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Description
Chalicotheres are some of the strangest mammals ever to walk our Earth and yet, despite their prehistoric appearance, these bizarre ungulates are one of the more recent groups of mammalian megafauna to radiate and diversify in the Cenozoic (starting in the very Late Oligocene) and they survived long enough to overlap with our own ancestors, all the way up to the first Homo erectus to reach Asia, circa 800,000 years ago. Chalicotheres are perissodactyls, the odd-toed ungulates, and more specifically, are classed as tapiromorphs, meaning their closest living relatives are tapirs and rhinos (including paraceratheres ), and more distantly, horses and the extinct palaeotheres and brontotheres (hippomorphs).

The family Chalicotheriidae first emerged during the Mid Eocene (45 mya), and by the Late Eocene-Early Oligocene, the very basal and fragmentary Schizotherium was widely distributed across Eurasia and is the type genus of one of the two chalicothere subfamilies; Schizotheriinae, who overall looked like typical if jacked-up basal perissodactyls but instead of hooves or hoof-like toes, these animals sported claws used for raking branches, essentially making them the ungulate version of the New World ground sloths, and several taxa also developed long necks to further help them with browsing , while the other subfamily, Chalicotheriinae, had shorter necks and blunter snouts but also much more exaggerated proportions, with very long front limbs and short hindlimbs, giving them a sloping back, and they started walking on their knuckles to protect their long, curved claws, much like a ground sloth or anteater. Their low-crowned teeth suggest that they favored soft leaves, and they even had ischial callosities on the pelvis indicating that they spent a lot of time sitting on their rumps while feeding, akin to gorillas and giant pandas, while schizotheriines would rear up like a ground sloth.

Seeing as they were the more basal subfamily, the schizotheriines were the first chalicotheres to grow large, starting with the Asian Borissiakia and the American Moropus circa 23 mya, who rank among the largest of their kind. This group was, in general, more diverse and widespread and a likely reason for that was their longer, higher-crowned teeth, which allowed them to eat tougher vegetation, leading to a more varied diet and the ability to live in various biomes, while chalicotheriines stuck to the forests and never ventured into North America over the Bering Strait. It's not clear just when the knuckle-walkers first diverged from the schizotheriines, but the oldest definitive member, known from extensive material from Kenya and Uganda (20-17 mya), is Winamia rusingense (previously classed as Chalicotherium rusingense), which was smaller than its descendants (as to be expected for a basal species) but already showed the key physical characteristics of the knuckle-walkers, such as the disproportionally long front limbs but still had a schizotheriine-like elongated snout. Some of the fossils from this region have recently been used to erect a second species of basal chalicotheriine; Iriritherium pyroclasticum (2020).

Obviously, the type genus of this group is Chalicotherium, starting with C. goldfussi, named by German naturalist Johann Jakob Kaup in 1833; a large, derived species (over 7 feet tall and more than a ton in weight) known from an extensive fossil record from the Late Miocene of Europe (circa 11-6 mya), and its smaller and slightly older relative, C. grande (500-600 kg) is also well represented in the fossil record, especially the Sansan site in southern France and from Chezoslovakia (with some 60 known specimens), but was later given its own genus; Anisodon, which also contains a few other purported knuckle-walkers from the Miocene-Pliocene of Eurasia.

Due to being the type genus, Chalicotherium was historically used as a wastebin taxon for various fragmentary chalicothere fossils, including alleged North American material that turned out to belong to brontotheres, and besides the European C. goldfussi, the only other definitive species of Chalicotherium is C. brevirostris; another large species known from some decent skull, jaw, and postcranial material from the Upper Miocene Tunggur Formation of Inner Mongolia , and some skull and jaw material from the slightly older Jiulongkou Formation, located in the Hebei Province, the latter representing the oldest known occurrence of the genus (circa 16-13 mya). Other known fauna from Tunggur include many typical Upper Miocene mammals such as the shovel-mouthed proboscidean Platybelodon, the barbourofelid Albanosmilus and machairodont Metailurus, the listriodontine suids Listriodon and Kubanochoerus, the giraffid Palaeotragus, the dog-like ursid Hemicyon, the midsized bear-dogs Gobicyon macrognathus and Amphicyon tairumensis, early hyena Percrocuta, and early horse Anchitherium.

