Paleoloxodon

''Palaeoloxodon'' is an extinct genus of elephant. The genus originated in Africa during the Early Pleistocene, and expanded into Eurasia at the beginning of the Middle Pleistocene. The genus 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 (''Palaeoloxodon antiquus'') 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 that evolved via insular dwarfism 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 (''Palaeoloxodon antiquus'') 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'' (which includes the Asian elephant).Davies, Paul; (2002
The straight-tusked elephant (''Palaeoloxodon antiquus'') in Pleistocene Europe
Doctoral thesis (Ph.D), UCL (University College London). In 1924, Matsumoto Hikoshichirō coined ''Palaeoloxodon'', and circumscribed it as a subgenus of '' Loxodonta'' (which include the living species of African elephants). 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 ''Paleoloxodon''. 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''.
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.Zhang, Steven
The Rise and Fall of The Straight-Tusked Elephants
June 2021 ''East Tennessee State University Natural History Museum & Gray Fossil Site Paleotalks'' Episode 49. Relevant content begins around 12:20 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 (''P. antiquus'') 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 (~60%) 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 (~36%) and to a lesser extent mammoths (~6%). 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: (note:mitochondrial genomes are only a single genetic locus and in contrast to the nuclear genome, do not necessarily accurately reflect true ancestral history)

List of species

Mainland species

* '' P. recki'' (Synonym: ''Elephas recki'') (East Africa), the oldest species and ancestor of all later species
* '' P. jolensis'' (Synonym: ''Elephas iolensis'') the last (late Middle-Late Pleistocene) representative of ''Palaeoloxodon'' in Africa
*'' P. antiquus'' (Synonym: ''Elephas antiquus'') (Straight tusked elephant) (Europe, Western Asia)
* '' P. huaihoensis'' (China)
* '' P. namadicus ''(Synonym: ''Elephas namadicus'') (Indian subcontinent, possibly also elsewhere in Asia), the largest in its genus, and possibly the largest terrestrial mammal ever
* '' P. naumanni ''(Synonym: ''E. namadicus naumanni'') (Naumann's elephant) (Japan, possibly also China and Korea)
* ''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'' (Crete)
* '' P. xylophagou'' (Cyprus)
* '' P. cypriotes'' (Cyprus)
* ''P. lomolinoi'' (Naxos)
* '' P. tiliensis'' (Tilos)
* '' P. mnaidriensis'' (Sicily and Malta)
*'' P. falconeri'' (Sicily and Malta)
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 was likely similar to the "splenius superficialis" found in Asian elephants, and which may have been an extension of the rhomboideus cervicis muscle) which wrapped around the top of the head to support it. The development of the crest is variable depending on the species, 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 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 (sockets) 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 molar teeth of ''Palaeoloxodon'' species typically show a "dot-dash-dot" wear pattern, 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 (high crowned) with a substantial number of lamellae (up to 19 to 21 on the third molar), 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 .

Ecology

Species of ''Palaeoloxodon'' are thought to have had similar social behaviour to modern elephants, with herds of adult females and juveniles, as well as solitary adult males. The African species of ''Palaeoloxodon'', as well as ''P. namadicus'' are suggested to have been grazers, while ''P. antiquus'' is suggested to have been a variable mixed feeder that consumed a considerable amount of browse.

Evolution

''Palaeoloxodon'' first unambiguously appears in the fossil record in Africa during the Early Pleistocene, around 1.8 million years ago as the species ''Palaeoloxodon recki ileretensis'' (it is contested whether earlier "''E. recki''" subspecies are related to ''Palaeoloxodon'').'''' ''P. recki'' was the dominant elephant in East Africa for most of the Pleistocene. A population of ''P. recki'' migrated out of Africa at the beginning of the Middle Pleistocene around 800,000 years ago, diversifying into the radiation of Eurasian ''Palaeoloxodon'' species, including ''P. antiquus,'' and ''P. namadicus.'' The precise relationships of the Eurasian taxa to each other are obscure. The arrival of ''P. antiquus'' in Europe co-incides with the extinction of the temperate adapted mammoth species ''Mammuthus meridionalis'' (sometimes called the southern mammoth) and its replacement by ''Mammuthus trogontherii'' (the steppe mammoth) in Europe, with the extinction of ''M. meridionalis'' possibly in part a result of competition with ''P. antiquus''. ''P. antiquus'' was able to disperse onto many islands in the Mediterranean, undergoing insular dwarfism and speciating into numerous distinct varieties of dwarf elephants. ''Palaeoloxodon'' fossils are abundant in China and are assigned to three species, ''P. namadicus, P. naumanni'' and ''P. huaihoensis.'' However, the relationships of Chinese ''Palaeoloxodon'' are currently unresolved and it is unclear how many species were present in the region.

Relationship with humans

Evidence of interaction with ''Palaeoloxodon'' by archaic humans extends back over 1 million years ago in Africa, with a number sites with ''Palaeoloxodon recki'' in Africa showing evidence of butchery. There is extensive evidence for butchery and to a lesser extent hunting of the European straight-tusked elephant by archaic humans like ''Homo heidelbergensis'' and Neanderthals. Evidence has been found for butchery of ''Palaeoloxodon turkmenicus'' by archaic humans in the Indian subcontinent. Based on the association of their remains with stone artefacts, it has been suggested modern humans encountered and butchered the Japanese '' P. naumanni'' and the Cyprus dwarf elephant '' P. cypriotes'' during the Last Glacial Period.

Extinction

The timing of the extinction of the last ''Paleoloxodon'' species in Africa, '' P. jolensis'', is uncertain. While often suggested to have gone extinct during the Late Pleistocene, most specimens of the species are poorly dated and dating of specimens from Kenya suggests that it went extinct there around 130,000 years ago, at the end of the Middle Pleistocene. Most Eurasian species of ''Palaeoloxodon'' became extinct towards the end of the Last Glacial Period. The youngest records of ''P. antiquus'' are from the Iberian Peninsula, dating to around 44-43,000 years ago, with footprints from the southern part of the peninsula possibly extending the record to 28,000 years ago. The youngest Japanese records of '' P. naumanni'' date to around 24,000 years ago. The timing of extinction of Chinese ''Palaeoloxodon'' and Indian ''P. namadicus'' is uncertain, but claims of a Holocene survival are not substantiated for either region. The youngest dates for the Sicilian dwarf elephant ''P.'' cf''. mnaidriensis'' are around 32-20,000 years ago, while those of Cyprus dwarf elephant '' P. cypriotes'' are around 12,000 years ago. The dwarf elephant '' P. tiliensis'' from the Greek island of Tilos is suggested by some authors to have survived as recently as 3,500 years Before Present (around 1500 BC) based on preliminary radiocarbon dating done in the 1970s, which would make it the youngest surviving elephant in Europe, but this has not been thoroughly investigated.

The extinction of ''Palaeoloxodon'' was part of the broader Late Quaternary extinction event, in which most larger mammals globally became extinct. Climate change, human hunting or a combination are generally thought to be the causes of the extinctions.

References

{{Taxonbar, from=Q1523445

Prehistoric elephants
Pleistocene proboscideans
Prehistoric placental genera
Fossil taxa described in 1924
Pleistocene mammals of Africa