Paleobiological and taphonomic study of two Miocene South Atlantic shelf bottom communities dominated by crabs

CASADIO, SILVIO; CRAWFORD, ROBERT; FELDMANN, RODNEY; GRIFFIN, MIGUEL; PARRAS, ANA; SCHWEITZER, CARRIE

Salt Lake City

Otro; The Geological Society of America Annual Meeting; 2005

The Geological Society of America

Paleobiological mapping studies, studying all faunal elements on two Miocene paleosurfaces, document some of the best preserved records of the shallow shelf communities of South America. The older paleosurface (early Miocene) crops out in the Monte León Formation, south of the Santa Cruz River, Argentina. The other paleosurface (early late Miocene) crops out in the Puerto Madryn Formation at Puerto Pirámide, Argentina. Both surfaces represent shallow shelf habitats ranging in depth from fair weather to storm wave base. The faunas are primarily preserved in life position; there is little abrasion and fragmentation, most multi-element remains are articulated, and bioerosion is moderate. These taphonomic conditions suggest that the organisms were preserved in their life habitats and that little post-mortem movement occurred. On the older paleosurface, only two crab species were recorded. These were overwhelmingly dominated by moderately small specimens of Chaceon peruvianus. Disarticulation of fossils on this paleosurface is very low and overall diversity is low. On the younger paleosurface, at least 19 taxa were recorded; six species of decapods, seven of bivalves, two of brachiopods, one of gastropods, one of bryozoans, one of barnacles, and one of fish. Oysters represent 19.8%, other bivalves 40.0%, and decapods 22.8% of the identified specimens. All of the crabs are concordant with stratification. The differences recorded in the communities of the two paleosurfaces may have been caused by a) different age, b) different latitude and/or, c) different paleoenvironment setting (i.e., temperature). The paleosurface in the Puerto Madryn Formation was originated during the latest Neogene warming event, while that in the Monte León Formation records the highest relative sea level and the influx of Antarctic waters onto the Argentine Continental Shelf. Change in thermal regime is the most likely cause of the faunal differences.