| Ortotherium | |
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| Illustration of the cast of the holotype dentary (MLP M 62). | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Pilosa |
| Family: | †Megalonychidae |
| Genus: | †Ortotherium Ameghino, 1885 |
| Species: | †O. laticurvatum
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| Binomial name | |
| †Ortotherium laticurvatum Ameghino, 1885
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| Synonyms | |
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Ortotherium ("upright beast") is a genus of megalonychid ground sloth from the Late Miocene (Huayquerian SALMA, around 9 to 6.8 million years ago) Ituzaingó Formation of Entre Rios Province, Argentina. Although many species were described, the only valid species of the genus is Ortotherium laticurvatum, with many species being junior synonyms. Ortotherium is known from very fragmentary material, all of which is material from the mandible (lower jaw) and teeth. The holotype (specimen a scientific name is applied to) of O. laticurvatum consists of an incomplete left dentary that had been unearthed from a series of sediments known as ‘Conglomerado osifero’ in Paraná, Argentina. Argentina paleontologist Florentino Ameghino named the species in 1885, though he would go on to name four more, invalid, species of the genus. One species however, O. brevirostrum, has been reclassified as Mesopotamocnus.
Due to Ortotherium being known from very scant fossils, little is definitively known about the animal. However, much can be inferred based on related taxa. Ortotherium was average-sized for a Huayquerian megalonychid, being around 65 kg (143.3 lbs) using the similarly proportioned genus Eucholoeops. Ortotherium has a long mandible, with large, rectangular molars preceded by a giant caniniform. It had long, robust forelimbs terminating in a series of enlarged claws, using a mix of quadrupedal and bipedal movement, possibly permitting tree climbing. The hindlimbs were large and pillar-like supported by a lengthy tail.
Being a megalonychid, Ortotherium was a herbivorous mammal that likely consumed a large amount of leafy plant material. Their jaws and teeth were built for a shearing and cutting method of mastication, bearing sharp teeth and a wide array of jaw movement. The Ituzaingó Formation that Ortotherium inhabited was subtropical, with a mix of swampy woodlands and more open grasslands present. Fossils of mangroves indicate parts of the formation bore a shoreline marine ecosystem along the warm, salty waters of the Paranaense Sea. This allowed for a wide array of fauna to inhabit the area, which included many different genera of ground sloths, "native ungulates", and rodents, in addition to a menagerie of birds, including the large carnivorous "terror birds", as well as fish and reptiles.