An exceptional cheloniid turtle, Osonachelus decorata nov. gen., nov. sp., from the Eocene (Bartonian) of Catalonia (Spain)

Osonachelus decorata nov. gen., nov. sp., is the first named chelonioid turtle from the Iberian Peninsula. This Eocene turtle was found in the marine sediments of the Vespella Marls Member (Vic-Manlleu Marls Formation, upper Bartonian) that occur in the Osona county (eastern Ebro Basin, Province of Barcelona, Catalonia, Spain). The area is rich in invertebrate, fish, and plant fossils but various vertebrates, including several chelonian taxa, have remained unpublished. This paper is the first dedicated to the fossil turtles of this area. Osonachelus nov. gen. is a large turtle with an impressive, massive skull revealing a specialized diet, a large and well-fenestrated shell (well ornamented dorsally and much lightened ventrally), and large paddles for high-sea locomotion. It is phylogenetically located among relatively advanced cheloniids, just basal to modern forms. Although apparent similarities in shell morphology exist with Allopleuron from the Maastrichtian of Holland, Osonachelus nov. gen. is distinct due to its masticatory apparatus, less usual among cheloniids and seemingly closer to that of the Lutetian Eochelone from Belgium. Based on the deeply concave and smooth lower jaw symphysis, its diet probably consisted of soft animals or plants rather than hard-food items. Therefore, it occupied a specialized ecological niche that is unique for the Eocene of the Iberian Peninsula.