Plesiadapis

Plesiadapis is one of the oldest known primate-like mammal genera which existed about 58–55 million years ago in North America and Europe.^[2][3] Plesiadapis means "near-Adapis", which is a reference to the adapiform primate of the Eocene period, Adapis. Plesiadapis tricuspidens, the type specimen, is named after the three cusps present on its upper incisors.

Plesiadapis Temporal range: Late Paleocene-Early Eocene[1] PreꞒ Ꞓ O S D C P T J K Pg N
Reconstructed skeleton of P. cookei at the University of Michigan Museum of Natural History
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Plesiadapiformes
Family:	†Plesiadapidae
Genus:	†Plesiadapis Gervais, 1877
Type species
†Plesiadapis tricuspidens

Taxonomy

Life restoration at MUSE - Science Museum in Trento

The first discovery of Plesiadapis was made by François Louis Paul Gervaise in 1877, who first discovered Plesiadapis tricuspidens in France. The type specimen is MNHN Crl-16, and is a left mandibular fragment dated to the early Eocene epoch.

This genus probably arose in North America and colonized Europe on a landbridge via Greenland. Thanks to the abundance of the genus and to its rapid evolution, species of Plesiadapis play an important role in the zonation of Late Paleocene continental sediments and in the correlation of faunas on both sides of the Atlantic. Two remarkable skeletons of Plesiadapis, one of them nearly complete, have been found in lake deposits at Menat, France.^[2] Although the preservation of the hard parts is poor, these skeletons still show remains of skin and hair as a carbonaceous film—something unique among Paleocene mammals. Details of the bones are better preserved in fossils from Cernay, also in France, where Plesiadapis is one of the most common mammals.

Classification

The following are possible shared derived features of Plesiadapiformes: maxillary-frontal contact in orbit, the presence of a suboptic foramen, an ossified external auditory meatus, the absence of a promontory artery, the absence of a stapedial artery, and a strong mastoid tubercle.^[4]

P. cookei compared to Notharctus tenebrosus (left), an early true primate. Both come from Eocene Wyoming, though the latter is slightly geologically younger.

Although the placement of the Plesiadapis lineage is still up for debate, the consensus in the 1970s was that they were closest to early tarsier-like primates.^[5] Plesiadapiformes have also been proposed as a nonprimate sister group to Eocene-Recent primates. A study done in 1987 linked Plesiadapiformes with adapids and omomyids through nine shared-derived features, six of which are cranial or dental: (1) auditory bulla inflated and formed by the petrosal bone, (2) ectotympanic expanded laterally and fused medially to the wall of the bulla, (3) promontorium centrally positioned in the bulla, and large hypotympanic sinus widely separating promontorium from the basisphenoid, (4) internal carotid entering the bulla posteriolaterally and enclosed in a bony tube, (5) nannopithex fold on the upper molars, and (6) loss of one pair of incisors.^[4]

In 2013, a phylogenetic analysis that includes also the basal primate Archicebus positions Plesiadapis firmly outside of the Primates, as a sister group to both Primates and Dermoptera.^[6]

Species[7]	Synonyms	Localities
P. tricuspidens (type species) Gervais, 1877	P. gervaisi Lemoin, 1887 P. rhemensis Rütmeyer, 1891 P. remensis Stehlin, 1916 P. trouessarti Schlosser, 1921	Cernay, France Lentille, France Berru, France Rilly, France
P. intermedius Gazin, 1971	Pronothodectes matthewi Gazin, 1956 Pr. simpsoni Gazin, 1956 Nannodectes intermedius Gingerich, 1974 Na. gazini Gingerich, 1974	Douglas Quarry, Montana Little Muddy Creek, Montana Keefer Hill, Wyoming Saddle, Wyoming
P. simpsoni Gazin, 1956	Pr. simpsoni Gazin, 1956 P. jepseni Gazin, 1956 Na. simpsoni Gingerich, 1974	Ledge, Wyoming West End, Wyoming
P. gidleyi Matthew, 1917	Nothodectes gidleyi Matthew, 1917 P. tricuspidens Teilhard, 1922 Na. gidleyi Gingerich, 1974	Mason Pocket, Colorado Joe's Bone Bed, Texas Black Peaks, Texas
P. fodinatus Jepsen, 1930	P. rubeyi Gazin, 1942 P. farisi Dorr, 1952 P. jepseni Gazin, 1956	Princeton Quarry, Wyoming Schaff Quarry, Wyoming Ravenscrag, Saskatchewan Titanoides, Wyoming Fossil Hollow, Wyoming Chappo, Wyoming Dell Creek Quarry, Wyoming
P. dubius Matthew, 1915	No. dubius Matthew, 1915 P. pearcei Gazin, 1956	Paint Creek, Wyoming Plateau Valley, Colorado Bearcreek, Montana Togwotee, Wyoming Chappo, Wyoming Clark Fork, Wyoming
P. anceps Simpson, 1936	P. jepseni Gazin, 1956 P. praecursor Gingerich, 1974	Tongue River, North Dakota Douglas Quarry, Montana Keefer Hill, Wyoming Scarritt Quarry, Montana Saddle, Wyoming Highway blowout, Montana
P. rex Gidley, 1923	Tetonius rex Gidley, 1923 No. gidleyi Simpson, 1927 P. paskapooensis Russel, 1964	Melville, Montana Ledge, Wyoming Cedar Point, Wyoming Battle Mountain, Montana Chappo, Wyoming Erikson's Landing, Alberta Love Quarry, Wyoming Twin Creek, Wyoming West End, Wyoming
P. churchilli Gingerich, 1974	P. farisi Krishtalka, 1975	Croc Tooth Quarry, Wyoming Airport, Wyoming Long Draw Quarry, Montana Badwater, California Sand Draw, Wyoming
P. simonsi Gingerich, 1974		Sand Draw, Wyoming
P. cookei Jepsen, 1930		Little Sand Coulee Area, Wyoming Paint Creek, Wyoming Red Creek, Wyoming Susan, Wyoming Verland's, Wyoming
P. insignis Piton, 1940	Sciurus feignouxi Launay, 1908 Menatotherium insigne Piton, 1940	Menat, France
P. walbeckensis Russel, 1964		Walbeck, Germany
P. remensis Lemoine, 1887		Cernay, France Lentille, France Berru, France
P. russelli Gingerich, 1974	P. daubrei Teilhard, 1922 Platychoerops daubrei Russell, 1967	Meudon, France

