Evolution of cetaceans - AbsoluteAstronomy.com

Cetacea

The order Cetacea includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea...

ns (whale

Whale

Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...

s, dolphin

Dolphin

Dolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in 17 genera. They vary in size from and , up to and . They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating...

s and porpoise

Porpoise

Porpoises are small cetaceans of the family Phocoenidae; they are related to whales and dolphins. They are distinct from dolphins, although the word "porpoise" has been used to refer to any small dolphin, especially by sailors and fishermen...

s) are marine mammal

Marine mammal

Marine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding. The level of...

descendants of land mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s. Their terrestrial origins are indicated by:

Their need to breathe air from the surface;
The bones of their fins, which resemble the limbs of land mammals
The vertical movement of their spine
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...

s, characteristic more of a running mammal than of the horizontal movement of fish.

The question of how land animals evolved into ocean-going leviathan

Leviathan

Leviathan , is a sea monster referred to in the Bible. In Demonology, Leviathan is one of the seven princes of Hell and its gatekeeper . The word has become synonymous with any large sea monster or creature...

s was a mystery until discoveries starting in the late 1970s in Pakistan

Pakistan

Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...

revealed several stages in the transition of cetaceans from land to sea.

Earliest ancestors

The traditional theory of cetacean evolution was that whales were related to the mesonychid

Mesonychid

Mesonychia are an extinct order of medium to large-sized carnivorous mammals that were closely related to artiodactyls and to cetaceans...

s, an extinct order of carnivorous ungulate

Ungulate

Ungulates are several groups of mammals, most of which use the tips of their toes, usually hoofed, to sustain their whole body weight while moving. They make up several orders of mammals, of which six to eight survive...

s (hoofed animals), which resembled wolves with hooves and were a sister group of artiodactyls (even-toed ungulates). These animals had unusual triangular teeth similar to those of whales. This is why scientists long believed that whales evolved from a form of mesonychid. But more recent molecular phylogeny

Molecular phylogeny

Molecular phylogenetics is the analysis of hereditary molecular differences, mainly in DNA sequences, to gain information on an organism's evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree...

data suggest that whales are more closely related to the artiodactyls, specifically the hippopotamus

Hippopotamus

The hippopotamus , or hippo, from the ancient Greek for "river horse" , is a large, mostly herbivorous mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae After the elephant and rhinoceros, the hippopotamus is the third largest land mammal and the heaviest...

. The strong evidence for a clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

combining cetaceans and artiodactyls is further discussed in the article Cetartiodactyla

Cetartiodactyla

Cetartiodactyla is the clade in which whales and even-toed ungulates have currently been placed. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. The term Cetartiodactyla reflects the idea that whales evolved within the artiodactyls...

. However, the anthracothere ancestors of hippos do not appear in the fossil record until millions of years after Pakicetus, the first known whale ancestor.

The molecular data is supported by the recent discovery of Pakicetus, the earliest proto-whale (see below). The skeletons of Pakicetus show that whales did not derive directly from mesonychids. Instead, they are artiodactyls that began to take to the water soon after artiodactyls split from mesonychids. Proto-whales retained aspects of their mesonychid ancestry (such as the triangular teeth) which modern artiodactyls have lost. An interesting implication is that the earliest ancestors of all hoofed mammals were probably at least partly carnivorous or scavengers, and today's artiodactyls and perissodactyls became herbivores later in their evolution. By contrast, whales retained their carnivorous diet, because prey was more available and they needed higher caloric content in order to live as marine endotherms. Mesonychids also became specialized carnivores, but this was likely a disadvantage because large prey was not yet common. This is why they were out-competed by better-adapted animals like the creodonts and later Carnivora

Carnivora

The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...

which filled the gaps left by the dinosaur

Dinosaur

Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

Indohyus

Indohyus
Indohyus
Indohyus is a genus of extinct artiodactyl known from Eocene fossils in Asia, purported to be approximately 48 million years old. A December 2007 article in Nature by Thewissen et al. used an exceptionally complete skeleton of Indohyus from Kashmir to indicate that raoellids may be the "missing...

is a small deer-like creature, which lived about 48 million years ago in Kashmir

Kashmir

Kashmir is the northwestern region of the Indian subcontinent. Until the mid-19th century, the term Kashmir geographically denoted only the valley between the Great Himalayas and the Pir Panjal mountain range...

. It belongs to the artiodactyls family Raoellidae
Raoellidae
Previously grouped with Helohyidae, Raoellidae is now a family in the Suborder Cetancodonta. It is found in Eocene of South and Southeast Asia....

