Apataelurus ("false cat") is an extinctgenus of saber-toothedplacental mammals from the extinct family Oxyaenidae, that lived in North America and East Asia from the early to middle Eocene, 48-40 million years ago.[1] This genus was defined by teeth that were well-adapted to a carnivorous diet. A distinct feature described was a long upper canine tooth that resembled a saber tooth. There are two species currently described: Apataelurus kayi, the type species, and Apataelurus pishigouensis, discovered in 1986.[2][3]
As a large, leopard-sized predator, Apataelurus dominated the Uinta Formation area. It was adapted to taking on large prey with more struggling motion tolerant muscles in its mouth, allowing it to attack large prey that would fight back. It was closely related to other Machaeroidinae, such as Diegoaelurus vanvalkenburghae. Apataelurus and other species within the Uinta Basin emerged during a major transition between the reduction in tropical zones and the increase in temperate and subtropical biomes.[4]Apataelurus was a more evolved member of Oxyaenidae, and lived in the middle to late Lutetian age.[1]
Discovery and naming
A. kayi was originally discovered by William Berryman Scott in Wagonhound Canyon at the Uinta Formation of the Uinta Basin, Utah. It was described and published in May 1938 as a "problematical, cat-like mandible".[2]A. kayi was described from two separate halves of a lower jaw, the left half containing two of the cheek teeth, and the right half with four cheek teeth.
Apataelurus originates from Greek, with "apat" (false or tricky) and "aelurus" (cat or feline). A. kayi was further described in A Remarkable Sabretooth-Like Creodont From the Eocene of Utah, also by W.B. Scott. A. kayi was named for American paleontologist J. Leroy Kay.[3]
The second species, Apataelurus pishigouensis, was discovered at the Hetaoyuan Formation in Henan, China in 1986 by Tong Yongsheng and Lei Yizhen.[5]A. pishigouensis was named for the Pishigou fossil site, where it was discovered by Tong and Lei.[6]A.pishigouensis was originally named Propterodonpishigouensis, under Hyaenodonta, but a study by S.P. Zack in 2019 reclassified the species into the genus Apataelurus and family Oxyaenidae.[7]
Description
Collected Apataelurus specimens consist exclusively of remains of the lower jaw. The most important find is an almost complete lower jaw, which contains part of the rear teeth. Based on the existing dental sockets, a dental formula with two incisors, one canine, four premolars, and two molars was likely present.[7]
A size comparison chart comparing the two identified species of Apataelurus.
The lower jaw was 14.9cm (5.9in) long and 2.7cm (1.1in) high below the first molar. Towards the front, the horizontal bony body became noticeably higher and ended in the area of the symphysis in a flange-like projection pointing downwards. Such projections are characteristic of predators whose upper canines were significantly elongated, as is the case, for example, in saber-toothed cats. They protected the canine tooth when the jaw was closed. A. pishigouensis and A. kayi share a well developed paraconid (a major cusp on the inner edge of the cheek teeth).[8]
The ascending ramus (lower part of the jaw, to which pterygoid muscles attach to) featured a deep masseteric fossa (flat bone surface) with sharp edges to which the masseter muscle attached. The articular process and coronoid process were both significantly reduced in size. Both the protrusion of the anterior segment of the mandible and the low position of the coronoid process were more pronounced in Apataelurus than in the closely related Machaeroides.[7]
A. pishigouensis differs from A. kayi primarily in its smaller size. The talonid of A. pishigouensis is smaller than that of A. kayi.[7]
Classification
Apataelurus is a genus from the extinct subfamily Machaeroidinae, within the extinct order Oxyaenidae, which is also in the extinct order Oxyaenodonta. According to phylogenetic studies, the clade Pan-Carnivora is split into two orders: Oxyaenodonta and Hyaenodonta.[9]
Diegoaelurus, a related Oxyaenid, had a very similar mouth and tooth structure to Apataelurus. However, Diegoaelurus had a short mandibular flange. Shown here is the holotype of Diegoaelurus.
Three Oxyaenid genera and four species have thus far been described: Machaeroides, with species M. simpsoni and M. eothen, Apataelurus, and Diegoaelurus.[10] Of the three genera, Machaeroides is the most primitive, with very few adaptations to the saber tooth dental form. Apataelurus and Diegoaelurus are similar in appearance, slightly more derived from M. eothen.[9] However, Diegoaelurus differed with its shorter mandibular flange, suggesting a more specialized carnivorous diet.[5]
Machaeroidinae
Diegoaelurus vanvalkenburghae
Apataelurus pishigouensis
Apataelurus kayi
The cladogram above shows the divergence of the three species.[9]
Paleobiology
A map showing the distribution of Machaeroidinae species across the western United States.
Due to the lack of specimens, there is little information about its paleobiology and behavior. Its detention and similarity to other "creodonts" and Machaeroides suggests that it had a carnivorous diet. It[9] was significantly larger than Diegoaelurus, a similar Oxyaenid predator. It likely hunted large prey, including uintatheres and brontotheriid perissodactyls. A. pishigouensis, which lived in now-China, likely hunted perissodactyls related to modern-day tapirs.[9] Almost all oxyaenodonts for which the body skeleton is at least partially known had an elongated body structure with short, powerful legs. Based on similar species’ skeletons, it can be assumed that Apataelurus had a plantar walk.[11]
The mandible of A. kayi is less reinforced from the top and bottom (dorsoventrally) of its skull than the two Machaeroides species. This difference could indicate that small Machaeroidinae fed on smaller prey, which were less able to inflict torsional stress upon the predator's jaws. A. kyai's mandible could have been less buttressed to allow for more torsion resistant motion (side to side or twisting motions) when dealing with larger and stronger prey.[12]
Apataelurus pishigouensis' lower mandible.
