The evolution of cetaceans
According to the most recent theories on the evolution of cetaceans, these animals evolved from terrestrial ancestors and then returned to aquatic life. Their terrestrial origins are indicated by the fact that they breathe air from the surface of the water; from the fact that the bones of their pectoral fins are homologous to those of terrestrial mammals; finally, from the fact that the movements of their backbone are vertical and characteristic of an animal adapted to running on land, rather than to the horizontal movements of fish.
The question of how these animals evolved to fully aquatic life remained unanswered for a long time, due to a lack of fossil records, until relatively recent discoveries made in Pakistan helped solve many mysteries and it is now possible to know.
The first ancestors
The traditional theory of the evolution of cetaceans was that these animals were related to the Mesonychidae (see fig. Above), an extinct order of carnivorous, wolf-like ungulates and akin to artiodactyls. These animals possessed unusual triangular teeth, similar to those of fossil whales. For this reason, researchers have long believed that cetaceans evolved from a mesonychid.
Since the early 1990s, molecular analyzes of large numbers of proteins and DNA sequences have questioned this theory, arguing that cetaceans were more closely related to artiodactyls, most likely to hippopotamids. This has led many scientists to propose the establishment of the taxon Cetartiodactyla, which includes both artiodactyls and cetaceans.
The discovery of Pakicetus, the first protobalena, supports the molecular data. The skeleton of the pachycetus shows that the cetaceans do not derive directly from the mesonychids. Instead, they are a form of artiodactyls that returned to the water after they separated from the mesonychids. In other words, proto whales were primitive artiodactyls that retained aspects of their mesonychid ancestors, such as triangular teeth, which modern artiodactyls have lost. An interesting consequence of this is that the early ancestors of modern artiodactyls were probably at least partially carnivorous, while today all artiodactyls are herbivores (hard to imagine a ferocious sheep, save perhaps the most aggressive wild boars). The Cetaceans, due to the greater availability of animal prey compared to vegetables in water, maintained the carnivorous diet of the mesonychids.
Pachycetidae, the first cetaceans
Pakicetidae were ungulates classified as the first cetaceans. They lived in the Eocene, about 53 million years ago. They were similar to dogs with hoofed legs and long, thick tails. They have been linked to cetaceans by the ears: the structure of the tympanic bubble consists only of the ectotympanic bone surrounding the eardrum. The shape of the ear region of Pakicetus is highly unusual and only resembles that of the whale skull, in which this character is diagnostic. It was initially thought that the ears of pachycetids were adapted to be able to hear underwater, but these animals were able to hear only on land. The ability to hear underwater possessed by cetaceans must therefore have evolved later. According to Dr. Thewissen, the teeth of the pachycetum resemble those of fossil whales, being similar to the teeth of a shark, with a triangular shape.
Thewissen then found the same ear structure possessed by the pachycetus, in the fossils of a small deer-like animal, the Indohyus, which lived about 48 million years ago in Kashmir. About the size of a domestic cat, this herbivore shared some of the characteristics of Cetaceans and showed signs of adaptation to aquatic life, such as the presence of a thick, heavy coating around the bones similar to that possessed by the bones of some modern creatures, such as hippos.
Ambulocetidae and remingtonocetidae
The most remarkable recent discoveries in Pakistan concern Ambulocetus, which resembled a cross between a crocodile and a mammal, about 3 meters long.
This animal was clearly amphibian, as evidenced by its hind legs more suited to swimming than to land movement. In any case, it is likely that this animal swam by moving its tail vertically, like do all modern aquatic mammals. It has been speculated that ambulocetes hunted like crocodiles, waiting for prey hiding under the water.
A smaller cousin of the Ambulocet was the Remingtonocetus, which was better adapted for aquatic life and likely hunted like today's sea otters.
Protocetides
Protocetidae formed a diverse and heterogeneous family that lived in Asia, Europe, Africa and North America.
Several genera belonged to it and some of them, such as Rodhocetus, are very well known. They had large forelimbs and hind limbs that could support body weight on land and it is likely that they were amphibians. It is not yet clear whether they possessed a caudal fin similar to that of today's Cetaceans.
Basilosaurids and durontids, completely marine cetaceans
Basilosaurus, discovered in 1847 and mistaken for a reptile and Dorudon, lived about 38 million years ago, had a shape very similar to that of a whale and lived entirely in the oceans.
