Monotoidea (Bivalvia, Tithonian) from northern Neuquén, Argentina.

DAMBORENEA, S.E.; LEANZA, H.A.

Mendoza, Argentina

Simposio; 3º Simposio Argentino del Jurásico; 2007

Comité Argentino de Estratigrafía-Jurásico

The extensive outcrops of the Vaca Muerta formation in the Neuquén Basin have yielded very few bivalves, and these are not well-known. The material was collected near the settlement of Huncal in central Neuquén, from calcareous levels within the black shales, about 250 m below the Huncal Member, 100 m above limestones with Substeueroceras and 170 m below a level with Acevedites. There are two main preservation types. Most of the material appears in shell pavements, with equally abundant right and left valves and a wide size range, in "sandwich concretions". These specimens are commonly complete but disarticulated, and show radial fractures which were probably produced during compaction, as suggested by their orientation at right angles to the shell margins. A few specimens were found on the outer part of large calcareous concretions within black shales, these are often articulated, complete shells, which preserve the original convexity of the valves and in some cases show a close relation to large ammonite shells. The material is described as a new bivalve species and genus referred to the superfamily Monotoidea. The ligament area, which is a key character for establishing relation ships within this group, is only imperfectly known. The right valve resilifer is triangular in shape, slightly prosocline. The new species is comparable to Malayomaorica species, but the shells lack radial ornament and also differ by details of the ears and ligamental area. This new genus is significant to understand the late Jurassic distribution of monotoidean bivalves, with mostly bipolar ranges at the time.