Anatomy and remains

Restoration

Plesiadapis is one of the most completely known early primatomorphs, with a significant amount of the skeleton known. Though, the skeleton is mostly known from P. gidleyi and the relatively late (derived) P. tricuspidens. The skull is overall reminiscent of a lemur, though it lacks the postorbital bars (a vertical bar bordering the posterior margin of the eye socket). The brain was probably relatively large compared to similarly sized contemporary mammals, namely the arctocyonids. The dental formula is usually 2.1.3.31.1.3.3, with two incisors, one canine, three premolars, and three molars in either half of the upper jaw; and one incisor, one canine, three premolars, and one molar in either half of the lower jaw. The incisors are quite long. Already, Plesiadapis had lost the first premolar from the mammalian common ancestor, but later primatomorphs would lose the second premolar as well. P. dubius consistently lacks the lower second premolar, and about half of P. rex specimens lack it too. P. gidleyi and European Plesiadapis lack the lower canines.^[7]

P. cookei cast of a specimen from Wyoming. At the AMNH.

The skeletal adaptations are consistent with a largely arboreal lifestyle in the trees. The sacrum of P. gidleyi is similar to that of the eastern gray squirrel, though lacking the strong spines. The humerus is robust and features a strong S-curve.^[7]

Nearly all of what is known about the anatomy of plesiadapiforms comes from fragmentary jaws and teeth, so most definitions of plesiadapiform genera and species are based on dentition. Plesiadapis' dentition shows a functional shift toward grinding and crushing in the cheek teeth as an adaptation towards increasing omnivory and herbivory.^[8] The skull of Plesiadapis is relatively broad and flat, with a long snout with rodent-like jaws and teeth and long, gnawing incisors separated by a gap from its molars. Orbits are still directed to the side, unlike the forward-facing eyeballs of modern primates that enable three-dimensional vision.^[7] Plesiadapis had mobile limbs that terminated in strongly curved claws, and it sported a long bushy tail which is preserved in the Menat skeletons. The way of life of Plesiadapis has been much debated in the past. Climbing habits could be expected in a relative of the primates, but tree-dwelling animals are rarely found in such high numbers. Based on this and other evidence, some paleontologists have concluded that these animals were mainly living on the ground, like today's marmots and ground squirrels.^[2] However, more recent investigations have confirmed that the skeleton of Plesiadapis is that of an adept climber, which can be best compared to tree squirrels or to tree-dwelling marsupials such as possums.^[7] The short, robust limbs, the long, laterally compressed claws, and the long, bushy tail indicate that it was an arboreal quadruped. Remains found showed that it had a body mass of around 2.1 kilograms (4.6 lb).^[9]

References

1. McKenna, M. C & S. K. Bell (1997). Classification of Mammals Above the Species Level. Columbia University Press. ISBN 0-231-11012-X.
2. Gingerich, P.D. (1976). "Cranial anatomy and evolution of early Tertiary Plesiadapidae (Mammalia, Primates)". University of Michigan Papers on Paleontology. 15: 1–141. hdl:2027.42/48615.
3. Rose, K.D. (1981). "The Clarkforkian Land-Mammal Age and mammalian faunal composition across the Paleocene-Eocene boundary". University of Michigan Papers on Paleontology. 26: 1–197. hdl:2027.42/48626.
4. Kay, Richard F.; Thewissen, J. G.; Yoder, Anne D. (1992). "Cranial anatomy of Ignacius graybullianus and the affinities of the Plesiadapiformes". American Journal of Physical Anthropology. 89 (4): 477–498. doi:10.1002/ajpa.1330890409.
5. Gingerich, P. D. (1975). New North American Plesiadapidae (mammalia, primates) and a biostratigraphic zonation of the Middle and Upper Paleocene. Ann Arbor: University of Michigan Museum of Paleontology.
6. Ni, X.; Gebo, D. L.; Dagosto, M.; Meng, J.; Tafforeau, P.; Flynn, J. J.; Beard, K. C. (2013). "The oldest known primate skeleton and early haplorhine evolution". Nature. 498 (7452): 60–64. Bibcode:2013Natur.498...60N. doi:10.1038/nature12200. PMID 23739424. S2CID 4321956.
7. Szalay, F. S. & Delson, E. (1979). Evolutionary history of the Primates. Academic Press. ISBN 9780126801507.
8. Fleagle, K. D. (1987). "The First Radiation—Plesiadapiform Primates". In Ciochon, R. L.; Fleagle, J. G. (eds.). Primate Evolution and Human Origins. Hawthorne: Aldine de Gruyter. pp. 41–51. ISBN 0-202-01175-5.
9. Fleagle, J. G. (1988). Primate Adaptation and Evolution. San Diego: Academic Press. ISBN 0-12-260340-0.

External links

• Mikko's Phylogeny Archive

• Paleontology portal