Platybelodon
Template:Short description Template:Automatic taxobox
Platybelodon (possibly "shovel tooth")<ref name=":1">Template:Cite journal</ref> is an extinct genus of amebelodontid proboscidean mammal, distantly related to modern-day elephants. Fossils are known from middle Miocene strata from parts of Asia and the Caucasus. The first specimens of Platybelodon, consisting of a partial skull, a nearly complete lower jaw, and multiple disarticulated remains, were discovered in the Tchokrak beds of north Caucasus, in the summer of 1927. The following year, Russian palaeontologist Alexey Borissiak described them, giving them the name Platybelodon danovi (now the type species of the genus). Several additional species have been described, including Platybelodon grangeri, named by Henry Fairfield Osborn in 1928.
Platybelodon is best known for its elongated, spoon-shaped lower jaw. While its upper incisors formed a pair of long, slender tusks (which were larger in males), the lower incisors instead formed flat, broad tusks, as in other amebelodontids. While initially suggested to be an adaptation for feeding on water plants, tooth wear patterns suggest that Platybelodon instead used its lower tusks to remove bark from trees and for cutting vegetation. Similarly, while originally believed to lack a trunk and to instead have a large upper lip similar to that of a hippopotamus, Platybelodon had a fairly large trunk which was likely used to grasp tree branches and other vegetation. While bones from the postcranial skeleton are rare, limb elements suggest that Platybelodon was more lightly built and possibly more agile than many other proboscideans.
TaxonomyEdit
History of discoveryEdit
In the summer of 1927, the remains of a proboscidean were recovered from the Miocene-age Tchokrak beds of the Kuban region (north Caucasus). The remains were fragmentary, consisting of the partial skull and a nearly complete mandible (lower jaw) of one specimen, and a few disarticulated teeth and bones of others.<ref name=":0">Template:Cite journal</ref> On April 4, 1928, Russian palaeontologist Alexey Borissiak briefly described the remains, assigning to them the binomial name Platybelodon danovi.<ref name=":3">Template:Cite journal</ref> A year after his initial description, Borissiak published a more detailed diagnosis of the relevant specimens. Borissiak believed that P. danovi represented a new branch of mastodonts, subsequently erecting a subfamily which he dubbed Platybelodontinae.<ref name=":0" />
On July 28, 1928, during the Central Asiatic Expeditions to Mongolia, Roy Chapman Andrews and Walter W. Granger came upon a series of mandibles and teeth in the Tung Gur locality (now the Tunggur Formation). The following year, Henry Fairfield Osborn assigned them to Platybelodon, assigning to them the name Platybelodon grangeri, after Granger. AMNH 26202, the partial mandible of an adult, was designated as the type specimen.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
In 1960, E. I. Beliajeva and L. K. Gabunia named a new species of Platybelodon, P. jamandzhalgensis, based on the mandible of a juvenile individual from the same locality as the P. danovi type.<ref>Template:Cite journal</ref> Thirteen years later, it was synonymised with P. danovi by Heinz Tobien.<ref>Template:Cite journal</ref><ref name=":4">Template:Cite journal</ref> In 1971, L. I. Alexeeva described a third species of Platybelodon, P. beliajevae, based on several molars recovered from the Oshi locality in western Mongolia.<ref>Template:Cite journal</ref>
A fourth species, P. tongxinensis, was described in 1978 by G. F. Chen, based on two molars from the Zhangenbao Formation in Tongxin County, China.<ref>Template:Cite journal</ref><ref name=":2">Template:Cite journal</ref> While briefly synonymised with P. danovi,<ref>Template:Cite book</ref> a 2022 paper by Shiqi Wang and Chun-Xiao Li reaffirmed its validity and attributed specimens formerly assigned to Gomphotherium to it. A fifth Platybelodon species, P. tetralophus, was named in the same paper, based on a partial mandible. Multiple specimens known from the Platybelodon Quarry of the Tunggur Formation, and some in the collection of the American Museum of Natural History from the Wolf Camp Quarry, were assigned to P. tetralophus.<ref name=":2" /> An as-yet unnamed species is known from Loperot, Kenya.<ref name=":02">Template:Citation</ref>