At the carnassial, the bite force of A. kayi was 3 and a half times greater than that of the smaller M. eothen. A. kayi had 89% of the bite force of Panthera pardus, a contemporary predatory mammal with similar mandibular length.[12] Studies showed that the mandible of Machaeroidinae is stronger at the canine teeth rather than the cheek teeth, indicating that they delivered powerful canine bites to kill their prey, like felids, rather than shallow bites like hyaenids.[12]
The relative mandibular force values of M. eothen and A. kayi are similar to those of conical-toothed felids. These conical-toothed felids, Machairodontinae, tended to have stretched out jaw muscles, which led them to develop a weaker bite.[13] From these mandibular properties, it can be assumed that Machaeroidinae delivered canine bites on prey that offered relatively less resistance than the prey of larger conical-toothed felids. As such, prey must have been held down before the saber-like canines were used to kill.[12] Machaeroides and Apataelurus had well-developed shearing teeth and long slicing canines and may have been similar to small to medium sized species of extant cats.[14][15]
Paleoecology
Apataelurus lived in a warm and humid climate, characterized by fluvial and floodplain environments. In the early Eocene, temperatures surged, hypothesized to be caused by greenhouse gas emissions, an equatorial warm saline deep water, or both. However, oceanic cooling began around 50 million years ago, cooling about 11-15 degrees Fahrenheit in the middle-late Eocene and a loss of tropical environments in North America.[16] Previous to the development of the fluvial environment, the Uinta Formation and the nearby Green River Formation were composed of lacustrine habitats. These lakes began to recede in the late middle Eocene, about the time that Apataelurus emerged.[16]
The Uinta Formation in this area comprises an interbedded sequence of silt, clay, and small amounts of gravel. The Uinta Basin was formed in the very Late Cretaceous during the Laramide uplift of the Uinta Mountains. This formation, alongside the Green River and Piceance Creek basins, began forming during the Laramide orogeny. The Laramide orogeny was a period of great tectonism in North America that began in the Late Cretaceous and continued until the late Eocene.[17]
Uinta Formation
The Uinta Formation.
The Uinta Formation is notable for its vast deposits of fossil mammals, which would come to define the Uintan Stage of the North American Land Mammal Age (NALMA).[18] The Uinta Formation has been formally divided into a lower Wagonhound Member, where Apataelurus was discovered, and an upper Myton Member (named for the nearby town of Myton).[4] Despite this formal classification, geologists and paleontologists organize the formation into three components: Uinta A, B, and C. These are ordered from lowest (oldest) to highest. Uinta A and B comprise Wagonhound Canyon and the areas nearby, while C represents the Myton Member.[19] Based on the presence of Uintan brontotherids, Uinta A is post-Bridgerian. The Bridgerian was an American faunal stage under NALMA that preceded the Uintan, ranging from 46.2 to 50.5 Ma. The Uintan NALMA was a major transition in mammalian evolution, as approximately 30% of modern mammalian families appear in the fossil record.[1] The Uintan NALMA was the end of a global greenhouse that had begun in the early Eocene. Tropical and rainforest biomes started to decrease, and subtropical and temperate habitats began to appear.[4] Primitive perissodactyls were replaced with perissodactyls more adapted to the subtropical conditions. Furthermore, rabbits made their first appearance alongside small rodents such as Paramys (Rodentia) and the genus Honrovits (Chiroptera).[20][21]
A diagram representing the stratigraphy of the Uinta Basin, Douglas Creek arch, and Piceance Basin. The Unita Formation is visible in the Tertiary period, above and to the left of the Green River Formation.
Apataelurus was likely near the top of the food chain, with hypercarnivorous adaptations and size advantages over other Oxyaenodonts. The Uinta Formation was dominated by perissodactyls such as Dolichorhinus, Metarhinus, and Sthenodectes.[22]
Hetaoyuan Formation
The Hetaoyuan Formation is a section of the Biyang Depression (Sag), Nanxiang Basin, which is a small intermountain faulted basin. The region is located within the Qin-and-Dabie Mountains orogenic belt in central China. The formation is divided into three sections, with the middle section (the Anpeng Deposits) displaying laminated oil shale, dolomite, and sodium carbonate minerals named nahcolite.[23]
The Anpeng Deposits are remarkably similar in age to the Green River and Piceance Creek Formations, which were inhabited by A. kayi.[24] The Biyang Sag has Cenozoic-age depositional systems consisting of braided deltas, slumped turbidite fans, and shallow and deep lakes.[25] During the entire Paleogene, rivers collected sediments and created geological deposits in the lakes, creating shallow sections. These deposits also created braided river deltas, establishing small islands for Eocene flora and fauna to travel between. This environment is strikingly alike to the riverine and lacustrine Uinta Formation, where A. kayi lived.[25]
References
1 2 3 Mauricio, Anton (2013). Sabertooth. Indiana University Press. pp.46, 88.
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