''Purgatorius'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of seven

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

eutheria Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials. Eutherians are distinguished from noneutherians by various phenotypic tra ...

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

typically believed to be the earliest example of a

primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter includin ...

or a proto-primate, a primatomorph precursor to the

Plesiadapiformes 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 t ...

, dating to as old as 66 million years ago. The first remains (''P. unio'' and ''P. ceratops'') were reported in 1965, from what is now eastern

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columb ...

's Tullock Formation (early Paleocene, Puercan), specifically at Purgatory Hill (hence the animal's name) in deposits believed to be about 63 million years old, and at Harbicht Hill in the lower

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pala ...

section of the

Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The formation stretches over portions of ...

. Both locations are in McCone County. They have also been found in the

Ravenscrag Formation The Ravenscrag Formation is a stratigraphic unit of early Paleocene age in the Western Canada Sedimentary Basin. It was named for the settlement of Ravenscrag, Saskatchewan, and was first described from outcrops at Ravenscrag Butte near the Fr ...

and widely discovered in the early Paleocene Bug Creek Group, along with leptictids. These deposits were once thought to be late Cretaceous, but it is now clear that they are Paleocene channels with time-averaged fossil assemblages. It is thought to have been rat-sized ( long and 1.3 ounces (about 37 grams)) and a diurnal insectivore, which burrowed through small holes in the ground. In life, it would have resembled a

squirrel Squirrels are members of the family Sciuridae, a family that includes small or medium-size rodents. The squirrel family includes tree squirrels, ground squirrels (including chipmunks and prairie dogs, among others), and flying squirrels. ...

or a tree shrew (most likely the latter, given that tree shrews are the closest living relatives of primates, and ''Purgatorius'' is considered to be the progenitor to primates). The youngest remains of ''Purgatorius'' date back to ~65.921 mya, or between 105 thousand to 139 thousand years after the K-Pg boundary.

Description of remains

Postcanine dentition of ''P. unio'' is documented by 13 dentulous, fragmentary mandibles, a fragmentary maxillary and more than 50 isolated teeth from Garbani Locality 80 km west of Purgatory Hill. ''P. ceratops'' is represented by an isolated lower molar found at Harbicht Hill, McCone County. The report of the occurrence of ''Purgatorius'' in the Late Cretaceous was based on an isolated, worn molar found in a channel filling that contains early Puercan fossils. It is also abundantly represented in Pu 2-3 local faunas in the northwestern interior, suggesting that it came into the area between 64.75 and 64.11 Mya. Fragmentary dentition from the Garbani Channel fauna from ''Purgatorius janisae'' shows that the lower dental formula was 3.1.4.3.

Dentition

The type specimen of ''P. unio'', a damaged upper molar, is essentially identical to teeth found at the Garbani Locality. Data from this sample support Van Valen and Sloan's identification of topotypic lower molars, and also demonstrate that the lower dentition of ''P. unio'' includes seven postcanines. The alveolus for the single root of P1, crown unknown, is smaller than those for the canine or P2. The second lower pre- molar is smaller than P3; both are two- rooted. The fourth lower premolar is submolariform. A metaconid is lacking, although on some teeth slight thickenings of the enamel are present in this region. Talonid cusps are slightly differentiated. The first and second lower molars are approximately the same length (M1, average length x=- 1.93 mm, N- 13; M2, x=2.00 mm, N- 9); M. is longer (x= 2.32 mm, N -7). Widths of talonids of M1.2 vary from less than to greater than widths of trigonids. Hypoconulid of M. is enlarged, salient, and on some teeth incipiently doubled by addition of a lingual cusp.

Ankle bones

Bones from the ankle are similar to those of primates, and were suited for a life up in trees.

Relationship

For many years, there has been debate as to whether ''Purgatorius'' is a primitive member of the primates or a basal member of the

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 ...

. Several characters of the dentition of ''Purgatorius'', which includes its incisor morphology, can ally it with later plesiadapiforms. The prism cross sections are highly variable with circular, horseshoe and irregular shapes, while the prisms of cheek teeth are radially arranged. Due to the fragmentary dentaries found in the Garbani Channel fauna from ''Purgatorius janisae'' the morphology of the canine and incisor alveoli suggest the derived gradient in the crown size of: I1>or = I2>I3 A phylogenetic analysis of 177 mammal taxa (mostly

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

and

Palaeocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pala ...

fossils), published in 2015, suggests that ''Purgatorius'' may not be closely related to

primates Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...

at all, but instead falls outside crown-group placentals - specifically as the sister taxon to ''

Protungulatum ''Protungulatum'' ('first ungulate') is a extinct genus of pan-euungulate mammals within extinct family Protungulatidae, and also one of the earliest known placental mammals in the fossil record, that lived in North America from the Late Creta ...

''. Similar results had been obtained in previous studies with far fewer species. But this study had been criticized and refuted by subsequent authors and their studies.

Notes

References

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External links

"Oldest primate fossils indicate our ancestors walked with dinosaurs "
Includes artist's illustration of ''Purgatorius mckeeveri'', a newly described species of early primate. ''New Atlas'', March 01, 2021 {{Taxonbar, from=Q136654 Cretaceous mammals Paleocene mammals Paleocene genus extinctions Cretaceous mammals of North America Paleocene mammals of North America Hell Creek fauna Fossil taxa described in 1965 Prehistoric primate genera