Patterns in Fossils

Question

Look at the figures below. Figure 1 is a basic cladogram of organisms. Figure 2 is a diagram of whale evolution, with the oldest ancestors at the top and the most recent prehistoric whale, the Basilosaurus, at the bottom. Figure 3 is the skeleton of the humpback whale found in oceans today.

FIGURE 1


From bottom to top, point A diverges in two lines. At the end of the left line is a camel. At the end of the right line is a cow. The right line has 3 branches extending from it. At the end of the first branch is a peccary. Just before the peccary, the line has a branch point, B, that extends to a pig. The second branch starts halfway up the right line at point C and extends to a hippo. Half way up the second branch is a branch point, D, that extends to a whale. The third branch starts at point E and starts just before the cow. At the end of the third branch is a deer.

FIGURE 2

From top to bottom, the illustration titled 'Evolution of the Whale' shows the skeletal structures of Pakicetus that lived 49 million years ago, Ambulocetus that lived 48 million years ago, Malacetus that lived 47 million years ago, Dorudon and Basilosaurus that lived 40 to 34 million years ago. Pakicetus had similar structures as a land animal with a long skull and large carnivorous teeth and does not look like a whale. Ambulocetus had short legs and a long body with an elongated skull and carnivorous teeth. The feet flattened, and the tail was longer. Malacetus had webbed feet with long digits. They had an elongated skull with carnivorous teeth and a short but thick tail. Dorudon had tiny hind limbs and a more fish-like body and elongated skull with carnivorous teeth. Basilosaurus had a streamlined fish-like body, elongated skull with carnivorous teeth and the hind limbs were very small.

FIGURE 3

The letters in the diagram represent anatomy that is found in modern humpback whales. Letter A represents the pelvic (hip) bone, letter B represents the vertebrae (backbone), letter C represents the ribs, letter D represents the phalanges (fingers), letter E represents the eye socket, and letter F represents the dorsal fin.

Constructed Response Question
Using Figure 3, identify two structures in humpback whales that suggest whale ancestors once walked on land and describe how the structures changed over time.


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Answer 1

In Figure 3 of the skeleton of the modern humpback whale, two structures that suggest whale ancestors once walked on land are the pelvic bones (label A) and the phalanges (label D).

1. Pelvic Bones (A):

   • Description: The pelvic bones in modern whales are significantly reduced in size and are not used for supporting hind limbs, as they are in terrestrial mammals.
   • Change Over Time: In ancestral whale species like Pakicetus and Ambulocetus, the pelvic bones were more developed and connected to limb structures that were used for walking on land. As these ancestors adapted to an aquatic lifestyle, the pelvic bones shrank and lost their functionality in locomotion, indicating a transition from land to water.

2. Phalanges (D):

   • Description: The phalanges in modern humpback whales are present but are greatly modified. In whales, they form part of the flippers but are not adapted for grasping or walking.
   • Change Over Time: In early whale ancestors, such as Ambulocetus and Malacetus, the phalanges were longer and more robust, functioning as part of limbs necessary for walking on land. Over time, these structures evolved to become shorter and more flattened, transitioning to their role in the flippers of modern whales. This change reflects the adaptation of whale ancestors to a fully aquatic environment where swimming replaced walking as the primary mode of mobility.

These structures highlight the evolutionary history of whales, demonstrating how adaptations to a marine lifestyle have modified their anatomy from terrestrial ancestors.