| Microsyops | |
|---|---|
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Plesiadapiformes |
| Family: | †Microsyopidae |
| Subfamily: | †Microsyopinae |
| Genus: | †Microsyops Leidy, 1872 |
| Species | |
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Microsyops is aplesiadapiform primate found in MiddleEocene inNorth America.[1] It is in the familyMicrosyopidae, aplesiadapiform family characterized by distinctive lanceolate lower first incisors.[2] It appears to have had a more developed sense of smell than other early primates.[3] It is believed to have eaten fruit, and its fossils show the oldest known dentalcavities in amammal.[4][5]
There are nine species ofMicrosyops that exist in the fossil record from the middleWasatchian (~53 million years ago) throughUintan (~42 million years ago) North American Land Mammal Ages.Microsyops is primarily known from theRocky Mountain region of theUnited States, though fossils have also been found inCalifornia andTexas.[6][7]
Body size diversity ofMicrosyops spans from the 700-gramMicrosyops cardiorestes to over 3000 grams forMicrosyops kratos, estimated using the dimensions of the upper and lower last premolar and first molar.[8][9] The diet ofMicrosyops is varied among the nine species, with the smaller-bodied and more primitive species, likeM. cardiorestes, likely able to exist by eating almost exclusively insects.[9] However, larger-bodied species, such asMicrosyops annectens orM. kratos, likely needed to expand their diets to include other food sources. This is also supported by wear facets on the molars ofM. annectens andM. kratos that are indicative of heavier shearing and crushing forces required of harder foods like fruits and nuts.[9] Another indication of expanded diets, away from strict insectivory, comes in the form of reportedcavities in a sample ofMicrosyops latidens where a sample of 1030 individuals included 77 specimens showing signs ofcavities. In this case, cavities are likely caused by a reliance on more sugary foods, such as fruits, moving away from strict insectivory.[10]
Consistent with other North American members of Microsyopidae,Microsyops has a lower central incisor that is enlarged, procumbent, and lanceolate. The expansive flattened surface of the lower central incisor is oriented towards the front of the tooth.[2]Microsyops has a lower dental formula of 1-0-3-3, with one incisor, no canine, three premolars, and three molars. The lower second premolar is single-rooted, and the third premolar is premolariform. The fourth lower premolar has a distinct metaconid, no paraconid, and a two-cusped talonid with a more fully-developed basin than in the closely relatedArctodontomys. Lower molars each have a small yet distinct paraconid, a semi-compressed trigonid, a developed mesoconid, and a small, twinned hypoconulid.[9]
The upper canine ofMicrosyops is double-rooted. The upper fourth premolar has a distinct metacone and a weak parastyle. Upper molars exhibit clear conules, in particular a distinct metaconule, unlike the condition in the closely relatedCraseops.[9] Additionally,Microsyops upper molars lack a postprotocingulum, in contrast to the condition found in most earlyPaleogene primates.[2]
The basicranium ofMicrosyops has been described in detail in order to determine its affinities with respect to other mammals. Based on basicranial features, the internal carotid artery which supplied blood to the brain ofMicrosyops was primitive with respect to both extinct and extant euarchontans. These features include a transpromontorial groove indicating an unreduced internal carotid artery and grooves marking the course for both stapedial and promontorial branches of the internal carotid artery (Silcox et al. 2020). Another characteristic that suggests Microsyops was primitive is the presence of unexpanded caudal and rostral tympanic petrosal processes.[11] Unlike other plesiadapiforms,Microsyops lacked a bony auditory bulla (Gunnel 1989, Silcox et al. 2020). Additionally,Microsyops lacks the specialized cranial morphology considered characteristic of crownscandentians anddermopterans.[11]
The most characteristic aspect of the cranial morphology of Microsyops is the presence of a postorbital process.[2][11] This trait is unlike the condition found in early Paleogene primates, which possess a full postorbital bar. However, Microsyops also differs from otherplesiadapiforms, which lack either apostorbital bar or process.[11] The postorbital process ofMicrosyops has been described as being superficially similar to that of dermopterans.[2]
Due to limited available material, very little is known about the postcranial morphology ofMicrosyops, and Microsyopidae in general.[2]
Microsyops was first described byJoseph Leidy in 1872. He compared lower jaw fragments, found by Dr. J. V. Carter in the Bridger Basin of southwesternWyoming, to the condylarthHyopsodus gracilis, named by ProfessorO. C. Marsh ofYale University. At the time he believed the fragments to represent the same animal and proposed the new binomial combination ofMicrosyops gracilis. Leidy later compared hisM. gracilis to Marsh'sLimnotherium elegans, which was originally described as a diminutive mammal and later as a primate.[12] He concluded they were the same but with L. elegans as a species of the genus Microsyops, and that his original Microsyops gracilis should be properly named Microsyops elegans.[9]
Microsyopinae and Uintasoricinae are subfamilies within the plesiadapiform family Microsyopidae.Microsyops is a genus of the subfamily Microsyopinae which also includes the generaArctodontomys,Megadelphus andCraseops. This subfamily includes the larger microsyopids. The subfamily Uintasoricinae includes the diminutive taxaNiptomomys,Uintasorex, andChoctawius. Microsyopidae is one of the longest-lived groups of plesiadapiforms, lasting 20 million years inNorth America from the late Paleocene to late Eocene (Silcox et al. 2021). Two families of plesiadapiforms, Microsyopidae and Paromomyidae, have representative taxa from the Uintan Land-Mammal Age (middle Eocene) while the Plesiadapidae and Carpolestidae disappeared at the end of the Paleocene.[13]
Recognized species of Microsyops includesM. elegans,M. annectens,M. scottianus,M. augustidens,M. kratos,M. latidens,M. cardiorestes,M. vicarius, andM. knightensis, withM. elegans being the type species.[13]
Some authors argue that microsyopids are plesiadapiforms while others suggest a dermopteran grouping.[14] However, the overall relationship between plesiadiforms and other living and fossil members of Euarchontoglires has been disputed. In a cladistic analysis including postcranial, cranial, and dental characteristics by Bloch et al. (2007),[15] microsyopids were found to be plesiadapiforms more distantly related to euprimates than plesiadapoids or paromomyoids, and without any special relationship to dermopterans.[14] However, while analyses support a euarchontan grouping, specific relationships of microsyopids to other plesiadapiforms, euprimates, scandentia, and dermoptera remain unresolved.[11] Microsyopids are generally thought to beeuarchontans, and some researchers consider them to be stem primates.[2]
Microsyopidae lived from the latePaleocene to the middleEocene in North America.[14] Conditions of theEocene supported extensive subtropical woodland and rainforest environments which facilitatearboreal lifestyles. This time is also characterized by thePaleocene-Eocene Thermal Maximum, displaying the highest temperatures of theCenozoic period. From this peak, steady temperature declines are displayed throughout the middle to lateEocene.[16]
A well-preserved skull ofMicrosyops annectens fromCarter Mountain in northwesternWyoming has been used to generate a virtualendocast viamicro-CT.[14] Cranial capacity has been estimated as 5.9 cm3, yielding anencephalization quotient (EQ) of 0.26-0.52 depending on different body mass estimates and the choice of equation used to estimate EQ.Microsyops has larger EQ thanPlesiadapis cookei, and falls in the lower range of estimates for early Paleogene primates. However, basicranial anatomy is remarkably primitive, because theauditory bulla was not ossified and there are only grooves, rather than bony tubes, for the intrabullar parts of the internal carotid artery and its dependencies.[11] The basicranial anatomy ofMicrosyops appears to be little changed from that of primitiveplacental mammals.
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