Apataelurus

Apataelurus Temporal range: early to middle Eocene
Lower jaw of A. kayi
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	†Oxyaenodonta
Family:	†Oxyaenidae
Subfamily:	†Machaeroidinae
Genus:	†Apataelurus Scott, 1938
Type species
†Apataelurus kayi Scott, 1938
Species
†Apataelurus kayi Scott, 1938 †Apataelurus pishigouensis (Tong and Lei, 1986)

Apataelurus ("false cat") is an extinct genus of saber-toothed hypercarnivorous placental 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. 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.

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. Apataelurus was a more evolved member of Oxyaenidae, and lived in the middle to late Lutetian age.

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 "problematic, cat-like mandible". 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. The second species, Apataelurus pishigouensis, was discovered at the Hetaoyuan Formation in Henan, China in 1986 by Tong Yongsheng and Lei Yizhen. A. pishigouensis was named for the Pishigou fossil site, where it was discovered by Tong and Lei. A. pishigouensis was originally named Propterodon pishigouensis, under Hyaenodonta, but a study by S.P. Zack in 2019 reclassified the species into the genus Apataelurus and family Oxyaenidae.

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.

The lower jaw was 14.9 cm (5.9 in) long and 2.7 cm (1.1 in) 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 that is almost as developed as the talonoid.

The ascending ramus featured a deep masseteric fossa with sharp edges to which the masseter muscle attached. The coronoid process was significantly reduced in size. The articular process, which ended below the dental plane, was also much smaller. The entire lower jaw reached a height of 1.8 cm (0.71 in). 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.

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.

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. Of the three genera, Machaeroides is the most primitive, with very few adaptations to the saber tooth dental form. Apataelurus and Diegoaelurus are similarly evolved, slightly more derived from M. eothen. However, Diegoaelurus differed with its shorter mandibular flange, suggesting a more specialized carnivorous diet.

Machaeroidinae	Machaeroides simpsoni Machaeroides eothen Diegoaelurus vanvalkenburghae Apataelurus pishigouensis Apataelurus kayi

The cladogram above shows the divergence of the three genera.

Paleobiology

Apataelurus pishigouensis' lower mandible.

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. Relying on relative jaw size, A. kayi was approximately the size of a leopard. It 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.

The mandible of A. kayi is less dorsoventrally buttressed (a bodily structure reinforced from the top to the bottom) than the two Machaeroides species. This difference could indicate that small Machaeroidinae fed on smaller prey, while Apataelurus preferred larger prey. Due to the fact that Machaeroidinae had more dorsoventrally buttressed mandibles, it is likely that they were more adapted to dealing with smaller prey that inflicted less torsional stress (side to side or twisting motions). A. kyai's mandible could have been less buttressed to allow for more torsion resistant motion when dealing with larger and stronger prey. A. kayi had 89% of the bite force of Panthera pardus, a contemporary predatory mammal with similar mandibular length.

Paleoecology

Apataelurus lived in a warm and humid climate, characterized by fluvial and floodplain environments. 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.

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). 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). 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 of Wagonhound Canyon and the areas nearby, while C represents the Myton Member. Based on the presence of Uintian brontotheres, Uinta A is post-Bridgerian.

Mammalian biostratigraphy

The Uintan NALMA was a major transition in mammalian evolution, as approximately 31% of modern mammalian families appear in the fossil record. 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. 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).

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.

Eocene Uinta ecosystem

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. In addition to perissodactyls, artiodactyls such as the pig-like Achaenodon inhabited the area. In lakes, rivers, and other fast moving waterways, Priscacara was likely present, a temperate bass found in Lake Uinta and Lake Gossiute, Wyoming.

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.

The Anpeng Deposits are remarkably similar in age to the Green River and Piceance Creek Formations, which were inhabited by A. kayi. The Biyang Sag has Cenozoic-age depositional systems consisting of braided deltas, slumped turbidite fans, and shallow and deep lakes. 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.

Eocene Hetaoyuan Formation ecosystem

In the Hetaoyuan Formation, A. pishigouensis was also likely at the top of the food chain, using its strong bite force and powerful jaws to take down larger and stronger prey. Alongside A. pishigouensis, Ctenodactyloid rodents such as Tamquammys and Viriosomys were dominant primary consumers. In addition to Ctenodactyloidea, Cricetidae, Zapodidae, and Cylindrodontidae fossils have been found within the formation, confirming their presence. In addition to being in the Hetaoyuan Formation, similar species were dispersed around most of modern-day central China.