Purgatorius
Encyclopedia
Purgatorius is the genus
of the four extinct
species
believed to be the earliest example of a primate
or a proto-primate, a primatomorph precursor to the Plesiadapiformes
. Remains were originally discovered in what is now eastern Montana
's (United-States) Tullock Formation (early Paleocene, Puercan}, specifically at Purgatory Hill (hence the animal's name) in deposits believed to be about 63 million years old. They have also been widely discovered in the early Paleocene Bug Creek fauna, along with other leptictid
. These deposits were once thought to be latest Cretaceous, but it is now clear that they are Paleocene channels with time-averaged fossil assemblages. It is thought to have been rat
-sized (6 in (15.2 cm) long and a few ounces (about 1–2 kg)) and a diurnal insectivore, who burrowed through small holes in the ground.
Currently it is considered one of the few plesiadapiform mammals primitive enough to have possibly given rise to both the plesiadapiformes and the higher primates. Though its classification below the superorder Euarchonta
remains uncertain, dental evidence and molar morphology indicate a close link with the primate order.
P. janisae exhibit some typical plesiadapiform specializations. Due to general morphology of the postcanine dentition of Purgatorius, it could be expected to characterized as a primitive member of the Primates. But, due to the specializations of its incsiors of P. janisae it should be considered as a basal member of the Pleasiadapiformes sensu lato.
According to the hypothesis of character change in euarchontan evolution, Purgatorius is a primitive primate.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of the four extinct
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally 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 recover may have been lost before this point...
species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
believed to be the earliest example of a primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
or a proto-primate, a primatomorph precursor to the Plesiadapiformes
Plesiadapiformes
Plesiadapiformes is an extinct order of mammals. It is either closely related to the primates or a precursor to them. Many are too derived to be ancestral to primates, but the earliest Plesiadapiformes have teeth that are strongly indicative of a common ancestor...
. Remains were originally discovered in what is now eastern Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
's (United-States) Tullock Formation (early Paleocene, Puercan}, specifically at Purgatory Hill (hence the animal's name) in deposits believed to be about 63 million years old. They have also been widely discovered in the early Paleocene Bug Creek fauna, along with other leptictid
Leptictidium
Leptictidium is an extinct genus of small mammals; together with macropods and humans, they are the only known completely bipedal mammals. Comprising five species, they resembled today's elephant shrews...
. These deposits were once thought to be latest Cretaceous, but it is now clear that they are Paleocene channels with time-averaged fossil assemblages. It is thought to have been rat
Rat
Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...
-sized (6 in (15.2 cm) long and a few ounces (about 1–2 kg)) and a diurnal insectivore, who burrowed through small holes in the ground.
Currently it is considered one of the few plesiadapiform mammals primitive enough to have possibly given rise to both the plesiadapiformes and the higher primates. Though its classification below the superorder Euarchonta
Euarchonta
The Euarchonta are a grandorder of mammals containing four orders: the Dermoptera or colugos, the Scandentia or treeshrews, the extinct Plesiadapiformes, and the Primates....
remains uncertain, dental evidence and molar morphology indicate a close link with the primate order.
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 northern Western interior, suggesting that it came into the area between 64.75 and 64.11 Mya. Due to fragmentary dentition from the Garbani Channel fauna from Purgatorius janisae proves 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.Relationship
For many years, there has been a large debate as to whether Purgatorius is a primitive member of the Primates or a basal member of the Plesiadapiforms. 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>I3According to the hypothesis of character change in euarchontan evolution, Purgatorius is a primitive primate.