Reappraisal of the fossil record of Polycotylidae (Sauropterygia: Plesiosauria) from the Upper Cretaceous of Chile

Abstract

The polycotylids, mostly short-necked plesiosaurs, were a singular clade of marine reptiles which fossil record is known at least from the Albian to the Maastrichtian, with an almost cosmopolitan geographical distribution. In southern continents, the presence of this group has remained poorly documented, and except for some specimens, most records are limited to fragmentary material. Reports on the presence of this group in South America are infrequent, particularly in Chile, and these have been preliminary described. In this work, a critical review of the fossil record of Upper Cretaceous polycotylids from Chile is carried out. Previous putative reports of the group in the Quiriquina Formation (Maastrichtian) of the Arauco Basin (central Chile) were exclusively supported by misidentified axial elements positively belonging to elasmosaurid plesiosaurs, and therefore, are dismissed here. On the contrary, the presence of Polycotylidae is confirmed in Campanian-Maastrichtian levels of the Dorotea Formation, in the Magallanes Basin. Additionally, specimens previously referred to Elasmosauridae from the Dorotea Formation, are here reassessed to Polycotylidae, adding a new locality with the presence of this group in Chile. The pattern of taxonomic diversity, including polycotylids together with the abundant presence of mostly non-aristonectine elasmosaurs in the Magallanes Basin is similar to that observed in upper Campanian-lower Maastrichtian localities of Argentinean Patagonia and Antarctica, but different from that observed in the upper Maastrichtian Quiriquina Formation, where aristonectine elasmosaurs dominate the plesiosaur fauna. Polycotylids of equivalent age have also been described in other regions of the Gondwana, like New Zealand. The presence of this group at the end of the Cretaceous at high latitudes in austral continents is relevant and suggest some Weddellian biogeographic pattern in this clade before the K/Pg extinction.