''Aphelops'' (Greek: "smooth" (apheles), "face" (ops), in a reference of lacking a horn) is an extinct genus of hornless

rhinocerotid A rhinoceros ( ; ; ; : rhinoceros or rhinoceroses), commonly abbreviated to rhino, is a member of any of the five extant taxon, extant species (or numerous extinct species) of odd-toed ungulates (perissodactyls) in the family (biology), famil ...

s endemic to North America. It lived from the Middle

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

to the early

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Teleoceras''.

Description

On the basis of skull size, the largest species of ''Aphelops'' is ''A. mutilus'' (which is the largest North American rhinoceros) and the smallest is the

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

''A. megalodus''. ''A. mutilus'' has been estimated to have weighed , and ''A. malacorhinus'' has been estimated at . ''Aphelops'' can be distinguished by other members of the

Aceratheriinae Aceratheriinae is an extinct subfamily of rhinoceros endemic to Asia, Africa, Europe, and North America, from the Oligocene through the Pliocene. It lived from 33.9 to 3.4 mya, existing for approximately . Taxonomy Aceratheriinae was named by D ...

by two traits: the arched top of the skull, and the long diastema (gap) between the second incisor (lower tusk) and first premolar. Many other aspects of its anatomy are typical of aceratheriines, including: the absence of a horn on the broad, unfused

nasal bone The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Eac ...

s; the reduced

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...

and lost first

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...

; a nasal incision (or notch below the nasal bones) reaching at least the level of the fourth

premolar The premolars, also called premolar Tooth (human), teeth, or bicuspids, are transitional teeth located between the Canine tooth, canine and Molar (tooth), molar teeth. In humans, there are two premolars per dental terminology#Quadrant, quadrant in ...

; a triangular-shaped skull when viewed from the rear; narrow

zygomatic arch In anatomy, the zygomatic arch (colloquially known as the cheek bone), is a part of the skull formed by the zygomatic process of temporal bone, zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the ...

es;

brachydont The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone tooth ...

or low-crowned teeth without

cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...

; upper molars bearing a fold of enamel known as an anterocrochet; and lower tusks that are subcircular in cross-section. More derived (specialized) species have larger teeth and longer, more slender nasal bones (also producing a larger incision). The various species also differ in the depth of the nasal incision: in ''A. megalodus'', it reaches the front of the fourth premolar; in ''A. malacorhinus'', it reaches the rear of that tooth; and in ''A. mutilus'', it reaches the first molar.

Palaeobiology

The lower tusks of ''Aphelops'' are informative about the sex and age of individuals, as in ''Teleoceras'': males have much thicker tusks, while older individuals have more strongly erupted tusks showing more extensive wear patterns. The stages in the eruption of the molar-like cheek teeth were identical to living rhinos. These factors were used by Matthew Mihlbacher in 2003 to analyze a

bone bed A bone bed is any Geology, geological stratum or deposition (geology), deposit that contains bones of whatever kind. Inevitably, such deposits are Sedimentary rock, sedimentary in nature. Not a formal term, it tends to be used more to describe esp ...

of ''Aphelops'' known as the Love Island bonebed; adults (between 35% and 68% of maximum lifespan) are most common, there appears to be no male bias, and young males are rare. This is unlike ''Teleoceras'', where young males are disproportionately preserved in bone beds. Such a difference may imply different social behaviour: young male ''Aphelops'' perhaps died less frequently from fighting, and used different kinds of ritualistic displays. ''Aphelops'' were probably not completely monogamous, with the degree of

sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

being suggestive of

polygyny Polygyny () is a form of polygamy entailing the marriage of a man to several women. The term polygyny is from Neoclassical Greek πολυγυνία (); . Incidence Polygyny is more widespread in Africa than in any other continent. Some scholar ...

Palaeoecology

''Aphelops'' is thought to have been a browser on C3 plants, like the modern

black rhinoceros The black rhinoceros (''Diceros bicornis''), also called the black rhino or the hooked-lip rhinoceros, is a species of rhinoceros native to East Africa, East and Southern Africa, including Angola, Botswana, Eswatini, Kenya, Lesotho, Malawi, Moza ...

, with its longer limbs adapted to traversing open, brushy country. Isotopic evidence suggests that, in the late Miocene, ''Teleoceras'' had limited intake on newly-emergent C4 grasses, but ''Aphelops'' continued to browse. This difference in feeding ecologies explains how they could coexist sympatrically across many environments for 13 million years. Both went extinct at the end of the

Hemphillian The Hemphillian North American Stage on the geologic timescale is a North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 10,300,000 to 4,900,000 years BP. It is usually considered t ...

North American land mammal age 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 ref ...

, most likely due to rapid climate cooling, increased seasonality and expansion of C4 grasses.

References

{{Taxonbar, from=Q773433 Zanclean extinctions Prehistoric mammals of North America Miocene mammals of North America Pliocene mammals of North America Miocene rhinoceroses Pliocene rhinoceroses Fossil taxa described in 1845 Taxa named by Richard Owen Ringold Formation Miocene Fauna