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.
DEVONIAN PROGRESS
The Devonian is one of the most outstanding of all periods from the viewpoint of life development. Dominance of the fishes is its greatest achievement, the invertebrates remaining about as they were and the higher vertebrates barely in evidence, but life on a large scale was no longer confined to the seas. Fresh-water fishes became prominent and land plants well established. The first forests appeared, with fern-like plants predominating although woody trees of several types and considerable size were included. It is quite possible that extensive land areas had been well supplied with vegetation during earlier times, but the delicate tissues of plants are far less likely to be preserved than the limy parts of animals. The fossil record, therefore, cannot be expected to reveal more than a suggestion of the progress made at this level of living. The story of plant life becomes much clearer in the next period when conditions were more favorable for the production and preservation of plant fossils.
Land animals of the time are almost unknown. A few snails and scorpions have been found, and some footprints made by early amphibians. Insects probably were in existence although the evidence is not quite clear on this point. The increasing number of fresh-water fishes, however, may be regarded as a sure indication that inland conditions were becoming more favorable for plant and animal inhabitants of all kinds.
The extent of development among the fishes cannot be accurately indicated by naming a few types, for it is mainly in the number of species and genera within the larger groups that progress is seen. In general it may be stated that the fishes of the period had not yet acquired the bony skeleton and typical form of familiar modern species. Skeletons were of cartilage, partly hardened in some instances by lime. Armor plates were customary with certain races but were not present among all fishes. Neither were these armored forms exceptionally large, as compared with living sharks. Although occasional giants appeared, the majority were small. Many were sluggish creatures with poorly-developed jaws, living as scavengers on sea and stream bottoms. Tail fins were usually unbalanced as in the sharks, or pointed and rounded rather than evenly forked.
Modernized Types of Fishes from Eocene Shales of Southwestern Wyoming