Learn About Cetacea

In this episode of the I Can't Sleep Podcast, fall asleep learning about Cetacea. Whales! More about whales. You know, those cuddly giant sea monsters that get really old. Good luck staying awake with this one. Happy sleeping!

Transcript

Welcome to the I Can't Sleep podcast,

Where I read random articles from across the web to bore you to sleep with my soothing voice.

I'm your host,

Benjamin Boster.

Today's episode is from a Wikipedia article titled,

Cetacea.

Cetacea,

From Latin,

Cetus,

Whale,

From ancient Greek,

Kaitos,

Huge fish,

Sea monster,

Is an infra-order of aquatic mammals belonging to the order Arteodactyla that includes whales,

Dolphins,

And porpoises.

Key characteristics are their fully aquatic lifestyle,

Streamlined body shape,

Often large size and exclusively carnivorous diet.

They propel themselves through the water with powerful up and down movement,

And their tail,

Which ends in a paddle-like fluke,

Using their flipper-shaped forelimbs to maneuver.

While the majority of cetaceans live in marine environments,

A small number reside solely in brackish water or freshwater.

Having a cosmopolitan distribution,

They can be found in some rivers and all Earth's oceans,

And many species inhabit vast ranges where they migrate with the changing of the seasons.

Cetaceans are famous for their high intelligence,

Complex social behavior,

And the enormous size of some of the group's members.

For example,

The blue whale reaches a maximum confirmed length of 29.

9 meters,

Which is 98 feet,

And a weight of 173 tons,

Making it the largest animal known ever to have existed.

There are approximately 89 living species split into two parvidores,

Odontoceti or toothed whales containing porpoises,

Dolphins,

Other predatory whales like the beluga and the sperm poorly understood beaked whales,

And the filter-feeding mysticeti or baleen whales,

Which includes species like the blue whale,

The humpback whale,

And the bowhead whale.

Despite their highly modified bodies and carnivorous lifestyle,

Genetic and fossil evidence places cetaceans as nested with even-toed ungulates,

Most closely related to Hippopotamus with the clade Whippomorpha.

Cetaceans have been extensively hunted for their meat,

Blubber,

And oil by commercial operations.

Although the International Whaling Commission has agreed to put a halt to commercial whaling,

Whale hunting is still going on,

Either under IWC quotas to assist the subsistence of Arctic native people,

Or in the name of scientific research,

Although a large spectrum of non-lethal methods are now available to study marine mammals in the wild.

Cetaceans also face severe environmental hazards from underwater noise pollution,

Entanglement in abandoned robes and nets,

Collisions with ships,

Plastic and heavy metals buildup,

To accelerating climate change,

But how much they are affected varies widely from species to species,

From minimally in the case of the southern bottlenose whale and the baiji,

Or Chinese river dolphin,

Which is considered to be functionally extinct due to human activity.

The two parvidors,

Baleen whales and toothed whales,

Are thought to have diverged around 34 million years ago.

Baleen whales have bristles made of keratin instead of teeth.

The bristles filter krill and other small invertebrates from seawater.

Gray whales feed on bottom-dwelling mollusks.

Rural family baleen optorhids use throat pleats to expand their mouths to take in food and sieve out the water.

Baleenids,

Right whales and bowhead whales,

Have massive heads that can make up 40% of their body mass.

Most mysticides prefer the food-rich colder waters of the northern and southern hemispheres,

Migrating to the equator to give birth.

During this process,

They are capable of fasting for several months,

Relying on their fat reserves.

The parvidor of odontocetes,

The toothed whales,

Include sperm whales,

Beaked whales,

Orcas,

Dolphins,

And porpoises.

Generally,

Their teeths have evolved to catch fish,

Squid,

Or other marine invertebrates.

Not for chewing them,

So prey is swallowed whole.

Teeths are shaped like cones,

Dolphins and sperm whales,

Spades,

Porpoises,

Pegs,

And other fish.