, and is believed to be the closest sister group of Cetacea
Cetacea
The order Cetacea includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea...

. About the size of a raccoon or domestic cat, this herbivorous creature shared some of the traits of whales, most notably the involucrum

Involucrum

Involucrum is a layer of new bone growth outside existing bone seen in pyogenic osteomyelitis. It results from the stripping off of the periosteum by the accumulation of pus within the bone, and new bone growing from the periosteum....

, which is the diagnostic characteristic of any cetacean, and is not found in any other species. Besides, it also showed signs of adaptations to aquatic life, including a thick and heavy outer coating which is similar to the bones of modern creatures such as the hippopotamus

Hippopotamus

, and reduces buoyancy so that they can stay underwater. This suggests a similar survival strategy to the African mousedeer

Chevrotain

Chevrotains, also known as mouse deer, are small ungulates that make up the family Tragulidae, the only members of the infraorder Tragulina. There are 10 living species in three genera, but there are also several species only known from fossils...

or water chevrotain

Hyemoschus aquaticus

The water chevrotain , also known as the fanged deer, is a small ruminant found in tropical Africa. It is the largest of the ten species of chevrotains, evolutionarily primitive even-toed ungulates which are similar to deer but are barely larger than small dogs...

which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes.

Pakicetidae

General

The pakicetid

Pakicetid

Pakicetids or Pakicetidae are a carnivorous mammal family of the suborder Archaeoceti which lived during the Early Eocene to Middle Eocene in Pakistan and existed for approximately ....

s are hoofed mammals that are the earliest whales, with Indohyus
Indohyus
Indohyus is a genus of extinct artiodactyl known from Eocene fossils in Asia, purported to be approximately 48 million years old. A December 2007 article in Nature by Thewissen et al. used an exceptionally complete skeleton of Indohyus from Kashmir to indicate that raoellids may be the "missing...

from family Raoellidae
Raoellidae
Previously grouped with Helohyidae, Raoellidae is now a family in the Suborder Cetancodonta. It is found in Eocene of South and Southeast Asia....

being the closest sister group.
They lived in the early Eocene

Eocene

The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

, around 53 million years ago. Their fossils were first discovered in North Pakistan in 1979, located at a river not far from the shores of former Tethys Sea. After the initial discovery, more fossils were found, mainly in the late-early Eocene fluvial deposits in northern Pakistan and northwestern India. Based on this discovery, pakicetids most likely lived in an arid environment with ephemeral streams and moderately developed floodplains millions of years ago. By using stable oxygen isotopes analysis, they were shown to drink fresh water. Their diet probably included land animals that approached water for drinking or some freshwater aquatic organisms that lived in the river.

Skull morphology

Pakicetids were classified as cetaceans mainly based on the structure of the auditory bulla

Auditory bulla

The auditory bulla is a hollow bony structure on the ventral, posterior portion of the skull of placental mammals that encloses parts of the middle and inner ear. In most species, it is formed by the tympanic part of the temporal bone.In extant primates, the structure is found in tarsiers,...

, which is formed from the ectotympanic bone only. The shape of the ear region in pakicetids is highly unusual and the skull is cetacean-like, although a blowhole is still absent at this stage. The jawbone of pakicetids also lacks the enlarged space (mandibular foramen

Mandibular foramen

The Mandibular foramen is an opening on the internal surface of the ramus for divisions of the mandibular vessels and nerve to pass.-Contents:...

) that is filled with fat or oil, which is used in receiving underwater sound in modern whales. They have dorsal orbits

Orbit (anatomy)

In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents...

(eye sockets facing up), which are similar to crocodiles. This eye placement helps submerged predators observe potential prey above the water. According to Thewissen et al., the teeth of pakicetids also resemble the teeth of fossil whales, being less like a dog's incisors, with a serrated triangular shape, similar to a shark's tooth, which is another link to more modern whales.
It was initially thought that the ears of pakicetids were adapted for underwater hearing, but, as would be expected from the anatomy of the rest of this creature, the ears of pakicetids are specialized for hearing on land. However, pakicetids were able to listen underwater, by using enhanced bone conduction, rather than depending on tympanic membrane like general land mammals. This method of hearing does not give directional hearing underwater.

