Are rudist bivalves cockles? How Pachyrsima grande Morris & Lycett, 1850 may jumble Mesozoic bivalve phylogeny

Are rudist bivalves cockles? How Pachyrisma grande Morris & Lycett, 1850 may jumble Mesozoic bivalve phylogeny.

Simon Schneider, CASP, 181A Huntingdon Road, Cambridge CB3 0DH, UK; e-mail: [email protected]

Pachyrisma grande and the family Pachyrismatidae are currently considered the most likely ancestors of the Hippuritida. The Hippuritida [vulgo ‘rudists’] are one of the most diverse bivalve groups of the Mesozoic with approximately 200 genus-level and 2,000 species-level names available. The origins of Pachyrisma are supposed to lie in the Megalodontidae, a group of large-sized Triassic lagoonal bivalves, predominantly occurring in the Tethyan realm.

Re-study of Pachyrisma grande has revealed several characters previously unknown or misinterpreted. In particular, these are the presence of radial external ornament on the posterior slope; a posterior lateral tooth PI; a pitted pallial line; the absence of a lunule; and the shape and position of the nymph. These features support the species as the closest known ancestor of the Hippuritida, but not as a descendent of the Megalodontidae. The cardioid hinge and the lack of a lunule suggest an origin close to Protocardia in the Cardioidea.

The proposed evolutionary relationships have wider implications for bivalve systematics: (1) Megalodontidae probably went extinct at the Triassic-Jurassic boundary. (2) Early cardioid evolution has to be revised. (3) Hippuritida are probably descended from Cardioidea via the Pachyrismatidae as a link. (4) Cardioidea are thus paraphyletic with regard to Hippuritida.