Amphimachairodus

Amphimachairodus is an extinct genus of large machairodonts. It is also a member of the tribe Homotherini within Machairodontinae and is most closely related to such species as Xenosmilus, Homotherium itself, and Nimravides. It inhabited Eurasia, Northern Africa and North America during the late Miocene epoch.

History and taxonomy
The genus Amphimachairodus was first proposed by Miklos Kretzoi for the species Machairodus palanderi.

Machairodus horribilis was first described in 1903 by Schlosser, who failed to correctly designate a holotype specimen, and thus the species was largely ignored until a 2008 paper redescribed the species and properly designated a lectotype for it. It was subsequently suggested to be reassigned to Amphimachairodus by Ruiz-Ramoni et al. (2019).

Amphimachairodus pliocaenicus was described in 1988 by Joan Pons-Moyà based on fossils from the early Pliocene, found on the Iberian Peninsula. But Ruiz-Ramoni et al. in 2019 considered the fossils too scarce to confirm its assignment to the genus.

Machairodus kurteni was described in 1992. The same paper also resurrected the previously-synonymized Pogonodon copei as Machairodus copei, and reassigned the subspecies Machairodus aphanistus taracliensis as Machairodus giganteus taracliensis.

Machairodus kabir was described in 2005, and reassigned to Amphimachairodus in 2007. The describing paper also considered the species Machairodus tingii, Machairodus leoninus, Machairodus taracliensis, and Machairodus palanderi synonyms or subspecies of "Machairodus" giganteus.

Amphimachairodus alvarezi was described by Ruiz-Ramoni et al. in 2019.

In 2023, a review of the genus considered species Amphimachairodus irtychensis a junior synonym of A. horribilis, and A. kurteni a synonym of A. palanderi.

The species Amphimachairodus hezhengensis was described in 2023.

Description
There was marked sexual dimorphism in A. giganteus, with males being much larger than females.

The species Amphimachairodus coloradensis, from the United States (formerly Machairodus coloradensis) was a significantly large animal, about 1.2 m at the shoulder, according to skeletal and life reconstructions, potentially making it one of the largest known felids. All Amphimachairodus species have a developed mandibular flange, however, A. colaradensis is distinguishable from A. giganteus and A. kurteni by subtle differences in the shape of the mandible and placement of lower carnassials.

In size and proportions, the Eurasian species A. giganteus was remarkably similar to a modern lion or tiger and had a shoulder height of 1.1 m. This species has a skull length of around 14 in. The African species A. kabir (formerly Machairodus kabir, from Arabic kabir = "big") is suggested to have weighed over 350 kg. This would make it comparable in size to Xenosmilus, Machairodus horribillis and slightly smaller than Smilodon populator. In 2022, this species was proposed to be reassigned to a separate genus, called Adeilosmilus.

Amphimachairodus was about 2 m long and probably hunted as an ambush predator. Its legs were too short to sustain a long chase, but it most likely was a good jumper. It probably used its canines to cut open the throat of its prey, severing the major arteries and possibly crushing the windpipe. Its teeth were rooted to its mouth and were not as delicate as those of most other saber-toothed cats of the time, which had extremely long canines that hung out of their mouths. The fangs of Amphimachairodus, however, were able to easily fit in its mouth comfortably while being long enough to be effective for hunting.

Skull
This specimen was from a large male A. giganteus with the skull measuring 14 in from the Late Miocene in China, comparable to a male lion or tiger. Deformation of the skull through natural fossilization processes has changed the shape slightly, making it asymmetrical, but overall it remains an excellent specimen for studying the cranial morphology of this particular genus and species.

