National Geographic, Nov. 2001

What are the characteristics of modern whales? MYSTICETES (baleen whales) above ODONTOCETES (toothed whales) below What 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.

Extinct land mammals with whale-like teeth 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

Pakicetus Skull and teeth only CLUE #3 ~50 mya Pakicetus Skull and teeth only Found by Philip Gingerich in Pakistan, 1983 Philip Gingerich

Basilosaurus Hind leg found by Gingerich in Egypt, 1990 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?

September 2001 The journal Science publishes whale evolution articles by Gingerich, Theweissen, and Kenneth Rose. Rodhocetus Look closely. What do you see in this artist’s depiction of Rodhocetus?

Astragalus Ankle Bones (in circles) Rodhocetus Astragalus Ankle Bones (in circles) Artiocetus Rodhocetus

Comparison of Ankle Bones astragali above calcanei below Primitive whales Mesonychians 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 Basilosaurus (~37 mya) had reduced but well-developed hind limbs Today’s sperm whale has vestigial hind limbs

NASAL DRIFT Pakicetus (~50 mya) had a land mammal’s nostrils at end of the snout Rodhocetus (~47 mya) swam the seas with nostrils intermediate between those of its ancestors and modern whales A modern gray whale’s blowhole is at the top of its head

HEARING AIDS Ambulocetus (~48 mya) heard directly through its ears In Basilosaurus (~37 mya) sounds were transmitted to middle ears as vibrations from lower jaw In modern toothed whales the melon directs sound at an object, and the lower jaw receives echoing reply