Asteroid trace fossils from Lower Cretaceous shallow- to marginal-marine deposits in Patagonia

Most stellate trace fossils of the ichnogenus Asteriacites are attributed to asterozoan producers in general and the majority is the result of the work of ophiuroids. The fossil record of asterozoans is scarce in South America, particularly for the Mesozoic. Asteriacites specimens found in shallow- to marginal-marine Lower Cretaceous (upper Hauterivian-lower Barremian) deposits in the Neuquen Basin (Patagonia, Argentina) exhibit sculpture and morphometry typical of asteroid producers. This is the second record of asteroids from the Lower Cretaceous of South America. The close association between these Asteriacites possibly produced by astropectinids and traces assignable to Siphonichnidae are suggestive of a predator-prey interaction, adding palaeoecological information for community-structure reconstruction of these deposits. For ichnotaxonomic evaluation, morphometric parameters of Asteriacites were elaborated using simple photogrammetric procedures applied on negative epirelief specimens and undertraces to define edges of the stellate trace fossils

<b>Mollusk shells for AAR: Dating the Quaternary shorelines along the coastline of Camarones, Chubut Providence, Argentina</b>

Argentinean Quaternary beach ridges are rich in marine fossil shells. However, the deposits lack material commonly used for the age constraint of coastal deposits, e.g., sediments for luminescence analysis or coral for U/Th analysis. To date, numerical ages have been determined by electron spin resonance and U/Th analysis of shells of the marine mollusk Ameghinomya antiqua, applications for which there are concerns due to the uptake of uranium following organism death. The amino acid racemization (AAR) geochronology method analyzes the extent of protein degradation within shells and provides an estimate of the time elapsed since the cessation of protein formation, commonly equated with the death of the organism. The application of the AAR method to mollusk shells in geochronological studies of Quaternary marine and coastal deposits is well-established, providing relative ages to stratigraphic sequences in the form of D/L values. In this study, we use samples of Ameghinomya antiqua to assess its suitability for AAR analysis in six Pleistocene field sites along the coastline of Camarones’ locality in Chubut Province, Argentina. These field sites represent beach ridges with maximum elevations ranging from ~9 to ~30 m aHT. The D/L values indicate deposits from at least two interglacial periods, consistent with previously published results, i.e., MIS 5 and MIS 11, although some discrepancies are present. The AAR results also show mixed-age deposits, indicating reworking of older deposits into younger ones. Our initial results show that AAR analysis of A. antiqua is able to discern interglacial deposits of differing ages and can be used to assess and complement ESR and U/Th results.Video presented in the EBRAM-CLAMA Congress 2023</p