A remarkable new genus of Mytilidae (Bivalvia) from the Lower Cretaceous of southwestern Gondwanaland

Abstract

The lower Cretaceous strata of the Neuquén 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.