Re-evaluation of the morphology and phylogeny of Diplocynodon levantinicum Huene & Nikoloff, 1963 and the stratigraphic age of the West Maritsa coal field (Upper Thrace Basin, Bulgaria)

Abstract

Diplocynodon levantinicum Huene & Nikoloff, 1963 was described based on few bone fragments from the West-Maritsa lignite basin of Central Bulgaria. Huene & Nikoloff, 1963 assumed a late Pliocene age, implying that this species represents the stratigraphically youngest crocodilian of Europe. In this current study, we re-evaluate the stratigraphy of the West-Maritsa Basin and conclude a late Oligocene age of ~26 Ma for the Kipra coal-seam, the fossiliferous horizon. Furthermore, topotypical and undescribed D. levantinicum specimens are accessible now and allowed for a deeper taxonomic and phylogenetic analysis. A comparison with other Diplocynodon species reveals D. levantinicum as a valid species, having (1) a long suborbital fenestra, (2) a very short dentary symphysis, (3) a large gap between the first and second dentary alveolus, (4) an occlusion pit in line with the tooth row posterior to the 14th dentary alveolus, (5) a sulcus lateral to the glenoid fossa and, (6) a lingual foramen for the articular artery situated entirely on the surangular. The phylogenetic analyses find D. levantinicum deeply nested inside the Diplocynodontinae subfamily. After the disappearance of the Paratethyan influence (Solenovian regional stage) in the Upper Thrace Basin this species has roamed during the late Oligocene extensive freshwater lake and swamp ecosystems represented by the Maritsa Formation. Diplocynodon levantinicum represents the only nominal Diplocynodon taxon of late Oligocene (Chattian) age.