Platybelodon
Platybelodon 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 Borissiako described them, giving them the name Platybelodon danovi. 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, 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.
Taxonomy
History of discovery
In the summer of 1927, the remains of a proboscidean were recovered from the Miocene-age Tchokrak beds of the Kuban region. The remains were fragmentary, consisting of the partial skull and a nearly complete mandible of one specimen, and a few disarticulated teeth and bones of others. On April 4, 1928, Russian palaeontologist Alexey Borissiak briefly described the remains, assigning to them the binomial name Platybelodon danovi. 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.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. 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.
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. Thirteen years later, it was synonymised with P. danovi by Heinz Tobien. 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.
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. While briefly synonymised with P. danovi, 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. An as-yet unnamed species is known from Loperot, Kenya.
Classification
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. Others, however, suggest that amebelodonts are an unnatural assemblage of gomphotheres which independently evolved a "shovel-tusked" tooth morphology. 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.The below cladogram reflects the results of Li et al., 2023.
Etymology
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, bel, and don ; thus they suggested that the name in full may translate to either "shovel tooth", "broad tooth", or "plate tooth".Description
Platybelodon was a fairly small proboscidean, being far smaller than North American taxa such as Amebelodon, though its precise dimensions are unclear. Postcranial elements from Platybelodon are extremely scarce. The pes 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 and several other limb bones, are known, and further support this hypothesis.Skull and dentition
The skull of Platybelodon, measured from the occipital border, is in length in a female specimen, HMV0023, and in length in a male specimen, HMV0940. The facial skeleton of Platybelodon was flat and wide, particularly towards the front. The premaxilla of HMV0940 measured in length. On the dorsal surface of the premaxilla is a rough surface, to which the maxillo-labialis would have attached. While it was initially suggested that a trunk was absent in Platybelodon, and that a hippopotamus-like upper lip was present instead, it seems likely that one was not only present, but increased in size throughout the genus' history. In males, this rough surface is broader than in females and juveniles; this, in conjunction with more posterior nasal bones, suggests that males had a stronger trunk. The neurocranium in Platybelodon was low and very elongated. The palate was extremely long and thin, 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. PlatybelodonPlatybelodon