A new asymmetric rhynchonellide from the Cretaceous of the Eastern Prebetic (Southeastern Spain)

Abstract

The rhynchonellide genus Cyclothyris M'Coy, 1844, is well represented in the Cretaceous of Europe (e.g., Owen, 1988). Nevertheless, the correct taxonomical classification, and therefore the stratigraphical range and the geographical distribution, of asymmetrical Upper Cretaceous species of Cyclothyris was very confusing until a few years ago, when the asymmetric forms of this genus were updated. This was the result of an intensive revision of numerous specimens of Cyclothyris stored in historical collections from Europe and Northern Africa (e.g., d'Orbigny and Coquand Collections, among others) and also the description of new species (Berrocal-Casero, 2020;Berrocal-Casero et al., 2020a, 2020b. For this previous research, it was essential to distinguish between the terms proposed by Fürsich and Palmer (1984) as 'facultative' type of asymmetry of the anterior commissure, in which the individuals of the same species can develop both symmetric and asymmetric shells [e.g., the Cenomanian Cyclothyris difformis (Valenciennes in Lamarck, 1819