Homalodotherium is an extinct genus of South American native ungulates in the order Notoungulata. Fossils of Homalodotherium have been found in the Middle Miocene (Friasian in the SALMA classification) Santa Cruz Formation of Argentina and the Río Frías Formation of Chile. The first specimen, a partial skull, was discovered by Robert Holiver Cunningham while on an expedition to Patagonia. By the time it had arrived in England, it was degraded to the point where only a few elements remained. Regardless, in 1870, it was given its genus name by Thomas Henry Huxley, and its species name (H. cunninghami, after its discoverer) by William Henry Flower. The name was misspelled repeatedly over the years, leading to the erection of the family Homalodontotheriidae to include it, though this has since been amended to Homalodotheriidae. Since, then four more species of Homalodotherium have been named. Homalodotherium was a fairly large animal, with a body length of 2 m (6.6 ft) and a body mass of 250–350 kg (550–770 lb). It had a proportionally small head with broad, flat, teeth, and, unusually for South American native ungulates, large claws on its forelimbs. Initially, it was suggested that these were used for digging. However, it is more likely that they were instead used for pulling vegetation to within the range of the mouth, which may have been supplemented by a prehensile upper lip (indicated by its retracted nasal bones). In these regards, Homalodotherium is convergent with chalicotheres, a lineage of perissodactyls that may have similarly browsed from trees.
Taxonomy
Early history In March 1863, the British Admiralty, at the behest of naturalist Charles Darwin and naval officer Bartholomew Sulivan, sent the latter to Patagonia to collect fossils from the banks of the Río Gallegos, along with his son and Scottish naturalist Robert Oliver Cunningham. At some point during this expedition, Cunningham recovered the skull of a large mammal. These, along with the other remains recovered from Patagonia, were returned to England; by the time they did so, the skull had degraded until only the teeth and part of the mandibular ramus remained. What remained came into the possession of English biologist and anatomist Thomas Henry Huxley. In 1870, Huxley presented the teeth to the Geological Society of London, whereupon he proposed the generic name Homalodotherium for them. Huxley gave the teeth to anatomist William Henry Flower of the Royal College of Surgeons, who, in 1872, wrote an abstract discussing them. In that abstract, he provided a species name for the new species, declaring it Homalodotherium cunninghami, in honour of its discoverer. Two years later, Flower published a paper describing the specimen in greater detail. Notably, he altered the spelling of the name, providing instead the combination Homalodontotherium cunninghami. He referred to several bone fragments found nearby and discussed whether or not they could be assigned to H. cunninghami, concluding that they could not. In an 1880 paper discussing the mammals of South America, H. Gervais and Florentino Ameghino discussed the taxon, referring to it by the name Homalodontotherium, following Flower's 1874 paper. In a later work, Ameghino would mistakenly attribute that spelling to Huxley, and erect the family Homalodontotheridae to include the genus. Later, in 1894, Richard Lydekker amended the family name by adding a second "i" (thus rendering it Homalodontotheriidae), and mistakenly attributed the original paper by Flower to 1884. Subsequently, Ameghino wrote again on the taxon, using the original name Homalodotherium but retaining Lydekker's spelling of the family name. In a subsequent paper, though, he would refer to the family repeatedly as Homalotheriidae (and occasionally referred to Homalodotherium by Homalotherium. In 1910, William King Gregory finally provided the family with the current spelling of its name, Homalodotheriidae, though subsequent authors would continue to misspell it in various fashions. The matter of nomenclature would not be fully resolved until 2017, whereupon Homalodotherium and Homalodotheriidae were determined to be the correct spellings for the generic and familial names respectively.
Other species In an 1891 paper, Ameghino described a new species of Homalodotherium, H. segoviae, based on a fairly complete skull. Three years later, he would go on to name two more species of Homalodotherium, H. crassum and H. excursum, both based on limb elements. In 1941, Alejandro F. Bordas described a fifth species, H. canepai, from the Rincón del Buque locality of southern Patagonia.
Evolution Homalodotherium is a member of the Notoungulata, an order of South American native ungulates (SANUs). Notoungulates were the most diverse SANU lineage, with over 150 described genera in 13 different families. Homalodotherium belongs to the family Homalodotheriidae, along with Asmodeus, Chasicotherium, and Trigonolophodon, though the exact placement of that family has long been unclear. Richard Lydekker, in 1894, suggested that homalodotheriids fell under the Astrapotheria, and were thus sister to Astrapotheriidae. In 1910, Henry Fairfield Osborn proposed a sister-group relationship with Notostylopidae, with both families falling under the suborder Homalodotheria. George Gaylord Simpson suggested instead that homalodotheres formed a group, Entelonychia, with Isotemnidae. In 2017, Juan D. Carillo and Robert J. Asher performed a phylogenetic analysis of Toxodontia. They recovered homalodotheres, specifically Asmodeus and Homalodotherium, as a group of their own, basal to the group that includes Toxodontidae.
The below cladogram shows the results of Carillo & Asher (2017):
Description
Size Homalodotherium had a skull length of around 40 cm (16 in), and a total length of around 2 m (6.6 ft). In 2010, Andrea Elissamburu estimated its body mass at 1,150 kg (2,540 lb). In 2012, though, Guillermo Cassini et al. recovered a lower mass of 400 kg (880 lb), based on craniodental anatomy. Darin A. Croft et al. agreed with the latter result. The results of Allison Nelson et al. using a dataset founded on modern mammals, suggested that Homalodotherium weighed around 250–350 kg (550–770 lb).
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