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Updated: June 13, 2025
But it is probable that vertebrates had been on the earth for ages before this in lowly types, which, being destitute of hard parts, would leave no record. The narrowing of the seas and the emergence of the lands which characterized the closing epoch of the Ordovician in eastern North America continue into the succeeding period of the Silurian.
We have somewhat obscure and disputed traces of ferns in the Ordovician, and, as they and the Horsetails and Club-mosses are well developed in the Devonian, we may assume that some of the sea-weeds had become adapted to life on land, and evolved into the early forms of the ferns, at least in the Cambrian period.
In parts, these Ordovician rocks are much folded, crushed and metamorphosed, and they are associated with schists and altered volcanic rocks which are probably pre-Cambrian. Between the eastern and central pre-Cambrian masses carboniferous rocks are found.
During the ages of the Ordovician, life made great advances. Types already present branched widely into new genera and species, and new and higher types appeared. Sponges continued from the Cambrian. Graptolites now reached their climax.
Secondary Cretaceous 7,200,000 years or Jurassic 3,600,000 " Mesozoic Triassic 2,500,000 " Primary Permian 2,800,000 years or Carboniferous 6,200,000 " Palaeozoic Devonian 8,000,000 " Silurian 5,400,000 " Ordovician 5,400,000 " Cambrian 8,000,000 "
It is interesting to note that in many species the segments of the thorax have now come to be so shaped that they move freely on one another. Unlike their Cambrian ancestors, many of the Ordovician trilobites could roll themselves into balls at the approach of danger. It is in this attitude, taken at the approach of death, that trilobites are often found in the Ordovician and later rocks.
CORALS developed along several distinct lines, like modern corals they secreted a calcareous framework, in whose outer portions the polyps lived. In the Ordovician, corals were represented chiefly by the family of the CHOETETES, all species of which are long since extinct. The description of other types of corals will be given under the Silurian, where they first became abundant.
KAINOZOIC OR TERTIARY: Pleistocene and Recent. Pliocene. Miocene. Oligocene. Eocene. MESOZOIC OR SECONDARY: Cretaceous. Jurassic. Triassic. PALAEOZOIC OR PRIMARY: Permian. Carboniferous. Devonian or old Red Sandstone. Silurian. Ordovician. Cambrian. A VILLAGE BUILT ON PILES IN A SWISS LAKE. Restored by Dr. Works of Art in Danish Peat-Mosses. Remains of three Periods of Vegetation in the Peat.
Below this Devonian limestone, its crystals sparkling in the sunshine, with its coral fossils, its fragments of crinoids, and its broken shells of brachiopods, down through the Devonian, the Silurian, the Ordovician, and the Cambrian rocks, down to the original crust formed when first the earth began to cool, if any there be remaining; all these miles of rocks are inorganic, built up of crystals.
Its GENERIC characters appeared in other species earlier in the Silurian and continued through the Devonian. Its FAMILY characters, represented in different genera and species, range from the Ordovician to the close of the Paleozoic; while the characters which it shares with all its order, the Zoantharia, began in the Cambrian and are found in living species.
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