, Department of Earth Sciences, School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham. Department of Palaeontology, The Natural History Museum, London
One of the most rapid and most important morphological diversifications of echinoids occurred during the late Early and Middle Jurassic. It was during this period that the major lines of irregular echinoid became established, leading on the one hand to the Cassiduloids and on the other, ultimately, to the Holasteroids and Spatangoids. These two groups are now morphologically highly differentiated, but precisely how this was achieved remains poorly understood. It is generally supposed that a small group of late Early-early Middle Jurassic irregulars, the Galeropygidae, are basal to both clades. While the Cassiduloid fossil record can be traced back with confidence to the Galeropygidae, thanks to an excellent fossil record of intermediaries, the origins of the Holasteroid-Spatangoid body plan is less well understood, and they have traditionally been linked to the Galeropygidae through a morphologically rather diverse Jurassic group, the disasteroids. There have been few previous attempts to produce a phylogeny documenting this critical period of divergence among the irregular echinoids, and none that take into consideration all of the pertinent taxa. Here, a comprehensive study of relevant material, coupled with new plating diagrams of many of these taxa, has been used to generate a cladistic analysis of the early irregular echinoid genera. This has confirmed the Galeropygidae as basal to both the Clypeidae and the collyritid-disasterid complex.