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Homotherium

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Homotherium
Temporal range: Early Pliocene to Late Pleistocene, 4–0.012 Ma[1]
Skeleton of H. serum from Friesenhahn cave, Texas Science & Natural History Museum, University of Texas at Austin, Austin, Texas.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Suborder: Feliformia
Family: Felidae
Subfamily: Machairodontinae
Tribe: Homotherini
Genus: Homotherium
Fabrini, 1890
Type species
Homotherium latidens
Owen, 1846
Other species
  • Homotherium ischyrus (Merriam, 1905)
  • Homotherium serum (Cope, 1893)
  • Homotherium venezuelensis Rincón et al., 2011
Synonyms
  • Dinobastis Cope, 1893

Homotherium is an extinct genus of scimitar-toothed cat belonging to the extinct subfamily Machairodontinae that inhabited North America, South America, Eurasia, and Africa during the Pliocene and Pleistocene epochs from around 4 million to 12,000 years ago.[1][2] It was one of the last surviving members of the subfamily alongside the more famous sabertooth Smilodon, to which it was distantly related. It was a large cat, comparable in size to a lion. In comparison to Smilodon, the canines of Homotherium were shorter (though still longer than those of living cats), and it was probably adapted to running down rather than ambushing prey, and is suggested to have engaged in cooperative hunting.

History and taxonomy

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Eurasia

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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]

Africa

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In 1972 a species Homotherium problematicum (originally Megantereon 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]

A third species, Homotherium africanum (originally Machairodus africanus), has also been included.[16][17][18]

Americas

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In 1905, Merriam described a new species Machaerodus ischyrus.[19] Subsequently, in 1918, Merriam reassigned it to a new genus Ischyrosmilus along with the new species Ischyrosmilus idahoensis.[20] The genus Dinobastis was originally named by Cope in 1893, with the type species Dinobastis serus.[21] 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.[22]

In 1966, Churcher named Dinobastis a junior synonym of Homotherium, and recombined D. serus as Homotherium serum.[23] In 1970, a new species Ischyrosmilus crusafonti was described from the early Pleistocene of Nebraska.[24] 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.[25] Up to five species have been recognised from North America: H. idahoensis, H. crusafonti, H. ischyrus, H. johnstoni, and H. serum,[26] 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.[27] In 2022, it was proposed that Homotherium venezuelensis be reassigned to the genus Xenosmilus.[28]

Evolutionary history

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The lineage of Homotherium is estimated (based on mitochondrial DNA sequences) to have diverged from that of Smilodon about 18 million years ago.[29] Homotherium has been suggested to have originated from African species of the genus Amphimachairodus.[30] 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 (~3.6-2.6 million years ago).[12] Specimens of Homotherium are known from Venezuela in northern South America, of an uncertain Early-Middle Pleistocene age.[31] On the African continent the genus disappeared about 1.5 million years ago, during the Early Pleistocene.[32] The latest records of Homotherium in Europe date to the late Middle Pleistocene, around 300-200,000 years ago,[33] with the exception of a single lower jaw bone from the North Sea which dates to around 28-30,000 years ago.[34] 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.[29] Homotherium serum became extinct in North America around 12,000 years ago as part of the end-Pleistocene extinction event of most large mammals across the Americas.[35]

Description

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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.[36][37] 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.[38]

H. venezuelensis skeleton

The large upper canines of Homotherium, which are distinctly flattened,[39] 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.[40]

Compared to Smilodon, the legs were proportionally longer, and the forelimbs were less powerfully built, being narrow and intermediate in form between those of cheetahs and lions. The neck was relatively long and had a high degree of flexibility, while the back along with the tail were relatively short. The claws were small and semi-rectractable, the dewclaw being large, with the second phalanges being less asymmetrical than those of lions, giving the feet a dog-like posture. The part of the humerus closest to the foot was narrow, with the olecranon fossa being strongly vertical. The hindfeet were held in a raised digitigrade posture. Homotherium likely walked with a posture intermediate between that of living big cats and hyenas, similar to that of canines.[39]

Diet and habitats

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H. serum restoration

Homotherium is suggested to have been adapted to the hunting of large prey.[39] The reduced claws, relatively slender limbs, and sloping back all appear to be adaptations for moderate speed endurance running in open habitats.[41][39] The running-adapted morphology of its forelimbs suggests that they were less useful than those of Smilodon or many living big cats in restraining prey and that the incisors instead played a greater role than in living cats, like in hyenas and canines.[39] It has been speculated based on its adaptation to open habitats and high levels of competition from other carnivores, that Homotherium probably relied on group hunting.[39]

Analysis of the genome of a Homotherium specimen found in permafrost in Yukon suggests that Homotherium experienced positive selection for genes related to respiration and the circulatory system, which may have been adapatations for endurance running. Positive selection for genes related to vision indicates that sight probably played an important role in hunting, suggesting that Homotherium was a diurnal (day active) hunter. Selection for genes related to congnition were tenatively suggested by researchers to possibly support the social hunting hypothesis.[42]

Isotopic analysis of H. latidens from in Spain dating to the Early Pleistocene, around 1.6 million years ago, suggests that at this locality H. latidens was an apex predator that hunted large prey in open habitats, and niche partitioned with the sabertooth Megantereon (a close relative of Smilodon) by hunting different prey.[43]

Analysis of specimens from Punta Lucero in northern Spain, dating to the early Middle Pleistocene (600-400,000 years ago), suggests that H. latidens at this locality exclusively consumed large prey, possibly including the giant deer Praemegaceros, and heavily overlapped in diet with the coexisting "European jaguar" Panthera gombaszoegensis.[44]

At the Friesenhahn Cave site in Texas, which dates to the Late Pleistocene, the remains of almost 400 juvenile mammoths were discovered along with numerous Homotherium serum skeletons of all ages, from elderly specimens to cubs.[45] 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.[46] Isotopic analysis of H. serum dental remains at Friesenhahn Cave have confirmed that at this locality it fed on mammoths, along with other C4 grazers in open habitats.[47]

Isotopic analysis of H. serum specimens from Eastern Beringia (now Alaska and Yukon) suggests that in this region the species was not a specialised mammoth predator, and consumed a variety of large prey, including bison, horse and reindeer, as well as also probably mammoths.[48]

See also

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References

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