This is a good time to stop and try to explain that this story, of
plants and animals of the past, has a firm foundation in today’s
facts—it is not a fantasy.
The methods by which geologists and paleontologists have established the
age, climate, and life of Morrison time cannot be described for you here
in detail. To attempt such a description would require too much space
and would probably seem dull to most readers. Perhaps the best approach
is to describe some features and explain how they contribute to our
knowledge.
The rocks that were deposited here in Morrison time tell us much of the
story. The sandstones were once stream sandbars or perhaps beaches
around lakes. The shale, siltstone, and clay were muddy stream or lake
bottoms. The discontinuous ledges of conglomerate probably represent
gravel bars formed during flood stages or at places where the stream
currents were very swift.
Just rocks you may say—but look closely. A piece of sandstone contains
grains of sand that differ from each other in size, shape, and
composition. Frequently these characteristics point to the source of the
sandstone and tell something about the conditions at the time it was
deposited. Chunks of black material are examined closely and prove to be
charcoal—carbonized remains of plants.
Microscopic examination of clay fragments reveals shards of volcanic
glass and ash that speak of active volcanoes. Sometimes these clays bear
the carbonized imprints of delicate plants that long ago sank to the
bottom of some lake or stream where they were buried and fossilized.
The fossils themselves are most important in reconstructing conditions
of the past. We find the shells of fresh-water clams in the sandstones
with dinosaur bones. Crocodile bones are also common. We are reasonably
sure, then, that these deposits of sand and mud were formed in rivers
and lakes when the climate was mild.
We reason by analogy. For example, fossil plants and animals have
counterparts or descendants in the world of today. We assume, in the
absence of contrary evidence, that the fossil animals lived like their
present-day counterparts. Although no birds, mammals, or pterosaurs have
been found in this quarry, they were probably living here with the
dinosaurs. It is possible, in fact probable, that some modern animals
and plants live in different environments than did their Morrison
ancestors and relatives but we have no way of knowing which ones they
were. We can only take the data available, arrange them as logically as
possible, and continue the search for more. Some may scoff at such
methods of reasoning yet they do provide good results. What other
methods can be used when the world under investigation lies millions of
years in the past?