A clam dies on a river delta and is covered by fine silt within two days. A second clam dies on an open beach and lies exposed for months. Why is only the first likely to become a fossil?

A biologist studies which parts of a dead deer might survive long enough to be preserved in sedimentary rock. Which part is most likely to be fossilized?

A fallen tree trunk is buried in volcanic ash, and for thousands of years groundwater carrying dissolved silica seeps through the wood. The result is a stone log that keeps the wood's cell pattern. How did it form?

Only a tiny fraction of the organisms that have ever lived left any fossil at all. Which explanation best accounts for that?

Hardened mud beside an ancient lake preserves a trackway of three-toed dinosaur footprints, but no bones are found with it. What can this trace fossil show that a bone could not?

An undisturbed road cut shows four layers. From the top down they are: P with mammoth bones, Q with early horse teeth, R with ammonite shells, and S with trilobites. Which statement is supported?

The cliff shown is undisturbed sedimentary rock. A fossil fern is collected from the lowest layer and a fossil pine cone from the highest layer. What can a geologist conclude?

The same species of small trilobite turns up in rock layers on three different continents. Why is a species that was widespread but existed for only a short time useful for relative dating?

In the cliff shown, the sedimentary layers are tilted at a steep angle instead of lying flat. What must have happened here?

Two different fossil animals are collected from the same single rock layer of the undisturbed cliff shown. What is best supported by that?

The rock layers in the cliff shown have been folded and partly turned over by mountain building. Why can a geologist not simply assume the top layer here is the youngest?

The fossil shown preserves feathers and a wishbone alongside teeth in the jaws, clawed fingers on the forelimbs and a long bony tail. What does this combination of features best support?

Tiktaalik roseae comes from rock about 375 million years old. It has fish scales and gills, and also a mobile neck, a flattened skull and sturdy fins containing shoulder, elbow and wrist bones. What does it provide evidence for?

Fossil whales such as Basilosaurus, from rock about 40 million years old, have small hind limbs with hip, thigh and foot bones that were far too weak to carry the animal's weight. What do these structures show?

Fossils of Hyracotherium, a dog-sized browser with four toes on each front foot, come from rock about 52 million years old. Fossils of the one-toed modern horse Equus come from much younger rock. Which statement is supported?

A bed of volcanic ash lies between two fossil-bearing rock layers, and a geologist needs the ash bed's age in years, not just its position in the sequence. Which measurement gives that figure?

Carbon-14 has a half-life of about 5,730 years and is useful back to roughly 50,000 years. Potassium-40 has a half-life of about 1.25 billion years. Which sample should be dated with potassium-40?

The fossil record is incomplete partly because soft-bodied organisms such as jellyfish and worms are rarely preserved.

Scientists have not found fossils for every step between two groups they think are related. What does such a gap in the record show?

A rock layer in Wyoming dated to about 52 million years ago contains fossils of freshwater fish, turtles and crocodile relatives. What does this evidence best support about the area at that time?