Evidence of Evolution

Basilosaurids and dorudontids lived in the late Eocene, approximately 35 and 41 million years ago. They are mainly known from the eastern United States and from Egypt, but were probably worldwide in their distribution. Basilosaurids were enormous (possibly up to 60 feet long) and had snake-like bodies. They had a tailfluke, but it is not clear whether that was the main propulsive organ. Dorudontids were proportionally more like dolphins. Both basilosaurids and dorudontids had complete hindlimbs that included a mobile knee and several toes. However these extremities were tiny, so small that they were certainly not important in aquatic propulsion.
Protocetids
The Protocetids were better-adapted for the water and lived around 45 million years ago. The best-known is Rhodocetus. The major Protocetid adaptation was the appearance of flukes (horizontal bars) on their tails, which enabled faster swimming. However, the skeletons of Rhodocetus indicate that they retained substantial hind legs. They lived in shallow seas, and may have had a similar lifestyle to seals, or even dolphins; it is not clear whether they ever came onto the land.

Basilosaurus and Dorudon

Characteristics of Ambulocetidae:
• Pterygoid process same height as braincase
• Medially places, laterally facing orbits
• Medium sized mandibular foramen

Remingtonocetides
Remingtonocetids are known from 49-43 million year old deposits in India and Pakistan. Their skeletons are similar to ambulocetids in retaining large hind limbs. They differ from ambulocetids in having smaller orbits (and eyes), long slender snouts, and a wide basicranium which places the ear regions far from the midline. The widely separated ears, combined with the small eyes, may have been an adaptation for enhanced emphasis on hearing to locate prey.

Characteristics of Remingtonocetides:
• Long rostrum
• Small orbits
• Convex posterior palate extending beyond the tooth row
• Ear region placed laterally with oblong... [continues]

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