MYSTICETES (baleen whales) above

ODONTOCETES (toothed whales) below

What are the characteristics of modern whales?

How do we know that whales have evolved?

Fossil evidence, which includes:

1) Whale ancestors with major structures that are homologous to tetrapods.

So what does a “homologous structure” mean?

Homologous structures are characteristics which are shared by related species because they have been inherited in some way from a common ancestor.

Vertebrate Forelimb Homologies

What pattern shows that they came from a common ancestor?

2) Evidence in LIVING WHALES of vestigial structures related to their ancestry.

What is a vestigial structure?

A structure or organ is vestigial if it has diminished in size or usefulness in the course of evolution.

Such features, though no longer useful (as far as we can tell), are presumed to have been useful in ancestral species.

Areas in red indicate sites where fossil whales have been found.

How does this relate to Geologic Uniformitarianism?

The shallow Tethys Sea is the likely site of early whale evolution

MYSTICETES – Modern Baleen Whales

What is this structure?

What does the presence of this

structure tell us about whale evolution?

Is it functional?

Dorudon ~36 mya

Scientists have found many fossils of primitive whales like Dorudon dated between 25 and 45 mya. No modern whales have been found in this time period.

Basilosaurus ~37 mya

Hind leg found by Gingerich in Egypt, 1990

Rodhocetus kasrani ~46 mya

Found by Gingerich in 1993. Has well-developed hip bones and large pelvis. No limb bones found. Vertebrae suggest strong tail muscles and flukes for swimming.

Rodhocetus and Artiocetus ~47 mya

In 2001 in Pakistan Gingerich finds important new whale fossils that include leg and foot bones.

What question might the foot bones answer?

Rodhocetus

September 2001

The journal Science publishes whale evolution articles by Gingerich, Theweissen, and Kenneth Rose.

Look closely. What do you see in this artist’s depiction of Rodhocetus?

~48 mya

Ambulocetus Found by Hans Thweissen in Pakistan in 1994. Fossil has teeth similar to Mesonychids and early whales. Each toe on the on the huge hind feet has a tiny hoof.

Ambulocetus natans - The 49-million year old walking, swimming whale had long hind and front legs, but had teeth and ears like more modern whales.

Hans Thewissen (pictured) says the whale “was on the fence between land and sea”.

Ambulocetus

Pakicetus ~50 mya

Skull and teeth only

Found by Philip Gingerich in Pakistan, 1983

Mesonychlids ~55 mya

Extinct land mammals with whale-like teeth

LOSING THEIR LEGSFour-legged Ambulocetus (~48 mya) could walk and swim

Today’s sperm whale has vestigial hind limbs

Basilosaurus (~37 mya) had reduced but well-developed hind limbs

NASAL DRIFT

Pakicetus (~50 mya) had a land mammal’s nostrils at end of the snout

A modern gray whale’s blowhole is at the top of its head

Rodhocetus (~47 mya) swam the seas with nostrils intermediate between those of its ancestors and modern whales

HEARING AIDS

Ambulocetus (~48 mya) heard directly through its ears

In modern toothed whales the melon directs sound at an object, and the lower jaw receives echoing reply

In Basilosaurus (~37 mya) sounds were transmitted to middle ears as vibrations from lower jaw