318 Evolution? The Fossils Say YES!
Northwest of North America), and the first walruses (the desmatophocines of the early
Miocene of North America and Prototaria of the early Miocene of Japan).
The best documented of these transitions is the evolution of walruses. The most primi-
tive walruses, such as the middle Miocene Proneotherium and Neotherium (fig. 13.19A), are
barely different from Enaliarctos, except that they are larger and more robust, and already
begin to show the size and canine tusk differences between males and females that is so
characteristic of walruses and not enaliarctines. Slightly later in the middle Miocene, we
find Imagotaria, which was about the size of the living walrus, with ear bones already
suited to underwater hearing. Imagotaria was beginning to develop the tusk-like canines
and simplified peg-like molars and premolars in its cheek teeth that all walruses have. By
the late Miocene and early Pliocene, there were at least eight different kinds of walruses
along the Pacific Rim, most of which looked like unusually large sea lions with short
to medium-sized tusks. They included Pontolis, Gomphotaria (fig. 13.19C), and Dusig-
nathus, with completely peg-like simplified cheek teeth, large lower tusks, and small
upper canine tusks, and Aivukus (fig. 13.19B), with slightly larger upper tusks, broad
cheek teeth, and a deep lower jaw with small canines. Aivukus apparently lived in shal-
low water as a bottom feeder and crushed its food like modern walruses do. Then, in
the latest Miocene and early Pliocene, the walruses spread through the Central American
seaway (there was no Panamanian land bridge until the middle Pliocene), up the Atlantic
Coast, and on to Europe and the Mediterranean. In the early Pliocene of Europe and
the African coast of the Mediterranean, we find fossils of Alachtherium, which looked
slightly more like a modern walrus, with large tusks, reduced peg-like molars, a deep
lower jaw with no canines, and only a few other cheek teeth. In almost every respect, it
is an ideal intermediate between the primitive walruses and the modern groups. Finally,
the late Miocene and Pliocene also yield an even better transitional form, Valenictus
(fig. 13.19D), which has long tusks (but still considerably shorter than the tusks of the
living walrus, Odobenus, fig. 13.19E), and almost no teeth in the lower jaw. These wal-
ruses also had the characteristic arching of the palate that we see in living walruses. This
arched palate, combined with the action of the tongue, allows living walruses to suck
their prey (mostly mollusks) into their mouths, where they then crush the shells and
suck out the contents.
If a creationist saw the end-members of this transitional series, they would not guess
the connection, until we put all the transitional fossils in between them and demonstrated
this dramatic example of land animals adapting to marine life. The walruses, in particular,
give us an amazing array of transitional forms from sea lion–like early forms to those with
intermediate conditions of the tusks and cheek teeth. Ironically, some of the best specimens
FIGURE 13.19. The evolution of walruses from primitive forms that resembled sea lions. (A) Proneotherium
from the early Miocene, which has short canines and relatively primitive teeth but several distinctive features
found only in walruses. (B) Aivukus from the late Miocene with larger canine tusks and simpler peg-like cheek
teeth. (C) The dusignathine walrus Gomphotaria, with its big upper and lower tusks. (D) The more advanced
walrus Valenictus, with upper tusks almost as large as those of the living species, a highly arched palate, and
greatly reduced cheek teeth. (E) The living walrus, Odobenus rosmarus. (Photo [C] courtesy L. Barnes; [B] after
Repenning and Tedford 1975, courtesy U.S. Geological Survey; all others courtesy T. Deméré)