Homotherium

In Eurasia

Cast of the fossil skulls of H. latidens (left) and H. serum (right).

The first fossils of this genus were described in 1846 by Richard Owen as the species Machairodus latidens.[3] The name Homotherium (Greek: ὁμός (homos, 'same') and θηρίον (therion, 'beast')) was proposed by Emilio Fabrini (1890), without further explanation, for a new subgenus of Machairodus, whose main distinguishing feature was the presence of a large diastema between the two inferior premolars. He further described two species in this new subgenus: Machairodus (Megantereon) crenatidens and Machairodus (Megantereon) nestianus.[4] In 1918, the species Homotherium moravicum was described by Woldřich.[5] In 1936, Teilhard de Chardin described the new species Homotherium ultimus based on fossils from the middle Pleistocene-aged site at Zhoukoudian.[6] In 1972, a species Homotherium davitašvlii was described based on fragmentary material found at Kvabebi in Georgia.[7] Further material from Odessa was tentatively assigned to this species in 2004.[8] In 1986, the species Homotherium darvasicum was described based on material from Kuruksay, Turkey.[9] In 1989, another species Homotherium tielhardipiveteaui was named based on fossils from Tajikistan.[10] In 1996, Homotherium hengduanshanense was described based on fossils from the Hengduan Mountains.[11]

There is currently only one recognised species Homotherium in Eurasia during the Late Pliocene-Pleistocene, Homotherium latidens; other species, including H. nestianus, H. sainzelli, H. crenatidens, H. nihowanensis, and H. ultimum, were proposed mainly on size differences, and do not appear to be distinct.[12]

In Africa

In 1972 a species Homotherium problematicus was named based on fragmentary material from Africa.[13] A second African species discovered in Ethiopia, Homotherium hadarensis, was described in 1988.[14] In 1990, Alan Turner challenged the validity of these two species, and later authors typically refrained from referring the African fossils to any specific species.[12] Although in 2015, further material from Dikika was tentatively referred to H. hadarensis.[15]

In the Americas

In 1905, Merriam described a new species Machaerodus ischyrus.[16] Subsequently, in 1918, Merriam reassigned it to a new genus Ischyrosmilus along with the new species Ischyrosmilus idahoensis.[17] The genus Dinobastis was originally named by Cope in 1893, with the type species Dinobastis serus.[18] In 1965, the species Ischyrosmilus johnstoni was described. In the same paper, it was noted that a comparative study of both Ischyrosmilus and Homotherium might conclude them as synonyms.[19]

In 1966, Churcher named Dinobastis a junior synonym of Homotherium, and recombined D. serus as Homotherium serum.[20] In 1970, a new species Ischyrosmilus crusafonti was described from the early Pleistocene of Nebraska.[21] In 1988, after some debate, the genus Ischyrosmilus was declared a junior synonym of Homotherium and all four species were reassigned to that genus as H. ischyrus, H. idahoensis, and H. johnstoni. The same paper also proposed keeping Dinobastis serus separate from Homotherium.[22] Up to five species have been recognised from North America: H. idahoensis, H. crusafonti, H. ischyrus, H. johnstoni, and H. serum,[23] while other authors suggest that there are only two species, with older Blancan specimens are assigned to the species H. ischyrus, while the younger ones are assigned to the species H. serum.[12]

In 2005, a new species Homotherium venezuelensis was described based on fossils from the Pleistocene of Venezuela.[24] In 2022, it was proposed that Homotherium venezuelensis be reassigned to the genus Xenosmilus.[25]

The lineage of Homotherium is estimated (based on mitochondrial DNA sequences) to have diverged from that of Smilodon about 18 million years ago.[26] Homotherium has been suggested to have originated from African species of the genus Amphimachairodus.[27] Homotherium first appeared during the Early Pliocene, about 4 million years ago, with its oldest remains being from the Odesa catacombs in Ukraine, and Koobi Fora in Kenya, which are close in age, making the origin location of the genus uncertain. The genus arrived in North America during the late Pliocene.[12] Specimens of Homotherium are known from Venezuela in northern South America, of an uncertain Early-Middle Pleistocene age.[28] On the African continent the genus disappeared about 1.5 million years ago, during the Early Pleistocene.[29] Homotherium was formerly thought to have become extinct in Eurasia during the late Middle Pleistocene, around 300,000 years ago, until the discovery of a single jaw bone from the North Sea which dates to around 28-30,000 years ago.[30] The mitochondrial genome of this specimen is nearly identical to specimens known from North America, suggesting that this may have represented a Late Pleistocene dispersal from North America, rather than a continuous undocumented occupation of the region.[26] Homotherium became extinct in North America around 12,000 years ago as part of the Quaternary extinction event along with most other large mammals in the Americas.[31]

