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Middle Miocene tests of Clypeaster from L’Arrabassada (Tarragona, north-east Spain) show evidence of intense endoskeletozoan colonization, preserved as borings and associated carbonate secretions that allow gastrochaenid bivalves to be identified as the colonizers. Two modes of occurrence have been recognized for these bivalve dwelling cavities; ‘intrastereom clavate borings’ which are restricted to the echinoid stereom, and ‘semi-endoskeletal dwellings’, which penetrate across the test wall and extend as carbonate crypts into the sediment fill of the internal test cavity. Their size, density and position rule out a syn-vivo relationship with the echinoids and demonstrate that colonization was post mortem. Because of the endurance of clypeasteroid skeleton and the pronounced bell-shaped morphology of Clypeaster, the tests of these echinoids provided the most suitable substrates for hard-bottom colonizers on an otherwise sandy seafloor. The scenario described from Tarragona can be extended to other Neogene and Quaternary strata elsewhere; there is ample evidence for the long-term utilization of tests of Clypeaster by gastrochaenid bivalves in shoreface palaeoenvironments.

Echinoid teeth are without doubt the most com- plex and highly specialized skeletal component to have evolved in echinoderms. They are biomechanically con- structed to be resilient and tough while maintaining a self- sharpening point. Based on SEM analysis of isolated tooth elements collected primarily from the Ordovician and Silu- rian of Gotland, we provide a detailed structural analysis of the earliest echinoderm teeth. Eight distinct constructional designs are recognized encompassing various degrees of sophistication, from a simple vertical battery of tooth spines to advanced teeth with multiple tooth plate series and a rein- forced core of fibres. These provide key data from which we reconstruct the early stages of tooth evolution. The simplest teeth are composed of stacked rod-like elements with solid calcite tips. More advanced teeth underwent continuous replacement of tooth elements, as a simple self-sharpening mechanism. Within echinoids tooth design was refined by evolving thinner, flatter primary plates with buttressing, allowing maintenance of a sharper and stronger biting edge. Despite the obvious homology between the lanterns of ophiocistioids and echinoids, their teeth are very different in microstructural organization, and they have evolved different self-sharpening mechanisms. Whereas echinoid teeth evolved from a biseries of mouth spines, ophiocistioid goniodonts evolved from a single series of mouth spines. Rogeriserra rep- resents the most primitive known battery of tooth elements but its taxonomic affinities remain unknown.

Detailed examination of isolated thecal plates belonging to three discocystinid edrioasteroids, Spiraclavus nacoensis Sumrall, Hypsiclavus kinsleyi Sumrall, and Giganticlavus bennisoni Sumrall and Bowsher, reveals striking similarity in morphology among these species. Stereom observed in the ambulacral floor plates indicates that ligamentous connective tissue opened the ambulacral cover plates and muscle tissue closed them. The ambulacral floor plates are interpreted as rigid supports for the oral surface with the interambulacral areas acting as flexible integuments of plates. The aboral surface is interpreted as flexible and highly contractile. All discocystinid thecal openings are consistent in morphology with adaptations for thecal pressurization. Extension and contraction of the theca was accomplished by pumping water in and out of an inflatable sac associated with the periproctial opening. The pedunculate zone is interpreted as passively expanding and contracting by relaxing of mutable collagenous tissue and stiffening when the theca was in the desired position. All of these features illustrate that discocystinid edrioasteroids have highly–evolved morphology and function.