As the Miocene came to a close, the chalicotheres' glory days were over, but the group persevered well into the Quaternary, with the schizotheriine Ancylotherium dying out in Eurasia in the Late Miocene-Early Pliocene (circa 5 mya) but surviving in Africa up until the Early Pleistocene (2.5 to 1.8 mya), while chalicotheriines (Nestoritherium and Hesperotherium) survived in Asia until just 750,000 years ago, along with fragmentary Pliocene finds from the Siwalik Hills (tentatively attributed to Chalicotherium and/or Anisodon). The African Ancylotherium hennigi was the last schizotheriine and numerous broken and partial specimens have been found across East Africa (including footprints), often together with our own ancestors (Australopithecus), the youngest fossils coming from the Shungura Formation in Ethiopia. Other animals found alongside it include Palaeoloxodon recki, Deinotherium bozasi (the last of the plesielephantiformes), the giraffid Sivatherium maurusium, the machairodonts Homotherium, Megantereon and Dinofelis, the giant baboon Dinopithecus, giant geladas (Theropithecus) the large warthog Metridiochoerus, the hyenas Pachycrocuta and Chasmaporthetes, and the jaguar-sized otter Enhydriodon.

While mostly associated with Pliocene Africa, the most complete Ancylotherium species is A. pentelicum, a Late Miocene taxon best known from Pikermi (Greece) but distributed throughout southeast Europe and Asia. It was another very large chalicothere, and its skull has an inflated frontal bone forming a modest-sized skull dome , a feature that is more exaggerated in its American cousin Tylocephalonyx, and weirdly enough, converged in the Late Miocene European chalicotheriine Kalimantsia, an animal that could apptly be described as a sloth-ungulate-pachycephalosaur combo.

Description
Chalicotheres are some of the strangest mammals ever to walk our Earth and yet, despite their prehistoric appearance, these bizarre ungulates are one of the more recent groups of mammalian megafauna to radiate and diversify in the Cenozoic (starting in the very Late Oligocene) and they survived long enough to overlap with our own ancestors, all the way up to the first Homo erectus to reach Asia, circa 800,000 years ago. Chalicotheres are perissodactyls, the odd-toed ungulates, and more specifically, are classed as tapiromorphs, meaning their closest living relatives are tapirs and rhinos (including paraceratheres ), and more distantly, horses and the extinct palaeotheres and brontotheres (hippomorphs).

The family Chalicotheriidae first emerged during the Mid Eocene (45 mya), and by the Late Eocene-Early Oligocene, the very basal and fragmentary Schizotherium was widely distributed across Eurasia and is the type genus of one of the two chalicothere subfamilies; Schizotheriinae, who overall looked like typical if jacked-up basal perissodactyls but instead of hooves or hoof-like toes, these animals sported claws used for raking branches, essentially making them the ungulate version of the New World ground sloths, and several taxa also developed long necks to further help them with browsing , while the other subfamily, Chalicotheriinae, had shorter necks and blunter snouts but also much more exaggerated proportions, with very long front limbs and short hindlimbs, giving them a sloping back, and they started walking on their knuckles to protect their long, curved claws, much like a ground sloth or anteater. Their low-crowned teeth suggest that they favored soft leaves, and they even had ischial callosities on the pelvis indicating that they spent a lot of time sitting on their rumps while feeding, akin to gorillas and giant pandas, while schizotheriines would rear up like a ground sloth.

