Sclerobiont communities associated to oysters and pectinids during the Oligocene/Miocene in southern Patagonia

PARRAS, A.

Santa Rosa

Simposio; 2º Simposio Latinoamericano de Icnología; 2013

INCITAP y UNLPam

Oysters and pectinids are calcitic mollusks that are among those with the highest chances of preservation. They also provide an excellent record of the associated sclerobiont communities. Rocks assigned to the San Julián, Monte León, Estancia 25 de Mayo, and Santa Cruz Formations (late Oligocene/early Miocene) exposed along the coast and western area of Santa Cruz Province (Patagonia, Argentina) carry an abundant fauna of these mollusks, offering an excellent opportunity to gain insight into the high-latitude hard-substrate communities in the southern hemisphere during that period. Diversity of encrusters and borers was compared among three bivalve species representing four successive intervals spanning ~10 Ma in the history of the Austral Basin. Crassostrea? hatcheri (Ortmann) built framework reefs in shallow shelf environments, and its valves form large biogenic and sedimentologic concentrations exposed in many areas of southern Patagonia. Data on 87Sr/86Sr rates in specimens from the San Julián Formation yielded a late Oligocene age (late Chattian) and an early Miocene age (early Burdigalian) for those from the Estancia 25 de Mayo Formation. Specimens of Reticulochlamys proximus (Ihering) presented mostly as biogenic concentrations, also from shallow-shelf environments, in the Monte León Formation rendered an early Miocene age (late Burdigalian). The other oyster, Crassostrea orbignyi (Ihering), built reefs in more marginal estuarine environments, and their monospecific concentrations are intercalated in the lowermost beds of the early Miocene Santa Cruz Formation. Richness analyses of the sclerobiont communities on C.? hatcheri, C. orbignyi and R. proximus, revealed the presence of a) endobionts: fungi, algae? (Clionolithes isp., Dendrina isp.), sponges (Entobia isp.), polychaetes (Maeandropolydora isp., Caulostrepsis isp., Trypanites isp.), phoronids (Talpina isp.), bivalves (Gastrochaenolites isp.), gastropods (Oichnus isp.), bryozoans (Pennatichnus isp., Pinaceocladichnus isp.), brachiopods (Podichnus isp.), cirripedians (Rogerella isp.); b) epibionts: coralline algae, polychaetes (Serpulidae indet.), bivalves (Ostreidae, Anomiidae), bryozoans (Leptichnus isp., Cheilostomata indet., Cyclostomata indet.), cirripedians (Balanomorpha indet.); and c) vagile organisms: gastropods (Radulichnus isp.). Results reveled that there are significant differences according to relative abundance among the four studied samples. These differences could be attributed to variations in the nature of the host organism (e.g., lifespan, shell thickness, size and ornamentation), and to differences in environmental conditions (including water depth, salinity and temperature) rather than to changes in sclerobiont diversity through time. In this sense, the sclerobiont community structure appears unaffected by the environmental disturbances occurring during the Paleogene/Neogene transition (e.g., Mi1 event). The relative abundance of borers in oysters is higher than that of encrusters; among them the most abundant are sponges and polychaetes, in agreement with other Cenozoic hard substrate communities studied elsewhere. However, this is not the case for pectinids, in which encrusters and borers occur in similar proportions. Clearly, more work is needed to better understand these diversity patterns. Future research on marine hard-substrate communities in the Cenozoic of Patagonia should include not only detailed bed by bed well-dated sampling, but also the examination of different host species in the same bed, and equivalent hard substrate in different environments.