Ajg 1 MzY5OTcxOTM5/details (a) Scientists found fossilised shells of one species of animal in the rock layers of a cliff The
age of each layer (A-E) is shown
Age of layer
in millions
of years
Shell
2
A
slit
5
B
2
c
8
D
E
Use the scale bar to give the diameter of the oldest shell
10
Diameter
mm
Describe two ways that the shell evolved (changed) over time
[2]
Species that fail to evolve may become extinct What is the meaning of the term
extinct
11
Scientists think that this species became extinct about 2 million years ago Give the
evidence in the diagram that supports this idea
111
Species evolve by natural selection
Gve the name of the scientist who first described evolution by natural selection
-
.

While it may seem obvious today, most people at the time did not believe that fossils were once part of living creatures. The reason was that the fossils of clams, snails, and other marine animals were found in tall mountains, miles from any ocean. Two schools of thought explained these fossils. Some religious writers believed that the shells were washed up during the Biblical flood. But this explanation could not account for the fact that fossils were not only found on mountains, but also within mountains, in rocks that had been quarried from deep below Earth’s surface. Seeking an alternate explanation, other writers proposed that the fossils had formed within the rocks as a result of mysterious forces. In other words, fossil shells, bones, and teeth were never a part of a living creature!

Steno had other ideas. For Steno, the close resemblance between fossils and modern organisms was impossible to ignore. Instead of invoking supernatural forces to explain fossils, Steno concluded that fossils were once parts of living creatures. He then sought to explain how fossil seashells could be found in rocks far from any ocean. As in the Tyrannosaurus rex Figure 11.10, fossils resemble living organisms.

Superposition of Rock Layers

Steno first proposed that if a rock contained the fossils of marine animals, the rock was formed from sediments that were deposited on the seafloor. These rocks were then uplifted to become mountains. Based on those assumptions, Steno made a remarkable series of conjectures that are now known as Steno’s Laws.

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