CLADES BUILT ON SHIFTING SANDS? THE JURASSIC FOSSIL RECORD OF IRREGULAR ECHINOIDS

What factors control the movement of a clade into a previously under-utilised facies? Random character innovation may by chance produce organisms better suited to the new environment. Alternatively, the under-utilised facies may previously have been poorly represented in the geological record, and a long, ‘hidden' evolutionary history of the clade in this environment may be inferred.

These two theories are explored using the Jurassic irregular echinoids as a case study. Irregular echinoids first appear in large numbers in the upper Toarcian and Aalenian of northwestern Europe. These first echinoids occupy agitated sediments in the sublittoral zone, but from the Bathonian, irregular echinoids are found exploiting marls and muds characteristic of more distal facies. New phylogenies of the pertinent groups are used in conjunction with detailed, stage-by-stage maps of the Jurassic of England and France to explore whether this is a real shift, triggered by character evolution, or reflects the poor preservation of distal facies prior to the Bathonian.