Exceptionally well-preserved silicified hippuritid rudist bivalves from the lower Maastrichtian of Puerto Rico

Abstract

Exceptionally well-preserved (silicified) hippuritid rudists occur in the El Rayo Formation (lower Maastrichtian) of south-western Puerto Rico. Three species belonging to three different genera are present: Caribbea muellerreidi (VERMUNT), Laluzia peruviana (GERTH) and Parastroma guitarti (PALMER). Acid digestion of the limestones has resulted in a collection with numerous three-dimensional left and right valves many with the preservation of the minute details of the pore system. The morphological features of each species are described, and many features are illustrated for the first time. The new material, coupled with descriptions from other studies, demonstrates that six genera of endemic hippuritids evolved in two separate radiations in the New World: an older radiation of forms that had pallial canals in their left valves (Barrettia, Whitfieldiella and Parastroma) and a younger radiation of forms lacking pallial canals in their left valves (Laluzia, Caribbea and Praebarrettia). The exquisite preservation also reveals that in these endemic New World hippuritids the sockets for the teeth consisted of slots into which ribs on the teeth fitted; this contrasts with Old World hippuritids that have true sockets formed from upfolds of the tabulae for the teeth. The distinctive morphology of the tooth sockets is here used to define a monophyletic subfamily for which the name Barrettiinae CHUBB is available.