Purgatorius is an extinct genus of eutherian mammal from the early Paleocene epoch of western North America. It is typically believed to be the earliest example of a primate or protoprimate (a primatomorph precursor to the Plesiadapiformes), dating to nearly 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's Tullock Formation (early Paleocene, Puercan). P. unio is from Purgatory Hill (hence the animal's name) in deposits believed to be about 63 million years old, and P. ceratops is a single worn tooth from Harbicht Hill in the lower Paleocene section of the Hell Creek Formation. Both locations are in McCone County, Montana. Harbicht Hill was once thought to be late Cretaceous, but it is now clear that it represents Paleocene channels with time-averaged fossil assemblages. Several more Montanan Purgatorius species have been named since the initial discovery, though fossils are still limited to teeth, jaw fragments, and a few ankle bones. Two Purgatorius species were also named from the Ravenscrag Formation of Saskatchewan, and an undetermined species is known from Corral Bluffs in Colorado.

Purgatorius is thought to have been rat-sized ( long and 1.3 ounces (about 37 grams)) and a diurnal insectivore. In life, it would have resembled a squirrel or a tree shrew (most likely the latter, given that tree shrews are one of the closest living relatives of primates, and Purgatorius is considered to be the progenitor to primates). The oldest remains of Purgatorius date back to 65.946–65.912 million years ago, or between 105 thousand to 139 thousand years after the K-Pg boundary.

Discovery and species

left|thumb|Jaw fragment of an unnamed Purgatorius species from the [[Willow Creek Formation of Alberta.]]

Purgatorius contains at least five valid species distributed throughout the early Paleocene of western North America. Buckley proposed a new species for the Simpson Quarry fossils, P. titusi, but several subsequent studies considered his species a junior synonym of P. unio. The youngest known Purgatorius fossils are teeth similar to P. unio and P. pinecreeensis found at Horsethief Canyon, a site in Garfield County, Montana dated to the Torrejonian 1 age. Further excavations have yet to reveal any more Purgatorius fossils at Harbicht Hill, and it is possible that P. ceratops is instead a tooth from a eutherian unrelated to Purgatorius. Garbani Channel is from the Puercan 3 land mammal age. 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.

The proximate cause of the tall protocone found in Purgatorius may have been a lingual bias in activator expression during the bud stage of dental development, which would have stablished a buccolingual growth axis.

Ankle bones

Bones from the ankle are similar to those of primates, and were suited for a life up in trees. 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<C. Isolated upper incisors referable from P. janisae exhibit some typical plesiadapiform specializations. Due to general morphology of the postcanine dentition of Purgatorius, it could be expected to be characterized as a primitive member of the primates. However, due to the specializations of its incisors of P. janisae it is considered by some investigators as a basal member of the Pleasiadapiformes sensu lato. Similar results had been obtained in previous studies with far fewer species.

References

Bibliography

  • "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 1, 2021