Boreal echinoids from the Tethys Realm: Infulaster and Hagenowia from the Upper Cretaceous of eastern Sardinia (Italy)

摘要

The occurrence of typical Boreal echinoid taxa in the Upper Cretaceous of Sardinia is reported and its implications discussed. These new records are unexpected, but not unprecedented, since previous studies have documented other benthic invertebrates with Boreal affinities from the Sardinian Cretaceous. Earlier papers have explained these occurrences by intermittent faunal exchange. This, however, fails to explain why they are absent from most Tethyan shelf deposits. Here we propose a new interpretation: these records could represent examples of a distribution mode widespread in present-day marine species, that of 'isothermal submergence'. This refers to distribution patterns where organisms occur at different depths at different latitudes. The palaeoenvironmental setting of Sardinia (pelagic limestones) and the Chalk Sea (epicratonic deposits) make such an interpretation plausible. A new species of cardiasterid heart urchin, Infulaster navicularis sp. nov., is described, as well as rostrum fragments of a Hagenowia species named in open nomenclature. The specimens were collected from the 'Lanaittu Marl' exposed in the Gorropu Canyon in eastern Sardinia, and are of Coniacian and Late Santonian age, respectively. The new Infulaster species is distinguished by a strongly elongated, non-cordiform outline, high, anteriorly projecting profile, narrow apical angle (c. 55°), and a plastron with two uniserially arranged sternal plates following the labrum. In form it is close to Hagenowia infulasteroides, but it differs from that taxon by its contiguous plastron and larger aboral ambulacral pores.