The Tremadocian rhynchonelliform brachiopod shell beds of NW Argentina from a palaeoecological view

MUÑOZ, DIEGO F.; BIGNON, ARNAUD; BENEDETTO, JUAN LUIS; WAISFELD, BEATRIZ G.

Puerto Madryn

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2018

Occupationof shallow environments by rhynchonelliform brachiopods is rare during theLower Ordovician. However, in the NW Argentina basin this group, especially Tarfaya purmamarcaensis (Benedetto2002), occupied high-energy environments as early as the Tremadocian. Thisspecies forms up to 15 cm thick monospecific concentrations in shoreface andoffshore transition settings, and polytypic pavements in relatively deeperwater offshore environments. Generation of such concentrations could be linkedto the population dynamics of T.purmamarcaensis. The geometric morphometrics analysis alloweddifferentiating four growth stages in the ontogeny of the species. The juvenilephases are absent in high-energy proximal environments while no representativesof the fourth phase (hypermature adults) have been found in the open platformdeposits. On the other hand, measurement of a large number of shells showedthat juveniles are scarce in the shoreface while adults are almost absent inthe offshore. Although concentrations are usually linked to physical processes(storm events), the low taphonomic indices of the shells suggest that transportwas not a highly influential factor. Morphological differences (i.e.development of cardinal canals, ribs incurved posterolaterally) in specimensfrom different environments suggest that the population dynamics could havebeen the main cause in generating different concentrations. According to thesource-sink model, high productivity of brachiopods in the shorefaceenvironment and a passive transport of larvae to the offshore might explain notonly the thicker shallow-water concentrations but also the differences in shellgrowth of populations inhabiting these environments.