Learn About Dolphin Evolution

Whales, dolphins, and porpoises are all members of the scientific order Cetacea.  They are divided into three suborders: 

1) Archaeoceti (extinct whales)

2) Odonotoceti (toothed whales)

3) Mysticeti (baleen whales)

Today's living whales, both toothed and baleen, have unusual looking skeletons.  Why do they have finger bones locked inside their pectoral fins  . . . yet have no bones whatsoever within their dorsal fins or tail flukes? 

Their bodies suggest a complex evolutionary history - and the fossil record demonstrates this to be true.  Archaeological evidence reveals that whales and dolphins evolved through time from four legged animals that were initially semi-aquatic, then mostly-aquatic, and finally to fully-aquatic marine mammals.

Pakicetus

The original forerunner is named Pakicetus and lived about 50 million years ago.  In those days, the region of Earth known now as the Middle East was then a vast marshland bordering a shallow sea.  Along the rivers and estuaries, Pakicetids had learned to feed on small fish.  This species was a member of a group of animals referred to as artiodactyls.   

Modern artiodactyls, (also called even-toed ungulates), include deer, sheep, cattle, and pigs.  Chromosomes, blood composition, fetal blood sugar, shape of the uterus, and structure of the tooth enamel are common traits shared by both whales and these ungulates.  DNA evidence links the hippopotamus as the ungulate most closely related to whales.

As Pakicetids perfected their fishing skills over millennia, natural selection favored the subtle adaptations that improved these animals’ foraging and feeding success.  Eventually succeeding generations would inherit specific traits that allowed for better swimming ability, as well as eyes and ears adapting to function better underwater.

Evolution can cause whole populations of animals to change genetically over long periods time.  This results in speciation, where one species can gradually split into two or more species and slowly give rise to new groups of animals.

50
Millions of Years
Ambulocetus

Evolution can cause whole populations of animals to change genetically over long periods time.  This results in speciation, where one species can gradually split into two or more species and slowly give rise to new groups of animals.    

This seems to be the case with another family of semi-aquatic mammals called Ambulocetids which existed about 48 million years ago.  They were much better swimmers than the Pakicetids due to the fact that their feet were webbed, legs were shorter, and their backbones (vertebrae) were adapted for undulating in an up and down motion in sync with their hind legs - swimming in a fashion comparable to today's otters.  Changes in the lower jaw and ear had enhanced their ability to hear underwater. 

48
Millions of Years
45 M
Protocetid

Around 45 million years ago, members of the Protocetid family were so well adapted to an aquatic life, that they were probably only coming ashore for reproduction and nurturing their young offspring - much the way seals and sealions do today.

Other changes that had taken place were the loss of hair, the development of insulating blubber, the ability to hear underwater, the eyes and kidneys adjusting to salt water, and the development of nasal plugs to keep water out of their airways while diving.  Nostrils were shifting further atop a skull that was becoming longer and narrower.

Hind legs were becoming much smaller and some species were likely to have been swimming through the water with tails similar to those of modern whales.  Called the flukes, these paddle-like structures provided the animals with a powerful way to propel themselves. 

Flukes are made of a fibrous connective tissue which doesn't fossilize as bone does  . . . so with only bones for us to study, it's difficult to know exactly when flukes first appeared and how their form may have evolved over time.       

45
Millions of Years
Dorudontid

By 40 million years ago, ancient whales were fully aquatic and propelled exclusively by flukes.  Flattening of the final few vertebrae indicate a foundation to which the connective tissue of the flukes had evolved.  The Dorudontids, for example, not only had flukes but their small pelvic bones were unattached to the backbone, and their tiny hind limbs could not serve any swimming function.  They may have appeared as small hind fins.  

40
Millions of Years
Squalodontid

33 million years ago, Squalodontids had a body looking very much like modern toothed whales.  The shape of their skull tells us that they were able to direct sound ahead of themselves - a prerequisite for echolocation.  Combined with enhanced underwater hearing, these animals were able to find food, keep track of each other, and avoid predators using reflected sound.  Echolocation has since become highly refined in modern toothed whales, (Odontocetes).

33
Millions of Years
Aetiocetid

While early Odontocetes were developing echolocation, a new group of whales was branching away from their toothy cousins and would never develop sonar.  These would come to be known as the baleen whales, (Mysticetes).   Baleen is a tough material that grows downward from their upper jaw and is used to filter small fish and crustaceans out of the sea.  

One of the earliest family groups were called Aetiocetids.  33 million years ago, these whales indicate how small baleen plates evolved and grew in the spaces in between teeth - eventually replacing teeth completely as we see in modern Mysticetes.

33
Millions of Years
15 M
Kentriodontid

Both toothed and baleen whale groups expanded and gave rise to multiple species over the next several million years - over 80 different species today!  At 15 million years ago, Kentriodontids closely resembled various modern dolphins.  

15
Millions of Years
Delphinidae

Today, we have 36 different species in the dolphin family - called Delphinidae.  

2016
Today