Huncalotis, an enigmatic new pectinoid genus (Bivalvia, Late Jurassic) from South America

Abstract

The extensive outcrops of the Late Jurassic–Early Cretaceous Vaca Muerta Formation black shales and marls in the Neuquen Basin have yielded very few bivalves, and these are not well known. The material described here was collected in central Neuquen, from late Tithonian calcareous levels within the black shales, between beds with Substeueroceras sp. and with Argentiniceras noduliferum (Steuer). The material is referred to the new genus Huncalotis and to the new species H. millaini. The strongly inequivalve shells, the ligamental area with a triangular slightly prosocline resilifer, the right valve with ctenolium and a very deep byssal notch, and the lack of radial ornamentation make the shell of this new genus strikingly similar to the Triassic pectinid Pleuronectites. This resemblance may be due to either phylogenetic relationships or convergence. Obliquipecten peruanum Rivera from central Peru is tentatively referred to Huncalotis. The Neuquen specimens are found in two main preservation types. Most of the material occurs in shell pavements, with equally abundant right and left valves and wide size range, within “beef” or “sandwich concretions”. These specimens are commonly complete but disarticulated, and show radial fractures orientated at right angles to the shell margins. A few specimens were found on the outside of large calcareous concretions within black shales; these are often articulated, complete shells, which preserve the original convexity of the valves. In some cases these articulated shells seem to be associated with large ammonite shells, suggesting an epibyssate (possibly also pseudoplanktonic) lifestyle.