Postcranial morphology

Pakicetids have long thin legs, with relatively short hands and feet which suggest that they were poor swimmers. To compensate for that, their bones are unusually thick (osteosclerotic), which is probably an adaptation to make the animal heavier to counteract the buoyancy of the water. According to a morphological analysis by Thewissen et al., pakicetids display no aquatic skeletons adaptation; instead they display adaptations for running and jumping. Hence pakicetids were most likely an aquatic wader.

Ambulocetidae

General

In 1994, Ambulocetus natans, which lived about 49 million years ago, was discovered in Pakistan. It was probably amphibious, and resembled the crocodile

Crocodile

A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

in its physical appearance. In the Eocene, ambulocetids inhabited the bays and estuaries of the Tethys Ocean in northern Pakistan. The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and littoral molluscs. Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed a range of water with different degree of salinity, with some specimens having no evidence of sea water consumption and others did not ingest fresh water at the time when their teeth are fossilized. It is clear that ambulocetids tolerated a wide range of salt concentrations. Hence, ambulocetids represent the transition phase of cetacean ancestors between fresh water and marine habitat.

Skull morphology

The mandibular foramen in ambulocetids had increased in size, which indicates that a fat pad was likely to be housed in the lower jaw. In modern whales, this fat pad in the mandibular foramen extends posteriorly to the middle ear. This allows sounds to be received in the lower jaw, and then transmitted through the fat pad to the middle ear. Similar to pakicetids, the orbits of ambulocetids are on the dorsal side of the skull, but they face more laterally than in pakicetids.

Postcranial morphology

Ambulocetids had relatively long limbs with particular strong hind legs, and they retained a tail with no sign of fluke

Whale

(the horizontal tail fin of modern cetaceans). Although they could walk on land, as well as swim, it is clear that they were not fast on either terrain. It has been speculated that Ambulocetids hunted like crocodiles, lurking in the shallows to snatch unsuspecting riparian prey and fish. 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 otter

Otter

The Otters are twelve species of semi-aquatic mammals which feed on fish and shellfish, and also other invertebrates, amphibians, birds and small mammals....

s, seals

Pinniped

Pinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...

and whale

Whale

s do. This is an intermediate stage in the evolution of cetacean locomotion, as modern whales swim by caudal oscillation (a way of swimming similar to caudal undulation, but uses energy more efficiently).

Remingtonocetidae

General

Remingtonocetids lived in middle-Eocene South Asia, about 49 to 43 million years ago. Compared to family Pakicetidae and Ambulocetidae, Remingtonocetidae
Remingtonocetidae
Remingtonocetidae is a family of early carnivorous freshwater aquatic mammals of the order Cetacea endemic to the coastline of the ancient Tethys Ocean during the Eocene living from 55.8—48.6 mya, existing for approximately ....

was a diverse family found in north and central Pakistan, and also western India. Remingtonocetids were also found in shallow marine deposits, but they are obviously more aquatic than ambulocetids. This can be seen from the recovery of their fossils from a variety of coastal marine environments, including near-shore and lagoonal deposits. It is shown that most remingtonocetids did not ingest fresh water, and had hence lost their dependency on fresh water relatively soon after their origin.

Skull morphology

The orbits of remingtonocetids face laterally and are small. This suggests that vision is not an important sense for them. The nasal opening, which will later evolve to become blowhole in modern cetaceans, is located near the tip of the long snout. The position of the nasal opening had remained unchanged since pakicetids. One of the notable features in remingtonocetids is that the semicircular canals, which are important for balancing in land mammals, had decreased in size. This reduction in size had closely accompanied the cetacean invasion of marine environments. According to Spoor et al., this modification of the semicircular canal system may represents a crucial ‘point of no return’ event in early cetacean evolution, which excluded a prolonged semi-aquatic phase.

Postcranial morphology

Compared to ambulocetids, remingtonocetids had relatively short fore and hind limbs. Based on their skeletal remains, remingtonocetids were probably amphibious whales that are well adapted to swimming, and likely to swim by caudal undulation only.

Protocetidae

General

The protocetid

Protocetidae

The protocetids form a diverse and heterogeneous group of cetaceans known from Asia, Europe, Africa, and North America. There were many genera, and some of these are very well known . Known protocetids had large fore- and hindlimbs that could support the body on land, and it is likely that they...

s form a diverse and heterogeneous group known from Asia, Europe, Africa, and North America. They lived in the Eocene, approximately 48 to 35 million years ago. The fossil remains of protocetids were uncovered from coastal and lagoonal facies in South Asia; but unlike previous cetacean families, their fossils uncovered from Africa and North America also include open marine forms. Hence they were probably amphibious, but more aquatic compared to remingtonocetids. Protocetids were the first whales to leave the Indian subcontinent and disperse to all shallow subtropical oceans of the world.
There were many genera among the family Protocetidae
Protocetidae
The protocetids form a diverse and heterogeneous group of cetaceans known from Asia, Europe, Africa, and North America. There were many genera, and some of these are very well known . Known protocetids had large fore- and hindlimbs that could support the body on land, and it is likely that they...

