Gomphotheriidae (Greek: γόμφος: "peg or joint"; plus θηρίον: "beast") is an extinct family of elephant-like mammals of the order Proboscidea, differentiated from elephants by the presence of an elongated lower jaw, which in most species bore a second pair of tusks.
Superficially, gomphotheres looked like elephants, i.e. massive bodies, pillar-like legs, and a head bearing an elongated proboscis and tusks; they were about the same size as modern elephants. The major difference was the layout of the skull. Characteristic was the elongation as compared to today's elephants, which was largely caused by the lower and less dome-like upper skull which had air-filled chambers, reducing the weight of the head as a whole while at the same time having an enlarged surface area for the attachment of the musculature needed to bear the weight of the four tusks. The occipital bone was short and vertical, and bore a trapezoidal cross section up to 30 inches wide. The nasal bone was located relatively high up on the front skull and had slightly elevated bone at the nostril as attachment points for the trunk.
The characteristic and extremely massive lower jaw reached a length of up to 4.2 feet and was thus considerably longer than in today's elephants. The longitudinal extension was mainly due to the formation of the lower tusks. The lower jaw as a whole was narrow, the two branches meeting at an angle of 40°. The height of the lower jaw body was just over 8 inches at the front of the molar; based on these teeth it is assumed that the diet of these animals was sparse vegetation from open environments, such as grasslands. The lower tusks themselves were either rounded or flat, depending on the species (such as the Amebelodontidae), and some have argued that the function of the jaw was similar to a shovel. The skull also bore a pair of larger tusks, which pointed either forward or downward; they were probably covered by a portion of the trunk, something not seen in the elephants of today. The trunk itself is theorized to have been short, and not extending much further than the tips of the lower tusks.
The spine was composed of 7 cervical, 20 thoracic, 4 lumbar, 3 sacral and 21 caudal vertebrae. Overall, the spine showed a slightly curved course, so that the back line of gomphotheres probably corresponded more to that of today's Asian elephant, the largest height of the animal was located above and behind the shoulder blades. The very high spinous processes on the front thoracic vertebrae, in addition to very large shoulder blades, served as attachment points of the powerful neck muscles. The limbs stood vertically under the body and thus looked like a column. The ratio of the longer upper limb to the shorter limb indicates gomphotheres had a slow and heavy mode of locomotion, which is typical of elephants.