A number of my fellow ScienceBloggers have covered the “Aquatic Elephant Hypothesis” lately (see here, here, and here), and even though I’m a little late to the party I thought that I’d throw in my two cents about the significance of ancient, waterlogged pachyderms.
The idea that the ancestors of elephants (including the two living genera Loxodonta and Elephas) were aquatic at some point in the past has been circulating for a number of years now, especially given the close relationship between sirenians (i.e. manatees and dugongs) and the proboscideans. The fossil proboscidean Moeritherium, the genus first being discovered in the Al Fayyum region of Egypt at the beginning of the 20th century and interpreted as having a hippo-like lifestyle around freshwater swamps, has also been marshaled as evidence for the past aquatic habits of elephant ancestors, but more robust evidence has been difficult to come by. A paper recently published in PNAS, however, has strengthened the argument that the ancestors of elephants may have been amphibious.
The big question that the researchers were trying to approach was whether or not the group Tethytheria (Sirenians and Proboscideans) is supported by more than just the relatively weak molecular evidence available so far. This would mean that some time during the Eocene in Africa, about 55 million years ago, there was a semi-aquatic common ancestor of the animals we now know as manatees and elephants. Moeritherium and another contemporaneous form called Barytherium are key players here, being sister groups to the ancestors of living elephants (rather than being the ancestors themselves, although more resolution is required in the placement of both). They are far too young to be the common ancestor (about 37 million years old), but resolving their lifeways can provide clues as to the habits of the ancestors of later elephants.
The sedimentary deposits that Moeritherium and Barytherium are found in indicate that they often died in bodies of fresh water, but whether they spent much (if any) of their time in such habitats is another question that cannot be ascertained just by looking at where the skeleton was deposited. The authors of the study decided to look at stable carbon and oxygen isotopes in the teeth of the fossils, a method that would indicate what these animals were eating. This technique has become increasingly common, especially in determining the presence (and abundance) of grazers vs browsers in ancient Africa, and it is possible to do this for the ancient proboscideans as the enamel of their teeth has been preserved. The results showed that the diets of Moeritherium and Barytherium were more similar to each other (especially in the oxygen isotope test) than to any of the other fossil mammals from the Fayuum deposits, and that their food preferences were biased towards aquatic plants.
Although the authors introduced a few caveats as to factors that could have influenced their data (i.e. large mammals that drink a lot of water take in a lot of oxygen in that water, which can influence the oxygen isotope values), the best fit for the data gathered is that both Barytherium and Moeritherium were semi-aquatic. Barytherium likely chewed through tough, near-shore plants with Moeritherium may have been spending more time in the water and eating different aquatic plants (which would explain the differences in the carbon isotope values between the two creatures).
The authors do not think that their study is the final word on the matter, however, and similar tests of Moeritherium and Barytherium teeth from different locations and ages will be required to support their hypothesis. Likewise, a study of older proboscideans will be required to determine when the semi-aquatic phase started, although more fossils are going to have to be uncovered in order to get a better idea of what some of the first proboscideans looked like. Still, this is an exciting area of research, the relationship between hyraxes, elephants, and manatees always being something of a head-scratcher, but it appears that researchers are beginning to unravel the evolutionary knot that has proven to be so difficult so far.
Liu, A.G., Seiffert, E.R., Simons, E.L. (2008). Stable isotope evidence for an amphibious phase in early proboscidean evolution. Proceedings of the National Academy of Sciences, 105(15), 5786-5791. DOI: 10.1073/pnas.0800884105