Gomphothere

Gomphotheres are an extinct group of proboscideans related to modern elephants. First appearing in Africa during the Oligocene, they dispersed into Eurasia and North America during the Miocene and arrived in South America during the Pleistocene as part of the Great American Interchange. Gomphotheres are a paraphyletic group ancestral to Elephantidae, which contains modern elephants, as well as Stegodontidae.

While the most well-known forms like Gomphotherium had long lower jaws with tusks, the ancestral condition for the group, some later members developed shortened (brevirostrine) lower jaws with either vestigial or no lower tusks, which survived considerably later than the long-jawed gomphotheres. This change made them look very similar to modern elephants, an example of parallel evolution. During the Pliocene and Early Pleistocene, the diversity of gomphotheres declined, ultimately becoming extinct outside of the Americas. The last two genera, Cuvieronius ranging from southern North America to northwestern South America, and Notiomastodon ranging over most of South America, continued to exist until the end of the Pleistocene around 12,000 years ago, when they became extinct along with most other American megafauna species following the arrival of humans.

The name "gomphothere" comes from Ancient Greek γόμφος (gómphos), "peg, pin; wedge; joint" plus θηρίον (theríon), "beast".

Gomphotheres differed from elephants in their tooth structure, particularly the chewing surfaces on the molar teeth. The teeth are considered to be bunodont, that is, having rounded rather than sharp cusps. They are thought to have typically chewed differently from modern elephants, using an oblique movement (combining back to front and side to side motion) over the teeth rather than the proal movement (a forwards stroke from the back to the front of the lower jaws) used by modern elephants and stegodontids, with this oblique movement being combined with vertical (orthal) motion that served to crush food. Like modern elephants and other members of Elephantimorpha, gomphotheres had horizontal tooth replacement, where teeth would progressively migrate towards the front of the jaws before they were taken place by more posterior teeth. Unlike modern elephants, many gomphotheres retained permanent premolar teeth though they were absent in some gomphothere genera.

Early gomphotheres had lower jaws with an elongate (longirostrine) mandibular symphysis (the fused front-most part of the lower jaw) and lower tusks, the primitive condition for members of Elephantimorpha. Later members developed shortened (brevirostrine) lower jaws and/or vestigial or no lower tusks, a convergent process that occurred multiple times among gomphotheres, as well as other members of Elephantimorpha. In Gomphotheriidae, these elongate mandibular symphysis tend to be narrow, while the lower tusks tend to be club-shaped. While the musculature of the trunk of longirostrine gomphotheres was likely very similar to that of living elephants, the trunk was likely shorter (probably no longer than the tips of the lower tusks), and rested upon the elongate lower jaw, though the trunks of later brevirostrine gomphotheres were likely free hanging and comparable to those of living elephants in length.

The upper tusks of primitive longirostrine gomphotheres typically gently curve downwards, and generally do not exceed 2 metres (6.6 ft) in length nor were heavier than 35 kilograms (77 lb) in life, though some later brevirostine gomphotheres developed considerably larger upper tusks. Upper tusks of brevirostine gomphotheres include those which are straight and upwardly curved. As is ancestral for elephantimorphs, many gomphotheres had a band of enamel running along the entire length of the lateral (facing outwards to the side of the animal) surface of the upper tusks from the tip to the root (or alternatively spiraling along the whole length if the tusk was twisted as in some gomphotheres), which was otherwise largely made of dentine like modern elephants. Another lateral enamel band is also present in the lower tusks. Such a feature is found in the tusks of early juvenile living elephants but is quickly lost and is not present in adults. Some gomphothere genera convergently lost the tusk enamel band in adults like living elephants.

Most gomphotheres reached sizes equivalent to those of those of the modern Asian elephant (Elephas maximus), though some gomphotheres reached sizes comparable to or somewhat exceeding African bush elephants (Loxodonta africana). The limb bones of gomphotheres like those of mammutids are generally more robust than elephantids, with the legs also tending to be proportionally shorter. Their bodies also tend to be more proportionally elongate than those of living elephants, resulting in gomphotheres being heavier than an elephant at the same shoulder height.

"Gomphotheres" are assigned to their own family, Gomphotheriidae, but are widely agreed to be a paraphyletic group. The families Choerolophodontidae and Amebelodontidae (the latter of which includes "shovel tuskers" with flattened lower tusks like Platybelodon) are often considered gomphotheres sensu lato, and sometimes considered subfamilies of Gomphotheriidae. Gomphotheres are divided into two informal groups, "trilophodont gomphotheres", and "tetralophodont gomphotheres". "Tetralophodont gomphotheres" are distinguished from "trilophodont gomphotheres" by the presence of four ridges on the fourth premolar and on the first and second molars, rather than the three present in trilophodont gomphotheres. Some authors choose to exclude "tetralophodont gomphotheres" from Gomphotheriidae, and instead assign them to the group Elephantoidea. "Tetralophodont gomphotheres" like Tetralophodon are thought to have evolved from "trilophodont gomphotheres", in particular having a close relationship with the Gomphotherium species G. steinhemense. "Tetralophodont gomphotheres" are suggested to be ancestral to Elephantidae, the group which contains modern elephants. It has been suggested that the Stegodontidae emerged from the species "Gomphotherium" annectens, which may fall outside of Gomphotheriidae proper.