Extraordinary fossorial adaptations in the oligocene palaeanodonts Epoicotherium and Xenocranium (Mammalia)

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Abstract
New fossils of the rare Oligocene mammals Xenocranium and Epoicotherium add information on their skulls and provide the first information on their postcranial skeletons. These epoicotheres, the latest surviving palaeanodonts, have numerous fossorial adaptations and must have been predominantly subterranean. Their skeletal specializations are similar to, and equal or surpass in degree of development, those of most living fossorial mammals. Principal modifications of the skull are the expanded, domed occiput with broad lambdoid crests, hypertrophy of the malleus-incus and related changes in other ear components, reduced eyes, and (in Xenocranium) a flaring, upturned, spatulate snout. The neck was strengthened by synostosis of the 2nd through 5th cervical vertebrae. The forelimb elements have exaggerated crests, processes, and fossae for muscles used in digging or in stabilizing certain joints. The scapula has a high, stout spine with bifid acromion, a “secondary spine,” and an expanded postscapular fossa for attachment of the teres major muscle. The humerus has an elongate pectoral crest, large lesser tuberosity, long entepicondyle, and large hooklike supinator crest. The enormous incurved olecranon process of the ulna provided insertion for the massive triceps and origin for the carpal and digital flexors, and the latter gained mechanical advantage by incorporating in its tendon a large carpal sesamoid. In the greatly shortened hand, digit three is largest, with its metacarpal and proximal phalanx fused and its claw-bearing ungual-phalanx very large. These traits indicate that Xenocranium and Epoicotherium were among the most specialized “rapid-scratch” diggers ever to evolve. Their remarkable convergence to chrysochlorids reflects a similar mode of digging, with extensive use of the snout for loosening and lifting soil when making shallow foraging burrows. For deeper burrowing, the forelimbs probably loosened the soil while the rear limbs moved it behind. Like many extant subterranean mammals, Xenocranium and Epoicotherium were essentially sightless, but they were specialized for low frequency sound reception. Their extinction may have been due to a combination of environmental change and competition with other fossorial animals, such as proscalopine insectivores and rhineurid amphisbaenians.