That elephants have an aquatic ancestry has been suspected for some time now. Moreover, the idea of elephant aquatic origins and elephant origins in general is part of a growing realization that many of the world’s aquatic mammals originated in a couple of regions of Africa that were for a very long time enormous inland seas (but that is another story I won’t cover here).
The earlier evidence came from observation of the ontogeny of the kidneys in elephants, during which the kidneys take on the characteristics that are found in aquatic mammals generally. That research was published in 1999, and is summarized here:
The early embryology of the elephant has never been studied before. We have obtained a rare series of African elephant (Loxodonta africana) embryos and fetuses ranging in weight from 0.04 to 18.5 g, estimated gestational ages 58-166 days (duration of gestation is ≈660 days). Nephrostomes, a feature of aquatic vertebrates, were found in the mesonephric kidneys at all stages of development whereas they have never been recorded in the mesonephric kidneys of other viviparous mammals. The trunk was well developed even in the earliest fetus. The testes were intra-abdominal, and there was no evidence of a gubernaculum, pampiniform plexus, processus vaginalis, or a scrotum, confirming that the elephant, like the dugong, is one of the few primary testicond mammals. The palaeontological evidence suggests that the elephant’s ancestors were aquatic, and recent immunological and molecular evidence shows an extremely close affinity between present-day elephants and the aquatic Sirenia (dugong and manatees). The evidence from our embryological study of the elephant also suggests that it evolved from an aquatic mammal.
Gaeth et al 1999
It is now well established that elephants arose during the early Eocene (about 55 million years ago) and share a taxonomic clade with the Sirenia and the Hyracoidea. I have spent a lot of time living among wild Hyracoidea, and I was just observing Sirenia the other day in the Yucatan, and I’ve gotta say, the resemblance is, well, totally non-existent. It is also well established that these creatures all arose in Africa, in an area that is now the arid regions of North Africa.
Elephants looked nothing like elephants during the Eiocene. They were smaller, did not have the big long trunk, had a long tubular cranium (as opposed to a tall, more rounded cranium).
The present study looks at early elephants, of the genera Barytherium and Moeritherium, and in particular looks at Oxygen and Carbon stable isotopes in the teeth of these early fossils.
To put it very simply, stable isotopes are versions of a given element that are very slightly different in the atomic nucleus. These are distinct from unstable isotopes, which over time decay radioactively, often changing from one element to another. Stable isotopes do not decay. For many purposes, one isotope is the same as the other (thus the term “iso” = same) but at the atomic and molecular level, which isotope a given atom is made up can matter, in a lot of ways. Since the discovery that isotopes matter in biological and ecological systems (in the 1970s, but with much of the really important work happening int he 1980s), isotope biologists (biologists who get chemistry and physics and have mass spectrometers at their disposal) have worked out the meaning of different isotopes in animal tissue, especially (but not limited to) enamel or other bony bits. (Tooth enamel is considered the most chemically stable tissue, with bone somewhat less stable, with respect to preserving the actual atomic composition … without mineral replacement … of the original animal.)
In essense, the carbon and oxygen isotope profiles of Barytherium and Moeritherium resemble those of acquatic and semi aquatic mammals.
Here we test the hypothesis of an aquatic ancestry for advanced proboscideans by measuring delta-18O in tooth enamel … The combination of low delta-18O values and low delta-18O standard deviations in Barytherium and Moeritherium matches the isotopic pattern seen in aquatic and semiaquatic mammals, and differs from that of terrestrial mammals. delta-13C values of these early proboscideans suggest that both genera are likely to have consumed freshwater plants, although a component of … terrestrial vegetation cannot be ruled out. The simplest explanation for the combined evidence from isotopes, dental functional morphology, and depositional environments is that Barytherium and Moeritherium were at least semiaquatic and lived in freshwater swamp or riverine environments, where they grazed on freshwater vegetation. These results lend new support to the hypothesis that Oligocene-to-Recent proboscideans are derived from amphibious ancestors.
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
Gaeth, A.P. Et Al. (1999) The developing renal, reproductive, and respiratory systems of the African elephant suggest an aquatic ancestry. Proc Natl Acad Sci U S A. 1999 May 11; 96(10): 5555-5558.