EXPERIMENTAL AND FIELD OBSERVATIONS ON THE PRESERVATIONAL BIAS OF BRACHIOPOD VALVES IN PRESENT-DAY ASSEMBLAGES OF BOUCHARDIA ROSEA FROM THE BRAZILIAN SHELF: PALEOECOLOGICAL IMPLICATIONS

In paleoecology valve ratios (dorsal/ventral; [DV/VV]) significantly different from the expected 50:50 ratio are accepted as evidence of a taphonomic bias. Usually, this bias is attributed to physical factors such as differential valve fragmentation, transport and/or settling. Modern brachiopod-rich assemblages from the Brazilian shelf are typically valve biased. Experimental data show that this type of bias is not only linked to extrinsic (environmental) factors, but also tied to intrinsic factors. Shells of Bouchardia rosea (the dominant species in the studied assemblages) are endopunctate in nature, with a thin prismatic layer (primary) and a thick fibrous-fabric secondary layer. SEM-analysis indicates that the prismatic layer is two times thicker in the case of the ventral valve, and the fibrous layer is ten times thicker in the case of the dorsal valve. Punctae density (punctae/mm2) is also variable, being denser in ventral valves and at the shell anterior end. Finally, dorsal valves are smaller, thicker, and heavier. To check the role of these intrinsic factors experimentally, 120 unaltered valves of B. rosea were cleaned, dried, weighed, measured, and placed in separated baths of 100ml of acetic acid solution (1%). Observations were recorded at 0.5h dissolution intervals for the first 3h, at 1h for the subsequent 6h, and then at 3h intervals until 58h. Dissolution rate (DR, %/h), dissolution half-life (DHL, h), total time for dissolution (TTD, h), and the percent of weight lost after 30 minutes (W30) were calculated: DR is higher for ventral valves (71.3 VV, and 58.2 DV), DHL (11.5 VV, and 24 DV) is shorter for ventral valves, and TTD (45 VV and 42 DV) and W30 (5.7 VV and 4.2 DV) are both higher for ventral valves. In sum, ventral valves are more prone to diagenetic-bias, since they are thinner, have a thicker, easier-to-dissolve primary shell layer, and denser punctae. Color alteration, bleaching, thinning of edges, roundness of shell sculptures, and chalky texture are signatures found in shells subjected to dissolution experiments. These data are consistent with field-based observations (Brazilian Platform) where brachiopod assemblages with dissolution signatures are dominated by dorsal valves. This study indicates that the valve bias may be driven by a complex interplay of both intrinsic and extrinsic factors.