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© 2005 MR THOMAS LINDSTRÍM, EMail: 73.thomas@gmail.com
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CETACEAN EVOLUTION

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.

The earliest basilosaurids are from the late middle Eocene of Pakistan. By the late Eocene, basilosaurids are virtually globally distributed with specimens known from New Zealand, Senegal, Europe, Jordan and common from Egypt and the southeastern United States.

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.