Chalicotheriidae
Family of extinct mammals
From Wikipedia, the free encyclopedia
Chalicotheriidae (from Ancient Greek χάλιξ (khálix), meaning "gravel", and θηρίον (theríon), meaning "beast") is an extinct family of herbivorous, perissodactyl mammals. Unlike living odd-toed ungulates, chalicotheriids bore large claws rather than hooves, although their dentition was adapted for browsing. The family is known from the Oligocene to the Pleistocene, reached its greatest diversity in the Miocene, and is known from Asia, Europe, Africa and North America. Asia appears to have been the main centre of diversification for the group.[2][3]
| Chalicotheriidae | |
|---|---|
| |
| Moropus elatus (Schizotheriinae) at the National Museum of Natural History, Washington, DC | |
 | |
| Life restoration of Anisodon (Chalicotheriinae) | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Perissodactyla |
| Superfamily: | †Chalicotherioidea |
| Family: | †Chalicotheriidae Gill, 1872[1] |
| Type genus | |
| †Chalicotherium Kaup, 1833 | |
| Subfamilies | |
| |
Chalicotheriids are usually divided into two subfamilies, Chalicotheriinae and Schizotheriinae, which differed in the skull, teeth and limbs.[2] Chalicotheriines generally had much longer forelimbs than hindlimbs and are usually interpreted as more specialised browsers of wooded habitats, whereas schizotheriines had more even limb proportions and appear to have occupied a broader range of environments. Derived schizotheriines also developed the distinctive fused phalanges known as the duplex bone.[3]
Microwear and mesowear studies indicate that chalicotheriids were browsers rather than grazers, feeding mainly on leaves and also on tougher plant foods such as bark, twigs, and in some taxa fruit or other hard items. Their unusual claws are therefore generally interpreted as part of a specialised browsing adaptation rather than evidence of a primarily digging lifestyle.[3][4][5]
History of discovery
The history of chalicothere research began in the early nineteenth century with the discovery of ungual phalanges near Eppelsheim in Germany. In 1822, Georges Cuvier interpreted these unusual claw bones as belonging to a gigantic pangolin. Johann Jakob Kaup later described chalicothere teeth from the same locality as the new genus Chalicotherium in 1833, but did not recognise that the teeth and claws belonged to the same kind of animal.[6]
The Miocene locality of Sansan in southern France played a central role in clarifying the nature of chalicotheres. Fossils from Sansan included postcranial remains named Macrotherium by Édouard Lartet in 1837 and cranial remains later referred by de Blainville to Anoplotherium in the 1840s. It was only after the discovery and description of a more complete skeleton by Henri Filhol in 1890 that the skull, teeth and unusual limb bones were firmly recognised as belonging to the same animal.[6]
Later work helped to resolve the complicated nomenclature of the classic European material, including the status of Macrotherium and the use of the name Anisodon for the Sansan chalicotheriines.[6] Because of their unusual combination of claws, browsing dentition and highly distinctive limb proportions, chalicotheres also became a notable case in the history of palaeontology, illustrating how difficult some fossil mammals were to reconstruct and compare with living animals.[7]
Description and functional morphology
Chalicotheriids were unusual among Perissodactyla in possessing large claws rather than hooves. Despite this, their dentition was that of browsing herbivores: the cheek teeth were low-crowned, and the lower incisors cropped vegetation against a toothless pad in the upper jaw.[8][9] Chalicotheriids ranged in size from small antelope-sized forms to animals comparable to large draft horses.[10] The family is generally divided into two subfamilies, Chalicotheriinae and Schizotheriinae, which differ in the skull, teeth, and appendicular skeleton.[9][11]
Skull and dentition
The skull and lower jaw differed noticeably between the two subfamilies. Schizotheriines generally had more slender mandibles, a tapered anterior horizontal ramus, and a relatively low symphysis, whereas chalicotheriines tended to have more robust jaws and, in at least some species, a longer diastema and a fuller complement of lower anterior teeth.[9][12] New mandibular material of Chalicotherium brevirostris showed that this species had a long snout, a long diastema, and three lower incisors plus a canine, revising earlier assumptions based on more fragmentary material.[12]
Forelimbs, claws, and stance
