The proboscidae diverged in to a number of different families. Mammoths and the existing elephant genera, Elephas and Loxodonta, both belong to the family elephantidae. All other families within the proboscidea are now extinct:

The Gomphotheres

The Gomphothereidae were the first proboscideans to truly resemble modern elephants, and may be ancestral to them and other families of proboscidean. One of the oldest examples is Palaeomastodon, known from fossils in the lower Oligocene in Egypt.

Palaeomastodon shows evidence of a trunk, if a rather short one, as well as two tusks curving downwards from the upper jaw and long, procumbent lower incisors. Like earlier proboscideans Palaeomastodon possessed multiple pairs of molar and premolar teeth, but like more recent proboscidean taxa the canine teeth were absent. It had long column-like legs like modern elephants, and is thought to have been a terrestrial animal. Palaeomastodon was smaller than living elephants, reaching about two meters at the shoulder.

The gomphotheres lived throughout the Oligocene, Miocene and Pliocene epochs. Advanced gomphotheres had a more developed trunk, but retained the same procumbent lower incisors and long shovel-like lower jaw. A typical example, platybelodon, was very widespread, found throughout Africa, Asia, Europe and North America. After the Northern and Southern American continents collided during the Pliocene Epoch the Gomphotheres spread to South America. The gomphotheres died out relatively recently in the Pleistocene epoch. The reason for their extinction is not known, but it has been proposed that excessive hunting by humans may have been a factor.

Isotopic evidence from gomphotheres found in South America suggests that they subsisted on a varied diet, depending on geographical location. Oxygen isotope ratios for tooth enamel, dentine and bone were used to indicate the relative concentration of carbon isotopes present when the animals were alive. These carbon isotope concentrations vary according to the types of plant that the gomphotheres ate, whether they employed the C3 or C4 carbon fixation pathway. Analysis of this data has suggested that some gomphotheres in Brazil subsisted entirely on C3 plants adapted to temperate climates, while those living on the La Carolina Peninsula in Ecuador fed on dry-climate C4 plants such as grasses. Gomphotheres living elsewhere in Ecuador and in Bolivia had a varied diet consisting of both plant types.

Mammutidae

The mammutidae were classified by Oliver Hay in 1922. Mammut, popularly known as the mastodon, was described in 1799 by Johann Blumenbach. The mammutidae retained multiple pairs of molar and premolar teeth observed in primitive proboscideans, but had lost all canines and lower incisors. Two upper incisors remained in the form of tusks similar to those of mammoths and living elephants. Members of the mammutidae are known from North and South America, Europe, Asia, Indonesia and Africa. The oldest known fossils of mammutids are from the early Oligocene of Congo.

The American mastodon was one of the most recent species of Mammut, persisting through the Pleistocene glaciations. Despite being smaller than the mammoths, the American mastodon may have born a physical resemblance and probably had a coat of thick hair. Dental morphology and Carbon isotope data indicate that mastodon were browsers, in contrast to other advanced proboscideans that were grazers.

When the bones of the American mastodon were first excavated in the early nineteenth century the concept of extinction was still highly controversial. The president of the United States at the time, Thomas Jefferson, expressed the belief that living mastodons would be found in unexplored parts of the Americas. The paradigm view today is that mastodon are extinct. The discovery of mastodon bones bearing blade marks suggest that they were exploited by humans, and studies of body size distribution before and after the arrival of humans in the Americas suggest that excessive hunting may have been a causal factor in their extinction. Mastodon remains have been found in association with spear points characteristic of a North American tribe of Homo sapien known as the Clovis, which are also believed to have preyed upon mammoths.

Stegodontidae

The stegodontidae were advanced proboscideans with a physical resemblance to living elephants, with a well developed trunk and column-like legs to support their large bodies. Stegolophodon, a primitive example of the stegodontidae known from Japan, possessed tusks on both upper and lower jaws. The more recent Stegodon lost the lower tusks, and indeed lower incisor teeth altogether. Stegodon was described in 1847 by Hugh Falconer and placed in a family of its own by Henry Fairfield Osborn in 1915. Fossils of Stegodontids are found from the upper Miocene until the Pleistocene Epochs. The Stegodontidae mostly occur in Asia, although genera returned to Africa several times.

The stegondontidae are represented among those probosicdean families with members that have undergone island dwarfism in Indonesia. The majority of Stegodon species are of similar size or larger than extant elephant species. Stegodon floresiensis is very much smaller, estimated to weigh about 300 kilograms. It is thought that the Stegodon on the island of Flores had fewer resources due to the islands' small area, thus causing them to adapt by reducing their own body size. Stegodon floresiensis remains bearing cut marks have been found associated with the remains of a dwarf human species, Homo floresiensis, suggesting that the dwarf Stegodon was prey for these humans.

Extinct Elephantidae

The Elephantidae have a number of extinct members. Primelephas is amongst the most primitive of true elephant genera, with tusks developing from incisor teeth in the upper and lower jaws. A considerable diversity in body size is expressed among the extinct members of the genus Elephas; the African Elephas recki, described by Dietrich in 1915, is amongst the largest of all elephants, some estimates suggesting over 4 meters in height. At the other extreme, a number of Elephas species are believed to have undergone insular dwarfism in the Mediterranean, giving rise to numerous dwarf elephant species on different islands. Species of Mammuthus and Palaeoloxodon are believed to have undergone similar dwarfism in the same area, and sometimes distinguishing the genera is difficult.

Reference:

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• Cerling, T.E., Harris, J.M. & Leakey, M.G. Browsing and grazing in elephants: the isotope record of modern and fossil proboscideans. Oecologia 120: 364-374 (1999)

• Melis, R., Palombo, M.R. & Mussi, M. Mammuthus lamarmorae (Major, 1883) remains in pre-Tyrrhenian deposits of San Giovanni in Sinis (Western Sardinia, Italy), The World of Elephants - International Congress, Rome (2001)

• Poulakakis, N., Parmakelis, A., Lymberakis, P., Mylonas, M., Zouros, E., Reese, D.S., Glaberman, S. & Caccone, A. Ancient DNA forces reconsideration of evolutionary history of Mediterranean pygmy elephantids. Biology Letters to the Royal Society 2: 451-454 (2006)

• Sanchez, B., Prado, J.L. & Alberdi, M.T. Feeding ecology, dispersal, and extinction of the South American Pleisocene gomphotheres (Gomphotheriidae, Proboscidea). Paleobiology 30: 146-161 (2004)

• Surovell, T.A. & Waguspack, N.M. How many elephant kills are 14? Clovis mammoth and mastodon kills in context. Quaternary International 191: 82-97 (2007)

• http://paleodb.org/cgi-bin/bridge.pl

• van den Bergh, G.D., de Vos, J., Aziz, F. & Morwood, M.J. Elephantoidea in the Indonesian region: new Stegodon findings from Flores, The World of Elephants - International Congress, Rome (2001)