Cranial anatomy of Hypisodus minimus (Artiodactyla: Ruminantia) from the Oligocene Brule Formation of North America

A [ACT study of two skulls of the small ruminant Hypisodus minimus from the White River Group (Late Eocene to Late Oligocene) of western North America revealed the following remarkable features: (i) downwards and inwards curved nasal bones that meet at the median line and form a bony tube, (ii) big orbitae supported by a closed orbital bar consisting almost entirely of the os frontale, (iii) a pair of indeterminate canals inside the corpus ossis basisphenoidalis, running in ventro-dorsal direction, and (iv) heavily inflated bullae tympanicae with fused paracondylar processes. The bullae are separated by a narrow slit from each other and are not in contact, as stated earlier. Large bullae enhance low-frequency hearing as seen in burrowing mammals such as the extant caviomorph rodent Dolichotis. In the inner ear, a secondary crus commune is missing and the cochlea shows 2.44 coils. In one skull the maxilloturbinalia, one undefined turbinal and the ethmoid bone (comprising two ethmoturbinalia, two frontoturbinalia, lamina semicircularis, and lamina cribrosa), as well as the anterior part of the lamina horizontalis are preserved.