Orthoceras fossils
are dated back to the Silurian Age 400 + million years ago and are the
earliest recognizable animals. They are ancestors to the modern day
squid. Even though they were among the earliest forms of life, this
class of nautiloid is still considered one of the most intelligent forms
of ocean life. The Orthoceras ranged in size from a few inches in length
to over six feet! They could swim as well as crawl on the ocean floor.
By filling the chambers in their shells with air they could float through
the seas propelling themselves by squirting jets of water. As they died
their shells accumulated upon the ocean floor, then covered by sediments
they transformed into stone over the ages. Having a long straight shell
their fossils have an iridescent gemlike quality when polished. These
Orthoceras specimens are from the Atlas Mountains in Morocco.
Although relatively simple
in form, ammonites are extremely rich in the information they
yield to scientists. These extinct marine animals, which thrived in
the Paleozoic and Mesozoic Eras, some 400 to 65 million years ago, were
cephalopods, and are thus related to squid, octopus, cuttlefish, and
the chambered nautilus. Ammonites were able to swim, thanks to the unique
construction of their shell, which was divided into a series of air
chambers. The air in the chambers provided buoyancy for the animal to
float; like modern cephalopods, they probably moved through the water
using jet propulsion. As different species of ammonites lived during
different time periods, scientists can use these animals to determine
the relative age of the rocks in which their fossils are found ("index
fossils"). Because ammonites lived exclusively in marine environments,
their presence also indicates the location of prehistoric seas.