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

Resumo

The external and internal features and the microstructure of the asymmetric rhynchonellides from the Albian–Cenomanian (Cretaceous) transition from the Alicante Province (Eastern Prebetic, Southeastern Spain) have been herein studied. Previous authors placed these rhynchonellides in Cyclothyris difformis, consequently attributing an unquestionable Cenomanian age (Upper Cretaceous) to the deposits in which they appear. The long dorsally concave crura and the leptinoid pattern microstructure of the shell confirm their attribution to the genus Cyclothyris. However, among other diagnostic criteria (e.g., ribbing pattern, relative width), C. difformis shows facultative type of asymmetry; while the forms studied here show obligate asymmetry. Therefore, the new species Cyclothyris ementitum sp. nov. is formally described, being characterized by the biconvexity of its shell, its obligate type of asymmetry and an ornamentation of around 25 ribs on each valve. Thus, the study and revision of these rhynchonellides has contributed to updating the record and distribution of the asymmetric Cretaceous rhynchonellides of the genus Cyclothyris. This work opens a new line of research to better understand the biostratigraphical calibration of the Cretaceous sediments from the Eastern Prebetic, and a new insight into the possible origin of the obligate asymmetry present in C. ementitum.