The Rise of Primitive Cetaceans
From Primitive Odontocetes to Delphinidae
It is believed that the family Mesonychidae, at the end of the Cretaceous was the point where cetaceans diverged from other mammals. During the late cretaceous, the family was widespread and diverse. There is strong evidence that that that family gave rise to modern ungulates, such as horses and pigs, in addition to cetaceans.
Mesonychidae were terrestrial mammals, occupying the planes of what is now Africa in the early Eocene. They had large bodies, and their dentition suggests that the family consisted of carnivors, herbivors, and omnivors. Fossils have been found in the sedimens deposited in estuaries and lagoons, leading to the conclusion that some mesonychids were evolving toward a more aquatic life.
The order Condylarthra was most likely a common ancestor between cetaceans and the artiodactyla, which gave rise to ungulates. Evidence for this includes support in the fossil record, and similar blood composition, fetal blood sugar, chromosomes, insulin, uterine morphology, and tooth enamel microstructure. These similarities, together with the skull morphology, leads to the conclusion that the Mesonychidae were probably ancestors of modern cetaceans.
Before the rise of the first cetaceans, terrestrial condylarthrans probably colonized the edges of the slow rivers that emptied into the southern and western Tethys Sea, which roughly corresponds to what is now the Mediterranean Sea and Persian Gulf. The assembly of these animals at water reserviors may have been in a similar manner to the mammals of the savanna tropics today, such as the hippopotamus. These creatures, though, were probably more aquatic and less bulky and specialized.
At first, the ancestors of cetaceans fed on molluscs and slow fish, but as the population grew and competition for resources increased, there was a need for the development of fast reflexes and teeth suitable for catching fast fish. The ability to escape from preditors was important, but that wasn't such a pressing issue.
Changes in the structure of the teeth facilitate the understanding of the fossil record. The members of the order condylarthra are thought to have given rise to archaeocetes, the first cetaceans, at the end of the Paleocene. It is possible that the intermediate between the Mesonychidae and the first cetaceans was a seal-like animal, coming to shore just for breeding, but feeding in the sea. It was around 50 million years ago that the colonisation of the sea occured.
The Rise of Primitive Cetaceans
During the Paleocene, what is now the Mediterranean Sea and the Arabian Gulf formed a semi-enclosed arm of the western part of the ancient Tethys Sea. It was probably in this area, around 50 million years ago, that the condylarths started to colonise the coastal fringes and swamps. They were likely exploiting the ecological niches left at the end of the Cretaceous by the extinct reptiles, including plesiosaurs, icthyosaurs, and others. The warm watesr of the Tethys Sea might have expanded during the Eocene due to the subsidence of Europe and increased volcanic activity in various parts of the world. This enlarged sea eventually became what is now the Atlantic and Indian Oceans.
Between the first cetaceans and their ancestors there is a lack of fossil information. It is possible that the transitional species was not very successful and widespread, so the few fossils that were actually formed are very isolated. It is also possible that the evolution from condylarthrans to cetacea was very rapid and localized geographically. Some evolutionists now believe that such rapid phyletic change in some animal orders is more common.
The world of the first primitive cetaceans, classified under the suborder Archaeoceti, was one of tropical vegitation, swamps, and advancing seas. These archaeocetes had elongated bodies and were mainly aquatic. Some were of a moderate size, while others may have grown up to 21 meters in length. They had reduced hindlimbs and long snouts, and they were well adapted to the shallow coastal fringes and open seas.
The oldest verifiable cetacean fossils are of the names Pappocetus luardi, from southern Nigeria, and Protoretus atavus from Egypt. Both were of the family Protocetidae, and were likely to have similar behavioral characteristics. A common name for the first cetaceans is Zeuglodonts, from the anachronistic generic name of one of the types, Zeuglodon. The bone structure of these animals was similar to the mammals of the late Cretaceous and early Eocene, with specialization for grasping fast prey such as fish.
During the Palaeocene and Eocene, there was a large amount of speciation among mammals, which led to a high degree of conflict for resources. Large populations developed and then collapsed due to the changes in the ecological balance brought on by their own existence. Since evolution usually occurs in unfavorable conditions where the size of the popluation is reduced, it was particularly accelerated during this period.
Natural selection at this time favored adaptations for the capture of fast-moving, agile fish rather than freshwater, estuarine molluscs and slow fish. The dentition of archaeocetes was heterodont, meaning that the incisors, cuspids, and molars were differentiated, as they are in terrestrial mammals. Like modern cetcea, these primitive cetaceans had dense ear bones, long palates, nostrils on the top of the snout, space around the ear bones for fat deposits, and air sacs to isolate the ear from the skull. The body was elongated and had a long tail, short neck, and reduced hindlimbs. The front limbs were paddle-shaped and there was a point of flexion in the tail vertebrae allowing up-down movement as well as side-side movement.