Beaked whales have long tusks,

Eggs,

Belugas,

Tusks,

Narwhals,

Or,

Variable,

Beaked whale males.

Female beaked whales' teeths are hidden in the gums and are not visible,

And most male beaked whales have only two short tusks.

Narwhals have vestigial teeths other than their tusks,

Which is present on males and 15% of females,

And has millions of nerves to sense water temperature,

Pressure,

And salinity.

A few toothed whales,

Such as some orcas,

Feed on mammals,

Such as pinnipeds and other whales.

Toothed whales have well-developed senses.

Their eyesight and hearing are adapted for both air and water,

And they have advanced sonar capabilities using their melon.

Their hearing is so well adapted for both air and water that some blind specimens can survive.

Some species,

Such as sperm whales,

Are well adapted for diving to great depths.

Cetacean bodies are generally similar to those of fish,

Which can be attributed to their lifestyle and habitat conditions.

Their body is well adapted to their habitat,

Although they share essential characteristics with other higher mammals.

They have a streamlined shape and their forelimbs are flippers.

Almost all have a dorsal fin on their backs,

But this can take on many forms depending on the species.

A few species,

Such as the beluga whale,

Lack them.

Both the flipper and the fin are for stabilization and steering in the water.

The body is wrapped in a thick layer of fat known as blubber.

This provides thermal insulation and gives cetaceans their smooth,

Streamlined body shape.

In larger species,

It can reach a thickness up to 1.

5 meters.

Whales have an elongated head,

Especially baleen whales,

Due to the wide,

Overhanging jaw.

Bowhead whale plates can be 9 meters long.

Their nostrils make up the blowhole,

With one in toothed whales and two in baleen whales.

The nostrils are located on top of the head,

Above the eyes,

So that the rest of the body can remain submerged while surfacing for air.

The back of the skull is significantly shortened and deformed.

By shifting the nostrils to the top of the head,

The nasal passages extend perpendicularly through the skull.

The teeth or baleen in the upper jaw sit exclusively on the maxilla.

The brain case is concentrated through the nasal passage to the front and is correspondingly higher,

With individual cranial bones that overlap.

In toothed whales,

Connective tissue exists in the melon as a head buckle.

This is filled with air sacs and fat that aid in buoyancy and biosonar.

The sperm whale has a particularly pronounced melon.

This is called the spermaceti organ and contains the eponymous spermaceti,

Hence the name sperm whale.

Even the long tusk of the narwhal is a vice-formed tooth.

In many toothed whales,

The depression in their skull is due to the formation of a large melon and multiple asymmetric airbags.

River dolphins,

Unlike most other cetaceans,

Can turn their head 90 degrees.

Most other cetaceans have fused neck vertebrae and are unable to turn their head at all.

The baleen of baleen whales consists of long,

Fibrous strands of keratin.

Located in place of the teeth,

It has the appearance of a huge fringe and is used to sieve the water for plankton and krill.

Sperm whales have the largest brain mass of any animal on earth,

Averaging 8,

000 cubic centimeters and 7.

8 kilograms in mature whales.

The brain-to-body mass ratio in some odontocetes,

Such as belugas and narwhals,

Is second only to humans.

In some whales,

However,

It is less than half that of humans,

0.

9% versus 2.

1%.

In cetaceans,

Evolution in the water has caused changes to the head that have modified brain shape,

Such that the brain folds around the insula and expands more laterally than in terrestrial mammals.

As a result,

The cetacean prefrontal cortex,

Compared to that in humans,

Rather than frontal,

Is laterally positioned.

Brain size was previously considered a major indicator of intelligence,

Since most of the brain is used for maintaining bodily functions,

Greater ratios of brain-to-body mass may increase the amount of brain mass available for cognitive tasks.

Elemetric analysis of the relationship between mammalian brain mass weight and body mass for different species of mammals shows that larger species generally have larger brains.

However,

This increase is not fully proportional.

