MIDDLE TRIASSIC MARINE BIVALVES OF THE NEW PASS RANGE, WEST-CENTRAL NEVADA: PALEOBIOGEOGRAPHIC IMPLICATIONS

A nearly continuous sequence of normal marine Middle and Upper Triassic rocks ranging from Anisian to Carnian age is exposed in South Canyon, on the western flank of the New Pass Range in Churchill County, Nevada. These rocks unconformably overlie Permian rocks of the Golconda allochthon and are believed to have been deposited on the allochthon after its emplacement on the continental margin in the late Permian or early Triassic. The fauna thus provides a rare biogeographical calibration point along the cratonal margin for judging the latitudinal affinities of faunas of comparable age on displaced terranes.

Previous studies of colonial corals, hydrozoans, and involutinid foraminiferans in the Ladinian part of this section have emphasized west Tethyan affinities. An ongoing systematic study of the bivalves is revealing a very different paleobiogeographic picture. Twenty bivalve species exhibit remarkable endemism at both the species (60%) and genus (29%) levels, and several species provide clear biogeographic ties with the Northern and especially Eastern Pacific Realms. One example is a new pectinid genus and species that occurs in association with "Tethyan" colonial corals but has congeners only in known cratonal rocks of NE British Columbia of Ladinian and Carnian age (the Mahaffy Cliffs and Lima? poyana faunas of McLearn). Another example higher in the section is a distinctive gigantic bakevelliid strongly resembling a species in the Tethyan Sephardic province (S. Spain, Algeria, Jordan). The Nevada form occurs in association with three ammonoid species that are the same as or comparable to Canadian species that characterize the Meginae to Sutherlandi Zones of the Ladinian as well as with species of Protrachyceras resembling Ladinian Tethyan species. Morphological details of the bivalve, however, indicate a sister-group relationship with the Sephardic form, not conspecificity, and suggest descent from an older, probably Anisian, common ancestor.

Past assessments of Tethyan affinities of this fauna may be biased by including only slowly evolving taxa. The more rapidly evolving bivalves, in contrast, are more sensitive indicators of biogeographic affinity and, with the ammonoids, demonstrate that the Easter Pacific Realm was an important center of speciation in the Ladinian.