PALEOBIOLOGICAL AND TAPHONOMIC STUDY OF TWO MIOCENE SOUTH ATLANTIC SHELF BOTTOM COMMUNITIES DOMINATED BY CRABS

Paleobiological mapping studies, incorporating all faunal elements on two paleosurfaces, document some of the best preserved records of shallow shelf paleocommunities in South America. Both expose remarkable accumulations of crabs. 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 are recorded. These are overwhelmingly dominated by moderately small specimens of Chaceon peruvianus. Disarticulation of fossils and overall diversity on this paleosurface are low. On the younger paleosurface, at least 19 taxa are recorded; six species of crabs, seven of bivalves, two of brachiopods, and one each of gastropods, bryozoans, barnacles, and 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 diversity of the communities of the two paleosurfaces, notably with reference to the crabs, may have been caused by different a) age, b) latitude and/or, c) paleoenvironmental setting (i.e. temperature). The paleosurface in the Puerto Madryn Formation formed during the latest Neogene warming event; that in the Monte León Formation records a highstand of 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.