Paleobiology of an extinct trochid genus endemic to the Andean region (Gastropoda, Early Jurassic)

DAMBORENEA, S.E.; FERRARI, S.M.

Mendoza, Argentina

Simposio; 3º Simposio Argentino del Jurásico; 2007

Comité Argentino de Estratigrafía - Jurásico

Jurassic gastropods from South America are poorly known. Among them the Trochidae, a diverse living family with a Triassic origin, was represented by several species, of which the most conspicuous and widely distributed are those belonging to the genus Lithotrochus Conrad. The genus is known in the Andean region from northern Peru to Patagonia, is endemic to this area and has a short time range (Sinemurian-Pliensbachian). In Argentina it had only been recorded from southern Neuquén, but it was also found in southern Mendoza and Chubut. Lithotrochus humboldtii (von Buch) was one of the first Mesozoic gastropod species to be described from South America. The presence of a smooth columellar lip links this genus with the Trochidae, yet its relationships are difficult to establish, due to lack of key protoconch and opercula characters, neither of them preserved in the analyzed material. Almost all previous descriptions and figures seemingly belong to the type species, for which there are at least two available names proposed by von Buch and d´Orbigny. Although found in many localities, its distribution is limited to sublittoral deposits, associated to a varied benthonic fauna, and often near coralliferous facies. They were probably grazing herbivores inhabiting firm substrates in well-lit and oxygenated waters. A detailed analysis of the shell morphology of the type species and its ontogeny shows that there is a change in the translation rate along the coiling axis as the shell grows, which could be related to a change in the organism life habits with time.