The Taxonomy and phylogeny of Elephant
The Taxonomy and phylogeny of Elephant. Elephants belong to the family Elephantidae, the sole remaining family within the order Proboscidea. which belongs to the superorder Afrotheria. Their closest extant relatives are the sirenians and the hyraxes, with which they share the clade Paenungulata within the superorder Afrotheria Elephants and sirenians are further grouped in the clade Tethytheria.
Three species of elephants are recognised; the African bush elephant (Loxodonta africana) and forest elephant (Loxodonta cyclotis) of sub-Saharan Africa, and the Asian elephant (Elephas maximus) of South and Southeast Asia. African elephants have larger ears, a concave back, more wrinkled skin, a sloping abdomen, and two finger-like extensions at the tip of the trunk. Asian elephants have smaller ears, a convex or level back, smoother skin, a horizontal abdomen that occasionally sags in the middle and one extension at the tip of the trunk. The looped ridges on the molars are narrower in the Asian elephant while those of the African are more diamond-shaped. The Asian elephant also has dorsal bumps on its head and some patches of depigmentation on its skin.
Among African elephants, forest elephants have smaller and more rounded ears and thinner and straighter tusks than bush elephants and are limited in range to the forested areas of western and Central Africa. Both kinds of elephant were traditionally considered to be the same species Loxodonta africana, but molecular studies have affirmed their status as separate species. In 2017, DNA sequence analysis showed that L. cyclotis is more closely related to the extinct Palaeoloxodon antiquus, than it is to L. africana, possibly undermining the genus Loxodonta as a whole.
Evolution and extinct relatives of elephant
Over 180 extinct members and three major evolutionary radiations of the order Proboscidea have been recorded. The earliest proboscids, the African Eritherium and Phosphatherium of the late Paleocene, heralded the first radiation.
The Eocene included Numidotherium, Moeritherium, and Barytherium from Africa. These animals were relatively small and aquatic. Later on, genera such as Phiomia and Palaeomastodon arose; the latter likely inhabited forests and open woodlands. Proboscidean diversity declined during the Oligocene.
One notable species of this epoch was Eritreum melakeghebrekristosi of the Horn of Africa, which may have been an ancestor to several later species. The beginning of the Miocene saw the second diversification, with the appearance of the deinotheres and the mammutids.
The former were related to Barytherium and lived in Africa and Eurasia, while the latter may have descended from Eritreum[ and spread to North America.
The second radiation was represented by the emergence of the gomphotheres in the Miocene, which likely evolved from Eritreum and originated in Africa, spreading to every continent except Australia and Antarctica.
Members of this group included Gomphotherium and Platybelodon. [The third radiation started in the late Miocene and led to the arrival of the elephantids, which descended from, and slowly replaced, the gomphotheres.
The African Primelephas gomphotheroides gave rise to Loxodonta, Mammuthus, and Elephas. Loxodonta branched off earliest around the Miocene and Pliocene boundary while Mammuthus and Elephas diverged later during the early Pliocene.
Loxodonta remained in Africa while Mammuthus and Elephas spread to Eurasia, and the former reached North America. At the same time, the stegodontids, another proboscidean group descended from gomphotheres, spread throughout Asia, including the Indian subcontinent, China, Southeast Asia, and Japan. Mammutids continued to evolve into new species, such as the American mastodon.
At the beginning of the Pleistocene, elephantids experienced a high rate of speciation. The Pleistocene also saw the arrival of Palaeoloxodon namadicus, the largest terrestrial mammal of all time. Loxodonta atlantica became the most common species in northern and southern Africa but was replaced by Elephas iolensis later in the Pleistocene.
Only when Elephas disappeared from Africa did Loxodonta become dominant once again, this time in the form of the modern species. Elephas diversified into new species in Asia, such as E. hysudricus and E. platycephus; the latter the likely ancestor of the modern Asian elephant.
Mammuthus evolved into several species, including the well-known woolly mammoth. Interbreeding appears to have been common among elephantid species, which in some cases led to species with three ancestral genetic components, such as the Palaeoloxodon antiquus. In the Late Pleistocene, most proboscidean species vanished during the Quaternary glaciation which killed off 50% of genera weighing over 5 kg (11 lb) worldwide.
Proboscideans experienced several evolutionary trends, such as an increase in size, which led to many giant species that stood up to 500 cm (16 ft 5 in) tall. As with other megaherbivores, including the extinct sauropod dinosaurs, the large size of elephants likely developed to allow them to survive on vegetation with low nutritional value. Their limbs grew longer and the feet shorter and broader.
The feet were originally plantigrade and developed into a digitigrade stance with cushion pads and the sesamoid bone providing support. Early proboscideans developed longer mandibles and smaller craniums while more derived ones developed shorter mandibles, which shifted the head’s centre of gravity.
The skull grew larger, especially the cranium, while the neck shortened to provide better support for the skull. The increase in size led to the development and elongation of the mobile trunk to provide reach.
The number of premolars, incisors and canines decreased. The cheek teeth (molars and premolars) became larger and more specialized, especially after elephants started to switch from C3-plants to C4-grasses, which caused their teeth to undergo a three-fold increase in teeth height as well as substantial multiplication of lamellae after about five million years ago.
Only in the last million years or so did they return to a diet mainly consisting of C3 trees and shrubs. The upper second incisors grew into tusks, which varied in shape from straight, to curved (either upward or downward), to spiralled, depending on the species.
Some proboscideans developed tusks from their lower incisors. Elephants retain certain features from their aquatic ancestry, such as their middle ear anatomy.
Dwarf species of Elephants
Several species of proboscideans lived on islands and experienced insular dwarfism. This occurred primarily during the Pleistocene when some elephant populations became isolated by fluctuating sea levels, although dwarf elephants did exist earlier in the Pliocene.
These elephants likely grew smaller on islands due to a lack of large or viable predator populations and limited resources. By contrast, small mammals such as rodents develop gigantism in these conditions. Dwarf proboscideans are known to have lived in Indonesia, the Channel Islands of California, and several islands of the Mediterranean.
Elephas celebensis of Sulawesi is believed to have descended from Elephas planifrons. Palaeoloxodon falconeri of Malta and Sicily was only 100 cm (3 ft 3 in) and had probably evolved from the straight-tusked elephant. Other descendants of the straight-tusked elephant existed in Cyprus.
Dwarf elephants of uncertain descent lived in Crete, Cyclades, and Dodecanese while dwarf mammoths are known to have lived in Sardinia. The Columbian mammoth colonised the Channel Islands and evolved into the pygmy mammoth.
This species reached a height of 120–180 cm (3 ft 11 in–5 ft 11 in) and weighed 200–2,000 kg (400–4,400 lb). A population of small woolly mammoths survived on Wrangel Island, now 140 km (87 mi) north of the Siberian coast, as recently as 4,000 years ago.
After their discovery in 1993, they were considered dwarf mammoths. This classification has been re-evaluated and since the Second International Mammoth Conference in 1999, these animals are no longer considered to be true “dwarf mammoths”.