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Resumen:
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The study of Upper Cretaceous Cyclothyris from Spain and the revision of asymmetrical rhynchonellides from numerous European collections have led to improve some systematical, biostratigraphical and palaeobiogeographical questions, allowing to update taxonomically this group. The species studied here are the following: Cyclothyris difformis (Valenciennes in Lamarck, 1819); Cyclothyris nekvasilovae sp. nov.; Cyclothyris? contorta? (d’Orbigny, 1847); Cyclothyris zahalkai Nekvasilová, 1973; Cyclothyris segurai Berrocal-Casero, 2020; Cyclothyris cardiatelia Berrocal-Casero, 2020; Cyclothyris claudicans (Coquand, 1879); Cyclothyris globata (Arnaud, 1877); and Cyclothyris? vesicularis (Coquand, 1860). Starting from the preliminary interpretation about the functional meaning of the asymmetry in C. cardiatelia, a hypothesis about the origin of the obligate asymmetry in Upper Cretaceous Cyclothyris has been proposed here, which implies a phylogenetic relationship between C. segurai, C. cardiatelia, C. globata and, possibly, C.? vesicularis.
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