National Geographic, Nov. 2001

MYSTICETES (baleen whales) above

ODONTOCETES (toothed whales) belowWhat are the characteristics of modern whales?

MYSTICETES – Modern Baleen Whales

What is this structure?

Is it functional?

What does the presence of this structure tell us about whale evolution?

CLUE #1 ~36 mya

Dorudon 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.

CLUE #2 ~55 mya

Mesonychlids

Extinct land mammals with whale-like teeth

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

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

CLUE #3 ~50 mya

Pakicetus Skull and teeth only

Found by Philip Gingerich in Pakistan, 1983

Philip Gingerich

CLUE #4 ~37 mya

Basilosaurus Hind leg found by Gingerich in Egypt, 1990

Philip Gingerich

CLUE #5 ~46 mya

Rodhocetus kasrani 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.

Philip Gingerich

CLUE #6 ~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.

Hans Thewissen with colleague Ellen Williams

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

CLUE # 7 Between 1983 and 1997 DNA and protein “finger printing” studies indicate artiodactyls such as deer, pigs, cattle, and hippos are closely related to whales.

Artiodactyls are mammals with an even number (2 or 4) of toes

Hippos

How many toes can you count in these artiodactyls?

CLUE #8 ~47 mya

Rodhocetus and Artiocetus 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?

Artiocetus

Rodhocetus

Rodhocetus

Astragalus Ankle Bones

(in circles)

Comparison of Ankle Bones

astragali above

calcanei below MesonychiansPrimitive

whales Artiodactyls

Fossil Hind Limb of Rodhocetus Showing Ankle Bones

1 Fossil Mesonychid astragalus 2 Modern artiodactyl astragalus 3 Fossil Rodhocetus astragalus

FAMILY TREE OF CETACEANS shows the descent of modern odontocetes and mysticetes from extinct ancestors

Branching diagrams illustrate various hypotheses of the relationship of whales to other mammals

Hippos = Hippopotamids Mesos = Mesonychids Artios = Artiodactyls other than hippos

LOSING THEIR LEGS

Four-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