| Torrejonia | |
|---|---|
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Primates |
| Family: | †Palaechthonidae |
| Genus: | †Torrejonia Gazin, 1968 |
Torrejonia is a genus of extinctplesiadapiform that belongs to the family Palaechthonidae. There are currently two species known,T. wilsoni andT. sirokyi.[1] This genus is present in the fossil record from around 62–58 Ma (late Torrejonian-Tiffanian North American Land Mammals Ages [NALMA]).[1][2][3][4] Species belonging to this genus are suggested to be plesiadapiforms based on adaptations observed in the skeletal morphology consistent with arboreal locomotor behavior.[1][2] Following the mass extinction event at theCretaceous–Paleogene boundary (K-Pg), a large diversity of plesiadapiform families were documented beginning at theTorrejonian NALMA.[4] Research has shown thatT. wilsoni is one of the largest palaechthonids (at 741g) and is reconstructed as being morefrugivorous than other palaechthonids.[3]
The extinction of dinosaurs at theCretaceous-Paleogene boundary sparked new opportunities for mammalian diversification.[2] The K-Pg boundary is marked by what is termed an “iridium anomaly” and can be seen in the United States in theSan Juan Basin ofNew Mexico and southwestColorado. For over a century, paleontologists have collected a plethora of different taxa from theNacimiento Formation, San Juan Basin, New Mexico, includingTorrejonian mammals.[2] However, primates from the Torrejon beds had not been known until they were first discovered in the Angel Peak Basin by Robert W. Wilson and a field crew in 1948.[1][2][3] In 1949, paleontologists C. Lewis Gazin and Franklin Pierce uncovered multiple specimens belonging to a new genus and named itTorrejonia wilsoni. Additional fossil material was uncovered in 1969 allowing G.F. Gunnell to draw a comparison to a later, larger species from the Tiffanian NALMA,Torrejonia sirokyi.[2]
The holotype ofT. wilsoni (USNM 25255) preserves the left ramus of the mandible with P3-M2 present.T. wilsoni is larger than the paromomyidParomomys maturus,Palaechthon alticuspis andPlesiolestes problematicus. The M2 is very close in size to the plesiadapidPronothodectes simpsoni.[1] More recent studies with additional fossil material gave researchers a better understanding of the dentition of this species. Theholotype is missing a metaconid, and a clear mesoconid. Portions of the jaw in specimen NMMNH P-54500 suggest a lower dental formula of 2-1-3-3 and that it housed a canine rather than a P1.[2] They have a lanceolate and dorsomedially oriented I1 and a canine that is considerably larger than the P2.[3]
Torrejonia belongs to the family Palaechthonidae and represents a group of plesiadapiforms thought to be the sister group of Paromomyidae.[2][3] Differences in skeletal morphology provide evidence for the splitting of these taxa into two separate families. The dental formula ofTorrejonia has been highly contested which has made its taxonomic placement unstable.Torrejonia was originally thought to belong to the family Paromomyidae as some dental characteristics share a close resemblance to species in this family.[1] Recent studies have shown thatTorrejonia belongs to the family Palaechthonidae.[2] Additionally, analysis of the partial skeleton ofT. wilsoni suggests that palaechthonids were arboreal and had capabilities for clinging and climbing on vertical supports like other plesiadapiforms.[4] The anterior dentition is similar to other plesiolestine palaechthonids such as Plesiolestes problematicus and Plesiolestes nacimienti, in having a flattened, broad, and dorsomedially oriented lower central incisor.T. wilsoni also has a large, lower canine and smaller second lower premolar similar to other plesiolestines,[3] thus makingT. wilsoni more closely related toparomomyids than toPlesiolestes nacimienti due to its arboreal nature.[3][4]
Given its morphology (i.e. forelimb, hindlimb), it has been suggested thatTorrejonia was well-adapted to an arboreal climbing lifestyle.[3][4] Analysis of the forelimb provides evidence for a mobile shoulder, a habitually flexed forearm, considerablesupination and pronation of the forearm and hand, and manual grasping. Analysis of the hindlimb suggests wide ranges of abduction and lateral rotation at the hip joint, a habitually flexed thigh and knee, and mobile ankle joints for inversion and eversion.[3]
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