Pachyrisma grande Morris & Lycett, 1850: redescription and assessment of its potential role as an ancestor to the Hippuritida

Pachyrisma grande Morris & Lycett, 1850, the type species of the genus Pachyrisma, from the Bathonian (Middle Jurassic) of western England is redescribed, a new lectotype is designated, and rich photographic illustration is provided. The validity of the family Pachyrismatidae is substantiated. The phylogenetic relationships of Pachyrisma grande are discussed. Based on reinterpretation of several shell characters and newly discovered features of ornamentation and hinge dentition, an origin in the Cardioidea rather than the Megalodontidae is suggested for Pachyrisma grande. These characters also support the species as the closest known ancestor of the Hippuritida, implying a potential origin of the Hippuritida in the Cardioidea. A phylogenetic analysis of Pachyrisma grande and presumed related taxa is needed to put the suggested evolutionary relationships to test.