, and some of these are very well known (e.g., Rodhocetus
Rodhocetus
Rodhocetus is one of several extinct whale genera that possess land mammal characteristics, thus demonstrating the evolutionary transition from land to sea.-Description:...

). Great variations in aquatic adaptations exist among them, with some probably able to support their weight on land, whereas others could not. Their supposed amphibious nature is supported by the discovery of a pregnant Maiacetus
Maiacetus
Maiacetus is a genus of early middle Eocene cetacean from Pakistan.-Paleobiology:The genus contains a single species Maiacetus inuus, first described in 2009 on the basis of two specimens, including a specimen which has been interpreted as a pregnant female and its fetus...

, in which the fossilised fetus was positioned for a head-first delivery, suggesting that Maiacetus gave birth on land.

Skull morphology

Unlike remingtonocetids and ambulocetids, protocetids have large orbits and are oriented laterally. Increasingly lateral facing eyes might be used to observe prey that live underwater, which are similar to modern cetacean. Furthermore, the nasal openings are large and are now halfway up the snout. The great variety of teeth suggests diverse feeding modes in protocetids.
In both remingtonocetids and protocetids, the size of mandibular foramen had increased. The large mandibular foramen indicates that the mandibular fat pad was present. However air-filled sinuses that are present in modern cetaceans, which function to isolate the ear acoustically to enable better underwater hearing, is still not present. The external auditory meatus (ear canal) which is absent in modern cetaceans is also present. Hence, the method of sound transmission present in them combines aspects of pakicetids and modern odontocetes. At this intermediate stage of hearing development, the transmission of airborne sound was poor due to the modifications of ear for underwater hearing; while directional underwater hearing was also poor compared to modern cetaceans.

Postcranial morphology

Some protocetids had short, large fore- and hindlimbs that are likely to be used in swimming, but the limbs give a slow and cumbersome locomotion on land. It is possible that some protocetids had flukes. However, it is clear that they are adapted even further to an aquatic life-style. In Rodhocetus, for example, the sacrum

Sacrum

In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...

(a bone that in land-mammals is a fusion of five vertebrae that connects the pelvis

Pelvis

In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...

with the rest of the vertebral column

Vertebral column

In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...

) was divided into loose vertebrae. However, the pelvis was still connected to one of the sacral vertebrae. The ungulate ancestry of these early whales is still underlined by characteristics like the presence of hooves at the ends of the toes in Rodhocetus.

Basilosauridae and Dorudontinae

General

Basilosaurids

Basilosauridae

Basilosauridae is family of extinct cetaceans that lived in tropical seas during the late Eocene.-Taxonomy:*Family Basilosauridae** Subfamily Basilosaurinae*** Genus Basilosaurus*** Genus Basiloterus** Subfamily Dorudontinae...

were discovered in 1840 and initially mistaken for a reptile, hence its name. Together with dorudontids

Dorudontinae

Dourdontinae are a group of extinct cetaceans that are related to Basilosaurus.- Classification :*Subfamily Dorudontinae** Genus Ancalecetus*** Ancalecetus simonsi** Genus Chrysocetus*** Chrysocetus healyorum...

, they lived in the late Eocene around 41 to 35 million years ago, and are the oldest known obligate aquatic cetaceans. They were fully recognizable whales which lived entirely in the ocean. This is supported by their fossils usually found in deposits indicative of fully marine environments, lacking any freshwater influx. They were probably distributed throughout the tropical and subtropical seas of the world. Basilosaurids are commonly found in association with dorudontids. In fact, they are closely related to one another. The fossilised stomach contents in one basilosaurid indicates that it ate fish.

Skull morphology

Although they look very much like modern whales, basilosaurids and dorudontids lacked the 'melon organ

Melon (whale)

The melon is an ovoid-shaped, fatty organ found in the forehead of all toothed whales , including dolphins and porpoises and believed to be used in echolocation....

