Miracinonyx
Miracinonyx, colloquially known as the American cheetah or the New World cheetah, is an extinct genus of felids belonging to the subfamily Felinae that was endemic to North America from the Pleistocene epoch and morphologically similar to the modern cheetah, although its apparent similar ecological niches have been considered questionable due to anatomical morphologies of the former that would have limited its ability to act as a specialized pursuit predator. The genus was originally known from fragments of skeletons, but nearly complete skeletons have been recovered from Natural Trap Cave in northern Wyoming.
Miracinonyx was originally established as a subgenus of the genus Acinonyx, with M. trumani assigned to as the type species. M. studeri was originally included as a second species, but it is more often listed as a junior synonym of M. inexpectatus. M. trumani was exclusive to the Rancholabrean age, while M. inexpectatus ranged from the late Blancan to Irvingtonian ages of North America.
Discovery and naming
The first fossils attributed to Miracinonyx were several isolated teeth from Port Kennedy Bone Cave from Pennsylvania, dating back to the Irvingtonian age. The American paleontologist Edward Drinker Cope originally considered these to be related to the spotted hyena, and described the material as Crocuta inexpectata in 1895. However, in his posthumous 1899 publication, he reclassified the specimens as Uncia inexpectata, considering them closely related to the snow leopard.More fossil material was recovered from deposits of similar age in Maryland and Arkansas, where in 1941 American paleontologist George Gaylord Simpson found them all belonging to the same animal, one he considered to be more closely related to the cougar, naming the animal as Felis ''inexpectata.
The type species, then known as Felis trumani, was described in 1969 by Phil C. Orr based on a complete skull from the Late Pleistocene found in Crypt Cave, Nevada. In 1979, Daniel B. Adams coined Miracinonyx as a new subgenus of the genus Acinonyx, assigning F. trumani to as the type species: Acinonyx trumani. The subgeneric name, Miracinonyx, is derived from the Latin word mirus and the genus Acinonyx which includes the modern cheetah.
The other species, then known as Felis studeri, was described by Donald E. Savage in 1960 based on a more complete material consisting of extensive postcranial and cranial material of the animal. A 1976 review of fossil pumas by Björn Kurtén considered F. inexpectata and F. studeri to be the same species, with F. inexpectata having priority. In 1979, Adams also included F. studeri as a second species of Miracinonyx, assigned to as Acinonyx studeri; while he recognized Kurtén's proposed synonymy, Adams considered F. inexpectata as a nomen dubium due to the holotype being a fragmentary isolated 4th premolar that prevents definitive classification. In 1990, Blaire Van Valkenburgh and colleagues elevated Miracinonyx to the genus level, and supported the synonymy of "F." studeri and "F." inexpectata, resulting in a new combination, M. inexpectatus''.
Taxonomy and evolution
Research into the American cheetah has been contradictory. It was originally believed to be an early cougar representative, before being reclassified in the 1970s as a close relative of the cheetah. This suggested that the ancestors of the cheetah diverged from the Puma lineage in the Americas and migrated back to the Old World, a claim repeated as recently as, and by However, other research by Barnett and Faurby, through examining mitochondrial DNA and reanalyzing morphology, has suggested reversing the reclassification: the American cheetah developed cheetah-like characteristics through parallel evolution, but it is most closely related to Puma and not to the modern cheetah of Africa and Asia. Moreover, Faurby notes that no Acinonyx fossils have been found in North America, and no Miracinonyx fossils elsewhere. However, posit that the supposed homoplasy between the genera is controversial, as it is asserted that is not necessarily any conclusive anatomical or genetic basis for dismissing a homologous relationship between Acinonyx and Miracinonyx. The veracity of the origin of the modern cheetah is also debated; however, Miracinonyx is believed to have evolved from cougar-like ancestors, regardless of whether in the Old World or the New World.The cougar and M. trumani are believed to have split from a cougar-like ancestor around three million years ago; where M. inexpectatus fits in is unclear, although it is probably a more primitive version of M. trumani.
Below is the phylogeny from when describing the earliest known fossil record of cougars in South America. As shown here, they found Miracinonyx to be a sister taxon to Puma.
