The freshwater coccolepids (Actinopterygii, Chondrostei) from the Jurassic of Patagonia

A. LÓPEZ.ARBARELLO; E. SFERCO; O.W.M. RAUHUT

Berlín

Congreso; Centenary Meeting of the Paläontologische Gesellschaft; 2012

Museum fuer Naturkunde

Due to a bias in the fossil record, Jurassic freshwater fish faunas are very scarce, and only a few of Late Jurassic age are known worldwide: the Almada Fauna from the Cañadón Calcáreo Formation in central Chubut province, Argentina, the Talbragar Fauna from the Talbragar Beds in Australia, and the fish assemblages from the Morrison Formation in the USA. The Almada Fauna is the only Late Jurassic freshwater fish fauna so far reported from South America. The Almada Fauna is preserved in the lowermost part of the Cañadón Calcáreo Formation (latest Oxfordian- Tithonian) and it is mainly composed by very well-preserved and abundant teleosts, representing a single species, Luisiella feruglioi, and numerous specimens representing a small chondrostean. Other actinopterygians,probably non-teleostean neopterygians,are also present, but extremely rare, and all specimens recovered so far are poorly preserved. Among these fishes, the chondrostean, which was originally named ?Oligopleurus groeberi, and later referred to the genus ?Coccolepis, was found to represent a new coccolepid genus. Coccolepid features of the new genus include the presence of sharply pointed denticles covering the dermal bones of the skull roof, maxilla, and scales,amioid type scales, nasal bones ornamented with thin concentric ridges, a small postrostral bone not reaching the rostral, and a large supracleithrum, which is as large or larger than the cleithrum. Distinct features of the new genus include the long and slender, but robust lower jaw, suboperculum, branchiostegal and gular plates ornamented with concentric ridges, gular plate traversed by a sensory canal, supracleithrum longer than cleithrum, a small and oval postcleithrum, and the presence of fringing fulcra only in the caudal fin. The cladistic analysis of the relationships of the new coccolepid genus shows that Coccolepidae (including Coccolepis bucklandi and "Coccolepis" groeberi, the latter representing a new genus) is the sister group of the Acipenseriformes within a monophyletic Chondrostei. Additionally, a survey of the fossil record of the family Coccolepidae shows a shift from marine to continental waters during the Late Jurassic, which is also observed in the fossil record of the acipenseriforms.