Seeing as they were the more basal subfamily, the schizotheriines were the first chalicotheres to grow large, starting with the Asian Borissiakia and the American Moropus circa 23 mya, who rank among the largest of their kind. This group was, in general, more diverse and widespread and a likely reason for that was their longer, higher-crowned teeth, which allowed them to eat tougher vegetation, leading to a more varied diet and the ability to live in various biomes, while chalicotheriines stuck to the forests and never ventured into North America over the Bering Strait. It's not clear just when the knuckle-walkers first diverged from the schizotheriines, but the oldest definitive member, known from extensive material from Kenya and Uganda (20-17 mya), is Winamia rusingense (previously classed as Chalicotherium rusingense), which was smaller than its descendants (as to be expected for a basal species) but already showed the key physical characteristics of the knuckle-walkers, such as the disproportionally long front limbs but still had a schizotheriine-like elongated snout. Some of the fossils from this region have recently been used to erect a second species of basal chalicotheriine; Iriritherium pyroclasticum (2020).

Obviously, the type genus of this group is Chalicotherium, starting with C. goldfussi, named by German naturalist Johann Jakob Kaup in 1833; a large, derived species (over 7 feet tall and more than a ton in weight) known from an extensive fossil record from the Late Miocene of Europe (circa 11-6 mya), and its smaller and slightly older relative, C. grande (500-600 kg) is also well represented in the fossil record, especially the Sansan site in southern France and from Chezoslovakia (with some 60 known specimens), but was later given its own genus; Anisodon, which also contains a few other purported knuckle-walkers from the Miocene-Pliocene of Eurasia.

Due to being the type genus, Chalicotherium was historically used as a wastebin taxon for various fragmentary chalicothere fossils, including alleged North American material that turned out to belong to brontotheres, and besides the European C. goldfussi, the only other definitive species of Chalicotherium is C. brevirostris; another large species known from some decent skull, jaw, and postcranial material from the Upper Miocene Tunggur Formation of Inner Mongolia , and some skull and jaw material from the slightly older Jiulongkou Formation, located in the Hebei Province, the latter representing the oldest known occurrence of the genus (circa 16-13 mya). Other known fauna from Tunggur include many typical Upper Miocene mammals such as the shovel-mouthed proboscidean Platybelodon, the barbourofelid Albanosmilus and machairodont Metailurus, the listriodontine suids Listriodon and Kubanochoerus, the giraffid Palaeotragus, the dog-like ursid Hemicyon, the midsized bear-dogs Gobicyon macrognathus and Amphicyon tairumensis, early hyena Percrocuta, and early horse Anchitherium.

As the Miocene came to a close, the chalicotheres' glory days were over, but the group persevered well into the Quaternary, with the schizotheriine Ancylotherium dying out in Eurasia in the Late Miocene-Early Pliocene (circa 5 mya) but surviving in Africa up until the Early Pleistocene (2.5 to 1.8 mya), while chalicotheriines (Nestoritherium and Hesperotherium) survived in Asia until just 750,000 years ago, along with fragmentary Pliocene finds from the Siwalik Hills (tentatively attributed to Chalicotherium and/or Anisodon). The African Ancylotherium hennigi was the last schizotheriine and numerous broken and partial specimens have been found across East Africa (including footprints), often together with our own ancestors (Australopithecus), the youngest fossils coming from the Shungura Formation in Ethiopia. Other animals found alongside it include Palaeoloxodon recki, Deinotherium bozasi (the last of the plesielephantiformes), the giraffid Sivatherium maurusium, the machairodonts Homotherium, Megantereon and Dinofelis, the giant baboon Dinopithecus, giant geladas (Theropithecus) the large warthog Metridiochoerus, the hyenas Pachycrocuta and Chasmaporthetes, and the jaguar-sized otter Enhydriodon.

While mostly associated with Pliocene Africa, the most complete Ancylotherium species is A. pentelicum, a Late Miocene taxon best known from Pikermi (Greece) but distributed throughout southeast Europe and Asia. It was another very large chalicothere, and its skull has an inflated frontal bone forming a modest-sized skull dome , a feature that is more exaggerated in its American cousin Tylocephalonyx, and weirdly enough, converged in the Late Miocene European chalicotheriine Kalimantsia, an animal that could apptly be described as a sloth-ungulate-pachycephalosaur combo.