Article: Implications of lantern morphology for the phylogeny of post-Palaeozoic echinoids
Aristotle's lanterns of the Liassic echinoids Diademopsis tomesii (Wright) and Eodiadema aff. minutum (Buckman) are described, and compared with those of living echinoids. Diademopsis has a primitive lantern and weakly grooved teeth. The ultrastructure of these teeth is well preserved, and it is shown how Recent grooved, keeled, and diamond-shaped teeth could be derived from a tooth of this construction. Eodiadema has diamond-shape teeth of a type previously known only in irregular echinoids, and its lantern is intermediate in form between the regular Diademopsis-typs of lantern and two types of irregular echinoid lantern (cassiduloid and holectypoid). The evolution of tooth and lantern in post-Palaeozoic echinoids is outlined, and is the basis for constructing a phytogeny. All living echinoids form a monophyletic group whose stem-group includes archaeocidarids and some miocidarids as its most advanced members. Cidarids are the monophyletic primitive sister group to the euechinoids and, within the Euechinoidea, echinothurioids are the primitive sister group to all others. Irregular echinoids are a monophyletic group with a stem group that includes Eodiadema as one of its members. Irregular echinoids are separated into three groups, eognathostomates, neognathostomates, and atelostomates. Eognathostomates, comprising pygasteroids and holectypoids, are the primitive sister group of all other irregular echinoids. Cassiduloids and clypeasteroids are grouped together as neognathostomates while spatangoids and holasteroids are placed in the atelostomates. Echinaceans are another monophyletic group whose most primitive members belong to the Pseudodiadematidae. Echinaceans and irregulars both evolved from an aulodont ancestor.