Virtual endocast of the Late Miocene Hoplitomeryx matthei (Artiodactyla, Hoplitomerycidae) and brain evolution in insular ruminants

Mammals often follow peculiar evolutionary trajectories on islands, with some Pleistocene insular large mammals exhibiting reduced relative brain size. However, the antiquity of this phenomenon remains unclear. Here, we report the first digital endocast of an insular artiodactyl, the five-horned ruminant Hoplitomeryx matthei from the Late Miocene Gargano palaeo-island (Apulia, Italy). We compare its brain morphology with that of extant and extinct relatives, including the early bovid Eotragus and the Mid-Miocene cervid Euprox, and investigate endocranial size and shape variation across 35 ruminant species. H. matthei displays a derived pecoran brain morphology, similar to that of bovids. This finding suggests that its ancestor, rather than deriving from an Oligocene member of Tragulina, was a Pecora and colonized Gargano no earlier than the Early Miocene. This is further supported by its encephalization quotient and the presence of a prominent marginal pole at the top of its endocasts, also found in Caprini. However, unlike the Balearian mouse goat Myotragus balearicus, H. matthei does not exhibit a reduced occipital region of the neocortex or olfactory bulbs. Instead, it underwent only a minor brain size reduction, highlighting distinct pathways of brain evolution in different island ecosystems. This study provides new insights into the biogeographic history of Hoplitomeryx and the palaeoneuroanatomy of insular mammals prior to the Quaternary.

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