''Apeomyoides savagei'' is a fossil

rodent Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...

from the

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 ...

of the United States, the only species in the genus ''Apeomyoides''. It is known from fragmentary jaws and isolated teeth from a site in the early

Barstovian The Barstovian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 16,300,000 to 13,600,000 years BP, a period of . It is usu ...

, around 15–16 million years ago, of

Nevada Nevada ( ; ) is a state in the Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the 7th-most extensive, ...

. Together with other species from scattered localities in the United States, Japan, and Europe, ''Apeomyoides'' is classified in the subfamily Apeomyinae of the extinct rodent family

Eomyidae Eomyidae is a family of extinct rodents from North America and Eurasia related to modern day pocket gophers and kangaroo rats. They are known from the Middle Eocene to the Late Miocene in North America and from the Late Eocene to the Pleis ...

. Apeomyines are a rare but widespread group that may have been adapted to a relatively dry habitat. As is characteristic of apeomyines, ''Apeomyoides'' was a large eomyid with high-crowned cheekteeth and a large gap between the

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 ...

s and cheekteeth. Furthermore, the cheekteeth—premolars and molars—approach a

bilophodont The molars or molar teeth are large, flat tooth, teeth at the back of the mouth. They are more developed in mammal, mammals. They are used primarily to comminution, grind food during mastication, chewing. The name ''molar'' derives from Latin, ...

pattern, with two distinct lobes. Other features distinguish ''Apeomyoides'' from other apeomyines, including the rectangular shape of the cheekteeth. The fourth lower premolar (p4) is larger than the molars behind it and has two roots, while the lower molars have three.

Taxonomy

''Apeomyoides'' is a member of the

, a diverse rodent family that is now extinct. Eomyids first appeared in the

Middle 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'', "da ...

(~47 million years ago, mya) in North America, where they existed until the end of the

(~5 mya). In Europe, they survived a little longer, until the end of the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Apeomyinae. The first apeomyine to be discovered was '' Apeomys tuerkheimae'', named in 1968 based on fossils from the

Early Miocene The Early Miocene (also known as Lower Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages: the Aquitanian age, Aquitanian and Burdigalian stages. The sub-epoch lasted from 23.03 ± 0.05 annum, Ma to ...

of Germany, and later discoveries have expanded the range of ''Apeomys'' and similar

taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

. In 1998, Oldřich Fejfar and colleagues reviewed ''Apeomys'' and identified a second, related genus '' Megapeomys'', from the Early Miocene of Germany, the Czech Republic, and Japan. These scientists also named a new eomyid subfamily, Apeomyinae, to house ''Apeomys'' and ''Megapeomys''. The Japanese ''Megapeomys'' was named as a separate species, ''Megapeomys repenningi'', in 2011. Further apeomyines have been identified in North America: ''Megapeomys bobwilsoni'' from the

Hemingfordian The Hemingfordian on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 20,600,000 to 16,300,000 years BP. It is usually considered to overlap the lates ...

of Nevada, ''Apeomyoides savagei'' from the

of Nevada, '' Zophoapeomys indicum'' from the

Late Oligocene The Chattian is, in the geologic timescale, the younger of two ages or upper of two stages of the Oligocene Epoch/ Series. It spans the time between . The Chattian is preceded by the Rupelian and is followed by the Aquitanian (the lowest stage ...

of South Dakota, and a possible second species of ''Zophoapeomys'' from the Late Oligocene of Nebraska. Another North American eomyid, '' Arikareeomys skinneri'', from the

Arikareean The Arikareean North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 30,600,000 to 20,800,000 years BP, a period of . It is usuall ...

of Nebraska, has also been reidentified as an apeomyine. In general, apeomyines are a widespread but generally rare group of dentally distinctive eomyids with a tendency towards large size. Fejfar and colleagues suggested that apeomyines were ecologically distinct from other eomyids and probably preferred a drier habitat. Although other scientists recognized Apeomyinae as a subfamily, in a 2008 summary of North American eomyids, Lawrence Flynn placed the group as a

tribe The term tribe is used in many different contexts to refer to a category of human social group. The predominant worldwide usage of the term in English is in the discipline of anthropology. This definition is contested, in part due to confli ...

, Apeomyini, within the subfamily Eomyinae. Flynn described ''Apeomyoides'' as the most derived apeomyine. ''Apeomyoides savagei'' was described as a new genus and species by Kent Smith, Richard Cifelli, and Nicholas Czaplewski in 2006. The generic name, ''Apeomyoides'', adds the Greek suffix ''-ides'', indicating similarity, to the name of the related genus ''Apeomys'', while the specific name, ''savagei'', honors Donald E. Savage for his work on fossil mammals and for assistance to Smith. In the same paper, these scientists argued in favor of apeomyine affinities for ''Arikareeomys''.

