Roveacrinids (Crinoidea) from the mid-Cretaceous of Texas: ontogeny, phylogeny, functional morphology and lifestyle

Abstract

New material of known roveacrinids from two Cretaceous sites in Texas is described. Upper Albian strata at Saginaw Quarry furnished rich material of Poecilocrinus latealatus and Roveacrinus pyramidalis; for these two forms cups can be combined with primibrachials and secundibrachials. Some cups of Orthogonocrinus apertus are also present. Juvenile cups of these three species demonstrate ontogenetic changes, which are most prominent in P. latealatus. At the adult stage this species has cups with dish-like lateral wings; the first primibrachials have an aboral bowl; axillary second primibrachials and proximal secundibrachials have a wide aboral bowl; and distal brachials carry spines. The arms lack pinnules. A pelagic lifestyle with a mouth-up position is assumed. In contrast, R. pyramidalis has rod-shaped or stick-like brachials without wings, and food grooves are lacking. The less rich lower Cenomanian strata at the Dottie Lynn site provided a number of cups and brachials of Roveacrinus spinosus. In addition, cups of R. peracutus and R. alatus are present. Styracocrinus Peck is considered synonymous with Roveacrinus Douglas. The double cavity in the cups of the various species is discussed; its proposed function as a hydrostatic organ seems conjectural. Two genera of Saccocominae of the Late Jurassic radiation are thought to be ancestors to similar genera of Roveacrinidae first appearing during the Albian radiation. Derivation of Poecilocrinus may be from Saccocoma, Discocrinus and Plotocrinus possibly from Crassicoma. The ancestry of species of Roveacrinus and Orthogonocrinus with conical cups and two distinct body chambers is unclear. The closely similar juvenile cups of Roveacrinus pyramidalis and Poecilocrinus latealatus suggest a common origin despite the widely diverging arm structure. A comparison of Roveacrinus pyramidalis with the Triassic somphocrinid Osteocrinus reveals similar rod-shaped, smooth and tall brachials lacking a distinct food groove. Based on their mass occurrence and wide distribution, species of Osteocrinus species are thought to have been pelagic. This is substantiated by the presence of cups and rod-shaped brachials in Ladinian black shales of southern China. While food of species of Poecilocrinus presumably consisted of coccolithophores and planktonic foraminifera, collection and transport of food particles by Roveacrinus pyramidalis is enigmatic, as is that of Osteocrinus.