Ignacius

Ignacius is a genus of extinct mammal from the early Cenozoic era. This genus is present in the fossil record from around 62-33 Ma (late Torrejonian-Chadronian North American Land Mammals Ages). The earliest known specimens of "Ignacius" come from the Torrejonian of the Fort Union Formation, Wyoming and the most recent known specimen of "Ignacius" was found in the Medicine Pole Hills (Chadron Formation) of North Dakota. "Ignacius" is one of ten genera within the family Paromomyidae, the longest living family of any plesiadapiforms, persisting for around 30 Ma during the Paleocene and Eocene epochs. The analyses of postcranial fossils by paleontologists suggest that members of the family Paromomyidae, including the genus "Ignacius", most likely possessed adaptations for arboreality.
Taxonomy.
There are currently four valid species within the genus "Ignacius": "I. frugivorus", "I. fremontensis", "I. clarkforkensis", and "I. graybullianus". There are also at least two undescribed species of "Ignacius" from the Arctic of Canada. The type species for the genus "Ignacius" is "I. frugivorus" and was found at the Mason Pocket locality in Colorado. The holotype specimen (AMNH 17368), published in 1921 by Matthew and Granger, consists of an upper jaw with the canine, fourth premolar, first molar, and second molar.
The genus "Ignacius" is situated within the family Paromomyidae and the order Plesiadapiformes. The relationship between plesiadapiforms and the modern group Euarchonta (Dermoptera, Scandentia, and Primates) is still under debate by paleontologists with some suggesting a closer relationship with primates, and others suggesting a closer relationship with dermopterans.
The genus "Ignacius" was found to be synonymous with the closely related genus, Phenacolemur, by G.G. Simpson in 1935 but revived by Bown and Rose in 1976.
Description.
"Ignacius", along with the other plesiadapiforms, possess large, procumbent lower incisors convergent with those found in rodents, however, unlike rodents, the incisors of plesiadapiforms were not continuously growing. Most species of "Ignacius" have a 1.0.1.3 lower dental formula although some specimens retain a small P3. Compared with other paromomyids, "Ignacius" has small P4s in relation to M1s with relatively low crowned upper and lower molars. The lower molars have reduced cusps, forming an almost continuous crest along the talonid basin. The jaw of "Ignacius" is deep in comparison to tooth crown height, a feature distinct to this genus. Upper molars possess strong crests in a distinct ‘v’ shape between the paracone and metacone.
Species.
"Ignacius" "frugivorus" was the first species of Ignacius to be described in 1921. "Ignacius frugivorus" and "I. fremontensis" are the oldest species of "Ignacius", both found in the Shotgun Local fauna of the Fort Union formation, Wyoming. The youngest known specimen of "Ignacius" was found in the Chadron formation of North Dakota and is represented by a single P4 (2). There is not enough material to identify this specimen to the species level. "Ignacius frugivorus" and "I. fremontensis" are distinguishable from one another based primarily on the size of lower fourth premolars. "Ignacius fremontensis" displays a significantly smaller P4 in relation to M1 when compared to I. frugivorus. "Ignacius clarkforkensis" is the largest described species in the genus and differs from other species in the retention of a single rooted P2 (5). "Ignacius graybullianus" has more quadrangular upper molars whereas the other three species are more triangular. Strong v-shaped postparacone/prematacone cristae are present in "I. graybullianus". These cristae are less obliquely oriented in the other three taxa.
Paleoenvironment and geographic range.
"Ignacius" occupied a broad geographic range during the early Cenozoic era in North America, ranging from Texas, to the Canadian Arctic. During the early Cenozoic, North America experienced multiple climatic changes with the warmest mean annual temperatures (around 16oC, 60oF) occurring during the Paleocene-Eocene Thermal Maximum (PETM) at around 55 Ma. During the Eocene Climatic Optimum (c. 53 Ma) there was a correlated diversification of flora creating a complex, warm-temperate to subtropical habitat over much of interior North America. The environment of Arctic Canada would have represented a temperate environment with winter temperatures rarely dropping below freezing. The late Cretaceous and early Tertiary saw a diversification of angiosperms providing the opportunity for arboreal mammals to utilize the fruit, seed, exudate, and flower resources of these plants. The environment of interior North America, extending into the Canadian Arctic, would have provided habitable ecosystems for these arboreal mammals to thrive and diversify.
Functional morphology.
Analysis of postcranial fossils of "Ignacius" suggest they were specialized for vertical climbing and clinging on tree trunks as well as for more agile arboreality than seen in other plesiadapiforms. Some researchers have also hypothesized based on analysis of hand bone morphology, that they possessed gliding adaptations like those of modern-day flying lemurs (Order Dermoptera). Further anatomical analysis of cranial synapomorphies, also suggest a close relationship between dermopterans and plesiadapiforms.
Evidence for vertical climbing can be seen in the morphology of the humerus, femur, and tarsal bones which are consistent with increased flexibility of the shoulder, elbow, hip, knee, and ankle joints. These features also suggest powerful flexion of the digits for grasping large tree trunks as well as small diameter support branches. The features listed above are seen in many plesiadapiforms, but some anatomical features set paromomyids apart in being more adept for arboreal living. Flexible lumbar vertebrae as well as increased surface area on the innominate and femur for the origin and insertion of gluteal muscles, suggest paromomyids were capable of powerful bounding across tree branches.
The large, procumbent incisors and reduced shearing crests of Paromomyids, especially "Ignacius" and "Phenacolemur", suggest a diet specialized for feeding on exudates, comparable to the adaptions seen in extant callitrichine primates and "Petaurus", a marsupial sugar-glider.