Since land floras and faunas had not yet become conspicuous the fossil
record for this period is limited to those areas which were invaded by
the sea. Apparently there was no such invasion of the present Colorado
region, for rocks of this age are not in evidence. If they exist at all
they are restricted to localized districts which are deeply buried under
sediments of later periods. There may have been no Silurian deposition
in this area, or such rocks may have been produced only to be destroyed
by elevation and consequent subjection to weathering and erosion during
a long interval of time, in which they were completely removed. In the
region of the Colorado Rockies there is no evidence of returning seas
until late Devonian time.
In other parts of the world, however, there was extensive deposition of
rock-making sediments in seas which were inhabited by algae and
invertebrates of the types previously described. Among the common
animals of the time there were still numerous species of brachiopods,
trilobites, corals, crinoids, and bryozoans. In addition to the
primitive cone-shaped, cup corals there were several advanced types but
the habit of building large reefs was not yet established.
“Sea scorpions,” really large crustaceans, flourished during Silurian
time, and late in the period there appeared a race of true scorpions
which lived on dry land or between high and low tides along the
seashore. These were smaller and much like modern descendants but
probably they did not wander far from the ocean shores where an
abundance of food was available. These little scorpions, the largest
measuring only two and a half inches in length, are the oldest
air-breathing land animals of the fossil record.
It was not until the period was well advanced that fishes became
numerous, and much of our knowledge of the beginning of an “Age of
Fishes” has been obtained from European fossils. Although fishes are
classed with the vertebrate or backboned animals there are large groups
which do not have bony skeletons but are provided instead with a simple
framework of cartilage. Among the earlier and more primitive types were
the ostracoderms or bony-skinned fishes with no internal bones and only
a small amount of bony substance in the armor-like plates and scales
which covered the forward portion of the body.
The ostracoderms comprise a small group of fishes about which very
little is known. They appear to have been inhabitants of fresh-water
streams as well as lagoons bordering the seas, and may have been related
to the small sharks of the time. They lived during the Ordovician,
Silurian, and Devonian periods, and left no descendants now recognized
among living creatures. A much larger type of armored fishes is known as
the arthrodires, a name which refers to a pointed neck and an
arrangement of the armor plates to permit a movement of the head. These
were the most ferocious fishes of the Silurian and Devonian seas, some
of them reaching a length of twenty feet though most were much smaller.
Their jaws were provided with formidable shearing and crushing plates
instead of teeth.