Evolution

Pakicetidea Family (the First Cetaceans)  

Externally, Pakicetids look nothing like a modern cetacean. They are more similar to a wolf with a long nose and tail. They have long thin legs, with relatively short hands and feet which suggest that they were poor swimmers. To compensate their bones are unusually thick which is probably an adaptation to make the animal heavier to counteract the buoyancy of the water. It is not thought that pakicetids displayed any aquatic skeletal adaptation, instead they display adaptations for running and jumping. Hence pakicetids were most likely aquatic waders. It is thought that Pakicetids  were a meat eater that sometimes ate fish. Although it had the body of a land animal, its head had the distinctive long skull shape of a whale’s and over time (as noted above), fossils also revealed that Pakicetus had an ear bone with a feature unique to whales and an ankle bone that linked it to artiodactyls, a large order of even-toed hoofed mammals.

Ambulocetidae Family (the First Marine Cetaceans)

Ambulocetid fossils suggest they were much larger than any Pakicetid, roughly the size of a large male sea lion.  They were vaguely crocodile-like mammals. The skull of Ambulocetus has a long snout, as evidenced by the long lower jaw and the eyes are located high on the skull. The fossils of ambulocetids are always found in near-shore shallow marine deposits. The tail vertebrae are robust, suggesting that the tail was muscular. It is suggested that Ambulocetus lived in water and was not a fast-moving predator they were an ambush predator, attacking prey that came close to it but not pursuing it over long distances.

This early whale has short and powerful and strong legs which made it easier to move in the water. They had five fingers in the hand and four toes in the foot with the feet being much larger than the hands.  They retained a tail with no sign of a fluke. The hindlimb structure of Ambulocetids shows that their ability to engage in terrestrial locomotion was significantly limited compared to that of contemporary terrestrial mammals, and likely did not come to land at all. This suggests that, on land, propulsion of the hindlimbs was powered by the extension of dorsal muscles. They probably swam by pelvic paddling (a way of swimming which mainly utilizes their hind limbs to generate propulsion in water) and caudal undulation (a way of swimming which uses the undulations of the vertebral column to generate force for movements), as otters, seals and modern cetaceans do.

Remingtonocetidae Family (Long-Snouted Cetaceans)

Remingtonocetids had long snouts and probably already actively hunted for fish, similar to today's otters. Their molars lost the crushing basins of pakicetids and ambulocetids. This suggests that the diet of Remingtonocetids is different from that of earlier cetaceans. In the genus Remingtonocetus, the eyes are very small, but the ears are large and set far apart on the skull, a feature that enhances directional hearing. The morphology of the sense organs suggests that hearing was important for Remingtonocetus but that vision was not. The nasal opening, which eventually becomes the blowhole in modern cetaceans, was located near the tip of the snout. The postcranial skeleton of Remingtonocetids shows that these whales had short legs but a very long powerful tail. They had well adapted to swimming, and likely to swim by caudal undulation only. One of the notable features in Remingtonocetids is that the semicircular canals, which are important for balancing in land mammals, had decreased in size.

Protocetidae Family (the Cetaceans that Conquered the Oceans)

Protocetids were probably amphibious, but more aquatic compared to Remingtonocetids. They were very good swimmers and had a shorter neck which was better suited for diving. They were the first cetaceans to leave the Indian subcontinent and disperse to all shallow subtropical oceans of the world. Protocetids are a diverse group, with approximately 15 genera described. They had great variety in such features as snout length and ear morphology. The great variety of teeth suggests diverse feeding modes in protocetids. A point of similarity among protocetids is the position and size of the eyes, which differentiates them from earlier cetaceans. The eyes are always large (unlike Remingtonocetids), face laterally (which might be used to observe underwater prey, and are similar to the eyes of modern cetaceans) and are set far from the midline of the skull under a thick flat skull roof called the supraorbital shield (unlike ambulocetids, pakicetids, and some Remingtonocetids. Also, unlike earlier cetaceans, the nasal opening is not at the tip of the snout. Instead, it is located further posterior on the snout, foreshadowing the formation of the blowhole of later whales. The blowhole in modern cetaceans is located between the eyes on the forehead, an adaptation for breathing while remaining submerged.

Basilosaurinae

Basilosaurinae had a snake-like body and ranged in size from 4 to 16 m (13 to 52 ft) and were fairly similar to modern cetaceans in overall body form and function. They had elongated vertebrae and long tails and had only tiny hind limbs. Their nostrils had already moved further back, but for breathing they probably still stuck out their head. The long, slender body was very suitable for hunting in shallow bays.

Dorudontinae 

Dorudontinae is shorter, as short as 4 m. They had dolphin-shaped bodies and swam by up-and-down motions of their tail fluke.  They also had tiny hind legs, but they were much smaller. The Dorudontinae are thought to be the origin of today's whales and dolphins.

Mysticeti

All modern baleen whales or Mysticetes are filter-feeders which have baleen in place of teeth, though the exact means by which baleen is used differs among species. The actual origin of the baleen is still not fully understood as the fossil record provides only a few clues on this process. 

It is suggested that the earlier varieties of baleen whales had very little baleen and relied mainly on their teeth.  The development of filter feeding may have been a result of worldwide environmental change and physical changes in the oceans. Using suction whales were freed from the need to grasp individual fish and allowed them to target smaller prey. It also meant that for the first time they were able to capture more than one prey item at once.  The earliest Mysticetes most likely looked different from the current living large and slow whales. They fed and swam in different ways, and their size was not as large in size as that of the living species.

Baleen is unlike any other feeding structure on the planet. Made of keratin, which is the same protein that makes up human hair and finger nails, baleen hangs from the roof of a whale’s upper jaw and is used to filter or sieve small fish and crustaceans from large gulps of water. As water rushes into the mouth of a whale, the small creatures easily pass through the comb-like bristles, but once a whale’s mouth is full of water, it flushes the water back out, trapping the tiny creatures in the wall of baleen to be swallowed whole.