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 tympanics

• Posterior of skull wide

Ambulocetids

Ambulocetids show more aquatic adaptations than Pakicetids, and probably filled an ecological niche similar to modern crocodiles. They are found in near shore environments and probably ambushed part of their prey in the shallows. They could move both on land and in water, and had robust jaws and teeth to handle large struggling prey. The post-cranial skeleton of Ambulocetids is well known thanks to a nearly complete skeleton of the species Ambulocetus natans that was found in northern Pakistan. Ambulocetids are only known from Eocene deposits of Pakistan, 49 million years ago

Characteristics of Pakicetidae:

• Dorsally placed orbits

• Palatine fissures present

• Nasal opening over incisors

• Hypoglossal foramen present and separated from jugular foramen

• Small mandibular foramen

• Lower molars with paraconid and metaconid and only a hypoconid on talonid basin

~ Whale Evolution ~

Whales live all their lives in the sea, and can never come to land, yet they are not fish, but mammals. They are warm-blooded, have small hairs covering their body, give birth to live young rather than eggs and they inhale air into their lungs rather than filter oxygen through gills. Modern whales are adapted perfectly to a permanent life in the sea.

• Their unique ears enable them to hear perfectly underwater

• They have no legs, moving instead with flippers and the flukes of their huge tails

• They do not have nostrils, having instead a single blowhole on top of their heads, which allows them to breath while mostly submerged

• They are able to give birth underwater, instead of having to come onto land like seals

• The Odontoceti can use ultrasound to hunt, while all whales communicate with a complex language of ‘songs’ that carry for miles underwater

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