Anatomical correlates and nomenclature of the chiropteran endocranial cast.

Abstract

Bats form a diverse group of mammals that are highly specialized in active flight and ultrasound echolocation. These specializations rely on adaptations that reflect on their morphoanatomy and have been tentatively linked to brain morphology and volumetry. Despite their small size and fragility, bat crania and natural braincase casts ("endocasts") have been preserved in the fossil record, which allows for investigating brain evolution and inferring paleobiology. Advances in imaging techniques have allowed virtual extraction of internal structures, assuming that the shape of the endocast reflects soft organ morphology. However, there is no direct correspondence between the endocast and internal structures because meninges and vascular tissues mark the inner braincase together with the brain they surround, resulting in a mosaic morphology of the endocast. The hypothesis suggesting that the endocast reflects the brain in terms of both external shape and volume has drastic implications when addressing brain evolution, but it has been rarely discussed. To date, only a single study addressed the correspondence between the brain and braincase in bats. Taking advantage of the advent of imaging techniques, we reviewed the anatomical, neuroanatomical, and angiological literature and compare this knowledge available on bat's braincase anatomy with anatomical observations using a sample of endocranial casts representing most modern bat families. Such comparison allows to propose a Chiroptera-scale nomenclature for future descriptions and comparisons among bat endocasts. Describing the imprints of the tissues surrounding the brain also allows to address to what extent brain features can be blurred or hidden (e.g., hypophysis, epiphysis, colliculi, flocculus). Furthermore, this approach encourages further study to formally test the proposed hypotheses.