''Xenocranium'' ("strange skull") is a

monotypic genus In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

of extinct epoicotheriid mammal whose fossils were recovered from late

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

of the

American state In the United States, a state is a constituent political entity, of which there are 50. Bound together in a political union, each state holds governmental jurisdiction over a separate and defined geographic territory where it shares its sove ...

Wyoming Wyoming () is a state in the Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the southwest, and Colorado to the s ...

. The single species is ''Xenocranium pileorivale''.

Etymology

The name ''Xenocranium'' comes from the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

words ''xenos'' and ''cranios'', meaning "strange" and "skull" respectively, after its unique skull arrangement. The specific epithet is derived from ''pileus'' meaning "hat" and ''rivale'' meaning "brook", in reference to the nearby township of Hat Creek,

U.S.A. The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

Description

''Xenocranium pileorivale'' possesses many traits indicative of a subterranean lifestyle, including small eyes, an upturned snout, muscular arms with large attachment points for the

triceps The triceps, or triceps brachii (Latin for "three-headed muscle of the arm"), is a large muscle on the back of the upper limb of many vertebrates. It consists of 3 parts: the medial, lateral, and long head. It is the muscle principally responsibl ...

teres major The teres major muscle is a muscle of the upper limb. It attaches to the scapula and the humerus and is one of the seven scapulohumeral muscles. It is a thick but somewhat flattened muscle. The teres major muscle (from Latin ''teres'', meaning ...

, and carpal and digital flexor muscles. The

dental formula Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiolog ...

is .

Palaeoecology

The holotype of ''Xenocranium pileorivale'' was recovered from the

Brule Formation The Brule Formation was deposited between 33 and 30 million years ago, roughly the Rupelian age ( Oligocene). It occurs as a subunit of the White River Formation in Nebraska, Colorado, North Dakota, South Dakota, and Wyoming. It is a sequen ...

, 160 feet below the top of the Oligocene outcrop in the area, which correlates with the

Chadronian The Chadronian age within the North American Land Mammal Ages chronology is the North American faunal stage typically set from 38,000,000 to 33,900,000 years BP, a period of . It is usually considered to fall within the Eocene epoch. The Chadroni ...

age to the

Orellan The Orellan North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), 34-32 million years ago. It is usually considered to fall within the Early Oligocene. ...

age under the

NALMA The North American land mammal ages (NALMA) establishes a geologic timescale for North American fauna beginning during the Late Cretaceous and continuing through to the present. These periods are referred to as ages or intervals (or stages when re ...

classification. Later on, this layer was found to be from Priabonian age of late

. Further remains have been found in the

White River Formation The White River Formation is a geologic formation of the Paleogene Period, in the northern Great Plains and central Rocky Mountains, within the United States. It has been found in northeastern Colorado, Dawes County in western Nebraska, Badlands ...

Nebraska Nebraska () is a state in the Midwestern region of the United States. It is bordered by South Dakota to the north; Iowa to the east and Missouri to the southeast, both across the Missouri River; Kansas to the south; Colorado to the sout ...

. The animals from these formations constitute the

White River Fauna The White River Fauna are fossil animals found in the White River Badlands of South Dakota, Wyoming, Colorado and Nebraska in the United States including Badlands National Park. The fossils have been found in the White River Formation, Chadron F ...

, which included predators like ''

Archaeotherium ''Archaeotherium'' ( grc, αρχαιοθήριον, meaning "ancient beast") is an extinct genus of entelodont artiodactyl endemic to North America during the Eocene and Oligocene epochs (35—28 mya), existing for approximately . ''Archa ...

'' and ''

Hyaenodon ''Hyaenodon'' ("hyena-tooth") is an extinct genus of carnivorous hyaenodont mammals from tribe Hyaenodontini, within subfamily Hyaenodontinae in family Hyaenodontidae,Malcolm C. McKenna, Susan K. Bell (1997."Classification of Mammals: Above the ...

'', and a large diversity of

herbivorous A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpar ...

mammals, such as the archaic horse ''

Mesohippus ''Mesohippus'' (Greek: / meaning "middle" and / meaning "horse") is an extinct genus of early horse. It lived 37 to 32 million years ago in the Early Oligocene. Like many fossil horses, ''Mesohippus'' was common in North America. Its shoulder hei ...

'', the cursorial rhinoceros '' Hyracodon'', and the very common "

oreodont Merycoidodontoidea, sometimes called "oreodonts" or "ruminating hogs", is an extinct superfamily of prehistoric cud-chewing artiodactyls with short faces and fang-like canine teeth. As their name implies, some of the better known forms were gener ...

" ''

Merycoidodon ''Merycoidodon'' ("ruminating teeth") is an extinct genus of herbivorous artiodactyl of the family Merycoidodontidae, more popularly known by the name ''Oreodon'' ("hillock teeth"). It was endemic to North America during the Middle Eocene to Mid ...

''.

Phylogenetic tree

The phylogenetic relationships of genus ''Xenocranium'' is shown in the following cladogram:

References

{{Taxonbar, from=Q19717449, from2=Q21369124 Palaeanodonta Prehistoric mammal genera