352 Evolution? The Fossils Say YES!
hands and feet! One could not imagine a better transitional form: a creature with all the skull
and skeletal features of manatees, yet it still has the ability to walk on land.
In many respects, Pezosiren is comparable to Ambulocetus, which is a beautiful example
of a walking whale making the transition to aquatic life, and to the enaliarctines, which are
the transition fossils linking terrestrial bears to seals and other pinnipeds. Just a few years
ago, we had no transitional forms to show how terrestrial ancestors of marine mammals
like sirenians, whales, and seals went back to the sea and became aquatic, and now we have
excellent fossil transitions for all three groups! The creationist websites have tried to dis-
credit Pezosiren, but reading their discussions just shows how laughably incompetent they
are in anatomy or paleontology. Their basic argument boils down to the idea that because it
was terrestrial, it couldn’t be a sirenian! They fail to notice all the uniquely sirenian features
of the skull (particularly in the snout and teeth, with their horizontal tooth replacement) and
especially the heavy ribs for ballast, showing that this creature was hippo-like with both
terrestrial and aquatic features—just as we would expect for a form making the transition
between terrestrial and aquatic lifestyles. To a creationist, it is either in the fully aquatic sire-
nian “kind” or it’s some other sort of terrestrial mammal, but their conceptual blinders make
it impossible for them to recognize a fossil that is perfectly intermediate between two of their
“created kinds.” Pezosiren is the stuff of creationist nightmares, because you couldn’t ask for
a better transitional fossil: half manatee, half walking land mammal.
Thus we have seen that the fossil record of hoofed mammals is full of transitional forms,
showing how nearly all the familiar large ungulates (horses, rhinos, giraffes, elephants, and
so on) evolved and how two groups of marine mammals (whales and sirenians) evolved from
land ancestors. Even more examples are given in my new book, The Princeton Field Guide to
FIGURE 14.21. The mounted skeleton of Pezosiren portelli, the manatee with feet rather than flippers, next to
Daryl Domning, who described and named it. (Photo courtesy Raymond L. Bernor)