Paper No. 23
Presentation Time: 1:30 PM-5:30 PM
TAPHONOMY OF RECENT ARTICULATE BRACHIOPODS FROM SUBTROPICAL, SILICICLASTIC-CARBONATE ENVIRONMENTS OF SOUTHERN BRAZIL
This taphonomic analysis focuses on unique brachiopod-mollusk assemblages from carbonate-siliciclastic shelf environments off the coast of Brazil. The assemblages are dominated by Bouchardia rosea, a calcitic, punctate terebratulid brachiopod. This study provides first rigorous taphonomic data for present day articulate brachiopods from sites analogous to low-latitude settings of the Paleozoic Era, in which brachiopods thrived. Three sedimentary environments of general area of the Ubatuba Bay, São Paulo (23°-24°S and 44°-46°W) were investigated: (1) sand-dominated beaches; (2) organic-rich, fine-grained (silt/sands) subtidal settings in protected areas of the bay; and (3) coarse biogenic subtidal settings of the outer portions of the bay. These settings represent a bathymetric cross-section of nearshore environments (from 0 to 35m). Grab (1/40m2), dredge, and beach samples, furnished a total of 913 brachiopod shells. Shell conditions were categorized into presence/absence grades, including (1) articulation, (2) valve type, (3) fragmentation, (4) abrasion, (5) edge modification, (6) color alteration (interior/exterior), (7) dissolution (i/e), (8) bioerosion (i/e), (9) encrustation (i/e), and (10) predation. Distinct taphonomic signatures that can be found in each of the three settings suggest that taphofacies with a remarkable spatial resolution can be delineated using brachiopod shells. We found that (1) discoloration, bioerosion and dissolution are the dominant processes in all settings; (2) disarticulation and fragmentation are common, especially in the beach and outer bay settings; (3) high rates of encrustation were observed in shells from the protected areas of the bay, which is remarkable considering that paleontological literature indicates that epibionts rarely settled on Paleozoic punctate brachiopods; (4) abrasion-derived alterations characterize shells from the beach setting; and (5) pedicle valves or the robust (hinge) shell fragments dominate beach assemblages, implying a pronounced preservational bias. Because B. rosea displays shell microstructure and mode of life believed to have been common among Paleozoic brachiopods, the taphonomic patterns described here should aid taphonomic studies of many brachiopod assemblages through the Phanerozoic fossil record.