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
Please don't have me killed for making a sudden veer into the Land of Woo.
I've always wondered why people would suggest that the mokele-mbembe (for the uninitiated, an amphibious mythological creature from the Sudan which is believed by some to be based on a real animal) was a sauropod when there are any number of proboscideans who would fit the description quite neatly -- and yet that hypothesis has never been proposed in the nutbar literature I love so dearly.
So far as I'm concerned the fantasy of a surviving shovel-tusker or Dinotherium is just as gratifying as a surviving Apatasaurus. More so, because it's easier to pretend that it's true.
Sean; I don't think I have the power to have anyone killed, nor do I think I'd have the desire to do so, so rest easy.
I remember seeing a bit about mokele-mbembe in some dinosaur documentaries for kids (the ones hosted by Eric Gordon and Gary Owens), and there was supposedly some picture and sound recording that they felt was more indicative of a dinosaur. From what I had heard some people also said that the mystery creature looked most like a sauropod, so I guess people have just been running with it.
I'd love to see a surviving Deinotherium or something similar, too, (in addition to some of the dinocerata, Andrewsarchus, etc.) but I guess mammals just don't sell as well as dinosaurs do.
Semi-aquatic early proscideans has been a working hypothesis for some time, hasn't it? I seem to remember that the BBC's Walking with Beasts series in 2001 offered a hippo like Moeritherium, and I'm sure the idea wasn't new then. This is mainly confirmatory, though none the less interesting for that.
The question is, how close to the surviving lines were these creatures? Could they have been an atypical offshoot, like herbivorous bears (pandas) and carnivorous parrots (kias), which don't tell us much about the group as a whole?
There was a minor radiation of *Moeritherium*-like, but tuskless, proboscidans in mid-Eocene Pakistan,the anthracobunids (this ranks with *Basilosaurus* as one of the all-time most confusing monikers, everybody who hears the name thinks the animal in question is an artiodactyl in general and an anthracothere in peculiar). They were also said to be amphibious. I don't know if the area where they were found was part of Asia or of India in the mid-Eocene, but in both cases the animals must have swum over from Africa.
Thanks for the article and links Brian. The hyrax, the elephant shrew, the Bering sea cow, the mammoth all shared close common ancestry is simply fascinating, considering the different morphologies and ecological niches.
Wierdness! So the mammals return to the sea, only to get all wrinkly and decide it's not worth it. :-)
Any idea where my favorite "Afrotheres," desmostylids, fit into the picture?
My favorite part is where they suggest that the hyraces were possibly semi-aquatic also !?! Now that's a newsworthy claim.
and now for something completely different...
(but keeping big teeth in mind)
Saber toothed crabs during the age of dinosaurs:
whence the claws became the jaws
I haven't read much about the mokele-mbembe, but I did once watch a TV documentary were they showed a picture of a rhino to the natives, who said that this was the mokele-mbembe. Apparently Rhinos could have existed in that area only a few thousand years ago, but got extinct because their habitat disappeared due to climate change. Sounds a lot more probable to me than surviving swamp-dwelling sauropods that look like in popular books from the 1950s.
I've seen desmostylians as closer to proboscideans, to sirenians, or the sister group to both. Here's a pdf (19.42 MB) of a 1986 paper on desmostylians that describes a new member of that group (Behemotops) and concludes that they were closer to proboscideans, and both originated in Asia.
They also mention a Late Paleocene mammal from China (Minchenella) that seems to be quite close to the common ancestor of both. Also contains a summary of what has been said about the position of desmostylians. (They were supposed to be multituberculates for some time!)
I don't know it any later studies have revised their results.
> They were supposed to be multituberculates for some time
And, by Sera (1954), to be dicynodonts. Sera thought the same of proboscidans and sirenians. In fact, the synapomorphies (obesity, tusks) are so obvious that one wonders how the elephants-are-mammals conspiracy could hide the truth for such a long time :-)
Oh yes, Sera. The same guy who thought that astrapotheres were partly arboreal (as I mentioned in a comment at Tet Zoo), and Palaeopropithecus was aquatic. He also developed a classification of simians in the 1920s, dividing them into six lineages, each of which included some platyrrhines, some catarrhines, and one human "race". At least, that's what I read about him, the astrapothere paper is the only one I've seen, and I couldn't understand most of it anyway, as I don't know Italian.
Obesity and tusks are of course also shared by dinocerates and their obvious* descendants, walruses. Oh, and hippos, and Odobenocetops. Probably all part of a clade Obesotuskotheria, the only mammals that descended neither from birds nor from amphisbaenians.
*At least to those who haven't been fooled by the "Odobenids are Obese Pinnipeds, Stupid" (OOPS) conspiracy, and have had their eyes opened by Steinmann (1908).
Other interesting facts...
Like sirenians, elephants lack gubernaculum.
no pampiniforn plexus, pleural cavity is absent,and embryonic nephrostome is seen.
One striking similarity ive noted is the apex of the heart is bifid in both of them, reinforcing this evolutionary story.
Some catarrhines, and one human "race". At least, that's what I read about him, the astrapothere paper is the only one I've seen, and I couldn't understand most of it anyway, as I don't know France.
Mammoths, dugongs, rock hyraxes and kin derive from antarctic-temperate Gondwana swamp hyraxes which fed on annual plant rhyzomes and perennial plant cambium tissues (parallel to arctic-temperate Eurasian giant beavers) with functional cutting incisors, then due to change to tropical African climate specialized on other foods with resulting change in dentition and niche specializations.
A similar parallel occurred in primate-anthropoid-hominoid LCA, eg. the lowland gorilla, Oreopithecus and Gigantopithicus foraged in wetlands eating plants but combined prehensile hands and teeth (not prehensile lips and tusks) to rake, dig and peel herbaceous vegetation, (this induced greater upright posture part-time).