A new view of an old enigma: Reassessing Gymnesicolagus gelaberti (Lagomorpha) from the Middle Miocene of Mallorca (Balearic Islands)

Over 40 years ago, a peculiar fossil lagomorph was described from the Middle Miocene of Mallorca (Balearic Islands, Spain). Gymnesicolagus gelaberti possesses enormous teeth with striking morphological features, making its systematic placement enigmatic. Here we revise the holotype and the currently available type material, and we describe additional, previously unreported specimens. Alongside classical comparative approaches, we performed micro-CT-based 3D reconstructions of selected specimens to investigate internal dental features relevant for taxonomic purposes. Several of the peculiar dental traits of G. gelaberti can be interpreted as insularity-driven modifications consistent with the Island Rule (increased size, hypsodonty/hypselodonty). Other traits provide relevant phylogenetic signals linking G. gelaberti to the European MP30-MN1 stem lagomorphs of the "Amphilagus antiquus-Titanomys group". Most notably, a unique and exceptionally rare p3 morphotype discovered in this mainland stock may explain the puzzling p3 morphotype of G. gelaberti as the fixation of a rare character under the founder effect. The evolutionary history of G. gelaberti shows notable similarities to that of Paludotona spp., other endemic insular stem lagomorphs of the western Mediterranean. Both genera are interpreted as deriving from pre-MN2 continental ancestors, are characterized by extended ghost intervals during their early evolutionary histories, and persisted in isolation within fragmented insular domains for several million years.