FERNANDEZ Diana Elizabeth
congresos y reuniones científicas
Nereites-like trace fossils in Lower Cretaceous marginal-marine environments from Patagonia: preservational variants and ichnotaxonomic discussion.
FERNÁNDEZ, DIANA E.; PAZOS, PABLO J.
Congreso; The Third International Congress on Ichnology (ICHNIA); 2012
Epichnial, winding to meandering structures with an inner area or median tunnel enveloped by an outer zone of reworked sediment, assignable to Nereites Macleay, are described. The material comes from the Lower Cretaceous Agrio and Mulichinco Formations (Neuquén basin, Patagonia, Argentina). It was found in marginal-marine deposits, with either wave ripple lamination or combined-flow structures, and sometimes in association with Asteriacites von Schlotheim or BoloniaMeunier. These burrows display three distinguishable morphotypes which, following previous authors? recommendations, are referred to informally as preservational variants of Nereites. In Morphotype A, a median meniscate tunnel and an outer lobate to diffuse zone is observed. This morphotype resembles N. missouriensis in its ?lobate form?. Morphotype B presents a lobate outer zone and a core of discrete uniserial pustules sometimes converging into a uniform tunnel. This morphotype is similar to Nereites missouriensis (Weller) var. phyllodocites Geinitz when discrete pustules are distinguishable, and to Nereites jacksoni Emmons when pustules converge. In Morphotype C only the core is preserved, as a uniserial chain of pustules. This morphotype resembles Nereites missouriensis var. uniserialis, or Nereites isp. preservational variation Neonereites uniserialis. Given its epichnial preservation, it might be considered as Nereites missouriensis in a ?pearl chain form?; the lack of a connecting string disregarded an assignment to Hormosiroidea Schaffer. Intergradation between morphotypes is observed. In agreement with previous authors, it is considered here that: a) Neonereites Seilacher is a preservational variant of Nereites; b) these preservational variants are not originated by changes in behaviour, but most likely by toponomic relationships and taphonomic bias; c) the most plausible producers are, in this case, arthropods.