Palaeoloxodon

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 some of the largest known species of elephants, over 4 m tall at the shoulders, 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 only 1 m in height, 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.

Taxonomy
Prior to the description of the genus, Palaeoloxodon species were initially placed in the genus Elephas (which includes the Asian elephant). In 1924, Matsumoto Hikoshichirō circumscribed Palaeoloxodon as a subgenus of Loxodonta (African elephants). It included the "E. antiquus—namadicus group", and he designated the Japanese "E. namadicus naumanni Mak." as its type species. Palaeoloxodon was often historically considered to be a subgenus or a synonym of Elephas due to morphological similarities between the two genera, though the results of morphological cladistic analyses finding Elephas and Palaeoloxodon to not be each others closest relatives led to this relationship being questioned by other authors. 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 had a complex hybridization history, with around 60% of its nuclear genome coming from a lineage most closely related to modern African elephants but which diverged before the split between the two living species, around 5% from Mammuthus and 36% deriving from L. cyclotis. 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 Pliocene and 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:

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) (Japan, possibly also China and Korea)
 * ?P. turkmenicus known from a single specimen found in the Middle Pleistocene of Turkmenistan, with possibly attributable remains known from Kashmir, validity uncertain.

Mediterranean island dwarfs
These Mediterranean insular dwarf elephant species are almost certainly descended from P. antiquus

Other indeterminate dwarf Palaeoloxodon species are known from other Greek islands, including Rhodes and Kasos.
 * 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)

Description
Most species of Palaeoloxodon are noted for their distinctive parieto-occipital crests present at the top of the cranium. The crest 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 4 m in length, and probably over 190 kg 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. While during the early evolution of the genus, the molar teeth had a low number of lamellae and were brachydont (low crowned), in Pleistocene species the teeth became very hypsodont (high crowned) and increased the number of lamellae, though the lamellae frequency is distinctly lower than that reached by advanced mammoth species.

Species of Palaeoloxodon varied widely in size. Large bulls of Palaeoloxodon recki, Palaeoloxodon antiquus, Palaeoloxodon namadicus and Chinese Palaeoloxodon are thought to have exceeded modern elephants in size, over 4 m tall at the shoulder and greater than 12 tonnes in body mass. In a 2015 study, one fragmentary unlocated femur of P. namadicus described in the 19th century was estimated to have belonged to an individual 5.2 m tall and 22 tonnes 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, were only 1 m tall as fully grown adults, with the bulls of P. falconeri only having an estimated body mass of 250 kg.

Ecology
Species of Palaeoloxodon are thought to have 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 0.8 to 0.6 Mya, 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 Mammuthus meridionalis and its replacement by Mammuthus trogontherii, suggesting that it might have shared a similar dietary niche and outcompeted the former. 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.

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 date to 32-20,000 years ago, while the youngest dates for the Cyprus dwarf elephant P. cypriotes are around 12,000 years ago. P. tiliensis from the Greek island of Tilos was suggested to have survived as recently as 3,500 years Before Present 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.

Relationship with humans
Remains of the species P. recki, P. antiquus, P. cypriotes and P. naumanni have been found associated at a number of sites with stone stools and/or with cut marks on their bones, suggesting that they were butchered, with some sites presenting clear evidence of hunting. These sites span from the Early Pleistocene, at least 1.3 but possibly as early as 1.6 million years ago to around 12,000 years ago, with the makers of the sites including both archaic humans (P. antiquus and P. recki) and modern humans (P. naumanni and P. cypriotes).