Palaeoloxodon


Palaeoloxodon is an extinct genus of elephant. It originated in Africa during the Early Pleistocene, and expanded into Eurasia at the beginning of the Middle Pleistocene. Palaeoloxodon contains the largest known species of elephants, with mature bulls over tall at the shoulders and over in weight, representing among the largest land mammals ever, including the African Palaeoloxodon recki, the European straight-tusked elephant and the South Asian Palaeoloxodon namadicus. P. namadicus has been suggested to be the largest known land mammal by some authors based on extrapolation from fragmentary remains, though these estimates are highly speculative. In contrast, the genus also contains many species of dwarf elephants on islands in the Mediterranean, some like Palaeoloxodon falconeri less than in shoulder height as fully grown adults, making them the smallest elephants known. The genus has a long and complex taxonomic history, and at various times, it has been considered to belong to Loxodonta or Elephas, but today is usually considered a valid and separate genus in its own right.

History of research and taxonomy

Remains of Palaeoloxodon species have probably been noted since ancient times where their remains like those of other fossil proboscideans were interpreted as those of giants or other mythical beings. In 1695, remains of a straight-tusked elephant were collected from travertine deposits near Burgtonna in what is now Thuringia, Germany. While these remains were originally declared by the Collegium Medicum in the nearby city of Gotha to be purely mineral in nature, Wilhelm Ernst Tentzel, a polymath in the employ of the ducal court of Saxe-Gotha-Altenburg, correctly recognised that they represented the remains of an elephant. Prior to 1845, the remains of Eurasian species of Palaeoloxodon were considered to be those of woolly mammoths. The earliest species of Palaeoloxodon to be described, the European straight-tusked elephant and the South Asian Palaeoloxodon namadicus, were named by British paleontologists Hugh Falconer and Proby Cautley in 1846-47. Prior to the description of the genus, Palaeoloxodon species were initially placed in the genus Elephas. In 1924, Matsumoto Hikoshichirō coined Palaeoloxodon, and circumscribed it as a subgenus of Loxodonta. It included the "E. antiquus—namadicus group", and he designated the Japanese "E. namadicus naumanni Mak." as its type species. Also in 1924, American paleontologist Henry Fairfield Osborn named the genera Sivalika and Pilgrimia, with the former covering the Asian species and the latter covering the African and Mediterranean island dwarf species of Palaeoloxodon. In 1931 Osborn named the genus Hesperoloxodon to include Palaeoloxodon antiquus. In a 1942 posthumous publication, Osborn recognised Sivalika and Pilgrimia as junior synonyms of Palaeoloxodon, while still recognising Hesperoloxodon as valid. This publication was the first to consider Palaeoloxodon a valid genus in its own right, an opinion followed by later authors such as Emiliano Aguirre in 1969. Later authors have considered Hesperoloxodon another synonym of Palaeoloxodon. Vincent J. Maglio in a 1973 publication controversially synonymised Palaeoloxodon with Elephas based on morphological similarities between the two genera. Later authors either considered Palaeoloxodon a valid genus or a subgenus of Elephas. Cladistic analyses finding Elephas and Palaeoloxodon to not be each other's closest relatives led to the placement of Palaeoloxodon species within Elephas to be questioned by other authors. By the 2010s Palaeoloxodon was widely regarded as a valid genus separate from Elephas.File:Palaeoloxodon phylogeny.svg|left|thumb|300x300px|Phylogeny showing the placement of Palaeoloxodon antiquus in relation to other elephantids based on nuclear genomes, after Palkopoulou et al. 2018, showing introgression from African forest elephants and mammoths
During the 19th and 20th centuries, species of Palaeoloxodon were subject to numerous phylogenetic hypotheses regarding their relationship to other elephantids. Some scholars like Hans Pohlig in 1891 and Osborn in 1935 considered the species of Palaeoloxodon to be closely related to African elephants, while others like Wolfgang Soergel in 1915 considered them to be closely related to mammoths. From the late 20th century to the first decade of the 21st century, Palaeoloxodon was usually considered to be closely related the Asian elephant and other members of the genus Elephas. In 2016, a study of the straight-tusked elephant mitochondrial genome and part of the nuclear genome found that the mitochondrial sequences were nested within the diversity of those of the African forest elephant, Loxodonta cyclotis, with the partial nuclear genome supporting P. antiquus as more closely related to L. cyclotis than the African bush elephant, L. africana. A later study published in 2018 by the same authors based on the complete nuclear genome revised these results, and suggested P. antiquus resulted from reticulate evolution and had a complex hybridization history, with the majority of its nuclear genome coming from a lineage more closely related to modern African elephants than to Asian elephants and mammoths, but which diverged before the split between the two living species, with significant introgressed ancestry from African forest elephants and to a lesser extent mammoths. The ancestry from L. cyclotis was more closely related to modern West African populations of the forest elephant than to other forest elephant populations, while the mammoth ancestry was basal to the split between woolly and Columbian mammoths, probably from shortly after the split between the ancestors of mammoths and Asian elephants. The hybridisation probably took place in Africa, where Palaeoloxodon was dominant for most of the Early Pleistocene, with the mammoth hybridisation suggested to have taken place earlier than the hybridisation with forest elephants.
Analysis of mitochondrial genomes, including Palaeoloxodon individuals from Northern China indicates Palaeoloxodon individuals harboured multiple separate mitochondrial genome lineages derived from African forest elephants, some being more closely related to some West African forest elephant groups than to others. It is unclear as to whether this is the result of multiple hybridisation events, or whether multiple mitochondrial lineages were introgressed in a single event. It has been found that mitochondrial genome of Chinese Palaeoloxodon specimens clustered with a P. antiquus individual from western Europe, which belonged to a separate clade than other sampled European P. antiquus specimens. The relatively low divergence between the mitochondrial genomes of the European P. antiquus individual and the Chinese Palaeoloxodon specimens may indicate that the populations of Palaeoloxodon across Eurasia maintained gene flow with each other, but this is uncertain.
Diagram of the relationships of elephant mitochondrial genomes, after Lin et al. 2023:

