Abstract
T he lower Cretaceous strata of the Neuquen Basin of Argentina are richly fossiliferous and characterized by diverse, primarily normal marine, molluscan-dominated faunas containing many interesting taxa. Among these, in the Lower Hauterivian part of the Agrio Formation at Cerro Mesa (Fig. 1), are diverse, coarsely ornate trigoniid bivalves of the genus Steinmanella Crickmay, 1930, and a new genus and species of ornate Mytilidae ( Nodomytilus trigonimimus ). The new taxon appears to be convergent in its surface sculpture on the Austral Gondwanian trigoniid Steinmanella transitoria (Steinmann, 1881). This may represent a rare case of convergence, or even mimicry, in the fossil record. The trigoniids have relatively low predation levels among bivalves (Kauffman, personal observation on fossil and recent populations). Convergence on the trigoniid morphotype could have been an adaptive advantage to frequently predated Mytilidae. The new taxon is described herein and compared to co-occurring Steinmanella . Figure 1 —Map showing the location of the fossil locality (*) yielding Nodomytilus trigonimimus n. gen. and sp The Cerro Mesa area in the Neuquen Basin contains extensively developed outcrops of the lower portion of the Agrio Formation (Weaver, 1931) which forms part of the Mendoza Group (Groeber, 1946). The Agrio Formation is characterized in this part of the Neuquen Basin by a shallow marine, storm-dominated, siliciclastic platform comprising shoreface facies, reflecting a high-energy depositional regime. It is composed of fine- to coarse-grained sandstones and siltstones with carbonate cement, interbedded with greenish-gray shales containing mostly reworked bivalves, gastropods, cephalopods, echinoids, and serpulid worms. Lenses of coquinoid limestones are also common (cf. Leanza, 1981b; Legarreta and Uliana, 1991). Within this stratigraphic context, Nodomytilus trigonimimus n. gen. and sp. occurs in association with the following taxa (see Leanza, 1993, 1998): Bivalves: Steinmanella (Transitrigonia) transitoria (Steinmann, 1881), S . ( T .) raimondii (Lisson, 1930), S …
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