| Uintasorex | |
|---|---|
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Plesiadapiformes |
| Family: | †Microsyopidae |
| Subfamily: | †Uintasoricinae |
| Genus: | †Uintasorex |
| Type species | |
| †Uintasorex parvulus Matthew, 1909 | |
| Species | |
Uintasorex is a genus ofprimate which lived inNorth America during theEoceneepoch.[1] Fossils belonging toUintasorex have been dated to theBridgerian andUintanstages, roughly 50.3 to 42 million years ago.
The genus name derives from the Latin word for "shrew" (-sorex), combined with a reference to theUinta Mountains where the holotype fossils were discovered.
Like other microsyopids, the most discussed feature ofUintasorex is its extremely tiny size. It is thought to have been smaller than themouse lemur, the smallest extantprimate.[2]Uintasorex was once thought to have been insectivorous based on its body mass and the principle ofKay's threshold,[2] which suggested that primates lighter than 500 grams tend to be insectivorous and those heavier than 500 grams are folivorous,[3] but the validity of this rule has come into question and can no longer be considered valid.
The hardness of theirenamel allowedUintasorexteeth to endure long enough after death to undergo fossilization, and much of what is known about thegenus comes from dental remains. A defining feature of microsyopids is a distinctive twinning between the hypoconulid-entoconid cusps of themolars, and within thefamily this feature is most developed inUintasorex. Well-defined crests on the upper and lower molars suggest the presence ofinterradicular fibers, a trait seen in other uintasoricinids.[4]Uintasorex sp. is distinguished fromU. parvulus by a larger tooth size.
The first specimen ofUintasorex (YPM VP 013519) was discovered by John W. Chew and L. Lamothe in July 1874.[5] The fossils were uncovered at the Henry's Fork locality of theBridger Formation inSweetwater County, Wyoming.[4] Other specimens ofUintasorex have been recovered from the Bridger Formation at theHypsodus Hill, Twin Buttes, and Tabernacle Butte locality, as well as theFriars Formation,Green River Formation,Tepee Trail Formation, andWasatch Formation.
Thetype species ofUintasorex isU. parvulus. Other species includeU. montezumicus[2] and an as-yet unnamed species tenatively known as "Uintasorex sp.".[4]U. montezumicus is defined by UCMP 104179, a tooth recovered from the Solstice Hill locality of the Friars Formation inCalifornia.[6]Uintasorex sp. is based on a collection of tinyUintasorex teeth recovered from the Green River Formation inUtah which went uncatalogued in the archives of theCarnegie Museum of Natural History until they were rediscovered and described byCharles L. Gazin in 1958.
Uintasorex was described byWilliam Diller Matthew in 1909 and assigned toApatemyidae because of its resemblance toApatemys,Phenacolemur,Trogolemur, and some other fossiltarsiers.[4]William King Gregory supported this classification when he proposed the orderSoricomorpha in 1910, as did Edward Troxell in June 1923.[7] In the coming decades, however, the genus was shuffled among a number of families includingAnaptomorphidae (Gazin 1958; Robinson 1966, 1968; Simons 1963; Simpson 1940, 1959);Chiromyidae (Teilhard 1922);Plesiadapidae (Scholosser 1923, Abel 1931); andPrimates,incertae sedis (Simpson 1945).[2] For much of the 20th century, there was much controversy over whether microsyopsids belonged to Primates orInsectivora, but the latter is now considered awastebasket taxon.[8]
In 1969 two genera,Uintasorex andNiptomomys, were reassigned byanthropologistFrederick Szalay toUintasoricinae, a newsubfamily withinMicrosyopidae.[4] The suggestion thatUintasorex had been a microsyopid was first privately put forward byDonald E. Russell in 1965, and the idea that the species represented a distinct family from the other taxa it was being grouped received its first mention as a footnote in a 1958 paper by Charles L. Gazin. The relationship was formally established when Szalay assigned previously-unstudied dental fragments to specimenAMNH 55664,Uintasorex teeth collected from the Tabernacle Butte locality in Wyoming, identifying the distinct dental features that are now considered ubiquitous in the family Uintasoricinae.Alveojunctus,Berruvius,Navajovius, andPalenochtha have also been included in Uintasoricinae.
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