Echinoid distribution and sequence stratigraphy in the Cenomanian (Upper Cretaceous) of southern England Andrew B. Smith 1 , Neale E. A. Monks 1, 2 & Andrew S. Gale 3 SMITH, A. B., MONKS, N. E. A. & GALE, A. S. 2006. Echinoid distribution and sequence stratigraphy in the Cenomanian (Upper Cretaceous) of southern England. Proceedings of the Geologists’ Association, 117, 207–217. Almost 3500 individual occurrences of echinoid have been recorded and correlated against a sequence stratigraphic framework for the entire Cenomanian and basal Turonian from three regions of southern England, representing onshore, mid-shelf and deeper-shelf habitats. There are marked differences in the composition and diversity of faunas both across the shelf at a single time interval and through time at the same locality, driven primarily by factors such as sedimentary facies, which are controlled by changing sea-levels. The ranges of individual taxa expand and contract across the shelf as sea-levels change. In mid-shelf environments more onshore taxa appear only near sequence bases, at times of lowest sea-level, while those from more outer shelf settings are found during highstand intervals, and this creates a cyclical pattern of diversity. By comparison with modern faunas, the Middle and Upper Cenomanian of the Sussex coast is likely to have been deposited in water depths of between 100 m and 150 m and the amplitude of sea-level change appears to increase through the Cenomanian. Key words: sea-level, diversity, diachroneity, faunal assemblages 1 Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK (e-mail: a.smith@nhm.ac.uk) 2 Present address University of Nebraska State Museum, Morrill Hall, Lincoln, Nebraska, 68588-0338, USA (e-mail: nmonks@mac.com) 3 School of Earth and Environmental Sciences, University of Greenwich, Medway Campus, Chatham, Kent ME4 4AW, UK (e-mail: a.s.gale@greenwich.ac.uk) 1. INTRODUCTION Echinoids have been an important constituent of the modern marine macrobenthic community ever since the Late Triassic, and today are found across the full spectrum of marine habitats, from the intertidal zone down to depths of more than 5000 m. Many factors control the distribution and preservation potential of individual taxa, the most important being nutrient availability, substrate type and turbulence (e.g. Nebel- sick, 1995, 1996). All three are directly or indirectly correlated with water depth. Life in rocky shore habitats requires special adapta- tions if epifaunal species are not to get torn off and crushed by powerful wave surge. This includes the development of large numbers of strong oral tube feet, the organs that are used for grip, stout spines for wedging into crevices and, in extreme cases, anen- casement of tessellated aboral spines (as in the Upper Jurassic Acrocidaris (Baumeister & Leinfelder, 1998) and the modern Colobocentrotus). Life in littoral sedi- ments is equally precarious because of the dangers of being washed out and crushed by wave surge and in these environments deep burial is a common strategy developed by infaunal irregular echinoids. Below the littoral zone to depths of approximately 30–50 m specialist algal grazers and herbivores are predominant amongst regular echinoids, and bulk sediment swallowers predominate amongst irregular echinoids. The latter ingest sediment more or less wholesale in order to exploit the small quantities of organic particles that it contains (Lawrence, 1987). However, as water depth increases, nutrient supply to the sea floor becomes significantly curtailed and the free organic material available in the sediment dimin- ishes, making bulk sediment swallowing unfeasible. With increasing depth and reducing light penetration, the stands of macroalgae also disappear and algal grazers are confined to areas where there is a supply of drift macroalgae. Consequently, in mid to outer shelf settings the echinoid fauna changes to one dominated by regular carnivore species and irregular phytodetri- tus browsers that mine organics at the sediment–water A version of this paper was presented orally at a joint meeting of the Geological Society of London, the Geologists’ Associ- ation and the Palaeontological Association: The life and work of Jake Hancock (1928–2004) held at the Geological Society, Burlington House, London, 14 October 2004. The meeting was convened by Professor John C.W. Cope, who has also been Guest Editor for the manuscripts arising from the meeting, now published in the Proceedings. Proceedings of the Geologists’ Association, 117, 207–217. 0016-7878/06 $15.00  2006 Geologists’ Association