Geoscience Reference
In-Depth Information
Archibald to be a result of a drop in sea level
36
]; why should the ter-
restrial realm be 'fragmented' by increasing the habitable area? An
increase in land should provide opportunities and space not previ-
ously available to land-dwelling organisms."
3
7
Peter Ward has said,
"We just do not know how a regression [drop in sea level] could kill
anything."
3
8
Of the theories on Benton's list, only massive volcanic activity
and extraterrestrial events meet Raup's criteria of being global,
infrequent, and lethal. Dinosaur specialist Dale Russell, reviewing
the evidence in 1979, thought volcanism was unlikely to provide
the answer because it tends to be gradual and episodic, rather than
sudden like the K-T extinction.
3
9
In Chapter 6 we saw how the
details of the Deccan eruptions fail to corroborate the volcanic
alternative. Now it is time to turn from theory to the dinosaur fossil
record.
D
INOSAUR
F
OSSIL
E
VIDENCE
We wish to learn how the dinosaurs died, not how they lived; there-
fore our interest is in their last few million years during the late Cre-
taceous. How many dinosaur species were living then, where are
their remains, and what do they tell us? According to paleontologist
Peter Dodson, only about 2,100 articulated bones of dinosaurs have
been collected, and they span 160 million years.
4
0
If spread evenly,
we would have one specimen for each 75,000 years, but in fact the
discovered remains of dinosaurs are highly clustered in time and
space. All the
Tyrannosaurus rex
specimens, for example, come from
Montana and the Dakotas. Therefore in the rock record there are
spans of millions of years during which we know dinosaurs lived but
of which we have no trace. Dodson reported that 336 recognized
dinosaur species have been identified, but that nearly 50 percent are
known only from a single fossil specimen. The 336 species belong
to 285 genera (remember that genus—plural, genera—is the next
taxonomic grouping above the species level), of which over 70 per-
cent occur only in one rock formation. Paleontologists have learned
that a typical genus has several species and that the species:genus
ratio therefore is usually well above 1:1. To find it so close to 1:1
(336:285) for the dinosaurs indicates that sampling has barely
scratched the surface. Recent experience confirms this conclusion,
for new dinosaur discoveries seem to pop up in the press every few
months. Surely many more dinosaur genera await discovery. Dale
Russell estimated that we have found only about 25 percent of the
