Preservation models of ophiuroids in epicontinental basins: Examples from a new Devonian echinoderm Lagerst?tte from Brazil

摘要

? 2022 Elsevier LtdDue to the low preservation potential of their multi-elemental endoskeletons, fossil ophiuroids can be used as a proxy for exceptional preservation conditions. Following this principle, this study investigates the taphonomic history of ophiuroid beds across three sites in southern Brazil, where Devonian epicontinental sequences from the Ponta Grossa Formation are well-represented. Through fieldwork, tomographic analysis, and description of 218 samples, five taphonomic grades were listed to represent the paleoecological and sedimentological histories of these ophiuroid beds. Three high-order taphonomic grades symbolize articulated ophiuroids suddenly buried in life, comprising sparse skeletons parallel to bedding planes, skeletal clusters oriented on specific horizons, or skeletons in escape postures inclined at low angles on bedding planes. Two low-order taphonomic grades include ophiuroids buried between hours to a few days after death, reflecting skeletons disarticulated in a normal sequence of microbial decay or skeletons selectively disarticulated by macrobenthic groups. These records suggest that fluvial discharges were the main burial mechanisms for ophiuroids, since they may transport freshwater-rich sediments, favoring the mass numbing of vagrant echinoderms. In contrast, storms were probably only a subordinate burial process, considering that they remobilize sediments without a key anesthetic for these echinoderms. Furthermore, carbonaceous compressions were recorded for the first time in fossil echinoderms through kerogenization of visceral organs in the disk of ophiuroids, apparently with associated pyritization. Considering the studied data, these ophiuroids probably thrived in different seafloors, although they have been preserved mainly in distal marine regions free from intense interference from waves, currents, and bioturbators. Due to the presence of well-articulated echinoderms and the exceptional preservation potential of other associated fossil groups, the Ponta Grossa Formation can be classified as a Konservat-Lagerst?tte of the Devonian of Brazil.