''Microsyops'' is a plesiadapiform primate found in Middle

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

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

. It is in the family Microsyopidae, a plesiadapiform 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 t ...

in a

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

. 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 Before Present, BP lasting . I ...

(~53 million years ago) through

Uintan The Uintan North American Stage is the North American faunal stage, typically set from 46,200,000 to 42,000,000 years before present lasting 4.2 million years. The Uintan Stage is a key part of the North American land mammal age, North American Lan ...

(~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 (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

, though fossils have also been found in

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

and

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

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 Period ( ; also spelled Palaeogene or Palæogene) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period Ma (million years ago) to the beginning of the Neogene Period Ma. It is the fir ...

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 an extinct basal pan-primates group, as sister to the rest of the pan-primates. The pan-primates together with the Dermoptera form the Primatomorpha. ''Purgatorius'' may not be a primat ...

, 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 strep ...

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 becoming a professor of natural history at Swarth ...

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

Wyoming Wyoming ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States, Western United States. It borders Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho t ...

, 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. A prolific fossil collector, Marsh was one of the preeminent paleontologists of the nineteenth century. Among his legacies are the discovery or ...

Yale University Yale University is a Private university, private Ivy League research university in New Haven, Connecticut, United States. Founded in 1701, Yale is the List of Colonial Colleges, third-oldest institution of higher education in the United Stat ...

. 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), ...

to the middle

in North America. Conditions of the

supported extensive subtropical woodland and rainforest environments which facilitate

arboreal Arboreal locomotion is the 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 (scansorial), but others are exclusively arboreal. The hab ...

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

Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

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 shee ...

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 ...

via

micro-CT In radiography, X-ray microtomography 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. It is similar to tomography and X-ray computed tomog ...

. 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 and predicted brain mass for an animal of a given size, based on nonlinear regre ...

(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 b ...

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 Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished ...

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