The forelimbs were the most distinctive part of chalicothere anatomy. Coombs interpreted chalicotheres as a perissodactyl lineage specialised for browsing on higher vegetation, with the hook-like manual digits used to pull branches within reach of the mouth rather than primarily for digging.[8] Schizotheriines retained more even limb proportions and were probably more typical quadrupedal browsers, although they also show adaptations consistent with rearing and branch-pulling.[8] In derived schizotheriines, the proximal and middle phalanges of the second manual digit fused to form the distinctive duplex bone.[11] This fusion immobilised the joint and is one of the clearest specialisations of the schizotheriine manus.[11] Chalicotheriines were more specialised in the forelimb, with proportionally longer forelimbs and a more unusual manus. A knuckle-supported forelimb stance has often been reconstructed for forms such as Chalicotherium, but this should be treated as an interpretation rather than an absolutely settled fact.[8] Recent work has suggested that both chalicothere subfamilies evolved ways of limiting movement in the digits, but that schizotheriines did so through regular phalangeal fusion, whereas chalicotheriines achieved digit immobilisation through a different joint structure rather than formation of a true duplex bone.[11]
Hindlimbs and posture
The hindlimbs and pelvis also differed between the two subfamilies. Schizotheriines had powerful hindlimbs and a body plan compatible with rearing while feeding, whereas chalicotheriines combined their elongated forelimbs with a postcranial skeleton that has often been interpreted as supporting upright and possibly seated feeding postures.[8] These reconstructions are based on comparative functional morphology and are best treated as well-supported interpretations rather than direct observations of behaviour.[8]
Subfamily contrasts
Taken together, the two chalicothere subfamilies represent different anatomical solutions to clawed browsing. Schizotheriines retained a more conventional quadrupedal stance and appear to have been functionally less specialised, while chalicotheriines evolved a more extreme forelimb-dominated body plan with greater emphasis on forelimb reach and branch manipulation.[8][9][11]
Palaeobiology
Diet
Dental microwear and mesowear studies indicate that chalicotheriids were browsers rather than grazers, and that grass did not form a significant part of their diet.[4] The family as a whole appears to have fed mainly on leaves, but different genera also show evidence for tougher or more abrasive browse, including bark, twigs, and fruit with hard seeds or pits.[4]
The two subfamilies do not seem to have been identical in diet. North American schizotheriines such as Moropus and Tylocephalonyx have microwear consistent mainly with leaf browsing, in some cases with a greater contribution from bark and twigs.[4] By contrast, several European chalicotheriines, especially Anisodon and Chalicotherium, show unusually abrasive microwear for brachydont browsers, suggesting regular consumption of resistant browse or hard fruit items rather than especially soft vegetation.[4] This is important because it contradicts the older assumption that chalicotheriines necessarily had the softest diets in the family.[4]
Diet could also vary within the family by genus and region. Metaschizotherium shows evidence of more fruit-browsing than Moropus, while the late Miocene Anisodon from Dorn-Dürkheim most likely fed on leaves and fruit, with no indication of substantial grit, dust, or grass in the diet.[4][13]
Locomotion and feeding posture
Functional interpretations of the postcranial skeleton indicate that chalicotheriids used their claws primarily in feeding rather than as digging tools.[8] Coombs interpreted the family as a lineage of specialised browsing perissodactyls in which the forelimbs and hooked claws were used to pull branches within reach of the mouth.[8]
Schizotheriines retained more even forelimb and hindlimb proportions and are generally interpreted as the less specialised of the two subfamilies.[8][9] Their powerful hindlimbs and pelvic morphology suggest that at least some could rear up while feeding, using the claws of the forelimbs to draw down branches.[8] Chalicotheriines developed a more extreme body plan, with much longer forelimbs and more unusual forefoot mechanics. A knuckle-supported or otherwise specialised forelimb stance has often been reconstructed for forms such as Chalicotherium, but this is best presented as a functional interpretation rather than a directly observed certainty.[8][14]
Habitat
Chalicotheriids are generally associated with wooded habitats, but the family did not occupy a single environment throughout its history.[8][4] Evidence from the early Miocene of the Lanzhou Basin suggests that schizotheriines such as Borissiakia lived in at least partly open woodland under relatively humid conditions.[9] Other schizotheriines are associated with forest, woodland, or treed habitats, including more open settings than are usually reconstructed for chalicotheriines.[14]
Chalicotheriines are more often linked to denser or more closed habitats, but the evidence is more nuanced than the older claim that they lived only in moist closed-canopy forest.[8][15] A chalicotheriine from the Miocene of Myanmar has been used to support the presence of an important closed-habitat component in the local palaeoenvironment, while the latest Central European Anisodon from Dorn-Dürkheim comes from a tropical savannah-to-woodland setting rather than a dense forest sensu stricto.[15][13] Recent work on Romanian and German localities has argued that the two chalicothere subfamilies probably differed in preferred environments, with schizotheriines plausibly more tolerant of open settings and chalicotheriines preferring denser, more wooded habitats.[14][3]
