Travel Reference
In-Depth Information
strange. The food chains as represented in the fossil record are not chains at all,
for on Gabriola the giant clams are the
only
fossils. There were surely many soft-
part organisms that left no fossil record, for there are many other signs of life,
such as abundant trackways and trails, but other than the very rare ammonite
fossils, it is a land of inoceramids. What is the nature of the trophic pyramid
here? Perhaps the inoceramids fed on plankton and thus were the equivalent of
the shallow-water clams. But there is a big problem with this scenario: The in-
oceramids appear to have lived at such great depths that they would have
starved to death. There is no plankton pasture at the bottom of the sea.
A great deal of paleontological research has addressed this question,
and the answer came from quite unexpected sources. The clues that helped
us solve the mystery of the inoceramids came from two separate discoveries,
one an act of genius, the other brought about completely by chance.
Taking the temperature of ancient seas
The act of genius occurred in Chicago in the early 1950s and was the cre-
ative work of a Nobel laureate from the University of Chicago, the chemist
Harold Urey. Urey's discovery derived from the study of isotopes. An isotope
is one of two or more atoms of a single element whose nuclei have the same
number of protons but different numbers of neutrons. In an earlier chapter
we saw how important the breakdown of radioactive isotopes is in geological
age determination. Other isotopes have proved equally useful in a variety of
geological investigations dealing with ancient environments. One of the
most useful tools is oxygen isotopic ratios. Urey discovered that by measur-
ing (with a mass spectrograph) the ratio of the very rare isotope oxygen-18
to the far more common oxygen-16 from calcareous shells, he could deduce
the temperature at which the shell formed. The warmer the temperature of
shell formation, the less oxygen-18 there was in the shells. As long as the
shell has not been altered, the same types of analysis can be performed on
fossil shells. Urey had devised a way to measure the temperatures of the an-
cient sea.
