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.). Differences appear when diversity among the four samples of bivalve-hosted sclerobiont communities is compared. Richness is higher in Oligocene C.? hatcheri and in R. proximus (nine eleven endobionts, seven epibionts and one vagile), and lower in Miocene C.? hatcheri (nine endobionts, three epibionts and one vagile). Instead, C. orbignyi contains only six eigth sclerobionts. Dominance index is very low for the four samples and the Simpson index is high, indicating a high evenness with non-dominant taxa. Chi-squared test was used to test the likelihood of samples with taxon abundance data being from the same community. Results reveled that there are significant differences according to relative abundance among the four studied samples (p