For felines, this skull is rather long, but rivaled by the skulls of the two largest species of extant cats: the lion and tiger. When compared with the skull of a regular lion, it is long and very narrow, particularly in the muzzle and width of the zygomatic arches. Its sagittal crest is well pronounced. Compared with other machairodonts, the canines are stout and capable of large amounts of stress. This characteristic is slightly remodeled in females, whose canines are slimmer and generally longer. Compared with females, the orbit of males are smaller, muzzles larger, the anterior-most portion of the nasal bones generally flare upwards slightly, and the downward slope of the dorsal edge of the skull in front of the orbit is not as pronounced, producing a straighter profile. Compared with the most well known machairodont Smilodon, commonly referred to as the "saber-toothed cat", the canines are much shorter, the facial portion again is much longer, and the teeth not reduced so far in number. Several machairodonts, namely Megantereon, bear flanges on the mandible, which are very reduced in A. giganteus though characteristics of the mandible associated with the flanges are present, particularly the lateral flattening of the anterior portion of the mandible, creating a cross section more square than semi-circular. The dental formula for this specimen is.

Paleoecology
Amphimachairodus giganteus was an inhabitant of woodlands and open floodplains as based on finds in Pikermi in Greece and Shanxi Province in China, indicating it had habitat preferences similar to modern lions in many respects. Specimens recovered from Turolian deposits indicate that the fauna living there was much the same, differing only by species in many cases.



Among the creatures it shared its environment with were bovids such as Parabos, Lutung monkeys, the proboscidean Anancus, the rhino Aceratherium, antelopes such as Tragoportax and Miotragocerus as well as gazelles and deer, a very large species of hyrax, early goats, various giraffes, camels such as Paracamelus, the horse Hipparion, a species of aardvark, the chalicothere Ancylotherium and the beaver-like Dipoides. Other carnivores it shared its territory with include the percrocutid Dinocrocuta, the bear Agriotherium, fellow machairodonts Metailurus and Paramachairodus and hyenas like Thalassictis.

The larger herbivores were likely common prey for Amphimachairodus, and it likely would have competed with Agriotherium for food, possibly yielding kills to the bear and possibly also stealing kills from hyenas such as Thalassictis and from Metailurus when the opportunity arose.

In North America, in places such as Coffee Ranch in Texas, Amphimachairodus coloradensis shared territory with Agriotherium as it had in Africa and Eurasia, but also shared territory with the feliform Barbourofelis and the canids such as Vulpes, Epicyon and Borophagus, and herbivores like the camels Aepycamelus and Hemiauchenia the pronghorn antelope Cosoryx, horses like Dinohippus, Neohipparion and Nannippus, the peccary Prosthennops and rhinoceroses like Teleoceras and Aphelops. At the Optima fossil site in Oklahoma isotopic analysis suggest a high degree of niche partitioning within the carnivore guild (Agriotherium, Borophagus, Eucyon, & the mustelid Pliotaxidea) with A. coloradensis having a preference for horses (61.4%) as opposed to camels, mastodons, pronghorns & rhinos (38.7%). A. coloradensis also had the lowest degree of moderate & heavy tooth wear, suggesting it primarily fed on soft tissues.

In the Djurab desert in northern Chad, Amphimachairodus kabir co-existed with fellow machairodonts Lokotunjailurus, Tchadailurus and early representatives of the genus Megantereon. In addition, animals such as crocodiles, three-toed horses, fish, monkeys, hippos, aardvarks, turtles, rodents, giraffes, snakes, antelopes, pigs, mongooses, foxes, hyenas, otters, honey badgers and the hominid Sahelanthropus dwelled here, providing ample food. Based on these and other fossils, it is theorized that the Djurab was once the shore of a lake, generally forested close to the shore with savannah-like areas some distance away. The great number of cat species in the environment indicates that there was significant prey and available niches for multiple species of large felids to coexist.

In the middle Miocene of the Tibetan Plateau, Amphimachairodus hezhengensis would have coexisted with a number of other large carnivores including two species of medium-sized bears, the barbourofelid Albanosmilus, and the huge hyena-like feliform Dinocrocuta. Potential prey species in the locality would have included rhinoceroses, pigs, deer, and medium-sized bovids. Other animals known from the area include skunks, mustelids, and four species of small to medium-sized hyena.