A restoration of Eritreum compared to the larger Gomphotherium. From Shoshani et al. (2009).
Before I loved dinosaurs, I loved elephants. I would run around the backyard with my little pith helmet on, firing my “elephant mover” to herd the imaginary pachyderms. (At the time I did not understand what guns did. When they went off in the documentaries I saw that the elephants moved, therefore guns were “elephant movers.”) It would only be much later, when I could properly appreciate the stout bones I saw in the halls of the American Museum of Natural History, that I would more fully appreciate the past history of some of my favorite animals.
A recent discovery made in ~27 million year old deposits in Eritrea helps fill in that history. Proboscideans, the group of mammals that contains living elephants and their many bizarre extinct relatives, have been around for about 55 million years, but one of the big questions has involved the evolution of the elephantimorphs (or elephants and extinct groups like mastodons, gomphotheres, and stegodons). How did the early elephantimorphs evolve, disperse, and replace the more archaic elephantiformes (creatures like Palaeomastodon, Moertheritherium, and Barytherium)? The new(-ish, 2006)* fossil from the land bordering the Red Sea sheds some new light on this question and may help connect the elephantimorphs to the elephantiformes.
*[Thanks to Darren for the correction. I was a little confused about publication dates because of the announcement of Eritherium azzouzorum, also announced in PNAS, last month. I will blog about Eritherium azzouzorum tomorrow.]
The lower jaw of Eritreum. From Shoshani et al. (2009).
The species, described in PNAS, is called Eritreum melakeghebrekristosi (say that ten times fast). It is entirely represented by parts of the lower jaw. Viewed from above, the foot-and-a-half-long jaw is Y-shaped, looking something like a fossil divining rod with molars in. In fact, it is the molars themselves that provide a lot of evolutionary details.
Modern elephants have a conveyor-belt arrangement of molars where one molar at a time is worn down and replaced. Eritreum exhibits a similar pattern. This particular specimen has empty sockets from molar 1, a functioning molar 2, and an erupting molar 3. Since the third molars were so far back and were not fully developed, as seen in modern elephants, the paleontologists hypothesized that as the tooth erupted it would have shifted forward behind molar 2, molar 1 having already fallen out by then. This means that Eritreum would have a fresh tooth to keep processing plants as the others became worn down and fell out.This would be important to animals that lived a long time, and indeed, the researchers calculated that this “subadult” specimen of Eritreum was already 26 years old!
The cladogram showing the placement of Eritreum, circled in red. From Shoshani et al. (2009).
This conveyor-belt like arrangement, in addition to other minute details of the molars, links Eritreum more closely with the elephantimorphs (like the gomphotheres) than the elephantiformes (like Palaeomastodon). Indeed, an evolutionary analysis placed Eritreum just outside the group containing mastodons, gomphotheres, and living elephants. It might even fall within this grouping, but more complete specimens will be required to know for sure.
As always, more work remains to be done, but as it stands now Eritreum indicates that east Africa was an important location for proboscidean evolution around 27 million years ago. It would have been a somewhat unfamiliar animal, standing only about four feet at the shoulder and having a long, flat head, but there would be no mistaking it for an elephant relative. Whether it is really a transitional form between the elephantiformes and elephantimorphs or is too derived to fill that role is still an open question, but by trying to confirm or refute these ideas we can learn quite a bit more about a major event in elephant evolution.
For more fossil elephants, see these posts
Shoshani, J., Walter, R., Abraha, M., Berhe, S., Tassy, P., Sanders, W., Marchant, G., Libsekal, Y., Ghirmai, T., & Zinner, D. (2006). A proboscidean from the late Oligocene of Eritrea, a “missing link” between early Elephantiformes and Elephantimorpha, and biogeographic implications Proceedings of the National Academy of Sciences, 103 (46), 17296-17301 DOI: 10.1073/pnas.0603689103