ClassificationEdit
The taxonomy of Platybelodon, and of amebelodontids as a whole, has been a matter of debate. Some studies recover amebelodonts as a monophyletic family, forming a sister group to gomphotheres.<ref>Template:Cite journal</ref><ref name=":6">Template:Citation</ref> Others, however, suggest that amebelodonts are an unnatural assemblage of gomphotheres which independently evolved a "shovel-tusked" tooth morphology.<ref>Template:Cite journal</ref> In 2023, Chunxiao Li et al. recovered a monophyletic Amebelodontidae. In their tree, Platybelodon formed a clade with Amebelodon, Aphanobelodon, Protanancus, and Torynobelodon, and Gomphotheriidae was a paraphyletic clade ancestral to elephantids.<ref name=":6" />
The below cladogram reflects the results of Li et al., 2023.<ref name=":6" />Template:Clade
EtymologyEdit
The full etymology of Platybelodon is obscure. In a paper attempting to translate the generic names of various extinct proboscideans into Chinese, Shi-Qi Wang, Chun-Xiao Li, and Xiao-Xiao Zhang divided the genus name into the Greek platy ("wide"), bel (presumably "shovel", which they supposed was in reference to the shape of its incisors), and don ("tooth"); thus they suggested that the name in full may translate to either "shovel tooth", "broad tooth", or "plate tooth".<ref name=":1" />
DescriptionEdit
Platybelodon was a fairly small proboscidean, being far smaller than North American taxa such as Amebelodon, though its precise dimensions are unclear.<ref name="probos_mass">Template:Cite journal</ref><ref name=":5">Template:Cite book</ref> Postcranial elements from Platybelodon are extremely scarce. The pes (hind foot) of an adult specimen suggests that most of its weight was supported by the fourth metatarsal, similar to certain artiodactyls. Such a shift indicates an unusual standing posture compared to other proboscideans, and that Platybelodon may have been more lightly-built and moved relatively quickly. Additional postcranial elements, such as a manus (front foot) and several other limb bones, are known (though undescribed), and further support this hypothesis.<ref>Template:Cite journal</ref>
Skull and dentitionEdit
The skull of Platybelodon, measured from the occipital border, is Template:Convert in length in a female specimen, HMV0023, and Template:Convert in length in a male specimen, HMV0940.<ref name=":4" /> The facial skeleton of Platybelodon was flat and wide, particularly towards the front.<ref name=":3" /> The premaxilla of HMV0940 measured Template:Convert in length. On the dorsal surface of the premaxilla is a rough surface, to which the maxillo-labialis (the principal motor muscles of the trunk in proboscideans<ref>Template:Cite book</ref>) would have attached.<ref name=":4" /> While it was initially suggested that a trunk was absent in Platybelodon, and that a hippopotamus-like upper lip was present instead,<ref name=":0" /><ref name=":5" /> it seems likely that one was not only present, but increased in size throughout the genus' history.<ref name=":4" /> In males, this rough surface is broader than in females and juveniles; this, in conjunction with more posterior (rearward) nasal bones, suggests that males had a stronger trunk. The neurocranium (braincase) in Platybelodon was low and very elongated.<ref name=":4" /> The palate was extremely long and thin,<ref name=":0" /> and arched slightly upward. The external nares, the opening for the nose, was wide and low, somewhat elliptical in shape. In males, the postorbital processes were very large, resulting in a broad and short dorsal table to the neurocranium; in females and juveniles, these processes were smaller, and the dorsal table was long and narrow.<ref name=":4" /> Platybelodon's mandible is very long, exceeding the skull in length.<ref>Template:Cite book</ref> Laterally (from the side), the body of the mandible was deep, steeply descending beyond the anterior (front) border of cheek teeth to form a wide, spoon-shaped mandibular symphysis.<ref name=":3" /><ref name=":4" /> The symphysis of P. danovi was more slender than that of P. grangeri.<ref name=":4" />
