Fig. 50.—Corals, representing the very simple subkingdom of animals called “Cœlenterates”: a, fossil shell of an individual “cup coral” found only in Paleozoic strata; b, a compound or “chain coral” skeleton found only in relatively old Paleozoic strata; and c, part of a modern coral colony showing living corals.
Corals comprise another important branch of the Cœlenterates. During the Cambrian period there were corallike sponges and possibly simple corals, but from the early Ordovician to the present true corals have been common, especially in the clearer, warmer seas. Their carbonate of lime skeletons have accumulated to help build up great limestone formations representing almost every geologic age from early Paleozoic time to the present. Paleozoic corals were in general notably different from those of later time. There were three main types including the compound “honeycomb” and “chain” types, and the solitary or compound “cup” type. They all had four, or multiples of four, radiating partitions; were rarely branched; and were generally large, some individual cup corals ranging in length from half an inch to a foot or more. Modern corals (beginning with the Mesozoic) have six or eight partitions; are nearly all profusely branched; and are mostly tiny individuals.
Echinoderms are all marine animals, including the so-called “starfishes,” which are not really fishes. They have body cavity, with digestive canal, low order nervous system, and a water circulatory system. Most of them have radially segmented shells or skeletons. The oldest fossil forms are found in Cambrian strata, these being very simple or primitive types, with a bladderlike head set on the end of a segmented stem, both head and stem having been supported by carbonate of lime. Such forms lived only to middle Paleozoic time. Ordovician strata contain representatives of all the main types of Echinoderms in well-fossilized forms.
Fig. 51.—Fossil Echinoderms or so-called “starfishes”: a, simple type known as the “stone lily” with head, stem, and roots intact from Silurian strata; b and c, irregular and regular higher type Echinoderms called “sea urchins” from Cretaceous strata.
A stemmed Echinoderm of special interest, first known from the Ordovician, has persisted to the present day. It is the so-called “sea lily” or “stone lily,” consisting of a complex, headlike portion attached to the sea bottom by a long segmented stem, the whole being supported by lime carbonate. They were very numerous during the Silurian, but they seem to have culminated in variety of species and numbers of individuals during the Mississippian period when they were exceedingly profuse. Hundreds of species of “stone lilies” are known from Mississippian strata alone, and in certain localities, as at Crawfordsville, Ind., and Burlington, Ia., the “stone lily” remains are so numerous that when living they must have literally forested parts of the sea bottom. From Mississippian time to middle Mesozoic time they occupied a relatively subordinate position when they again developed in great profusion. The Mesozoic forms were distinctly more like those of to-day, and it scarcely seems credible that any creature could have contained such a multiplicity of hard parts, more than 600,000 segments having been counted in a single fossil from Jurassic strata. The “sea lilies” of to-day are relatively unimportant.
The familiar five-pointed “starfishes,” so common along our seacoasts, are first known from the Ordovician, and they persisted through the many millions of years to the present time with remarkably little change. The so-called “sea urchins” live in rounded, segmented lime-carbonate shells bristling with movable spines. “Sea urchins” are first known from the Ordovician, but they did not become abundant and diversified until Mesozoic time, when many of them took on a very modern aspect.
Worms are known to have existed ever since late Proterozoic time, as proved by the occurrence of tracks, borings and more rarely delicate impressions on rock surfaces. Because of their softness they have rarely been well fossilized and are, therefore, of no great evolutionary or geological importance.