Mammalian Explosion 317there is no shortage of diversity of fossils or transitional forms between nearly every fossil
group known.
Finally, one of the most amazing of all transitions in mammals is the origin of pinni-
peds (seals, sea lions, and walruses). At one time, paleontologists thought that seals were
related to weasels and that sea lions and walruses were related to bears, but recent cladis-
tic and molecular phylogenetic analyses (Wyss 1987, 1988) have shown conclusively that
all pinnipeds are monophyletic and closely related only to bears (fig. 13.15). And there
are beautiful transitional fossils that link them. Oligocene deposits of Europe yield bears
known as amphicynodontids, which were terrestrial animals yet had many features that
link them to pinnipeds. Lower Miocene beds of California and Oregon yield the enaliarc-
tines, which are the first truly marine relatives of seals and sea lions (Mitchell and Tedford
1973; Barnes 1989; Berta et al. 1989; Berta and Ray 1990). Although they retained many
primitive skull features seen in the bearlike amphicynodontids, they also have some spe-
cializations of seals and sea lions, including enlarged eyes, an enlarged nasal cavity for
regulating the temperature of the blood as they swim, and larger openings for the muscles
that control their lips and whiskers. They also have reduced their olfactory lobes of the
brain (since the sense of smell is not very important to aquatic mammalian predators) and
improved the drainage of blood to their brains as an aid to diving. Their bodies (fig. 13.18)
also had rudimentary flippers and streamlined shapes, so they would definitely remind us
of the living seals, although they were very primitive looking and their flippers were clearly
not as advanced as seen in modern pinnipeds. However, their bodies were still not as fully
aquatic as the later seals and sea lions. Instead, they may have had lifestyles not too differ-
ent from that of the sea otter.
Not long after the enaliarctines, we find the first members of all the living pinniped
groups in the late early to middle Miocene, including the first true seals (Pontophoca, Prae-
pusa, and Cryptophoca in the middle Miocene of Europe and Leptophoca in the middle Mio-
cene of North America), the first sea lions (Pithanotaria in the middle Miocene of the Pacific
FIGURE 13.18. The earliest known relative of the seals, Enaliarctos, which was seal-like in many features but did
not have the fully specialized skull, nasal region, or ear region of modern seals and sea lions. Their webbed
feet and hands were more like the condition in otters and not fully modified into flippers. (Courtesy A. Berta)