Early Jurassic protobranch bivalves from Chubut, Argentina

Abstract

Knowledge on Early Jurassic marine fossil invertebrates from Argentina is very uneven. Particularly, faunas from Chubut Province received less attention and were thought to be poorer than those from the Neuquén Basin. Nevertheless,an updated revision of bivalves shows that some groups, such as the Protobranchia, were relatively more diverse in Chubut than in Neuquén during late Pliensbachian?early Toarcian times. Only four species are known from the southern areas of the Neuquén Basin, while seven species (three of them new) are here described from Chubut, belonging to the families Nuculidae, Nuculanidae, Polidevciidae, and Malletiidae. The new species are: Palaeonucula feruglioi n. sp., Ryderia tehuelchana n. sp., and Phaenodesmia? piatnitzkyi n. sp. The material is well preserved and allows a detailed analysis of their hinge regions and other internal characters. The presence in Chubut of Palaeoneilo? patagonidica (Leanza) is confirmed, which, along with Praesaccella ovum (Sowerby), are the only two species in common with the Neuquén Basin. The new species Ryderia tehuelchana, belonging to a genus that had a wide global distribution between Rhaetian and Toarcian times, but which was known from South America only for upper Triassic deposits, is of special paleobiogeographic interest. These bivalves are restricted to normal marine environments, and due to their detritivorous feeding habits they only live in organic-rich bottoms as very active shallow burrowers.