H. serum size comparison

Homotherium reached 1.1 m (3 ft 7 in) at the shoulder and weighed an estimated 190 kg (420 lb) and was therefore about the size of a male lion.[32][33] Homotherium had shorter upper canines than other machariodonts such as Smilodon or Megantereon, but these were still longer than those of extant cats. The incisors and lower canines of Homotherium formed a powerful puncturing and gripping device, and its large canine teeth were crenulated. The jaws of Homotherium may have been adapted to clamp and hold prey while inflicting damage with the canine teeth, due to comparable amounts of trabecular bone present in skulls of the genus to those of the modern lion.[34]

H. venezuelensis skeleton

The large upper canines of Homotherium were likely hidden by the upper lips and gum tissues of the lower lips jaw similar to extant cats, unlike the larger upper canines of Smilodon. This hypothesis is further supported by comparable space between the canines and mandible at full closure of the jaws to modern cats; while Smilodon has significantly more space in this respect, likely for soft tissue to fit between the canine and mandible.[35]

The visual cortex in the brain of Homotherium was large and complex, similar to the modern cheetah, implying that it relied heavily on vision during the hunt.

H. serum restoration

The decline of Homotherium could be a result of the disappearance of large herbivorous mammals like mammoths in America at the end of the Pleistocene. In North America fossil remains of Homotherium are less abundant than those of its contemporary Smilodon. For the most part it probably inhabited higher latitudes and altitudes and therefore was likely to be well adapted to the colder conditions of the mammoth steppe environment. The reduced claws, relatively slender limbs, and sloping back all appear to be adaptations for endurance running in open habitats.[36]

Genomic analysis supports the hypothesis that Homotherium was social and well-adapted to life as a pursuit predator. The study also revealed that this genus of machairodont was most likely diurnal, and would have mainly hunted in daylight.[37]

African Homotherium species seem to have hunted early Pleistocene species of Deinotherium, likely preferring to target the more vulnerable adolescents or calves in a herd. Due to their saber-teeth, an attack on such thick-skinned prey would have likely been significantly easier and less time-consuming compared to a similar hunt on modern elephants by lions.[38]

At the well known Friesenhahn Cave site in Texas, the remains of almost 400 juvenile mammoths were discovered along with numerous Homotherium skeletons of all ages, from elderly specimens to cubs. Based on this fossil site, Homotherium was likely a social predator that would have been specialized in hunting young mammoths and that subsequently dragged the kills into secluded caves to eat in relative peace. Homotherium also seemed to have retained the excellent nocturnal vision typical of most cats, and hunting at night in the arctic regions where many Homotherium have been found would have been a prime hunting method.[39] The sloped back and powerful lumbar section of Homotherium's vertebrae suggest a bear-like build, and thus that these animals could have been capable of pulling formidable loads; further, broken upper canines - a common injury in fossils of other machairodonts such as Machairodus and Smilodon that would have resulted from struggling with their prey - is not seen in Homotherium, perhaps because their social groups would completely restrain prey items before any of the cats attempted to kill the target with their saber teeth. Moreover, the bones of the young mammoths found in Friesenhahn Cave show distinctive marks matching the incisors of Homotherium, indicating that they could efficiently process most of the meat on a carcass and that the mammoths had been deposited in the caves by the cats themselves and not by scavengers. Examination of the bones also indicates that the carcasses of these juvenile mammoths were dismembered after being killed by the cats before being dragged away, suggesting that Homotherium would disarticulate their kill to transport it to a safe area such as a hidden lair or den and prevent competitors such as dire wolves and American lions from usurping the carcass.[40]

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