An extinct "anchor-tusked" proboscidean

A recent restoration of Deinotherium from Markov, et al (2001) "A reconstruction of the facial morphology and feeding behavior of the deinotheres." The World of Elephants, International Congress, Rome 2001. Taken from The World We Don't Live In.

Poor Deinotherium. Although it is one of the most readily recognizable proboscideans (the larger group to which modern elephants belong) it has long been treated as an uninteresting distant cousin of more popular fossil elephants like woolly mammoths. During the early 20th century, especially, it was often seen as something of a failed evolutionary experiment outside the "main line"* of elephant evolution, yet its downward-curved lower tusks have generated considerable interest.

*[There is no actual "main line" of evolution for any lineage. The phrase erroneously suggests that there is some sort of predetermined evolutionary pathway and casts extinct lineages as losers in "life's race."]

In terms of their general shape individuals of the genus Deinotherium, which persisted from about 14 million years ago to about a million years ago, would have looked much like living elephants. Deinotherium was an especially large proboscidean with a stout body held up by columnar limbs, but its head was very different from that of more familiar living elephants. Indeed, Deinotherium did not have long upper tusks. Instead it had a pair of enlarged lower incisors that hooked downwards and backwards. What could it possibly been have using these strange tusks for?

The skull of Deinotherium, as printed in the Encyclopaedia Britannica.

During the 19th century Deinotherium was often envisaged as an aquatic animal. Surely those lower tusks must have been heavy, and some naturalists thought that the only way a Deinotherium could have supported a head adorned with such armaments was if it had lived in water. The 1859 popular compendium Curiosities of Science stated;

The family of herbivorous Cetaceans [i.e. sirenians] are connected with the Pachydermata of the land by one of the most wonderful of all the extinct creatures with which geologists have made us acquainted. This is the Dinotherium, or Terrible Beast. ... It appears to have lived in the water, where the immense weight of these formidable appendages [i.e. tusks] would not be so inconvenient as on land. What these tusks were used for is a mystery; but perhaps they acted as pickaxes in digging up trees and shrubs, or as harrows in raking the bottom of the water.

As the opening line indicates, it had already been recognized that elephants were closely related to sirenians like manatees and dugongs. This hypothesis remained controversial for some time, but if Deinotherium was aquatic it could serve as an "intermediate type" between the two groups. This does not necessarily suggest an evolutionary relationship. An aquatic Deinotherium could just as easily be understood in terms of an unbroken chain of creatures created by God, thus filling a gap in nature.

Deinotherium, as restored in Hitchcock's Elementary Geology.

Nevertheless, placing Deinotherium in an aquatic habitat did not fully answer the question of what it was using its tusks for. While many naturalists thought that Deinotherium used its tusks to dig in the muck of freshwater habitats, there was another more fanciful, but complementary, hypothesis. It was not centered on feeding, but on what such an aquatic animal might do if it wanted to take a nap.

In his Geology and mineralogy considered with reference to natural theology (1837) the English geologist William Buckland looked to Deinotherium to help elucidate the wonders of God's "creative design." With an elephant-like body but tapir-like teeth the beast was clearly an "important extinct link" between the two groups, yet it represented some anomalies. The shoulder-blade, for instance, looked very similar to the same bone in moles, so much so that Buckland speculated that Deinotherium dug for food with its forefeet. The beasts teeth could then be used as a "pick-axe" or "horse-harrow" to assist in this task. More than that, the tusks could have been used as a kind of anchor. Buckland wrote;

The tusks of the Dinotherium may also have been applied with mechanical advantage to hook the head of the animal to the bank, with the nostrils sustained above the water, so as to breathe securely during sleep, whilst the body remained floating, at perfect ease, beneath the surface : the animal might thus repose, moored to the margin of a lake or river, without the slightest muscular exertion, the weight of the head and body tending to fix and keep the tusks fast anchored in the substance of the bank; as the weight of the body of a sleeping bird keeps the claws clasped firmly around its perch. These tusks might have been farther used, like those in the upper jaw of the Walrus, to assist in dragging the body out of the water; and also as formidable instruments of defence.

It is unfortunate that Buckland did not commission an illustration of Deinotherium reclining in an ancient swamp, but his ideas about the animal proved to be quite popular. In an article that appeared in the magazine Boys' and girls' bookshelf (collected in a 1917 volume) in the early 20th century, for example, author C.F. Holder imagined a scene where a Deinotherium slumbering at the water's edge was attacked by humans;

In answer to this, we find that the huge animal [Deinotherium] was a water-lover, and probably made its home on the banks of streams, living a life similar to that of the hippopotamus. With this knowledge, a use for these great recurving incisors is readily seen. They were used as pickaxes to tear away the earth and dig out the succulent vegetation that it fed upon ; and at night, when partly floating, they might have been buried in the bank, forming veritable anchors for the living and bulky ships. When attacked by its–perhaps human–enemies, we can imagine the great creature struggling from the mire, lifting itself to dry land by striking its tusks into the ground and using them to hoist its ponderous body to the bank.

Deinotherium, under attack from human hunters, as restored in Boys' and girls' bookshelf.

Buckland's vision of Deinotherium was also repeated in The Monthly Repository (1837), Hogg's Weekly Instructor (1845), Hitchcock's Elementary Geology (1847), Denton's Our planet, its past and future (1873), Ingersoll's The Life of Animals (1907), and probably many other sources. Yet how does it hold up today? Contrary to what Buckland thought, the head of Deinotherium appears to have been relatively light when compared to other fossil elephants. Deinotherium also had a longer neck, perhaps indicating that it had a greater range of motion with its head and did not require as long a trunk as modern elephants. Debates about trunk length aside, there is no evidence to suggest that it was confined to a watery habitat or ever dragged itself about on its lower teeth.

Unfortunately I am not personally aware of any recent studies of the tusks of Deinotherium like those carried out on the "shovel tuskers" Amebelodon and Platybelodon. If Deinotherium was using its teeth for feeding, like stripping bark off trees or digging in the ground, its teeth would bear distinctive markings. To test some of these centuries-old ideas we need to go back to the teeth.

Then again, must Deinotherium have used its recurved tusks for feeding? Not necessarily. Since we cannot observe a living Deinotherium it is difficult to reconstruct its behavior and paleobiology, and as we learned with the Amebelodon and Platybelodon just because a certain anatomical form resembles a tool it does not mean that such a form was used for the same function as that tool. Likewise, even if we were able to figure out how Deinotherium used its tusks it would not mean that we fully understood how such a tusk arrangement evolved. For now the tale of how the tusk arrangement of Deinotherium evolved and how it might have used them appears to be a mystery, though it is not one that is beyond our ability to more fully understand.