Typically,

The brain mass only increases in proportion to somewhere between the two-thirds power,

Or the square of the cube root,

And the three-quarters power,

Or the cube of the fourth root,

Of the body mass.

Comparison of a particular animal's brain size with the expected brain size based on such an analysis provides an encephalization quotient that can be used as an indication of animal intelligence.

The neocortex of many cetaceans is home to elongated spindle neurons that,

Prior to 2019,

Were known only in hominids.

In humans,

These cells are thought to be involved in social conduct,

Emotions,

Judgment,

And theory of mind.

Cetacean spindle neurons are found in areas of the brain homologous to where they are found in humans,

Suggesting they perform a similar function.

The cetacean skeleton is largely made up of cortical bone,

Which stabilizes the animal in the water.

For this reason,

The usual terrestrial compact bones,

Which are finely woven,

Cancellous bone,

Are replaced with lighter and more elastic material.

In many places,

Bone elements are replaced by cartilage and even fat,

Thereby improving their hydrostatic qualities.

The ear and the muzzle contain a bone shape that is exclusive to cetaceans with a high-density resembling porcelain.

This conducts sound better than other bones,

Thus aiding biosonar.

The number of vertebrae that make up the spine varies by species,

Ranging from 40 to 93.

The cervical spine found in all mammals consists of seven vertebrae which,

However,

Are reduced or fused.

This fusion provides stability during swimming at the expense of mobility.

The fins are carried by the thoracic vertebrae,

Ranging from 9 to 17 individual vertebrae.

The sternum is cartilaginous.

The last two to three pairs of ribs are not connected and hang freely in the body wall.

The stable lumbar and tail include the other vertebrae.

Below the caudal vertebrae is the chevron bone.

The front limbs are paddle-shaped with shortened arms and elongated finger bones to support movement.

They are connected by cartilage.

The second and third fingers display a proliferation of the finger members,

A so-called hyperphalange.

The shoulder joint is the only functional joint in all cetaceans except the Amazon River Dolphin.

Collarbone is completely absent.

Cetaceans have a cartilaginous fluke at the end of their tails that is used for propulsion.

The fluke is set horizontally on the body,

Unlike fish,

Which have vertical tails.

Cetaceans have parallel hearts.

Blood oxygen is distributed effectively throughout the body.

They are warm-blooded,

I.

E.

They hold a nearly constant body temperature.

Cetaceans have lungs,

Meaning they breathe air.

An individual can last without a breath from a few minutes to over two hours,

Depending on the species.

Cetacea are deliberate breathers who must be awake to inhale and exhale.

When stale air warmed from the lungs is exhaled,

It condenses as it meets colder external air.

As with the terrestrial mammal breathing out on a cold day,

A small cloud of steam appears.

This is called the spout and varies across species in shape,

Angle,

And height.

Species can be identified at a distance using this characteristic.

The structure of the respiratory and circulatory systems is of particular importance for the life of marine mammals.

The oxygen balance is effective.

Each breath can replace up to 90% of the total lung volume.

For land mammals,

In comparison,

This value is usually about 15%.

During inhalation,

About twice as much oxygen is absorbed by the lung tissue as in a land mammal.

As with all mammals,

The oxygen is stored in the blood and the lungs,

But in cetaceans it is also stored in various tissues,

Mainly in the muscles.

The muscle pigment myoglobin provides an effective bond.

This additional oxygen storage is vital for deep diving.

Since beyond a depth around 100 meters,

The lung tissue is almost completely compressed by the water pressure.

The stomach consists of three chambers.

The first region is formed by a loose gland and a muscular forestomach,

Missing in beaked whales.

This is followed by the main stomach and the pylorus.

Both are equipped with glands to help digestion.

A bowel adjoins the stomachs,

Whose individual sections can only be distinguished histologically.

The liver is large and separate from the stomach.

The kidneys are long and flattened.

The salt concentration in cetacean blood is lower than that in seawater,

Requiring kidneys to excrete salt.

