Bromidechinus, a new Ordovician echinozoan (Echinodermata), and its bearing on the early history of echinoids Andrew B. Smith and Jeremy J. Savill ABSTRACT: A new genus and species of primitive echinozoan, Bromidechinus rimaporus , is described from the Upper Ordovician of Oklahoma, USA. This has a unique plate arrangement. There is a single perradial series of imperforate plates bounded on either side by a column of perforate ambulacral plates. A double column of interambulacral plates separates ambulacral zones. The sparse record of Ordovician echinozoans is reviewed and cladistic analysis suggests that Bromidechinus represents a lineage that diverged prior to the split between bothriocidarids and main-line echinoids. This leads to a revised interpretation of the earliest stages of morphological evolution of echinoids. KEY WORDS: cladistic analysis, phylogeny Echinoidea, one of the ®ve classes of echinoderm, today com- prises more than 800 extant species. They are a distinctive and easily recognisable clade with a characteristic tessellated skeleton composed of calcite plates covered in spines. This skeleton, or test, is typically globular in form. Except for a couple of highly derived groups, all have a complex internal dental apparatus known as the Aristotle’s lantern that is used in feeding. Recent morphological and molecular evidence (Littlewood & Smith 1995; Littlewood et al 1997; Mooi & David 1997; Sumrall & Sprinkle 1998) places echinoids as sister group to holothurians, and the Echinoidea and Holothuroidea together form the clade Echinozoa. Holothurians have reduced their body-wall skeleton to small plates or microscopic elements, and have the poorest of body fossil records (Smith & Gallemi 1991). Luckily, their spicules are not uncommon in sediment samples and from this much can be determined about their evolutionary history (Gilliland 1992). Echinoids ®rst appeared in the Ordovician, whereas undis- puted holothurians did not appear until the late Silurian. How- ever, neither the origins of the Echinozoa nor the split of holothurians from echinoids is properly understood. It is thus quite possible that forms described as echinoids from the Ordo- vician represent a mixture of stem-group Echinozoa and basal members of the Echinoidea and Holothuroidea. Indeed, there has been considerable dispute over which of the Ordovician forms represent true echinoids. The root of the problem has been that the earliest echino- zoans fall into two very distinct groups based on their overall test morphology, one exempli®ed by Bothriocidaris and the other by lepidocentrids such as Aulechinus. Bothriocidaris is a small, globular form with thick tessellate plates (Fig. la). Transactions of the Royal Society of Edinburgh: Earth Sciences, 92, 137±147, 2001 Figure 1 The two contrasting morphologies of early Palaeozoic echinoids, as exempli®ed by Bothriocidaris and Aulechinus : (a) Bothriocidaris pahleni Schmidt, BMNH E83655, early Caradoc of Estonia, apical view, £ 5; (b) Aulechinus grayae Bather & Spencer, BMNH E40522a, late Ashgill, Girvan, Ayrshire, lateral view of internal mould, £ 1 . 5.