''Microsyops'' is a

plesiadapiform Plesiadapiformes (" Adapid-like" or "near Adapiformes") is a group of Primates, a sister of the Dermoptera. While none of the groups normally directly assigned to this group survived, the group appears actually not to be literally extinct (in ...

primate found in Middle

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

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

. It is in the family Microsyopidae, a

family characterized by distinctive lanceolate lower first incisors. It appears to have had a more developed sense of smell than other early primates. It is believed to have eaten fruit, and its fossils show the oldest known dental

cavities Cavity may refer to: Biology and healthcare *Body cavity, a fluid-filled space in many animals where organs typically develop **Gastrovascular cavity, the primary organ of digestion and circulation in cnidarians and flatworms *Dental cavity or too ...

in a

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

. There are nine species of ''Microsyops'' that exist in the fossil record from the middle

Wasatchian The Wasatchian 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 55,400,000 to 50,300,000 years BP lasting . It is usually co ...

(~53 million years ago) through

Uintan The Uintan 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 46,200,000 to 42,000,000 years BP lasting . It falls within the ...

(~42 million years ago) North American Land Mammal Ages. ''Microsyops'' is primarily known from the

Rocky Mountain region The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico in ...

of the

United States 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 territorie ...

, though fossils have also been found in

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

and

Texas Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...

Diet and body size

Body size diversity of ''Microsyops'' spans from the 700-gram ''Microsyops cardiorestes'' to over 3000 grams for ''Microsyops kratos,'' estimated using the dimensions of the upper and lower last premolar and first molar. The diet of ''Microsyops'' is varied among the nine species, with the smaller-bodied and more primitive species, like ''M. cardiorestes'', likely able to exist by eating almost exclusively insects. However, larger-bodied species, such as ''Microsyops annectens'' or ''M. kratos'', likely needed to expand their diets to include other food sources. This is also supported by wear facets on the molars of ''M. annecten''s and ''M. kratos'' that are indicative of heavier shearing and crushing forces required of harder foods like fruits and nuts. Another indication of expanded diets, away from strict insectivory, comes in the form of reported

in a sample of ''Microsyops latidens'' where a sample of 1030 individuals included 77 specimens showing signs of

. In this case, cavities are likely caused by a reliance on more sugary foods, such as fruits, moving away from strict insectivory.

Morphology

Dental morphology

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. ''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 related ''Arctodontomys''. Lower molars each have a small yet distinct paraconid, a semi-compressed trigonid, a developed mesoconid, and a small, twinned hypoconulid. The upper canine of ''Microsyops'' 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 related ''Craseops''. Additionally, ''Microsyops'' upper molars lack a postprotocingulum, in contrast to the condition found in most early

Paleogene The Paleogene ( ; British English, also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period, geologic period and system that spans 43 million years from the end of the Cretaceous Period million yea ...

primates.

Cranial and postcranial morphology

The basicranium of ''Microsyops'' 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 of ''Microsyops'' 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. 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 crown scandentians and dermopterans. The most characteristic aspect of the cranial morphology of Microsyops is the presence of a postorbital process. This trait is unlike the condition found in early Paleogene primates, which possess a full postorbital bar. However, Microsyops also differs from other

plesiadapiforms Plesiadapiformes (" Adapid-like" or "near Adapiformes") is a group of Primates, a sister of the Dermoptera. While none of the groups normally directly assigned to this group survived, the group appears actually not to be literally extinct (in ...

, which lack either a

postorbital bar The postorbital bar (or postorbital bone) is a bony arched structure that connects the frontal bone of the skull to the zygomatic arch, which runs laterally around the eye socket. It is a trait that only occurs in mammalian taxa, such as most streps ...

or process. The postorbital process of ''Microsyops'' has been described as being superficially similar to that of dermopterans. Due to limited available material, very little is known about the postcranial morphology of ''Microsyops'', and Microsyopidae in general.

Taxonomy

''Microsyops'' was first described by

Joseph Leidy Joseph Mellick Leidy (September 9, 1823 – April 30, 1891) was an American paleontologist, parasitologist and anatomist. Leidy was professor of anatomy at the University of Pennsylvania, later was a professor of natural history at Swarthmore ...

in 1872. He compared lower jaw fragments, found by Dr. J. V. Carter in the Bridger Basin of southwestern

Wyoming Wyoming () is a U.S. state, state in the Mountain states, 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 south ...

, to the condylarth ''Hyopsodus gracilis'', named by Professor

O. C. Marsh Othniel Charles Marsh (October 29, 1831 – March 18, 1899) was an American professor of Paleontology in Yale College and President of the National Academy of Sciences. He was one of the preeminent scientists in the field of paleontology. Among h ...

