithic organisms, such as bacteria and fungi, were determined as Morphotype 3 on bone remains. In addition, the presence of pyrite was recognized both in the microperforations as well as in the vascular channels, of which some were modified, indicating an early stage of bone decomposition. The bioerosive traces allowed a preliminary interpretation of some stages of a paleoecological succession. The mobile scavenger stage could be represented by indeterminate tracks similar to those produced by scavengers, although more detailed studies are required to confirm this. The opportunistic enrichment stage could be represented by Osedacoides isp., although this has not yet been confirmed. The modified vascular channels filled with pyrite would be related to the first stages of bacterial bone deterioration or the sulfophilic stage. While the traces assigned to Trypanites isp. and, if Entobia isp. is confirmed, could represent the reef stage. The presence of both macroscopic and microscopic perforations made it possible to record part of the components of the endolithic communities, that lived associated with the bone remains of the studied plesiosaur, indicating that they were exposed for a short time, although long enough, under conditions of low energy and rate of sedimentation, to allow the colonization of these communities.