Evidence of Evolution
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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
The mammalian order Cetarcea is divided into three sub-orders:
1. Odontoceti or toothed whales, which includes sperm whales, killer whales and dolphins.
2. Mysticeti or baleen whales, including blue whales, humpback whales and right whales. Baleen whales live by filtering plankton through a hairy sieve-like substance called baleen. Both the Odontoceti and Mysticeti orders are still living today
3. Older and more primitive Eocene Archaeoceti or archaic whales, which evolved from land mammals and gave rise to recent Odontoceti and Mysticeti.
The first question which must be answered is why leave the land? The answer to this question can be found among some communities of sheep that live on the cost of Scotland. These wild, goat-like-sheep have lived on the coast for hundreds of years, and like to eat seaweed and kelp. They like seaweed so much that they are often observed swimming out into shallow waters to find it. Perhaps if we returned in ten million years, the descendants of these sheep would be seal-like or even whale-like creatures. And if herbivorous creatures are prepared to brave the seas for food, it would be even more attractive for those that were able to eat fish.
The first fossil evidence for early whales arrived with the 1840 the discovery in Egypt of Basilosaurus, an enormous, 40-million-year-old creature with a long, serpentine body which was very whale-like in appearance, but also had tiny, useless hind legs indicative of a land-based origin. More evidence for the land-based ancestors of whales arrived with rhe discovery of the Mesonychids, and extinct type of mammal that flourished between 60 and 30 million years ago. Mesonychids were hoofed animals (ungulates), but unlike all other known ungulates they were meat eaters. Their most important feature was their unusual triangular teeth. The only animal with similar teeth to these are the whales.
Scientists have been studying whale DNA