' that allows their descendants to use echolocation

Animal echolocation

Echolocation, also called biosonar, is the biological sonar used by several kinds of animals.Echolocating animals emit calls out to the environment and listen to the echoes of those calls that return from various objects near them. They use these echoes to locate and identify the objects...

as effectively as modern whales. They had small brains; this suggests they were solitary and did not have the complex social structure of some modern cetaceans. The mandibular foramen of basilosaurids and dorudontids now cover the entire depth of the lower jaw as in modern cetaceans. Their orbits face laterally, and the nasal opening had moved even higher up the snout, closer to the position of blowhole in modern cetaceans. Furthermore, their ear structures are functionally modern, with the major innovation being the insertion of air-filled sinuses between ear and skull. Unlike modern cetaceans, basilosaurids retain a large external auditory meatus.

Postcranial morphology

Both basilosaurids and dorudontids have skeletons that are immediately recognizable as cetaceans. A basilosaurid was as big as the larger modern whales, up to 18 m (59.1 ft) long; dorudontids were smaller, about 5 m (16 ft) long. The large size of basilosaurids is due to the extreme elongation of their lumbar vertebrae. They had a tail fluke, but their body proportions suggest that it swam by caudal undulation and that the fluke was not the propulsive organ. In contrast, dorudontids had a shorter but powerful vertebral column. They too had a fluke, and unlike basilosaurids, they probably swam similarly to modern cetaceans, by using caudal oscillations.
The forelimbs of basilosaurids and dorudontids were probably flipper-shaped, and the external hind limbs were tiny and are certainly not involved in locomotion. Their fingers, on the other hand, still retain the mobile joints of their ambulocetid relatives. The two tiny but well-formed hind legs of basilosaurids which were probably used as claspers when mating; they are a small reminder of the lives of their ancestors. Interestingly, the pelvic bones associated with these hind limbs was now no longer connected to the vertebral column as it was in protocetids. Essentially, any sacral vertebrae can no longer be clearly distinguished from the other vertebrae.

Ancestry of modern Cetacea

Both basilosaurids and dorudontids are relatively closely related to modern cetacean, which belong to order Odontoceti and Mysticeti. However, according to Fordyce and Barnes, the large size and elongated vertebral body of basilosaurid preclude it from being the ancestor of the two modern orders. As for dorudontids, there are some species within the family that do not have elongated vertebral bodies, which might be the immediate ancestors of Odontoceti and Mysticeti.

Early echolocation

Toothed whales (Odontocetes) echolocate by creating a series of clicks emitted at various frequencies. Sound pulses are emitted through their melon-shaped foreheads, reflected off objects, and retrieved through the lower jaw. Skulls of Squalodon
Squalodon
Squalodon is an extinct genus of whales, belonging to the family Squalodontidae. Named by Grateloup in 1840, it was originally believed to be an iguanodontid dinosaur but has since been reclassified. The name Squalodon comes from Squalus, a genus of shark...

show evidence for the first hypothesized appearance of echolocation. Squalodon lived from the early to middle Oligocene

Oligocene

The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

to the middle Miocene

Miocene

The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

, around 33-14 million years ago. Squalodon featured several commonalities with modern Odontocetes. The cranium was well compressed, the rostrum telescoped outward (a characteristic of the modern suborder Odontoceti), giving Squalodon an appearance similar to that of modern toothed whales. However, it is thought unlikely that squalodontids are direct ancestors of living dolphins.

Early baleen whales

All modern mysticetes

Baleen whale

The Baleen whales, also called whalebone whales or great whales, form the Mysticeti, one of two suborders of the Cetacea . Baleen whales are characterized by having baleen plates for filtering food from water, rather than having teeth. This distinguishes them from the other suborder of cetaceans,...

are large filter-feeding or baleen

Baleen

Baleen or whalebone is a filter-feeder system inside the mouths of baleen whales. The baleen system works when a whale opens its mouth underwater and then water pours into the whale's mouth. The whale then pushes the water out, and animals such as krill are filtered by the baleen and remain as food...

whales, though the exact means by which baleen is used differs among species (gulp-feeding with balaenopterids, skim-feeding with balaenids, and bottom ploughing with eschrichtiids). The first members of some modern groups appeared during the middle Miocene

Miocene

. These changes may have been a result of worldwide environmental change and physical changes in the oceans. A large scale change in ocean current and temperature could have initiated the radiation of modern mysticetes, leading to the demise of the archaic forms. Generally it is speculated the four modern mysticete families have separate origins among the cetotheres

Cetotherium

Cetotherium is a genus of the extinct cetaceans from the family Cetotheriidae .-Known species:...

