The brachiopods were commonest of all animals representing this period, however, and their wide distribution, together with their known preference for warm waters, is taken to be an indication of mild temperatures prevailing over a large portion of the earth. Land plants are indicated by spore-bearing forms related to the ferns and mosses. Impressions of such plants have been found in Europe but, since most of the known rocks of this age were formed in seas, the marine algae are more abundant as fossils.
In the Colorado area, rocks of Ordovician time are exposed only in mountainous areas where they have been lifted high above their original levels. They are not especially rich in fossils although they have produced some fish remains which are of interest in that they suggest an age of vertebrates which is just ahead.
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.