List of species

  • P. recki , the oldest species and ancestor of all later species
  • P. jolensis the last representative of Palaeoloxodon in Africa
  • P. antiquus
  • P. huaihoensis
  • P. namadicus , the largest in its genus, and possibly the largest terrestrial mammal ever
  • P. naumanni
  • P. turkmenicus known from a specimen found in the Middle Pleistocene of Turkmenistan in Central Asia, as well as a specimen from the Kashmir Valley in the northwest Indian subcontinent.

    Mediterranean island dwarfs

These Mediterranean insular dwarf elephant species are almost certainly descended from P. antiquus
  • P. creutzburgi
  • P. xylophagou
  • P. cypriotes
  • P. lomolinoi
  • P. tiliensis
  • P. mnaidriensis
  • P. falconeri
Other indeterminate dwarf Palaeoloxodon species are known from other Greek islands, including Rhodes and Kasos.

Description

Many species of Palaeoloxodon are noted for the distinctive parieto-occipital crest, a bone growth at the top of the skull above the nasal opening which projects forwards and overhangs the rest of the skull. The crest probably functioned to anchor muscle tissue, including the splenius as well as an additional muscle layer called the "extra splenius" which wrapped around the top of the head to support it. The development of the crest is depending, growth stage and gender, with females and juveniles having less developed or absent crests. The crest likely developed as a response to the large size of the head, which in proportional and absolute terms are the largest in size of any proboscideans. The crest shows differences in development depending on species, with the earliest species P. recki as well as the Japanese P. naumanni and Central Asian P. turkmenicus only having weakly developed crests, while P. antiquus, P. namadicus and Chinese Palaeoloxodon have strongly developed crests. It is thought that the weakly developed morphology of the POC is the ancestral condition in Palaeoloxodon, with the strongly developed "namadicus morph" having evolved following the migration of Palaeoloxodon into Eurasia.
The skull is proportionally short and tall, with the premaxillary bones containing the tusks being flared outwards. The tusks have relatively little curvature, and are proportionally large, and somewhat twisted, with the tusk alveoli being divergent from each other at least in Pleistocene species. These tusks could reach in length, and probably over in weight in the largest species, larger than any recorded in modern elephants.
The lamellae of molar teeth of Palaeoloxodon species typically show a "dot-dash-dot" wear pattern when they are found early in their wear life cycle, with the enamel folds concentrated into a major central structure at the midline of the tooth, which are flanked by smaller folds on either side, and the crowns of the tooth are generally proportionally narrow. The teeth are typically very hypsodont with a substantial number of lamellae, though the lamellae frequency is distinctly lower than that reached by advanced mammoth species. The morphology of the teeth varies little between non-dwarf Eurasian Palaeoloxodon species, meaning that they generally cannot be distinguished based on tooth morphology alone.
Species of Palaeoloxodon varied widely in size. Fully grown bulls of Palaeoloxodon recki, Palaeoloxodon antiquus, Palaeoloxodon namadicus and Chinese Palaeoloxodon grew substantially larger than living elephants, with some mature bulls exceeding tall at the shoulder and in body mass, making them some of the largest known terrestrial mammals to have ever lived. In a 2015 study, one fragmentary unlocated femur of P. namadicus described in the 19th century was estimated to have belonged to an individual tall and in weight, exceeding the estimates for the otherwise largest known land mammals, the paraceratheres. However, this estimate is highly speculative and the author suggested that it should be "taken with a grain of salt". In contrast, some of the island dwarf species are the smallest elephants known. The smallest species, P. cypriotes and P. falconeri, only reached tall as fully grown adults, with fully grown adult bulls of P. falconeri having an estimated body mass of only.