Much of my current research is focused on the early evolution
of cetaceans (whales and dolphins). Modern cetaceans are fully aquatic mammals
that have no hind limbs, flippers for forelimbs, and fusiform bodies, among
other traits. I am very interested in the evolutionary development of these
aquatic attributes. Before describing my research in particular, a little background
information is necessary.
Modern cetaceans are divided into two groups. Odontoceti (toothed whales) are
the toothed whales. These include dolphins, porpoises, beaked whales, sperm
whales, narwhals, and others. These cetaceans often have teeth that are simple
pegs (or other forms that are nevertheless highly modified from early mammal
teeth). Odontocetes also use echolocation to find their prey and to sense their
surroundings underwater. Mysticeti (mustached whales) are the baleen whales.
These animals have no teeth but rather have structures that hang from their
upper jaws that they use to filter food from the water. These structures are
made of baleen, which is composed of the same material as your fingernails.
Fossil representatives of these modern groups go back to the Oligocene epoch.
Early members of these groups are more primitive than the modern representatives
(odontocetes with primitive-looking teeth, mysticetes with teeth), but they
are part of these modern groups. Prior to the Oligocene, during the Eocene
epoch, there were no odontocetes or mysticetes, only Archaeocetes (ancient
whales). Archaeocetes retain many features of more primitive mammals in their
skulls, teeth, vertebral columns, and limbs relative to their modern descendants.
My research has focused on archaeocetes from the middle and late Eocene, protocetid
and Basilosaurids.
Protocetids
While the type specimen of Natchitochia jonesi is only a set of 13 vertebrae
and some rib fragments, the vertebrae are distinct from any other known archaeocete.
Also, the set of vertebrae includes a sacral vertebra. The sacral vertebrae
support the innominata, which together form the pelvis, which holds up the
hind limb. In later archaeocete whales (basilosaurids) there is no connection
between the innominata and the vertebral column. In some earlier whales,
the sacrum is composed of 3 or 4 sacral vertebrae (remmingtonocetids, Rodhocetus).
In Natchitochia, the sacrum was composed of one sacral vertebra, which is
also the case in Protocetus, but Protocetus is much smaller than Natchitochia.The
type specimen of Eocetus wardii includes thoracic and lumbar vertebrae, ribs,
a piece of the snout, and much of the right innominate. The lumbar vertebrae
of Eocetus are interesting in that they are somewhat elongate, not unlike
those of later Basilosaurus. Also, the exterior texture of the bone is "pock
marked" with the openings of tiny vascular canals that feed into the
multiple layers of cortical bone. Similar texture can be seen in specimens
of Basilosaurus. Features like this might lead one to think they are related,
but the innominate of Eocetus is quite different from that of Basilosaurus,
and the type specimen of Eocetus (Eocetus schweinfurthi) shows that the teeth
of Eocetus are like those of other protocetids, rather than like those of
basilosaurids (see below).
Basilosaurids
Basilosaurids are the youngest group of archaeocete whales. By the late Eocene,
all other groups of archaeocetes became extinct and only basilosaurids. Based
on this observation and the sharing of many derived morphological features
with early mysticetes and early odontocetes, basilosaurids are thought to have
given rise to modern cetaceans. Basilosaurids all share the loss of the third
upper molar, the presence of accessory denticles on their cheek teeth, a high
number of lumbar vertebrae, highly reduced hind limbs that do not have a bony
connection to the vertebral column, and dorsoventrally flattened tail vertebrae
indicating the presence of a tail fluke.
Basilosaurids are divided into two groups, Basilosaurinae and Dorudontinae. Basilosaurines include the genera Basilosaurus and Basilosterus. Dorudontines include Dorudon, Pontogeneus, Zygorhiza, Saghacetus, Ancalecetus, and a new species soon to be named by Philip D. Gingerich and myself. Basilosaurines all have elongate posterior thoracic, lumbar, and anterior caudal vertebrae, a feature that dorudontines lack.
Basilosaurids have many anatomical features that indicate they were fully
aquatic. Their hind limbs are extremely reduced and lack a bony attachment
to the vertebral column. Their fore limbs, are formed into flippers, with limited
mobility at the wrist and elbow. They also have broad, fan-shaped scapulae
like those of modern cetaceans. Basilosaurids have short necks, although all
seven cervical vertebrae remain free and unfused, unlike many modern cetaceans
that fuse some of the cervical vertebrae. Also, basilosaurids have their posterior
caudal vertebrae dorsoventrally flattened. This feature is only found in modern
mammals that have a tail fluke (cetaceans and sirenians) and the pattern of
shape change along the vertebral columns of dorudontines is very similar to
the pattern seen in modern cetaceans.
