High latitude Early Jurassic pectinaceans: clues to the origin of living deep-sea scallops?

DAMBORENEA, S.E.

Córdoba, Argentina

Jornada; Reunión de Comunicaciones Científicas de la Asociación PaleontológicaArgentina; 1996

Asociación Paleontológica Argentina

Living deep-sea pectinaceans include thin-shelled, small-sized scallops with the following morphologies: a) internally-ribbed Propeamussiidae (such as Parvamussium Sacco and Propeamussium De Gregorio), b) internally smooth Propeamussiidae (such as Bathypecten Schein-Fatton, Cyclopecten Verrill and Similipecten Winckworth) and c) very small Pectinidae (such as Delectopecten Stewart and Hyalopecten Verrill). These three main morphological types (but in larger shells) are also found among neritic Early Jurassic fossil pectinaceans: a) Parvamussium is an ubiquitous genus almost cosmopolitan in distribution during the Jurassic; b) Kolymonectes Milova and Polubotko is restricted to high latitudes (both northern and southern) and was referred to the internally smooth propeamussiids on the basis of examination of Argentine material; and c) Ochotochlamys Milova and Polubotko, recently discovered in Patagonia and also antitropical in distribution, clearly belongs to the Hyalopecten group of pectinids. The relations of Kolymonectes and Ochotochlamys to stocks previously thought to appear during the Tertiary indicate that the origin of some of these deep-sea pectinaceans is considerably older and should be extended at least to the Triassic. Furthermore, it is evident that the environmental range of these pectinaceans has been severely restricted or shifted from neritic during the Jurassic to bathyal-abyssal during the Cainozoic. As a result of the "Mesozoic marine revolution" these pectinaceans, with very limited competitive and antipredator capabilities, seem to have been relegated to safer places of the deep seas. Their geographical range also changed from high to lower latitudes. As has been proposed for other invertebrates, the high latitude shelfs may have been sites of origin of taxa known to inhabit modern assemblages in lower latitudes but in deeper waters.