Rudist Evolution, Ecology and Environments

Peter Skelton
Open University, United Kingdom ([email protected])

The bizarrely shaped sessile epifaunal bivalves known as “rudists” make their first appearance in the Oxfordian (Upper Jurassic) of the Mediterranean Tethys. By the close of the Jurassic they had spread across the Tethyan Realm and became among of the most characteristic occupants of its shallow carbonate platforms during the Cretaceous.
From the outset they were characterized by an outer shell layer of fibrillar prismatic calcite covering a thick aragonitic inner shell with massive dentition. Attachment to the substrate was by the left or the right valve, according to taxon. During their 93 Myr history, ended by catastrophic extinction at the close of the Cretaceous, they underwent a series of evolutionary radiations, punctuated by mass extinctions, which were closely tied with the episodic growth and demise of the platforms. In primitive forms, the umbones grew spirally, like rams’ horns, due to the constraint of an external ligament that migrated backwards throughout life. They lived either attached to other shells or partially embedded in sediment, as spiralling “clingers,” often associated with corals.
In the Tithonian, some smaller forms attaching by the right valve invaginated the ligament and could thus grow uncoiled tubular valves. This innovation sparked an evolutionary diversification of ecological morphotypes ranging from partially embedded “elevators”, occupying areas of net positive sediment accumulation, to large “recumbents” lying prone in areas of sediment bypassing. While elevators spread from outer to inner platform environments, forming laterally extensive biostromes in the latter, recumbents occupied energetic platform margins.
Extinction episodes struck the outer platform associations most intensely, and those of the early Aptian and terminal Cenomanian, in particular, witnessed the temporary disappearance of the predominantly aragonitic recumbents. Because the surviving elevators and clingers usually had significantly thicker calcitic outer shell layers, involved in mutual attachment, faunal turnover at these times was thus also associated with distinct shifts in the calcite/aragonite ratio of rudist shells, with interesting petrological implications.

AAPG Search and Discovery Article #90073 © 2007 AAPG Foundation Distinguished Lecturer Series 2007-2008