Biology Now, 2e

Figure 11.6

Skeletons and body sizes of modern whales and fossil ancestors

These reconstructed skeletons from modern whales (top) and various ancestors are in chronological order.

Q1: What is the general definition of a fossil?

Q2: How are the ancestors of modern whales different from their present form?

Q3: What is meant by the term “intermediate fossil” when referring to the fossil record?

Balaena (baleen whale)

Dorudon

Rodhocetus

Ambulocetus

Pakicetus

Indohyus

Indohyus lived on land about 47 mya

and is an extinct whale “cousin,” not a

direct ancestor. Indohyus and

Pakicetus share a common ancestor.

The oldest whale ancestor,

Pakicetus, lived on land 50 mya.

By 40 mya, Dorudon

was fully aquatic.

In Rodhocetus, the body was stream-

lined, and the front legs were shaped

like flippers for swimming 48 mya.

It took roughly 15 million years for whale ancestors to make

the transition from life on land to life in the water. Compare

the whale ancestors with Balaena, a modern baleen whale.

Relative body size, to scale, is indicated by the blue shadows

next to each animal, or by overlaying as in the modern whale.

Ambulocetus had strong, well-developed legs

and was probably semiaquatic, living at the

water’s edge and hunting 48–50 mya in much

the same way that a crocodile does today.