Description
It has been proposed that Miracinonyx is an instance of parallel evolution with the cheetah of the Acinonyx genus. Recent studies, however, suggest that it was not as specialized in chasing as the cheetah, as it retained retractable claws and could supinate its forelimbs to grapple with prey, which may have diminished its ability to run fast compared to the true cheetah. Instead, it was more closely related to the cougar; M. trumani might have employed a hunting behavior without modern analogue, which may not have relied on speed as much as a cheetah. M. trumani also had a brain similar to a cougar's, albeit with cheetah-like adaptations in the auditory and visual cortexes, and therefore was not as cognitively prepared for the predatory behavior of true cheetahs. Moreover, it had underdeveloped paranasal sinuses which prevents the brain from overheating in Acinonyx during high-speed chases, though this may reflect the colder temperatures of Pleistocene North America compared to Africa and Iran. Additionally, the injuries that led to the death of a sub-adult Miracinonyx according to a 2022 research article by John-Paul Michael Hodnett et al suggest that Miracinonyx felids, like extant cats such as the puma, regularly fought one another; instances of conspecific aggression among Acinonyx cheetahs are relatively rare. Miracinonyx species were also larger than a modern cheetah and similar in size to a modern North American cougar. The body mass was typically around, with a head-and-body length of, tail length around, and shoulder height of. Large specimens could have weighed more than. Miracinonyx most likely preyed on mountain goats, horses and especially pronghorns.It is often assumed that pronghorns evolved to evade Miracinonyx, but this is still debated. Recent research has suggested that the pronghorn's speed may have been shaped by multiple ecological pressures beyond a single predator, including prehistoric wolves, North American lions, and environmental factors such as open grasslands. In the Neogene period, in North America, grassland environments, especially those of C4 carbon fixations, were replacing forested lands in North America as well as C3 plants, causing extensive changes in vegetational structure. As a result, artiodactyls and perissodactyls generally evolved with hypsodont dentition for eating C4 grasses and distally longer limbs for easier movement within open environments. In comparison, true pursuit predators didn't even exist until the Plio-Pleistocene, a more recent development of carnivorans compared to the ungulates that already were built for speed some 20 million years before. This general analysis, however, does not preclude the pronghorn's unusually high speed being a specific adaption to predation pressure from cheetahs, as isotopic analysis does indicate pronghorn were their preferred prey, making up an estimated 40% of their diet.
Behavior
Miracinonyx likely hunted cursorial prey such as Antilocapra, the extinct Tetrameryx and Stockoceros. Other prey would include horses, bighorn sheep and mountain goats. Fossils from Arizona show that M. trumani was a territorial animal, with evidence of pathologies being found on some of the bones.M. inexpectatus was more similar to the cougar in build than M. trumani. Fossil remains of M. inexpectatus found in Hamilton Cave in West Virginia show that this creature lived with and competed with other large cats like jaguars and the saber-toothed cat . M. trumani was more cursorial and cheetah-like than M. inexpectatus, but unlike cheetahs, both species retained fully retractable claws. This genus likely hunted in a way with no modern analogues as its elbow-joint morphology was intermediate between A. jubatus and P. concolor''.
Fossil distribution
Fossil remains of Miracinonyx have been found across the United States and as far south as Mexico. Remains have also been unearthed in western Canada. Fossils of M. trumani have been found in Arizona, New Mexico, Florida, Wyoming, Colorado, Nebraska, South Carolina, Maryland, and Pennsylvania. The most complete finds of M. trumani are from the Natural Trap Cave in northern Wyoming. The fossils unearthed in Arizona have offered us a better understanding of this enigmatic felid.Fossils of M. inexpectatus have also been found in Florida, Texas, Colorado, Georgia, South Carolina, Pennsylvania, West Virginia, Maryland, and California. Some fossil remains from sites in California, including Rancho La Brea and the McKittrick Tar Seeps, were originally misidentified as Puma concolor, but have since been reclassified as Miracinonyx trumani. This revision suggests a broader geographic and ecological range for the species in late Pleistocene California. In 2022, the skeletal remains of a M. inexpectatus were retrieved from a cave in southwestern Virginia.
M. inexpectatus lived between the late Blancan and Irvingtonian ages, while M. trumani was exclusive to the Rancholabrean age, with the youngest specimen assigned to as cf. M. trumani recovered from the Haystack Cave. Based on the shaft fragments of an unidentified mammal from stratum levels 3 and 4, the most reliable radiocarbon dating of this site is estimated at 19,659-16,604 calibrated years BP, which has been suggested to be the latest occurrence of M. trumani.