348 Evolution? The Fossils Say YES!
Now that we have seen that most of the popular large hoofed mammals—horses, rhinos, cam-
els, giraffes, and whales—have excellent fossil records that document transitional forms going
all the way back to the Cretaceous, we need to look at one more group: the elephants and their
relatives. Elephants, too, have an excellent fossil record in the late Oligocene and more recent
rocks, because mastodonts left Africa about 18 million years ago and migrated among all the
northern continents (fig. 14.18). Unfortunately, we are somewhat handicapped because most
of their early evolution took place in Africa, and we have a relatively poor fossil record in
Africa before the early Oligocene. Nevertheless, we can trace their lineage back from the mod-
ern Asian and African elephants and their extinct relatives, the mammoths and mastodonts,
through more primitive lineages with a wide variety of tusks and different lengths of trunks.
Some (the anancines) had two huge long straight tusks protruding from their skulls, while
others (the stegotetrabelodonts) had four long straight tusks; others (the deinotheres) had two
tusks that curled down from the lower jaw, and still others (the amebelodonts) had their lower
tusks flattened into large shovel-like blades. Going back farther into the early Oligocene, the
famous Fayûm beds of Egypt (source of the archaeocete whales with tiny hind limbs) also
produce very primitive, small mastodonts with short jaws and even shorter tusks, known
as Palaeomastodon and Phiomia. In the early Oligocene, the various lineages of proboscideans
(elephants, mammoths, and mastodonts) are very primitive and hard to tell apart, typical of
the early stages of an evolutionary radiation (figs. 14.19 and 14.20). These primitive forms can
be traced back to the ultimate transitional fossil, Moeritherium, from the late Eocene of Egypt.
Superficially, it looked more like a tapir or a pygmy hippo than an elephant and probably only
had a short proboscis, not a long trunk. But a close look at the skull shows that it had very
short tusks in the upper and lower jaws, the teeth of a primitive mastodont (not those of a tapir
or hippo), and the details of the ear region and other part of the skull (such as the condition of
the jugal bones in the zygomatic arch) are unique to the Proboscidea as well.
All of these fossils have been known for decades, but in the last few years, paleontologists
have found even older and better transitional forms. There is the 1984 discovery of an even
more primitive proboscidean, Numidotherium, from the early Eocene of Algeria (Mahboubi et al.
1984). Although the specimen is very incomplete, it already had the high forehead, the retracted
nasal opening (indicating a short proboscis), short upper tusks, mastodont-like teeth, and the
lower front jaw is beginning to develop a broad scoop, a diagnostic feature of mastodonts.
It was only a meter tall (3 feet) at the shoulder, smaller even than Moeritherium, yet it already
had the limb characteristics found in later, larger mastodonts. In 1996, Gheerbrant and others
reported the discovery of an even earlier proboscidean, Phosphatherium, from the late Paleocene
of Morocco. The fossil consists only of a partial skull (typical of the poor preservation of mam-
mal fossils in the Paleocene worldwide), but the teeth already show the distinctive mastodont
pattern at the very beginning of proboscidean evolution. Thus we now have fossils to trace
modern elephants continuously back through many different transitional forms to forms that
are almost 60 million years old, and that brings us almost to the time when all the hoofed mam-
mal lineages diverged. Other fossils from the Paleocene of North Africa, such as Daouitherium
and Eritherium, are even more primitive, although known only from teeth and jaws.
But proboscideans are not the only members of this clade. In his groundbreaking 1975
paper on the cladistic classification of mammals, Malcolm McKenna suggested that the clos-
est living relatives of the elephants was the sirenians or “sea cows,” better known to us as the
manatees and dugongs. Although sirenians are aquatic forms with flippers and a broad tail
fluke and no hind limbs that look superficially nothing like elephants, all the evidence shows