Encyclopedia of Evolution.pdf - Online Reading Center

It is usually easy to tell which structures in sedimentary
rocks are fossils and which are inorganic in origin, but not
always. There was some controversy about whether a “shell”
that was found in a Russian fossil deposit was actually a shell
or a percussion fracture from the drill bit that extracted the
sample.
Unfortunately for researchers, fossils are usually found
in fragments and have to be put back together. Examples
include:
• Plants often shed their organs separately, or are torn apart
during fossilization. Thus it is often impossible to tell
which leaf grew in connection with which trunk.
• The same thing can happen to animals. The largest animal
of the Cambrian seas, Anomalocaris, was formerly
described as three separate animal species. The animal’s
body had been described as a sea-cucumber; its mouth had
been described as a jellyfish; and its front legs had been
described as headless shrimp. The true identity of these
fragments was revealed only when a fossil was found that
had these fragments together.
• One of the most abundant kinds of Paleozoic fossils was
the conodont (cone-tooth) (see fishes, evolution of).
Everybody suspected that it was an animal tooth but
nobody could find the animal. In 1983, after many decades
of research, a conodont was found in association with a
soft-bodied vertebrate, and then only from a faint minnowlike
depression.
• When fossils are found individually, it is often impossible
to determine which of them belonged to the same species.
Juvenile dinosaurs have sometimes been classified as a separate
species from the parents. When whole herds or populations
of dinosaurs are found fossilized together, it may be
possible to determine which ones were the juvenile forms.
There are a few huge deposits of fossils, such as the hills
of diatomaceous earth near Lompoc, California, and the white
hills of Dover in England, which consist of billions of shells
of tiny single-celled organisms. A huge graveyard of dinosaur
bones, turned to stone, can be seen at Dinosaur National
Monument in Colorado. Numerous skeletons of rhinoceroses
and horses that died from breathing ash from a volcanic eruption
12 million years ago are preserved, in exquisite detail and
still in their original arrangement, at Ashfall Beds State Historic
Park in Nebraska (see figure on page 167).
Trace fossils are the preserved evidence of the activities
of the organisms. This includes worm burrows and animal
footprints. Worm burrows at the beginning of the Cambrian
period are the earliest signs of the rapid evolution of animal
life in the Phanerozoic Eon (see Cambrian explosion). Dinosaur
footprints have allowed the analysis of how large the
dinosaurs were and how fast they ran, which has fueled the
suggestion that some of them, at least, were warm-blooded.
Perhaps the most convincing evidence that Australopithecus
afarensis walked on two legs was the discovery, by Mary
Leakey, of a trail of remarkably human footprints produced
by this species (see Leakey, Mary; australopithecines).
Fossils can be used to reconstruct the climate in which
the organisms lived.
fossils and fossilization
• Plants with large leaves today grow in moist areas, and
plants with smaller leaves in drier areas. Serrated leaf margins
allow modern plants to cool more efficiently than
plants with smooth margins, without the evaporation of as
much water. The study of leaf shapes can help reconstruct
ancient temperature and moisture conditions.
• The pollen of seed plants has an outer covering that is very
resistant to decay, and usually each genus of plant has its
own visually distinguishable kind of pollen. Even when all
other parts of a plant have decomposed, the pollen (though
dead) may still be recognizable. Pollen from mud deposited
during the most recent ice age, for example, indicates
that spruce trees (now found only far north and on mountaintops)
grew as far south as Oklahoma, and that much
of what is now Amazonian rain forest was grassland. The
study of ancient pollen is called palynology.
The isotope ratios of fossils may also help determine the
climate in which they grew (see isotopes).
These rhinoceroses died million years ago and were buried in volcanic
ash in what is now Ashfall Beds State Historic Park in Nebraska. It is rare
for such fossils to be found with all the bones still articulated, as in these
specimens. (Photograph by Stanley A. Rice)