The first cetaceans were not likely as well adapted physiologically to a marine existance as modern cetaceans. Even later specimens were limited to warm water only, and the archaeocetes could only sustain short, shallow dives. The many limitations in the body plan made competition with the more advanced cetaceans during the Oligocene impossible. Although dominant in the Eocene, diversity fell during the Oligocene, although the Oligocene was known for low diversity in general, especially in the western South Pacific. The last remains of this suborder were from the early or early Middle Miocene in France. The date of these remains is not confirmed.
During the Oligocene, between 38 and 25 million years ago, archaeocetes were replaced by members of at least four families: the Agorophiidae and Squalodontidae, which were primitive odontocetes, and the Aetiocetidae and Cetotheriidae, early mysticetes. Life became even more aquatic for the cetaceans during this time, as the external nostrils shifted backwards, structures formed to seal the animals from the water, and the long, mobile neck, functional hindlimbs, and, eventually, most of the pelvic girdle were lost. Any remaining pelage that associated these creatures with their terrestrial ancestors was lost as well. The body became more torpedo-shaped and a dorsal fin developed. The latter was likely common to all species at first, but was later lost by some, such as the rightwhale dolphins. Horizontal tail flukes also developed during this period.
Any functions that were not useful for a marine existance were selected out of the cetaceans very quickly. The evolution was very aggressive, with many adaptations for survival, including resistance to the accumulation of hemoglobin, tolerance of low levels of oxygen, a hypodermal blubber layer for the storage of nutrients, sophisticated control of the body temperature, and telescoping of the front of the skull.
The most primitive odontocetes still had heterodont dentition, but by the late Oligocene, the teeth had been modified in some specimens to form long rows of many sharp, uniform teeth with single roots and conical crowns, a condition known as homodonty. Almost all present-day dolphins have homodont dentition, a notable exception being the Risso's dolphin. Some highly derived odontocetes, notably the Narwhal, lost or reduced parts of their dentition, or developed specialized teeth. The odontocetes underwent extreme modifications to the design of the skull, adapting for acoustic scanning and diving. The melon and modern nasal passages developed. The ability to perform echolocation probably developed when the skull was telescoped, and was probably practised by the earliest odontocetes. There were already some adaptations that aided this ability, as the isolation of the ear bones with fat bodies and air sacs allowed for directional hearing.
The earliest true odontocetes were of the family Agorophiidae. They were short-beaked whales, with triangular shark-like teeth. These gave rise to squalodonts, the behavior of some of whom might have resembled that of the killer whale, although morphologically they were quite different. Most were large, with bodies at least three meters in length. Their skulls were almost completely telescoped.
Squalodonts gave rise to a family of primitive dolphins, traditionally called Eurhinodelphinidae, but now termed Rhabdosteidae. These creatures had extremely long snouts, and most species were of approximately three meters in length. Fossils are from the Early and Middle Miocene and are found in Europe, North America, South America, and the freshwater deposits of Australia. The skulls were fully telescoped, with many homodont teeth. However, the skulls were still symmetrical at this time, unlike modern delphinids.
From Primitive Odontocetes to Delphinidae
During the early Miocene, the melon developed in its modern form and the acoustic systems were refined. Various dolphin-like families, now extinct, flourished. The Squalodelphinidae were the earliest known odontocetes with asymmetrical skulls. Two taxa have been identified: Squalodelphis, from northern Italy, and Diochotichus, from Argentina. These creatures were small, probably less than three meters in length, and had many homodont teeth. The skulls were telescoped in a manner similar to the Rhabdosteidae and many derived species of Squalodonotidae. Some believe that the beaked whales evolved from this species, but there is limited evidence to support this.
In modern times, the superfamily Delphinoidea contains most of the living cetacean species, including the families Delphinidae, Phocoenidae, and Monodontidae, as well as the now defunct families Albireonidae and Kentriodontidae, which are thought to be ancestors. Most members of the family Kentriodontidae were very small, with lengths only up to about two meters. They had short beaks and many homodont teeth. Although diverse during the Middle and Late Miocene in both the Atlantic and Pacific Oceans, there are no specimens less than ten million years old.
The families Delphinidae, Phoceonidae, and Monodontidae are genetically very similar, with only a 10-15% variation in C-heterochromatin. Almost all have the same chromosome number, 2n=44. The one exception is the orca, with only one pair of t chromosomes, which has short arms and satellite structures. The m chromosomes are also very small in the Orca.