This allows the animals to drink seawater.

Cetacean eyes are set on the sides rather than the front of the head.

This means only species with pointed beaks,

Such as dolphins,

Have good binocular vision forward and downward.

Tear glands secrete greasy tears,

Which protect the eyes from the salt in the water.

The lens is almost spherical,

Which is most efficient at focusing the minimal light that reaches deep water.

Odontocetes have little to no ability to taste or smell,

While mysticetes are believed to have some ability to smell because of their reduced but functional olfactory system.

Cetaceans are known to possess excellent hearing.

At least one species,

The Tucuxi or Guiana dolphin,

Is able to use electroreception to sense prey.

The external ear has lost the pinna,

Visible ear,

But still retains a narrow ear canal.

The three small bones or ossicles that transmit sound within each ear are dense and compact,

And differently shaped from those of land mammals.

The semicircular canals are much smaller relative to body size than in other mammals.

A bony structure of middle and inner ear,

The auditory bola,

Is composed of two compact and dense bones,

The periodic and tympanic.

It is housed in a cavity in the middle ear.

In the odontoceti,

Apart from in the fisiterids,

The cavity is filled with dense foam and completely surrounds the bola,

Which is connected to the skull or the ear canal.

This may isolate the ear from sounds transmitted through the bones of the skull,

Something that also happens in bats.

Cetaceans use sound to communicate,

Using groans,

Moans,

Whistles,

Clicks,

Or the singing of the humpback whale.

Odontoceti are generally capable of echolocation.

They can discern the size,

Shape,

Surface characteristics,

Distance,

And movement of an object.

They can search for,

Chase,

And catch fast-swimming prey in total darkness.

Most odontoceti can distinguish between prey and non-prey,

Such as humans or boats.

Captive odontoceti can be trained to distinguish between,

For example,

Balls of different sizes or shapes.

Echolocation clicks also contain characteristic details,

Unique to each animal,

Which may suggest that toothed whales can discern between their own click and that of others.

Mysticeti have exceptionally thin,

Wide basilar membranes in their cochlea without stiffening agents,

Making their ears adapted for processing low infrasonic frequencies.

The initial karyotype includes a set of chromosomes from 2n equals 44.

They have four pairs of telocentric chromosomes,

Whose centromeres sit at one of the telomeres,

Two to four pairs of subtelocentric and one or two large pairs of submetrocentric chromosomes.

The remaining chromosomes are metacentric.

The centromere is approximately in the middle and are rather small.

All cetaceans have chromosomes 2n equals 44,

Except the sperm whales and pygmy sperm whales,

Which have 2n equals 42.

Cetaceans are found in many aquatic habitats.

While many marine species,

Such as the blue whale,

The humpback whale,

And the orca,

Have a distribution area that includes nearly the entire ocean,

Some species occur only locally or in broken populations.

These include the vaquita,

Which inhabits a small part of the Gulf of California,

And Hector's dolphin,

Which lives in some coastal waters in New Zealand.

River dolphin species live exclusively in freshwater.

Many species inhabit specific latitudes,

Often in tropical or subtropical waters,

Such as Bride's whale or Rizzo's dolphin.

Others are found only in a specific body of water.

The southern right whale dolphin and the hourglass dolphin live only in the Southern Ocean.

The narwhal and the beluga live only in the Arctic Ocean.

Sower bees,

Beet whale,

And the climbing dolphin exist only in the Atlantic,

And the Pacific white-sided dolphin and the northern straight dolphin live only in the North Pacific.

Cosmopolitan species may be found in the Pacific,

Atlantic,

And Indian Oceans.

However,

Northern and southern populations become genetically separated over time.

In some species,

This separation leads eventually to a divergence of the species,

Such as produced in the southern right whale,

North Pacific right whale,

And the North Atlantic right whale.

Migratory species' reproductive sites often lie in the tropics and their feeding grounds in polar regions.

32 species are found in European waters,

Including 25 toost and 7 baleen species.

