Fossil dating activity
Of course, this is a very general pattern and does not explain why certain creatures that lived on the bottoms of oceans, like trilobites, make their first appearance in the Cambrian (505-540 Ma) while other creatures that live on ocean bottoms, like crabs and lobsters, don't appear until the beginning of the Cretaceous (65-145 Ma).
Why would creatures that would seem to share the same general environment while alive be so widely separated in the fossil record if they did indeed live at the same time and in pretty much the same location?
However, science is suppose to do just that - challenge ideas. Because often what seems obvious initially does not turn out to be true.
The scientific method is all about testing and retesting theories since no theory is ever proven by science.
Generally, species that use a wide range of resources or tolerate a variety of abiotic conditions can establish more populations in more places than comparable species with relatively narrow niches (Brown, 1995; Cook and Quinn, 1995).
Differences in the ability of species to distribute themselves across space have distinct consequences for the structure of communities.
Sites that encompass a greater area tend to have more species (Rosenzweig, 1995).
Many of the layers also show a certain specialization.
Some layers contain mostly fish fossils while others contain land-dwelling creatures such as dinosaurs.