Estructuras biogénicas de crustáceos en el Mioceno de la cuenca del Golfo San Jorge: implicancias paleobiológicas y evolutivas

CARMONA, N.B.; BUATOIS, L.A.

PUBLICACION ESPECIAL - ASOCIACION PALEONTOLOGICA ARGENTINA

Año: 2003 vol. 9 p. 97 - 108

0328-347X

 Trace fossils made by decapod crustaceans are very common in Cenozoic marine deposits of Patagonia. The ichnogenus Ophiomorpha is particularly abundant in the Lower Miocene Chenque Formation, Chubut Province. At the locality of Punta Delgada, this ichnotaxon occurs in middle to lower shoreface deposits. Ophiomorpha consists of a tridimensional burrow system, with a predominance of horizontal tunnels. Walls are made up of conical sedimentary pellets, which function as structural reinforcements for the burrows. Burrow roofs commonly contain thicker walls than burrow floors. The burrow fill is typically laminated and passive due to gravitational processes. Locally, certain sections of the laminae may be confused with active meniscate backfill by ingestion and excretion of a deposit feeder. This ichnotaxon is interpreted as recording the activity of infaunal suspension-feeding decapods, comparable to those from the Thalassinidea infraorden (e.g. Callichirus major (Say), formely Callianassa major). Two recent species of the Thalassinidea infraorden, Anacalliax argentinensis (Biffar) and Callianassa brachiophthalma (Milne-Edwards), inhabit modern shallow environments in Patagonia. In contrast to recent suggestions, Ophiomorpha, Thalassinoides and Spongeliomorpha are here considered separate ichnogenera because they represent particular morphologies that reflect specific ethological responses to the environment. The Mesozoic radiation of decapod crustaceans was a major event in shallow-marine environments. In post-Paleozoic ecosystems, the decapod burrows became dominant in shallow-marine benthic communities, producing a remarkable increase in depth and intensity of bioturbation. This study shows that Miocene shallow-marine environments had reached levels of bioturbation comparable to those of modern times.