NEW PALEOCENE RHYNCHONELLIDE BRACHIOPODS FROM THE POTRERILLOS FORMATION, NORTHEAST MEXICO

Two new species of the rhynchonellide brachiopod Probolarina are described from a Paleocene limestone lentil associated with a diapir in La Popa basin, northeastern Mexico. Both species are generally larger than other species of Probolarina found in the western hemisphere. New species A differs from new species B and previously described species based on its thicker proportions and more prominent, narrow costae. The brachiopods are abundant in the lentil and are distributed uniformly along its length, in addition to occurring in clusters. Both species were located in distinct zones, with species A collected several feet above species B.

The brachiopods were collected from the Upper Mudstone Member of the Potrerillos Formation. Abundant mollusk and echinoid fragments; benthic foraminifera, corals, small gastropods and bryozoa were also present. Previous paleontological reports from the black shales of this member include the nautiloid Cimomia haltomi Aldrich, an index fossil for the Paleocene in the Parras and La Popa basins. The lentil represents a shallow marine, subtidal environment, adjacent to a gypsum diapir. The lentil contains a very pure limestone and formed on the topographic rise of the diapir during episodes of low sediment input to the basin.

This is the oldest record for the genus in the western hemisphere, as the only other Paleocene species of this genus was reported from New Zealand. It is the first report of brachiopods for the Difunta Group from which a diverse marine fossil fauna has been recorded. Previous reports of Probolarina in the western hemisphere consisted of a sparse number of individuals and species of Eocene age.

Probolarina is one of a number of brachiopod genera that appear to represent early Cenozoic diversification among the articulated Brachiopoda. The Terebratellidae have previously been identified as having undergone significant radiation during the Cenozoic; rhynchonellides may have undergone modest radiation too. The Cretaceous-Tertiary transition appears to have had little effect on diversity among the Brachiopoda.