INTRODUCTION

Echinoids have been one of the success stories of the Mesozoic and Tertiary. Since the Triassic, echinoids have been increasing in diversity, expanding to occupy new habitats and exploit new sources of food. Today there are approximately 900 extant species, more or less equally divided between regular (pentaradiate forms with the periproct enclosed by the apical system and lying at the opposite pole to the mouth) and irregular (secondarily bilateral forms with the periproct displaced out-side the apical system) taxa. Echinoids are found in all marine habitats, from the Poles to the Equator and from the intertidal zone to more than 5000 m depth (Smith, 1984).

Many important changes have taken place in the evolution of echinoids over the past 145 million years. Diversities of echinoid clades have waxed and waned relative to one another, and the range of niches occupied by echinoids in marine benthic communities has expanded significantly. Specialist feeding strategies have been evolved and life history strategies have also been profoundly altered in certain groups. Unfortunately, in most cases insufficient work has been done to establish whether extrinsic, environmental factors are involved in driving these changes. However, in a few cases we do seem to have an environmental correlate. Major changes in preservational potential and perceived diversity probably correlate to rates of sea-level change, and a marked shift in favour of lecithotrophic (i.e. planktic, non-feeding) or brooded development was possibly driven by increasing climatic seasonality.