Transitional Fossils
Multicellular life began in the ocean, and transitioned to land. Some species, such as the whale, returned back to the water. How do we know this was the case? What was this transition like? We can use transitional fossils to find the answers to those questions.
A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.
For example, we know that aquatic life, such as fish, were around before terrestrial life. For a long time, we knew this was the case, but did not know exactly how the transition from water to land was made. That is, until we found the fossil remains of tiktaalik.
A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.
For example, we know that aquatic life, such as fish, were around before terrestrial life. For a long time, we knew this was the case, but did not know exactly how the transition from water to land was made. That is, until we found the fossil remains of tiktaalik.
Tiktaalik was a transitional species, bridging the gap between land and water. It had some features of land animals, and some of aquatic animals. Tiktaalik has lobed (foot-like) fins with ankle bones. Its skull is shaped similar to an amphibian or reptile, with its eyes on the top of its head. Tiktaalik has scales like a fish, but lung-like structures to breathe.
Transitional fossils are critical when tracing the evolution of whales from land mammals. Scientists have found fossil remains of many transitional species that bridge the gap between land mammal and modern cetacean.
The oldest transitional species discovered by scientists is called Pakicetus. Pakicetus, which lived about 52 million years ago, looks like a wolf or dog, but has many adaptations that caused it to successfully live in water for part of its life.
The oldest transitional species discovered by scientists is called Pakicetus. Pakicetus, which lived about 52 million years ago, looks like a wolf or dog, but has many adaptations that caused it to successfully live in water for part of its life.
Pakicetus had the following characteristics:
Scientists found that characteristics in ancestral whale fossils changed drastically over time.
The following was seen:
The movement of nostrils to the top of the head, eventually turning into a modern-day blowhole for whales.
- Dense bones (in order to successfully stay underwater)
- Eyes pointed to the top of its head (for hunting purposes)
- Fused earbones (for enhanced underwater hearing)
- Partially webbed back feet
Scientists found that characteristics in ancestral whale fossils changed drastically over time.
The following was seen:
The movement of nostrils to the top of the head, eventually turning into a modern-day blowhole for whales.
The transition of legs. Toes became longer and more fin-like, and hind legs shifted back, and eventually shrank into a vestigial organ.
Below are pictures of transitional whale forms. Use these pictures, and your packet, to arrange the forms in a logical order, and explain why you have arranged them in that way.
After you've done this, move on to DNA Evidence to see how scientists use genetics to determine the ancestry of whales!