Evolution of Dolphins
- Before Cetacea
- The Rise of Primitive Cetaceans
- Early Odontocetes
- 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
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
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
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
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
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
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
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.