Many species of whales migrate on a latitudinal basis to move between seasonal habitats.

For example,

The gray whale migrates 10,

000 miles round-trip.

A journey begins at winter birthing grounds in warm lagoons along Baja California and traverses 5,

000 to 7,

000 miles of coastline to summer feeding grounds in the Bering,

Chukchi,

And Beaufort Seas off the coast of Alaska.

Conscious breathing cetaceans sleep but cannot afford to be unconscious for long because they may drown.

While knowledge of sleep and wild cetaceans is limited,

Tooth cetaceans in captivity have been recorded to exhibit unihemispheric slow-wave sleep,

USWS,

Which means they sleep one side of their brain at a time so that they may swim,

Breeze consciously,

And avoid predators and social contact during their period of rest.

A 2008 study found that sperm whales sleep in vertical positions just under the surface in passive shallow drift dives,

Generally during the day during which whales do not respond to passive vessels unless they are in contact,

Leading to the suggestion that whales possibly sleep during such dives.

While diving,

The animals reduce their oxygen consumption by lowering the heart activity and blood circulation.

Individual organs receive no oxygen during this time.

Some rorquals can dive for up to 40 minutes,

Sperm whales between 60 and 90 minutes,

And bottlenose whales for two hours.

Diving depths average about 100 meters.

Species such as sperm whales can dive to 3,

000 meters,

Although more commonly 1,

200 meters.

Most cetaceans are social animals,

Although a few species live in pairs or are solitary.

A group known as a pod usually consists of 10 to 50 animals,

But on occasions such as mass availability of food or during mating season,

Groups may encompass more than 1,

000 individuals.

Interspecies socialization can occur.

Pods have a fixed hierarchy,

With the priority positions determined by biting,

Pushing,

Or ramming.

The behavior in the group is aggressive only in situations of stress,

Such as lack of food,

But usually it is peaceful.

Contact swimming,

Mutual fondling,

And nudging are common.

The playful behavior of the animals,

Which is manifested in air jumps,

Somersaults,

Surfing,

Or fin hitting,

Occurs more often than not in smaller cetaceans,

Such as dolphins and porpoises.

Males in some baleen species communicate via whale song,

Sequences of high pitch,

Sequences of high pitch sounds.

These songs can be heard for hundreds of kilometers.

Each population generally shares a distinct song,

Which evolves over time.

Sometimes an individual can be identified by its distinctive vocals,

Such as the 52 hertz whale that sings at a higher frequency than other whales.

Some individuals are capable of generating over 600 distinct sounds.

In baleen species such as humpbacks,

Blues,

And fins,

Male-specific song is believed to be used to attract and display fitness to females.

Pod groups also hunt,

Often with other species.

Many species of dolphins accompany large tunas on hunting expeditions,

Following large schools of fish.

The orca hunts in pods and targets belugas and even larger whales.

Humpback whales,

Among others,

Form in collaboration bubble carpets to herd krill or plankton into bait balls before lunging at them.

Cetacea are known to teach,

Learn,

Cooperate,

Scheme,

And grieve.

Smaller cetaceans such as dolphins and porpoises engage in complex play behavior,

Including such things as producing stable underwater toroidal air core vortex rings,

Or bubble rings.

The two main methods of bubble ring production are rapid puffing of air into the water and allowing it to rise to the surface,

Forming a ring or swimming repeatedly in a circle,

And then stopping to inject air into the helical vortex currents thus formed.

They also appear to enjoy biting the vortex rings so that they burst into many separate bubbles and then rise quickly to the surface.

Whales produce bubble nets to aid in herding prey.

Larger whales are also thought to engage in play.

The southern right whale elevates its tail fluke above the water,

Remaining in the same position for a considerable time.

This is known as sailing.

It appears to be a form of play and is most commonly seen off the coast of Argentina and South Africa.

Humpback whales also display this behavior.

Self-awareness appears to be a sign of abstract thinking.

