Geology Reference
In-Depth Information
Muscular support:
Building an exoskeleton provides a framework for muscles to at-
tach themselves to. This function is the same one your internal skeleton provides.
Because an exoskeleton provides structure for muscle attachments, it allows an
organism to grow larger than it would without such a supporting structure.
Protection from predators:
Possibly the most important advantage a shell
provides is protection from other animals that may attack and devour a soft-bod-
ied creature.
While all of these are great reasons to build an exoskeleton, scientists are not
certain which advantage started the evolutionary trend toward external hard
parts. Evidence in the fossil record of creatures with damaged shells indicates
that they were being hunted, attacked, and probably eaten by predators. For some
scientists, this fact is enough to conclude that predation was the driving force for
the evolution of exoskeletons at the beginning of the Paleozoic.
When roly polys ruled the world
The first
shelly fauna,
or animals with exoskeletons, were tiny creatures; their shells
were only a few millimeters in size. But it didn't take long for other animals to follow the
shelly trend. The most famous creature of the Paleozoic — possibly its mascot — is the
trilobite.
Burgess Shale
The Burgess Shale is a rock formation in British Columbia containing preserved remains of creatures
from the Cambrian period about 540 million years ago. It was discovered in 1909, but its importance
wasn't realized until the 1960s. More than 60,000 fossils were recovered, many of which were arthro-
pods (trilobites and similar organisms), but also specimens of other animal groups from the Cambrian
Explosion. The best part of the Burgess Shale is that the fine-grained sediments of the shale preserved
detailed features as well as complete specimens of soft-bodied organisms that would not have been
preserved under other conditions. Geologists think an underwater mudflow covered the seabed and all
the creatures in a sudden event, providing a snapshot in time of an undersea community of organisms
from 540 million years ago.
You can visit the Burgess Shale site, now considered a World Heritage site, in Yoho National Park, Brit-
ish Columbia.
