EVOLUTIONARY PATTERNS AMONG THE OTARIOID PINNIPEDS
EAT 2005 abstracts
LAWRENCE G. BARNES

Otarioidea (sea lions, fur seals, walruses & extinct relatives) are
secondarily aquatically-adapted mammals in the order Carnivora. The otarioid
pinnipeds are a monophyletic clade that first appeared in the North Pacific
in Late Oligocene. Otarioids underwent 4 major & sequential evolutionary
diversifications, each one replacing a previous one.
Stem members of each otarioid radiation were small in body size, piscivorous
judging by their dentitions, and retained the ³ursid loop² of the internal
carotid artery situated within an embayment in the basi-occipital bone.

1) the primitive Late Oligocene to Middle Miocene subfamily Enaliarctinae of
the family Otariidae. Small by comparison with most later otarioids,
enaliarctines retained vestigial carnassial cheek teeth, traces of their
terrestrial ursoid carnivoran ancestry.

2) the late Early Miocene to Late Miocene Desmatophocidae. The family
Desmatphocidae includes the earlier & less specialized subfamily
Desmatophocinae & the later-appearing, larger, deep-diving & enormous-eyed
members of the subfamily Allodesminae. Desmatophocids might have been
specialized squid-feeders.

3) the family Odobenidae, which in the latest Miocene & Pliocene were the
most diverse otarioids, and only the later members of which have tusks. Stem
odobenids are early Middle Miocene Imagotariinae which are small piscivores
with unmistakable odobenid basicranial & limb features. Through the Late
Miocene, imagotariines retained their basic body plan, and some became very
large, probably resembling the living S.American sea lion, Otaria. From the
imagotariines evolved the Late Miocene ³pseudo-walruses² of the odobenid
subfamily Dusignathinae, whose later members had multiple pairs of tusks,
the various late Miocene & Pliocene ³hyper-walruses² such as Valenictus,
which had osteosclerotic & pachyostotic limb bones, & the last-appearing
Pliocene to Recent true walruses of the subfamily Odobeninae, represented
now only by the relict Odobenus.

4) the sea lions & fur seals of the otariid subfamily Otariinae. Clearly
derived from the Enaliarctinae, they apparently have an Early & Middle
Miocene ghost lineage. Otariines first appear in Late Miocene time
(Pithanotaria), diversified during the Pliocene & Pleistocene, and appear to
be now at their maximum diversity, represented by the many species of fur
seals & sea lions.

With the exception of the Enaliarctinae, each evolutionary radiation of
Otarioidea was characterized by increasing body size & specialization of
feeding methods. The most successful otarioid adaptations include the
development of homodont but non-specialized dentitions that have allowed
feeding on a variety of prey items (fishes, cephalopods, arthropods). This
undoubtedly explains the persistence of the rel.generalized living Otariinae
subsequent to the extinctions of the more specialized desmatophocines,
allodesmines, dusignathines, valenictines & odobenines.