While most famous forms such asGomphotherium had long lower jaws with tusks, theancestral condition for the group, some later members developed shortened (brevirostrine) lower jaws with either vestigial or no lower tusks and outlasted the long-jawed gomphotheres. This change made them look very similar to modern elephants, an example ofparallel evolution. During thePliocene andEarly Pleistocene, the diversity of gomphotheres declined, ultimately becoming extinct outside of the Americas. The last two genera,Cuvieronius ranging southern North America to western South America, andNotiomastodon ranging over most of South America, continued to exist until the end of the Pleistocene around 12,000 years ago, when they became extinctalong with many other megafauna species following thearrival of humans.
The name "gomphothere" comes fromAncient Greekγόμφος (gómphos), "peg, pin; wedge; joint" plusθηρίον (theríon), "beast".
Gomphotheres differed from elephants in theirtooth structure, particularly the chewing surfaces on themolar teeth. The teeth are considered to bebunodont, that is, having rounded rather than sharp cusps.[1] 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,[2] with this oblique movement being combined with vertical (orthal) motion that served to crush food.[3] Like modern elephants and other members ofElephantimorpha, 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[4] though they were absent in some gomphothere genera.[5]
Lower jaw ofGomphotherium angustidens (bottom) showing elongatemandibular symphysis and lower tusks at their tips
Molar of a modern African elephant (Loxodonta) for comparison
Early gomphotheres had lower jaws with an elongate (longitostrine)mandibular symphysis (the fused front-most part of the lower jaw) and lower tusks, theprimitive condition for members ofElephantimorpha. 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.[5] In Gomphotheriidae, these elongate mandibular symphysis tend to be narrow, while the lower tusks tend to be club shaped.[6] 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.[3] The lower tusks and long lower jaws of primitive gomphotheres were likely used for cutting vegetation, with a secondary contribution in acquiring food using the trunk, while brevirostrine gomphotheres relied primarily on their trunks to acquire food similar to modern elephants.[6] The upper tusks of primitive longirostrine gomphotheres typically gently curve downwards,[3] and generally do not exceed 2 metres (6.6 ft) in length and 35 kilograms (77 lb) in weight,[7] though some later brevirostine gomphotheres developed considerably larger upper tusks.[3][7] Upper tusks of brevirostine gomphotheres include those which are straight[3] and upwardly curved.[8]
Most gomphotheres reached sizes equivalent to those of those of the modernAsian elephant (Elephas maximus), though some gomphotheres reached sizes comparable to or somewhat exceedingAfrican bush elephants (Loxodonta africana).[9] The limb bones of gomphotheres like those ofmammutids 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.[9][10]
"Gomphotheres" are assigned to their own family, Gomphotheriidae, but are widely agreed to be aparaphyletic group. The familiesChoerolophodontidae andAmebelodontidae (the latter of which includes "shovel tuskers" with flattened lower tusks likePlatybelodon) are sometimes considered gomphotheressensu lato,[11][12][13] though some authors argue that Amebelodontidae should be sunk into Gomphotheriidae.[14] 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.[11] Some authors choose to exclude "tetralophodont gomphotheres" from Gomphotheriidae, and instead assign them to the groupElephantoidea.[11] "Tetralophodont gomphotheres" are thought to have evolved from "trilophodont gomphotheres", and are suggested to be ancestral toElephantidae, the group which contains modern elephants, as well asStegodontidae.[15]
While the North American long jawed proboscideansGnathabelodon,Eubelodon andMegabelodon been assigned to Gomphotheriidae in some studies[5][16] other studies suggest that they should be assigned to Amebelodontidae (Eubelodon, Megabelodon) or Choerolophodontidae (Gnathabelodon).[6]
Gomphotheres are generally supposed to have been flexible feeders, with the various species having differingbrowsing, mixed feeding andgrazing diets, with the dietary preference of individual species and populations being shaped by local factors such as climatic conditions and competition.[17] Analysis of the tusks of a maleNotiomastodon individual suggest that it underwentmusth, similar to modern elephants.[18]Notiomastiodon is also suggested to have lived in social family groups, like modern elephants.[19]
Gomphotheres originated in Afro-Arabia during the mid-Oligocene, with remains from theShumaysi Formation in Saudi Arabia dating to around 29-28 million years ago. Gomphotheres were uncommon in Afro-Arabia during the Oligocene.[20] Gomphotheres arrived in Eurasia after the connection of Afro-Arabia and Eurasia during the Early Miocene around 19 million years ago,[21] in what is termed the "Proboscidean Datum Event".Gomphotherium arrived in North America around 16 million years ago,[22] and is suggested to be the ancestor of later New World gomphothere genera.[23] "Trilophodont gomphotheres" dramatically declined during the Late Miocene, likely due to the increasingC4 grass-dominated habitats,[21] while during the Late Miocene "tetralophodont gomphotheres" were abundant and widespread in Eurasia, where they represented the dominant group of proboscideans.[24] During the Late Miocene, the "tetralophodont gomphothere"Tetralophodon is suggested to have given rise to the family Elephantidae in Afro-Arabia.[25] All trilophodont gomphotheres, with the exception of the AsianSinomastodon, became extinct in Eurasia by the beginning of thePliocene,[26] along with the global extinction of the "shovel tusker" amebelodontids.[27] The last gomphotheres in Africa, represented by the "tetralophodont gomphothere" genusAnancus, became extinct around the end of the Pliocene and beginning of the Pleistocene.[28] The New World gomphothere generaNotiomastodon andCuvieronius dispersed into South America during thePleistocene, around or after 2.5 million years ago as part of theGreat American Biotic Interchange due to the formation of theIsthmus of Panama, becoming widespread across the continent.[29] The last gomphothere native to Europe,Anancus arvernensis[30] became extinct during the Early Pleistocene, around 1.6–2 million years ago[31][32]Sinomastodon became extinct at the end of theEarly Pleistocene, around 800,000 years ago.[33] From the latter half of the Early Pleistocene onwards, gomphotheres were extirpated from most of North America, likely due to competition withmammoths andmastodons.[17]
The extinction of gomphotheres in Afro-Eurasia has generally been supposed to be the result the expansion of Elephantidae andStegodon.[26][34] The morphology of elephantid molars being more efficient than gomphotheres in consuming grass, which became more abundant during the Pliocene and Pleistocene epochs.[34] In southern North America, Central America and South America, gomphotheres did not become extinct until shortly after the arrival of humans to the Americas, approximately 12,000 years ago, as part of theLate Pleistocene megafauna extinctions of most large mammals across the Americas. Bones of the last gomphothere genera,Cuvieronius andNotiomastodon, dating to shortly before their extinction have been found associated with human artifacts, suggesting that hunting played a role in their extinction.[29]
