The Tethyan oyster Pycnodonte (Costeina) costei (Coquand, 1869) in the Coniacian (Upper Cretaceous) of the Iberian Basin (Spain): Taxonomic, palaeoecological and palaeobiogeographical implications

Oysters were among the most important groups of marine invertebrates in Late Cretaceous biotas of the Tethyan Realm. During the major eustatic sea-level highstands of this interval, large areas of northern Africa and southern Europe were characterized by carbonate shelf environments with conditions favourable for the widespread dispersal of highly diverse marine ecosystems, which included many ostreid bivalves. In this palaeoecological setting, Pycnodonte (Costeina) costei (Coquand, 1869) is one of the most widespread representatives of the subfamily Pycnodonteinae, extending from Central Asia and Egypt to southern France and the Iberian Peninsula.This taxon is characterised by a strong convex left valve with an expanded posterior flange, suggesting that this species was a free-lying recliner on infralittoral soft substrates. The species is also remarkable due to its morphological plasticity, with ecophenotypes characterised by extremely variable left valve ornamentation. The assemblages of P. (C.) costei studied herein occur in middle to upper Coniacian outer-shelf facies within the northern and central sectors of the Iberian Basin (Spain), associated with transgressive and highstand system tracts. The species occurs mostly in autochthonous concentrations of several tens of individuals, cemented or oriented side by side and colonised by epizoans and borers. Endolithic sponges related to the ichnogenus Entobia were especially abundant, infesting many specimens to a great extent.These new finds in the Iberian Basin also improve the known palaeogeographic range of P. (C.) costei. Its distribution can be related to current circulation along the Tethys Ocean, firstly from its original area in Central Asia to North African shelves and then to the Iberian Basin and France, across the proto-Atlantic Lusitanian area. This dispersal model inferred by these currents also explains much of the Tethyan affinities found in the Late Cretaceous marine faunas of Iberia.