The basilosaur was up to 18 m long, while the durodon was comparable in size to that of a modern dolphin, about 5 m.
Appearance of echolocation
Squalodon's skull shows evidence for the first appearance of echolocation. These animals lived from the Oligocene to the Miocene, about 33-14 million years ago.
The skull was well compressed and the rostrum telescopic, giving these animals the appearance of modern odontocetes. However, it is thought that these animals are unlikely to have had anything to do with the ancestors of modern dolphins.
First Misticeti
It is the opinion of experts that the cradle of the Mysticetes was the western South Pacific, where the fertile deposits of zooplankton in the Oligocene layers and the presence of fossils of the first progenitor forms suggest a possible evolution of the baleen and an adaptation to a diet centered on the use of a filter. From here there would then have been irradiation in the Pacific and Indo-Pacific along lines of high productivity in the late Cenozoic, even if there is reason to think that the original distribution area of the fin whales was centered in the warm and temperate waters of the North Atlantic. The first whales appeared in the lower Oligocene or possibly the upper Eocene (between 39 and 29 million years ago).
The oldest cetacean associated with the mysticetes is Llanocetus, whose fossil remains have been found in Antarctica in layers of the upper Eocene. The first whales possessed real teeth, inheritance of their ancestors, unlike the baleen present in today's species. The Oligocene species Aetiocetus cotylalveus is considered an important intermediate step between toothed and baleen whales. This species was discovered in 1964 in Oregon. In the early 90's in Australia the fossils of Janjucetus hunderi were recovered, then described in 2006; this animal possessed sharp teeth and is supposed to have hunted fish and squid, as well as larger prey (perhaps sharks or other cetaceans). These fossils indicate that ancient whales were predators, and only after millions of years did they evolve into species without sharp teeth similar to those we know today.
A more recent study (Deméré et al., 2008) identified palatal holes (imprints of blood vessels in the bones, which connect the baleen to the jaws) in the palate of a toothed mysticet, Aetiocetus weltoni. Scientists believe that this ancient whale possessed both baleen and teeth, and is an example of an intermediate adaptive role between primitive (and toothed) mysticets and more advanced toothless mysticets. Another animal with teeth and baleen is Mammalodon of the Miocene.
The first whales exclusively equipped with baleen appeared in the upper Oligocene (such as Eomysticetus and Micromysticetus). Probably these whales could not yet use echolocation, as no evidence has been found in any fossil of these animals, preserved in the skulls and in the ear region, showing the adaptations associated with echolocation (instead present in current forms).
Between the Oligocene and Miocene the mysticetes underwent considerable evolutionary radiation, and separated into two main branches: on the one hand a clade including fin whales (Balaenopteridae), with a more slender body, and on the other a group in which there are also real whales (Balaenidae), with a huge and swollen head. During the Pliocene, whales continued to increase their specializations. Among the most important genera of this evolutionary period are the primitive fin whales Archaebalaenoptera and Eobalaenoptera, the "tiny" whale Balaenella, the primitive Titanocetus found in San Marino and the ancient gray whale Eschrichtioides.
The family of cetoterids (Cetotheriidae), in the past considered ancestral to all mysticetes, is currently considered closer to fin whales than to real whales.
First Odontoceti
During the Miocene, echolocation developed into its current form, and several families of dolphin-like animals appeared during this period.
Early Odontocetes include Kentriodon and Hadrodelphis. These possessed symmetrical skulls and are probably the ancestors of modern odontocetes. They lived from the late Oligocene to the late Miocene. They fed on small fish and other nectonic organisms. It is thought that they used echolocation and lived in herds.
Evolution of the skeleton
Today the hind limbs of the cetaceans have disappeared and only vestigial bones hidden inside the body remain. They are used as anchors for the genital muscles.
While the first cetaceans such as Pakicetus possessed nasal openings at the tip of the snout, in later species, such as Rodhocetus, the openings "slipped" on the top of the head.
The nostrils of modern cetaceans have changed into blowholes. The ears began to deepen and in the basilosaur the middle ear began to receive vibrations from the jaw. Modern Odontoceti use melon for the production of sounds used for echolocation.