. Modern baleen whales, Balaenopteridae (rorquals and humpback whale, Megaptera novaengliae), Balaenidae (right whales), Eschrichtiidae (gray whale, Eschrictius robustus), and Neobalaenidae (pygmy right whale, Caperea marginata) all have derived characteristics presently unknown in any cetothere.

Early dolphins

During the early Miocene

Miocene

(about 20 Ma), echolocation

Animal echolocation

developed in its modern form. Various extinct dolphin-like families flourished. Early dolphins include Kentriodon
Kentriodon
Kentriodon is an extinct species of toothed whale. Fossils have been found in Europe, Asia, New Zealand, and the United States.Kentriodon was the most diverse of all the kentriodontids, which include three named species and five undescribed species...

and Hadrodelphis. These belong to Kentriodontidae, which were small to medium-sized toothed cetaceans with largely symmetrical skulls, and thought likely to include ancestors of some modern species. Kentriodontids date to the late Oligocene

Oligocene

to late Miocene. Kentriodontines ate small fish and other nectonic organisms; they are thought to have been active echolocators, and might have formed schools. Diversity, morphology and distribution of fossils appear parallel to some modern species.

Skeletal evolution

Today, the whale hind parts are internal and reduced. Occasionally, the genes that code for longer extremities cause a modern whale to develop miniature legs (known as atavism

Atavism

Atavism is the tendency to revert to ancestral type. In biology, an atavism is an evolutionary throwback, such as traits reappearing which had disappeared generations before. Atavisms can occur in several ways...

).

Whereas early cetaceans such as the Pakicetus
Pakicetus
Pakicetus is a genus of extinct terrestrial carnivorous mammal of the family Pakicetidae which was endemic to Pakistan from the Eocene .Pakicetus existed for approximately...

had the nasal openings at the end of the snout

Snout

The snout, or muzzle, is the protruding portion of an animal's face, consisting of its nose, mouth, and jaw.-Terminology:The term "muzzle", used as a noun, can be ambiguous...

, in later species such as the Rodhocetus
Rodhocetus
Rodhocetus is one of several extinct whale genera that possess land mammal characteristics, thus demonstrating the evolutionary transition from land to sea.-Description:...

, the openings had begun to drift toward the top of the skull. This is known as nasal drift.

The nostrils of modern whales have become modified into blowhole

Blowhole (biology)

In biology, a blowhole is the hole at the top of a cetacean's head through which the animal breathes air. It is homologous with the nostril of other mammals. As whales reach the water surface to breathe, they will forcefully expel air through the blowhole. Not only is air expelled, but mucus and...

s that allow them to break to the surface, inhale, and submerge with convenience. The ears began to move inward as well, and, in the case of Basilosaurus, the middle ears began to receive vibrations from the lower jaw. Today's modern toothed whales use the 'melon organ

Melon (whale)

The melon is an ovoid-shaped, fatty organ found in the forehead of all toothed whales , including dolphins and porpoises and believed to be used in echolocation....

', a pad of fat, for echolocation

Animal echolocation

External links

For a review of whale evolution, see

Timeline of Whale Evolution - Smithsonian Ocean Portal
Cetacean Paleobiology – University of Bristol
BBC: Whale's evolution
BBC: Whale Evolution – The Fossil Evidence
Hooking Leviathan by Its Past by Stephen Jay Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....
Whale Origins, Thewissen Lab, Northeastern Ohio Universities College of Medicine
Digital Library of Dolphin Development, Thewissen Lab
Research on the Origin and Early Evolution of Whales (Cetacea), Gingerich, P.D., University of Michigan
Pakicetus inachus, a new archaeocete (Mammalia, Cetacea) from the early-middle Eocene Kuldana Formation of Kohat (Pakistan). Gingerich, P.D., 1981, Museum of Paleontology, The University of Michigan
Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls, Nature 413, 277–281 (20 September 2001), J. G. M. Thewissen, E. M. Williams, L. J. Roe and S. T. Hussain
Evolution of Whales segment from the Whales Tohorā Exhibition Minisite of the Museum of New Zealand Te Papa Tongarewa

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Earliest ancestors

Indohyus

Pakicetidae

General

Skull morphology

Postcranial morphology

Ambulocetidae

General

Skull morphology

Postcranial morphology

Remingtonocetidae

General

Skull morphology

Postcranial morphology

Protocetidae

General

Skull morphology

Postcranial morphology

Basilosauridae and Dorudontinae

General

Skull morphology

Postcranial morphology

Ancestry of modern Cetacea

Early echolocation

Early baleen whales

Early dolphins

Skeletal evolution

See also

External links