Self-awareness,

Although not well defined,

Is believed to be a precursor to more advanced processes such as metacognition reasoning,

Thinking about thinking,

That humans exploit.

Dolphins appear to process self-awareness.

The most widely used test for self-awareness in animals is the mirror test,

In which a temporary dye is placed on an animal's body and the animal is then presented with a mirror.

Researchers then explore whether the animal shows signs of self-recognition.

Critics claim that the results of these tests are susceptible to the clever Hans effect.

This test is much less definitive than when used for primates.

Primates can touch the mark or the mirror while dolphins cannot,

Making their alleged self-recognition behavior less certain.

Skeptics argue that behaviors said to identify self-awareness resemble existing social behaviors,

So researchers could be misinterpreting self-awareness or social responses.

Advocates counter that the behaviors are different from normal responses to another individual.

Dolphins show less definitive behavior of self-awareness because they have no pointing ability.

In 1995,

Martin and Sarracus used video to test dolphin self-awareness.

They showed dolphins real-time footage of themselves,

Recorded footage,

And another dolphin.

They concluded that their evidence suggested self-awareness rather than social behavior.

While this particular study has not been replicated,

Dolphins later passed the mirror test.

Collective decisions are an important part of life as a cetacean for the many species that spend time in groups,

Whether these be temporary,

Such as the fission-fusion dynamics of many smaller dolphin species,

Or long-term stable associations,

As seen in killer whale and sperm whale matrilines.

Little is known about how these decisions work.

Those studies have found evidence,

Messy consensus decisions in groups of sperm whales,

And leadership in other species like bottlenose dolphins and killer whales.

Among cetaceans,

Whales are distinguished by an unusual longevity compared to other higher mammals.

Some species,

Such as the bowhead whale,

Can live for up to three years,

And some species,

Such as the bowhead whale,

Can reach over 200 years.

Based on the annual rings of the bony aortic capsule,

The age of the oldest known specimen is male,

Determined to be 211 years at the time of death.

Upon death,

Whale carcasses fall to the deep ocean and provide a substantial habitat for marine life.

Evidence of whale falls in present-day and fossil records shows that deep-sea whale falls support a rich assemblage of creatures,

With a global diversity of 407 species,

Comparable to other narrative biodiversity hotspots,

Such as cold seeps and hydrothermal vents.

Deterioration of whale carcasses happens through three stages.

Initially,

Organisms such as sharks and hangfish scavenge the soft tissues at a rapid rate,

Over a period of months and as long as two years.

This is followed by the colonization of bones and surrounding sediments,

Which contain organic matter,

By enrichment opportunities,

Such as crustaceans and polychaetes throughout a period of years.

Finally,

Sulfophyllic bacteria reduce the bones,

Releasing hydrogen sulfide,

Enabling the growth of chemoautotrophic organisms,

Which in turn support organisms such as mussels,

Clams,

Limpids,

And sea snails.

This stage may last for decades and supports a rich assemblage of species,

Averaging 185 per site.

Brucellosis affects almost all mammals.

It is distributed worldwide,

While fishing and pollution have caused porpoised population density pockets,

Which risks further infection and disease spreading.

Brucelloseti,

Most prevalent in dolphins,

Has been shown to cause chronic disease,

Increasing the chance of failed births and miscarriages,

Male infertility,

Neurobrucellosis,

Cardiopathies,

Bone and skin lesions,

Strandings,

And death.

Until 2008,

No case had ever been reported in porpoises,

But isolated populations have an increased risk and consequentially a high mortality rate.

The direct ancestors of today's cetaceans are probably found within the Dorodontidae,

Whose most famous member,

Dorodon,

Lived at the same time as Bacillus aureus.

Both groups had already developed the typical anatomical features of today's whales,

Such as hearing.

Life in the water for a formerly terrestrial creature required significant adjustments,

Such as the fixed bola,

Which replaces the mammalian eardrum,

As well as sound conducting elements for submerged directional hearing.

