Mystemys giganteus n. gen. et sp.: an enigmatic and rare cricetid from the Terre Rosse M013 fissure filling (Gargano, Southeastern Italy).

Mystemys giganteus is a new strongly endemic cricetid that occurs only in the recently discovered M013 Terre Rosse fissure filling of the Gargano area. This species was presented alongside the other taxa of the M013 fissure in a previous paper and it is here described in detail. Although the material includes only five molars, Mystemys is exceptional for its very large size (length of M1: 6.23 mm) and peculiar morphology. The molars are heavy built with bulged and robust cusps, tubercular crests, narrow valleys, very thick and crenulated enamel. The morphological features of Mystemys are very carefully described, with special focus on the evolutionary modifications induced by isolation. The strong endemic modifications as well as the peculiar combination of features distinguish this cricetid from all the other mainland species, obscuring its phylogenetic relationship. Nevertheless, the systematic comparisons with the mainland cricetid genera suggest that this insular taxon should have derived from a Miocene mainland ancestor with a primitive “Democricetodon-like” or “Cricetodon-like” morphological pattern. This conclusion constrains the age of the potential mainland ancestor to a chronological interval spanning MN1–MN9. This time slice is not suitable for the dispersal of the ancestor of Gargano Muridae, thus reinforcing the idea that the Terre Rosse fauna is the result of multiple dispersal events.