Lithiotis (Bivalvia) bioherms in the Lower Jurassic of east-central Oregon, U.S.A

Abstract

The epifaunal, rudist-like bivalve Lithiotis formed extensive bioherms during the Pliensbachian and Toarcian ages of the Early Jurassic. The best developed bioherms in North America occur in the Pliensbachian Robertson Formation of east-central Oregon, a unit deposited in a shallow sea transgressing across the Suplee shelf. A typical bioherm consists of three biofacies that coexisted laterally and replaced each other vertically. The reef flank assemblage is the most diverse consisting of Nerinea gastropods, terebratulid brachiopods and Lithiotis debris in a calcarenite or calcilutite matrix. The death assemblage consists of tightly packed, current-aligned Lithiotis valves with gastropods and brachiopods much reduced in numbers. The core of the reef is built almost exclusively of in situ Lithiotis forming a bafflestone with a calcilutite matrix. Lithiotis inhabited a stressful environment inimical to most invertebrates since diversity is very low and there is no evidence of bioerosion. During the Pliensbachian, Lithiotis formed an endemic centre encompassing western North America and the Tethyan Ocean, linked through central America and the central Atlantic by the Hispanic Corridor. By the close of the Pliensbachian it had disappeared from North America but in the Toarcian made its first appearance in South America. By the end of the Toarcian, Lithiotis was extinct.