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 feruglioin. sp.,Ryderia tehuelchanan. sp., andPhaenodesmia?piatnitzkyin. sp. The material is well preserved and allows a detailed analysis of their hinge regions and other internal characters. The presence in Chubut ofPalaeoneilo?patagonidica(Leanza) is confirmed, which, along withPraesaccella ovum(Sowerby), are the only two species in common with the Neuquén Basin. The new speciesRyderia 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.UUID:http://zoobank.org/41225cf2-b7de-4ff0-ad20-ce0d738799a9

Revision of Jurassic Protobranch Bivalves from Gebel Maghara, northern Sinai, Egypt

“…Therefore, he placed Palaeonucula as a subgenus in Nucula , and accepted Nuculoma as a genus. The general shell morphology of Palaeonucula is similar to Nucula Lamarck, but the shell is non-nacreous and has smooth shell margins (Damborenea and Pagani, 2019, p. 929). In addition, species of Nucula should be restricted to forms with radial ribbing on the shell surface, which clearly distinguishes them from both Palaeonucula and Nuculoma (Duff, 1978, p. 23).…”

“…6E, F) assigned Leda decorata to Ryderia . According to Damborenea and Pagani (2019), the records of Ryderia from the Middle Jurassic of Egypt are doubtful and, therefore, they suggested that Douvillé's species probably belongs to the genus Nuculana . Based on the diagnostic features of Ryderia (a very long, narrow rostrum, a well-developed umbonal ridge, a well-developed corselet, an entire pallial line, and fine commarginal striae; Hodges, 2000, p. 40, 45), the present material (138 well-preserved specimens) can be placed in Ryderia without any doubt (Fig.…”

“…In addition, the oldest well-documented records of Ryderia are from Rhaetian beds (e.g., Ivimey-Cook et al, 1999) and it extends to the lower Toarcian (e.g., Fürsich et al, 2001). Damborenea and Pagani (2019, p. 934) pointed out that Middle Jurassic records of Ryderia are doubtful. The latter view is not accepted here, because Dietze et al (2021, p. 48, pl.…”

“…11, figs. 5, 6) from the Toarcian of France and Ryderia tehuelchana Damborenea and Pagani, 2019 (p. 934, figs. 5.1–5.5, 5.18, 7.4) from the Lower Jurassic of Chubut (Argentina) display a different ornamentation, particularly on the rostrum.…”

“…Praesaccella has a wide geographic distribution; it has been recorded from the Lower Jurassic of Chile (Aberhan, 1994, p. 10), Argentina (Damborenea, 1987; Damborenea and Pagani, 2019), Middle Jurassic of western India (Cox, 1940; Kanjilal and Singh, 1973; Jaitly et al, 1995), Spain (Delvene, 2001), Japan (Hayami, 1961), and Toarcian or Bajocian of East Africa (Cox, 1965). The species is recorded from Middle Jurassic rocks (upper Bajocian–Bathonian) of Egypt for the first time (Fig.…”