Taphonomy of Bouchardia rosea (Rhynchonelliformea, Brachiopoda) shells from Ubatuba Bay, Brazil: implications for the use of taphonomic signatures in (paleo)environmental analysis

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Braquiópodes (Rhynchonelliformea, Bouchardioidea) neógenos da bacia de Pelotas (RS) e seu significado paleoambiental

The occurrence of brachiopods in Cenozoic rocks of the Pelotas Basin is known since 1862. In spite of that, detailed systematic and taphonomic studies are still missing. Investigations made a half century ago, have suggested that these brachiopods could belong to Bouchardia cf. zitteli, a species found in the San Julian Formation, Late Oligocene, Argentina. Our data suggest that those brachiopods may resemble Bouchardia transplatina. In the Uruguayan portion of the Pelotas Basin B. transplatina is known in rocks of the Camacho Formation, Miocene. In addition, small recrystallized shells of brachiopods were also recovered from three Petrobras boreholes (2PJ-1-RS, 2PN-1-RS, and 2GA-1-RS) from the Pelotas Basin. Brachiopods come from the interval of 130 to 150 meters within the Miocene Henryhowella evax Zone. Despite the degree of taphonomic modiication of those brachiopod shells they indubitably belong to Bouchardia sp. This is noteworthy for various reasons: 1- Bouchardia is a brachiopod with warm water afinities. Presently, extant members of this genus are unknown in latitudes up to 34[degree]S, with the main records at 23[degree]S. 2- Although occurring in depths down to 200 meters, the living member (Bouchardia rosea) of this genus is most abundant in shallow platformal, nutrient-rich waters. 3- The occurrence of Bouchardia in the Miocene of the Pelotas Basin indicates that, at least to the interval of Henryhowella evax Zone, warm waters of the Brazilian currents prevail. This interpretation is in strong accordance with other paleoeoceanographic and paleoclimatic data offered by various groups of co-occurring microfossils, such as ostracodes and foraminifers.

Biotic interaction between spionid polychaetes and bouchardiid brachiopods: Paleoecological, taphonomic and evolutionary implications

Shells of Bouchardia rosea (Brachiopoda, Rhynchonelliformea) are abundant in Late Holocene death assemblages of the Ubatuba Bight, Brazil, SW Atlantic. This genus is also known from multiple localities in the Cenozoic fossil record of South America. A total of 1211 valves of B. rosea, 2086 shells of sympatric bivalve mollusks (14 nearshore localities ranging in depth from 0 to 30 m), 80 shells of Bouchardia zitteli, San Julián Formation, Paleogene, Argentina, and 135 shells of Bouchardia transplatina, Camacho Formation, Neogene, Uruguay were examined for bioerosion traces. All examined bouchardiid shells represent shallow-water, subtropical marine settings. Out of 1211 brachiopod shells of B. rosea, 1201 represent dead individuals. A total of 149 dead specimens displayed polychaete traces (Caulostrepsis). Live polychaetes were found inside Caulostrepsis borings in 10 life-collected brachiopods, indicating a syn-vivo interaction (Caulostrepsis traces in dead shells of B. rosea were always empty). The long and coiled peristomial palps, large chaetae on both sides of the 5th segment, and flanged pygidium found in the polychaetes are characteristic of the polychaete genus Polydora (Spionidae). The fact that 100% of the Caulostrepsis found in living brachiopods were still inhabited by the trace-making spionids, whereas none was found in dead hosts, implies active biotic interaction between the two living organisms rather than colonization of dead brachiopod shells. The absence of blisters, the lack of valve/site stereotypy, and the fact that tubes open only externally are all suggestive of a commensal relationship. These data document a new host group (bouchardiid rhynchonelliform brachiopods) with which spionids can interact (interestingly, spionid-infested sympatric bivalves have not been found in the study area despite extensive sampling). The syn-vivo interaction indicates that substantial bioerosion may occur when the host is alive. Thus, the presence of such bioerosion traces on fossil shells need not imply a prolonged post-mortem exposure of shells on the sea floor. Also, none of the Paleogene and Neogene Bouchardia species included any ichnological evidence for spionid infestation. This indicates that the Spionidae/ Bouchardia association may be geologically young, although the lack of older records may also reflect limited sampling and/or taphonomic biases.