Possible behaviour
Direct evidence for chalicothere behaviour is limited, so most reconstructions are inferential.[8] The best-supported behavioural interpretations are feeding-related: branch-pulling with the forelimbs, rearing in at least some schizotheriines, and more orthograde or specialised feeding postures in chalicotheriines.[8]
Evidence for social structure is much weaker. Chalicotheres are generally rare elements in many fossil assemblages, and some species show marked size variation that has been interpreted as sexual dimorphism; this is well documented in Moropus elatus and has also been discussed for other taxa.[16][14] However, rarity in the fossil record does not by itself demonstrate solitary behaviour, and there is currently no secure basis for reconstructing detailed social systems across the family.[14]
Systematics and taxonomy
Chalicotheres are part of the order Perissodactyla, which includes modern equines, rhinoceroses, and tapirs, as well as extinct relatives like brontotheres.[17] As the early evolution of perissodactyls is still unresolved, their closest relatives among other perissodactyl groups is obscure.[18] They are generally placed as part of the clade Ancylopoda alongside their close relatives Lophiodontidae. Many studies considered them as closer to Ceratomorpha (which includes tapirs and rhinoceroses) than Equoidea.[19][20] A 2004 cladistic study alternatively recovered Ancylopoda as sister to all modern perissodactyls (which includes Equoidea and Ceratomorpha), with the brontotheres as the most distantly related within the order Perissodactyla.[21]
Chalicotheriidae is the family that includes the Oligocene and later chalicotheres, and it is generally divided into two subfamilies, Chalicotheriinae and Schizotheriinae. This division is based chiefly on cranial, dental and postcranial characters. Chalicotheriines typically have more robust jaws, a longer mandibular symphysis, low-crowned cheek teeth and the retention of lower canines, whereas schizotheriines generally have more slender mandibles, a shorter symphysis and more derived cheek teeth.[9][2]
Recent treatments place Winamia, Chalicotherium, Kalimantsia, Anisodon, Nestoritherium and Hesperotherium in the Chalicotheriinae.[9] Classification within the subfamily has long been debated, however, and revisions have altered the status of several classic names. A nomenclatural review supported Anisodon as the appropriate genus for the Sansan species traditionally associated with Chalicotherium and Macrotherium, and a cladistic analysis of middle and late Miocene chalicotheriines recovered two main clades while also noting that the relationships of the group remain unstable pending revision of earlier taxa and the addition of more postcranial data.[22] Later work revalidated Nestoritherium and suggested that Hesperotherium may be nested within it.[23]
Schizotheriinae includes Schizotherium, Borissiakia, Phyllotillon, Moropus, Tylocephalonyx, Metaschizotherium and Ancylotherium.[9] Recent authors continue to follow the broad framework established by Margery Coombs for the diagnosis and internal relationships of the subfamily.[9] Some names within the group remain problematic: for example, the taxonomy of Phyllotillon has been regarded as controversial because of the scarcity of material, and some authors have suggested restricting the genus to the Bugti collections of Pakistan until better fossils are available.[9]
The early African chalicotheriine formerly called Butleria rusingensis has been renamed Winamia rusingensis because Butleria was preoccupied.[9] The generic placement of "Chalicotherium" pilgrimi remains unsettled in the literature, and some African late Neogene material has been referred to "Chemositia" tugenensis with caution rather than full confidence in the generic assignment.[2]
Genera
| Genus | Subfamily | Representative age | Main region(s) | Notes |
|---|---|---|---|---|
| Winamia | Chalicotheriinae | Early Miocene | East Africa | Replacement name for Butleria, which is preoccupied; usually represented by W. rusingensis.[9] |
| Chalicotherium | Chalicotheriinae | Late Middle to early Late Miocene | Europe | In recent narrow usage, usually centred on C. goldfussi; several species historically placed here have been reassigned elsewhere.[22][24] |
| Kalimantsia | Chalicotheriinae | Late Miocene (Turolian) | Balkans (especially Bulgaria) | Based on material from the Kalimantsi area of Bulgaria; a distinct late Miocene Balkan chalicotheriine.[25][3] |
| Anisodon | Chalicotheriinae | Middle to Late Miocene | Europe and the Balkans | Re-established as distinct from Chalicotherium in modern treatments; includes the Sansan species and some southeastern European material.[22][26][13] |