Yale University Yale University is a private research university in New Haven, Connecticut. Established in 1701 as the Collegiate School, it is the third-oldest institution of higher education in the United States and among the most prestigious in the wo ...

. At the time he believed the fragments to represent the same animal and proposed the new binomial combination of ''Microsyops gracilis''. Leidy later compared his ''M. gracilis'' to Marsh's ''Limnotherium elegans'', which was originally described as a diminutive mammal and later as a primate. 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. Microsyopinae and Uintasoricinae are subfamilies within the plesiadapiform family Microsyopidae. ''Microsyops'' is a genus of the subfamily Microsyopinae which also includes the genera ''Arctodontomys'', ''Megadelphus'' and ''Craseops''. This subfamily includes the larger microsyopids. The subfamily Uintasoricinae includes the diminutive taxa ''Niptomomys'', ''Uintasorex'', and ''Choctawius''. Microsyopidae is one of the longest-lived groups of plesiadapiforms, lasting 20 million years in

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. Recognized species of Microsyops includes ''M. elegans'', ''M. annectens'', ''M. scottianus'', ''M. augustidens'', ''M. kratos'', ''M. latidens'', ''M. cardiorestes'', ''M. vicarius'', and ''M. knightensis,'' with ''M. elegans'' being the type species. Some authors argue that microsyopids are plesiadapiforms while others suggest a dermopteran grouping. 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), microsyopids were found to be plesiadapiforms more distantly related to euprimates than plesiadapoids or paromomyoids, and without any special relationship to dermopterans. However, while analyses support a euarchontan grouping, specific relationships of microsyopids to other plesiadapiforms, euprimates, scandentia, and dermoptera remain unresolved. Microsyopids are generally thought to be euarchontans, and some researchers consider them to be stem primates.

Paleoenvironment

Microsyopidae lived from the late

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...

to the middle

in North America. Conditions of the

supported extensive subtropical woodland and rainforest environments which facilitate

arboreal Arboreal locomotion is the Animal locomotion, locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. Th ...

lifestyles. This time is also characterized by the Paleocene-Eocene Thermal Maximum, displaying the highest temperatures of the

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

period. From this peak, steady temperature declines are displayed throughout the middle to late

Notable fossils

A well-preserved skull of ''Microsyops annectens'' from

Carter Mountain Carter Mountain () is in Shoshone National Forest in the U.S. state of Wyoming. Carter Mountain slopes gently up from the Bighorn Basin to the east but has steep cliffs on its western face. The region is well known for large herds of bighorn sheep, ...

in northwestern

has been used to generate a virtual

endocast An endocast is the internal cast of a hollow object, often referring to the cranial vault in the study of brain development in humans and other organisms. Endocasts can be artificially made for examining the properties of a hollow, inaccessible sp ...

via

micro-CT X-ray microtomography, like tomography and X-ray computed tomography, uses X-rays to create cross-sections of a physical object that can be used to recreate a virtual model (3D model) without destroying the original object. The prefix ''micro-'' ...

. Cranial capacity has been estimated as 5.9 cm, yielding an

encephalization quotient Encephalization quotient (EQ), encephalization level (EL), or just encephalization is a relative brain size measure that is defined as the ratio between observed to predicted brain mass for an animal of a given size, based on nonlinear regressi ...

(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 than '' Plesiadapis cookei'', and falls in the lower range of estimates for early Paleogene primates. However, basicranial anatomy is remarkably primitive, because the

auditory bulla The tympanic part of the temporal bone is a curved plate of bone lying below the squamous part of the temporal bone, in front of the mastoid process, and surrounding the external part of the ear canal. It originates as a separate bone (tympanic ...

was not ossified and there are only grooves, rather than bony tubes, for the intrabullar parts of the internal carotid artery and its dependencies. The basicranial anatomy of ''Microsyops'' appears to be little changed from that of primitive

placental Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupial Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsup ...

mammals.

References

{{Taxonbar, from=Q28455296 Plesiadapiformes Prehistoric primate genera Eocene primates Ypresian life Lutetian life Bartonian life Wasatchian Bridgerian Uintan Eocene mammals of North America Fossils of the United States Paleontology in California Paleontology in Colorado Paleontology in Montana Paleontology in New Mexico Paleontology in North Dakota Paleontology in Texas Paleontology in Utah Paleontology in Wyoming Fossil taxa described in 1872 Taxa named by Joseph Leidy