Description

''Apeomyoides savagei'' is a large eomyid, though not as large as ''Megapeomys lindsayi'' and ''M. bobwilsoni''. ''Megapeomys repenningi'' from Japan is similar in size, but its cheekteeth are not as high-crowned. ''A. savagei'' shows a series of traits that are characteristic for the apeomyines: high-crowned cheekteeth with thick enamel that are bilophodont (divided into two lobes) in form and a very long

diastema A diastema (plural diastemata, from Greek διάστημα, space) is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars. More colloquially, the condition ...

(gap) between the lower incisor and cheekteeth. However, the cheekteeth are higher-crowned than those of other apeomyines, including ''Apeomys'', ''Megapeomys'', and ''Arikareeomys'', and they are rectangular in shape, while other apeomyines have barrel-shaped teeth. Similarly, the oldest apeomyine, ''Zophoapeomys'', is smaller and has lower-crowned cheekteeth. The known material of ''Apeomyoides'' consists of a number of fragmentary

mandibles In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

(lower jaws) and isolated cheekteeth. The length of the first and second lower molars (m1 and m2) ranges from 1.74 to 2.58 mm, the width from 2.08 to 2.33 mm. The fourth upper premolar (P4) has not been recorded, but there is a specimen of its

deciduous In the fields of horticulture and Botany, the term ''deciduous'' () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, ...

precursor (DP4). This tooth is characterized by four major cusps (protocone, paracone, hypocone, and metacone) and lophs or crests (protoloph, mesoloph, metaloph, and posteroloph), separated by synclines or valleys. The first and second upper molar (M1 and M2) are almost square and similar in size and structure to the DP4. An additional loph on M1 and M2, the entoloph, is incomplete in ''Apeomyoides'', but more prominent in both ''Megapeomys bobwilsoni'' and ''Arikareeomys''. The four lower cheekteeth—the fourth lower premolar (p4) and first through third lower molars (m1–m3)—are high-crowned teeth. Like the upper teeth, they bear four cusps (metaconid, protoconid, entoconid, and hypoconid), four lophs (metalophid, mesolophid, hypolophid, and posterolophid) and three valleys. Each of the lower cheekteeth lacks an additional loph, the ectolophid, which is present in ''Arikareeomys''. The p4 is larger than any of the molars and longer than it is wide. Among the molars, m1 and m2 do not differ appreciably from each other and are a little wider than they are long, while m3 is a little smaller and its back side is more rounded and narrower. Syncline IV, which is located at the back of the tooth, between the hypolophid and posterolophid, is closed at the margins; this valley is open in ''Megapeomys bobwilsoni''. Syncline IV also opens into the centrally located syncline III; this opening is absent in ''Arikareeomys''. There are two roots under p4 and three under each of the molars, fewer than in ''Megapeomys bobwilsoni'', which shows three under p4 and four under the molars. On the mandible, the diastema is very large and the incisor is procumbent (projecting forward), which distinguishes ''Apeomyoides'' from most eomyids apart from ''Megapeomys''. There is a masseteric scar (associated with the jaw muscles) from below the m1 forward to a point in front of p4, below the

mental foramen The mental foramen is one of two foramina (openings) located on the anterior surface of the mandible. It is part of the mandibular canal. It transmits the terminal branches of the inferior alveolar nerve and the mental vessels. Structure The ...

, an opening in the jawbone. In ''Apeomys'' and ''Megapeomys'' this scar only reaches to the level of the front root of p4. The mental foramen is very small and opens in the diastema, near the ventral shelf of the scar; in ''Apeomys'' and ''Megapeomys'' it is located near the dorsal shelf. Further foramina are present on the lingual (inner) surface of the bone, below the cheekteeth. Apeomyoides-fig3

Distribution and ecology

''Apeomyoides savagei'' is from the Eastgate local fauna in the Monarch Mill Formation of Churchill County, Nevada. This fauna dates to the early

land mammal age, around 15–16 mya. This makes ''Apeomyoides'' the youngest known apeomyine, and its occurrence helps close a gap in the known geographic range of North American apeomyines between ''Megapeomys bobwilsoni'' elsewhere in Nevada and ''Arikareeomys'' in Nebraska. Other eomyids found at Eastgate include species of '' Leptodontomys'' and '' Pseudotheridomys''.

References

Literature cited

* * * * * * {{Taxonbar, from=Q16974346 Geomyoid rodents Miocene rodents Prehistoric mammals of North America Prehistoric monotypic mammal genera Prehistoric rodent genera