So, the story told in both sections is: 1) the blue fossils lived first, 2) the green fossils appeared at the same time as the last blue fossils (so they co-existed for a little while), 3) the blue fossils disappeared, 4) the green fossils disappeared, and 5) the red fossils appeared last. Notice that most biozones contain only one characteristic fossil, but that we can also define one containing two fossils (the blue oyster and green belemnite) bounded on the appearance of one and the disappearance of the other.
If we assume that the same biozones in different areas are the same age (i.e., that the same fossils appeared and disappeared in both areas at about the same time; again, I'll discuss this assumption below), we have now learned a little bit more then we knew before.
This time scale divides up the history of the Earth into a few really big chunks of time (eons), which are in turn subdivided into increasingly small units of time (eras, periods, and epochs)..based almost entirely on fossils.
Rather, modern apes and humans (which are apes too..with it) are descended from ape species which are now extinct.
Fossil apes we discover which live millions of years ago are different from modern species, and we can distinguish them looking at their bones (especially their teeth).
Area C and Area D respectively seem to be showing us rocks older and younger than we have seen before.
Also notice that in Section D, there is a yellow coral (the "C"-shaped fossil) which occurs with the highest green belemnites and lowest red snails.
We didn't see this yellow coral in Areas A and B, so it may have lived only in Area D.
Let's now use biostratigraphic correlation to tie all four sections together: Notice that we can recognize two different zones in Area D using the yellow coral using appearances and disappearances, but that we have to approximate where the boundaries to these zones are in Area B.
Although the numeric dates (which I'll talk about in the next post) shift around a bit, the basic framework, based on fossils, has changed little in the past 150 years.
Geologists and paleontologists primarily use the fossils of marine organisms like corals, cephalopods (squid and their relatives), and microscopic plankton to correlate strata; marine organisms tend to have an easier time dispersing across the entire world since the oceans and seas are almost all connected, and as a result the same (or at least, closely related) species may be found in completely different parts of the world.
Let's talk briefly about the assumption on which biostratigraphic correlation in based: how can we be sure that when we find the same fossils in different areas, that they are really the same age? This is also part of the answer to the common creationist question: "if we are descended from apes, why are there still apes around?
After all, don't we have organisms around today that are "living fossils" and have been around for millions of years? " The answer is "because we aren't descended from modern day apes".
In the last blog, I discussed the Law of Superposition.