Original paper

Thecal (oral zone) elements in cystoids from Spain

Cid, María Dolores Gil; Rincón, Juan Manuel García

Abstract

In the Middle Ordovician of the Hesperic Massif, there is a great abundance of echinoderms in siliciclastic rocks. We especially highlight thecae of cystoids (diploporites). At present, the different study methods used on diploporites are based fundamentally on determinations carried out from incomplete specimens. We have checked the frequency with which aboral extremes have been used to obtain systematic determinations. We agree that this morphological part presents a high degree of variability that may correspond with adaptations to ways of life. Different genera or even species present similar designs of the aboral structure, which is not the case with the oral surface. Therefore, we understand that diagnoses should preferably be based on oral features. Diploporites from Navas de Estena mostly belong to Sphaeronitidae and Aristocystitidae. The genus Calix (Aristocystitidae) is the most cited diploporite in Spanish palaeontological literature, and the taxonomical features used in its determination are mainly based on the shape and size of the theca, shape, size and disposition of thecal plates, structure and disposition of the breathing pores and additional thecal elements. To date, taxobases of diploporites have included elements which are highly variable and which present few specific differences making them unreliable. It is therefore advisable to study the different components of the oral surfaces, as well as the rest of the thecal elements such as tubercles and protuberances arranged in lines, size and design of these elements.

Keywords

echinodermscystoidsordovicianspaindiploporita