Their wrists were stiffened and probably contributed to the typical build of flippers.

The hind legs existed,

However,

But were significantly reduced in size and with a vestigial pelvis connection.

The fossil record traces the gradual transition from terrestrial to aquatic life.

The regression of the hind limbs allowed greater flexibility of the spine.

This made it possible for whales to move around with the vertical tail hitting the water.

One of the oldest members of the ancient cetaceans is Pachycidas from the middle Eocene of Pakistan.

This is an animal the size of a wolf,

Whose skeleton is known only partially.

It had functioning legs and lived near the shore.

This suggests the animal could still move on land.

The long snout had carnivorous dentition.

The transition from land to sea dates to about 49 million years ago,

With the Ambulocetus running whale also discovered in Pakistan.

It was up to three meters long.

The limbs of this archaeoseed were leg-like,

But it was already fully aquatic,

Indicating that a switch to a lifestyle independent from land happened extraordinarily quickly.

The snout was elongated with overhead nostrils and eyes.

The tail was strong and supported movement through water.

Ambulocetus probably lived in mangroves and brackish water,

And fed in the riparian zone as a predator of fish and other vertebrates.

Dating from about 45 million years ago were species such as Indocetus,

Cuchicetus,

Rhodocetus,

And Androsypheus,

All of which were adapted to life in water.

The hind limbs of these species were regressed,

And their body shapes resemble modern whales.

Protocetidae family member Rhodocetus is considered the first to be fully aquatic.

The body was streamlined and delicate,

With extended hand and foot bones.

The merged pelvic lumbar spine was present,

Making it possible to support the floating movement of the tail.

It was likely a good swimmer,

But could probably move only clumsily on land,

Much like a modern seal.

Since the late Eocene,

About 40 million years ago,

Cetaceans populated the subtropical oceans and no longer emerged on land.

An example is the 18-meter-long Basilosaurus,

Sometimes referred to as Zugladon.

The transition from land to water was completed in about 10 million years.

The Wadi al-Hitan whale valley in Egypt contains numerous skeletons of Basilosaurus,

As well as other marine vertebrates.

Molecular biology,

Immunology,

And fossils show that cetaceans are phylogenetically closely related with the even-toed ungulates.

Whales' direct lineage began in the early Eocene,

Around 55.

8 million years ago,

With early anteodactyls.

Most molecular biological evidence suggests that hippos are the closest living relatives.

Common anatomical features include similarities in the morphology of the posterior molars and the bony ring on the temporal bone,

Bulla,

And the involucre,

A skull feature that was previously associated only with cetaceans.

Since the fossil record suggests that the morphologically distinct hippo lineage dates back only about 15 million years,

Cetacea and hippos apparently diverged from a common ancestor that was morphologically distinct from either.

The most striking common feature is the talus,

A bone in the upper ankle.

Early cetaceans,

Archaeocetes,

Show double castors,

Which occur only in even-toed ungulates.

Corresponding findings are from Tethys Sea deposits in northern India and Pakistan.

The Tethys Sea was a shallow sea between the Asian continent and northward-bound Indian Plate.

Molecular and morphological evidence suggests that artiodactyls,

As traditionally defined,

Are paraphyletic with respect to cetaceans.

Cetaceans are deeply nested within the artiodactyls.

The two groups together form a clade,

A natural group with a common ancestor,

For which the name cetaceodactyla is sometimes used.

Modern nomenclature divides artiodactyla or cetaceodactyla into four subordinate taxa,

Camelids,

Pigs and beckeries,

Ruminants,

And hippos plus whales.

Within cetacea,

The two parvidors are baleen whales,

Which owe their name to their baleen,

And toothed whales,

Which have teeth shaped like cones,

Spades,

Pegs,

And tusks,

And can perceive their environment through biosonar.

Reviews

I was awake till the end with this one as it was really interesting but as is the case with lots of these I fall asleep quite quickly after they’ve finished.