Platybelodon's inner pair of upper incisors were very reduced, and did not reach the front of the jaw.<ref name=":3" /> The second pair was large, forming a set of long, slender tusks, rounded in cross-section and covered in a thick enamel layer.<ref name=":4" /> The lower incisors formed large, flat tusks, measuring Template:Convert at their widest.<ref name=":3" /><ref name=":5" /> The width of these tusks exceeds that observed in other amebelodont genera.<ref>Template:Citation</ref> Like Torynobelodon, but unlike all other amebelodonts, these lower tusks had dentin tubules, microscopic channels running from the pulp to the enamel border.<ref name=":4" /> The cheek teeth of Platybelodon, the premolars and molars, were bunolophodont, having rounded cusps and prominent ridges running transversely along (across) their occlusal surfaces.<ref name=":3" /><ref name=":5" /> The second premolars of both the upper and lower jaws were very reduced.<ref name=":4" />
PalaeobiologyEdit
EcologyEdit
Platybelodon was originally believed to have fed in the swampy areas of grassy savannas, using its teeth to shovel up aquatic and semi-aquatic vegetation. Borissiak believed that Platybelodon used its spoon-shaped mandible to retain large quantities of water plants, and that it would have removed mud and other detritus using its enlarged upper lip.<ref name=":0" /> However, wear patterns on the teeth suggest that it used its lower tusks to strip bark from trees, and may have used the sharp incisors that formed the edge of the "shovel" more like a scythe, grasping branches with its trunk and rubbing them against the lower teeth to cut it from a tree.<ref name=":4" /><ref name="lambert1992">Template:Cite journal</ref> Adults in particular might have eaten coarser vegetation more frequently than juveniles.<ref>Template:Cite journal</ref> However, in 2023, it was suggested that Platybelodon may have extensively grazed in addition to feeding from trees. Its adaptations may have evolved as a result of a regional drying event, and the expansion of open ecosystems. Due to the flexibility of its lifestyle, Platybelodon may have outcompeted the other longirostrine (long-snouted) proboscideans of its environment. Platybelodon became extinct in northern China at the end of the Middle Miocene, perhaps as a result of unfavorable climatic change.<ref name=":6" />
Growth and sexual dimorphismEdit
Template:Multiple image Platybelodon exhibited strong sexual dimorphism. Males had larger and more robust upper tusks, indicating a strong role in agonistic behaviours (i.e. combat between males), more posteriorly located nasal bones, suggested to indicate a more well-developed trunk, a longer mandibular symphysis, and a short temporal fossa with a higher arch. The teeth were also somewhat sexually dimorphic, with males having narrower and longer third molars (both upper and lower). Juveniles have mandibular symphyses with a length-width ratio intermediate between that of males and females. The sexual dimorphism observed in Platybelodon is overall quite similar to that observed in Gomphotherium. In 2016, Shiqi Wang and Tao Deng noted that this implies a period of intense sexual selection in proboscidean evolution, particularly in trilophodont gomphotheres. This may have originated as far back as basal proboscideans such as Phiomia. Later proboscideans, such as modern elephants, ceased to be as sexually dimorphic.<ref>Template:Cite journal</ref>
ReferencesEdit
Further readingEdit
- Harry Cox, Colin Harrison, R.J.G. Savage, and Brian Gardiner. (1999): The Simon & Schuster Encyclopedia of Dinosaurs and Prehistoric Creatures: A Visual Who's Who of Prehistoric Life. Simon & Schuster.
- Jordi Agusti and Mauricio Anton. (2002): Mammoths, Sabertooths, and Hominids. Pg.90, Columbia University Press.
- Jayne Parsons.(2001): Dinosaur Encyclopedia. Pg.260, Dorling Kindersley.
- David Norman. (2001): The Big Book Of Dinosaurs. Pg.420-421, Welcome Books.
- Hazel Richardson.(2003): Dinosaurs and Other Prehistoric Animals (Smithsonian Handbooks). Pg.173, Dorling Kindersley.