Palaeoloxodon

In 1924, Matsumoto Hikoshichirō circumscribed Palaeoloxodon 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.[1]

Palaeoloxodon was later thought to be a subgenus of Elephas, but this was subsequently abandoned.[4] In 2016, a mitochondrial DNA study of the straight-tusked elephant (P. antiquus) found that it was more closely related to the African forest elephant, Loxodonta cyclotis, than the African bush elephant, L. africana. The authors suggested that this invalidated the genus Palaeoloxodon as currently recognized.[5] A second study published in 2018 based on the nuclear genome suggested P. antiquus had a complex hybridization history, with over 60% of its Nuclear DNA coming from a lineage closest to but outside the two extant Loxodonta species, around 6% from Mammuthus and 30% 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. The hybridisation probably took place in Africa, where Palaeoloxodon was dominant for most of the Pliocene and Early Pleistocene.[6]

Further analysis of mitochondrial genomes, including Palaeoloxodon individuals from Northern China found that Palaeoloxodon individuals harboured multiple separate mitochondrial genome lineages, 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. The analysis found that the mitochondrial genome of the 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.[7]

Diagram of overall relationships between elephant genera based on nuclear genomes (ignoring hybridisation) after Palkopoulou et al. 2018:[6]

Diagram of the relationships of elephant mitochondrial genomes, after Lin et al. 2023:[7]

Life restoration of P. namadicus

Skeleton of Palaeoloxodon antiquus

Partial skull of Palaeoloxodon namadicus, showing the parieto-occipital crest at the top of the skull

Most species of Palaeoloxodon are noted for their distinctive parieto-occipital crests present at the top of the cranium, which was used to anchor the splenius as well as possibly the rhomboid muscles to support the skull, which is proportionally large in comparison to other elephants. The development of the crest is variable depending on the species, growth stage and gender, with females and juvenliles having less developed or absent crests.[10] The skull is proportionally short and tall.[11] The tusks have relatively little curvature, and are proportionally large,[10] and somewhat twisted, with the tusk alveoli (sockets) being divergent from each other at least in derived species.[11]

Third molar teeth of Palaeoloxodon huaihoensis

The molar teeth of Palaeoloxodon species typically show a "dot-dash-dot" wear pattern,[12] 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.[13] 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.[11]

Size comparison of the dwarf elephant Palaeoloxodon falconeri, one of the smallest elephants known

Species of Palaeoloxodon varied widely in size. Large bulls of P. recki, P. antiquus and P. namadicus are thought to have been over 4 metres (13 ft) tall at the shoulder and greater than 12 tons in body mass, exceeding the size of the largest known African bush elephants. 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 metres (17 ft) and 22 tons 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".[2] In contrast, some of the island dwarf species are the smallest elephants known. The smallest species, P. cypriotes and P. falconeri, were only 1 metre (3.3 ft) tall as fully grown adults,[14][15] with the bulls of P. falconeri only having an estimated body mass of 250 kg (550 lb).[15]

Skeleton of Palaeoloxodon recki, the earliest species of Palaeoloxodon

Palaeoloxodon first appears in the fossil record in Africa during the early Pliocene, around 4 Mya as the species Palaeoloxodon recki. P. recki was the dominant elephant in East Africa for the Pliocene and most of the Pleistocene. A population of P. recki migrated out of Africa between 0.8 and 0.6 Mya, diversifying into the radiation of Eurasian Palaeoloxodon species, including P. antiquus, P. namadicus, and P. naumanni, the precise relationships of the Eurasian taxa to each other are obscure.[16] 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.[16] 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.[17] However, the relationships of Chinese Palaeoloxodon are currently unresolved and it is unclear how many species were present in the region.[10]

The last Paleoloxodon species in Africa, P. iolensis, is suggested to have become extinct at the end of the Middle Pleistocene, around 130,000 years ago.[18] Most Eurasian species of Palaeoloxodon became extinct towards the end of the last glacial period. The youngest record of P. antiquus are footprints from the southern Iberian Peninsula, dating to approximately 28,000 years ago.[19] The youngest Japanese records of P. naumanni date to around 24,000 years ago.[20] 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.[21][22][23] The youngest dates for the Sicilian dwarf elephant P. cf. mnaidriensis date to 32-20,000 years ago,[24] while the youngest dates for the Cyprus dwarf elephant P. cypriotes are around 12,000 years ago.[25] 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.[26]