Apataelurus

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^[1] 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.^[1] 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]

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]

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 slowly 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]

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.^[5] 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]

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]

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]