| Nestoritherium | Chalicotheriinae | Late Miocene to Early Pleistocene | Asia | Revalidated by Chen et al. (2012); includes Chinese late Miocene material and species formerly referred to Chalicotherium.[23][27] |
| Hesperotherium | Chalicotheriinae | Early to Middle Pleistocene | China | Erected for the last known Chinese chalicotheres; Chen et al. (2012) suggested it may be nested within Nestoritherium.[28][23] |
| Schizotherium | Schizotheriinae | Oligocene | Eurasia | Generally treated as the most primitive schizotheriine; best known from Oligocene Eurasian material.[9] |
| Borissiakia | Schizotheriinae | Late Oligocene to Early Miocene | Asia | Best known from Kazakhstan; some early Miocene Chinese material has been compared with or referred to this genus.[9] |
| Phyllotillon | Schizotheriinae | Early Miocene | Pakistan; possibly Europe | Taxonomically controversial; some authors restrict the genus to the Bugti material until better fossils are available.[9] |
| Moropus | Schizotheriinae | Miocene | Mainly North America; also reported from Eurasia | The best-known North American schizotheriine; Li et al. (2022) note a primarily North American distribution but possible early Miocene Eurasian occurrences.[9] |
| Metaschizotherium | Schizotheriinae | Early to Middle Miocene | Europe | A European schizotheriine; the status of some referred species remains under discussion.[9] |
| Tylocephalonyx | Schizotheriinae | Miocene | North America | North American dome-skulled schizotheriine.[9] |
| Ancylotherium | Schizotheriinae | Late Miocene to Pleistocene | Old World | The latest surviving schizotheriine; common in late Miocene Eastern Mediterranean faunas and persisting later in the Old World.[9][3] |
Evolution, biogeography and fossil record
The superfamily Chalicotherioidea first appeared in the Eocene in Asia, with the earliest chalicotheriids proper appearing during the following Oligocene.[29] By the late Oligocene, they had divided into schizotheriines and chalicotheriines.[10] Chalicotheriids had a broad Old World distribution Asia, Europe, Africa and North American distribution.[2] The family reached its greatest diversity in the Miocene.[2]
Asia appears to have been the principal centre of diversification for the family.[2] Early and middle Neogene records from China, Kazakhstan and surrounding regions show that both chalicotheriines and schizotheriines were well established there, and Asian records continue into the latest history of the group.[9] The early Miocene Lanzhou Basin fauna, for example, preserves a schizotheriine close to Borissiakia, while later Asian assemblages include advanced chalicotheriines such as Nestoritherium and Hesperotherium.[9]
Europe preserves a particularly important Miocene record of the family, including classic middle Miocene localities such as Sansan and a long later Neogene history in Central and southeastern Europe.[14] Schizotheriines were widespread in Europe during the middle Miocene, especially in the genus Metaschizotherium, whereas chalicotheriines such as Anisodon and later Balkan forms such as Kalimantsia are characteristic elements of later Miocene faunas.[14][3] Late Miocene localities in Romania and the Eastern Mediterranean are especially important because they document coexistence of the two subfamilies in the same broad region.[3]
African chalicotheriids are less diverse in the published record than those of Eurasia, but the continent preserves both early and late members of the family.[2] Coombs and Cote noted early Miocene African chalicotheriines and later Neogene material assigned with varying degrees of confidence to chalicotheriines and schizotheriines, showing that Africa remained part of the family's range after the European record declined.[2] North American records, by contrast, are dominated by schizotheriines, especially Moropus and Tylocephalonyx, and indicate that the family dispersed successfully into the continent even though the chalicotheriine branch is not a characteristic part of the North American record.[2] A Panamanian record also shows that small-bodied schizotheriines reached Central America during the early Miocene.[30]
The palaeobiogeographic history of the family also shows that the two subfamilies did not always share the same distribution or ecological settings.[3][14] Recent work has emphasised that confirmed coexistence of chalicotheriines and schizotheriines in the same localities is relatively uncommon, especially after the late Miocene.[3]
Never common animals in the fossil record, the chalicotheres declined from the late Neogene onwards, disappearing from North America and Europe by end of the Miocene.[31][32] The youngest chalicotheres are the chalicotheriines Hesperotherium from the Early Pleistocene of China,[33] Nestoritherium from the Early Pleistocene of Myanmar,[34] as well as the schizotheriine Ancylotherium from the Early Pleistocene of Eastern and Southern Africa,[35] also possibly known from the Early Pleistocene of China.[36] In China, Hesperotherium is suggested to have survived until the near the end of the Early Pleistocene, around 1 million years ago.[37][38]
