Geology: The Science of the Earth's Crust

by William J. Miller

Fig. 52.—Fossil brachiopods belonging to the subkingdom of animals known as “Molluscoids”: a, b, c, forms characteristic of the Ordovician, Devonian, and Triassic periods, respectively.

The subkingdom Molluscoids has been richly represented by both the so-called “sea mosses” and brachiopods. The “sea mosses” form colonies of tiny mosslike tufts, resembling corals outwardly, though they are much more highly organized. They have been common from Ordovician time to the present, their carbonate of lime skeletons often having contributed to the building of limestone formations. Brachiopods always have two external shells or valves, in most cases working on a hinge, and also a pair of long, spiral-fringed arms associated with the soft part of the animal inside the shells. They differ from the other type of bivalve (e.g., clam, oyster) in that they are symmetrical with reference to a plane passed through the middle of the shells at right angles to the hinge line. They have rarely grown to be more than a few inches long. A few scant brachiopod remains are known from the late Proterozoic, but throughout known geologic time they reached their greatest development in the Paleozoic era, more especially in the Devonian period. Combining number of species and number of individuals, the brachiopods probably hold the record of all important groups of fossil animals, more than 7,000 species being known. Many layers of rock are filled with their shells ([Plate 14]). Since the close of the Paleozoic they have fallen off notably, and are now represented by relatively few small forms. From the standpoint of evolution it is interesting to note that in very early Paleozoic time the brachiopods were mostly small, of relatively simple organization, and their thin shells were not joined by hinges. Later they became larger and more complex and their thicker shells worked on hinges. Nearly all the Paleozoic forms had long, straight hinge lines, which made it difficult for their enemies to open them. Along with the change to narrower, curved hinge lines came the decline of the tribe. They have been of great value to the geologist in subdividing the geological column of strata into its many formations.

The Mollusks, which are more highly organized than the Molluscoids, have more or less distinctly developed heads and locomotive organs. Many thousands of species are now extinct, the classes of most geological importance being represented by clams, snails, and the pearly nautilus. Most of them have shells and gills for breathing. The members of the simplest group, well represented by the clam tribe, possess two similar shells working on hinges, so that in this regard they are much like brachiopods, but, unlike the latter, they are not symmetrical with reference to a plane at right angles to the hinge line. Cambrian strata contain the oldest known of the fossil forms where they are small, relatively thin-shelled, and rare. In marked contrast to the brachiopods these bivalves have rather steadily increased in numbers of species and individuals to the present time, now being represented by thousands of forms. During the Mesozoic era they greatly out-numbered the brachiopod bivalves and took on a more distinctly modern aspect, when the oyster tribe and closely related types were prominently developed. Culmination in size and thickness of shell seem to have been reached in early Cenozoic time, strata of that age in certain places, for example in Georgia and southern California, being filled with oyster shells 10 to 20 inches long and 4 to 6 inches thick! In addition to their gigantic size and thickness, many of the shells were fluted or ribbed, and so they represented an extreme type of defensive armor among the lower animals.

Snails have existed from the earliest Paleozoic era to the present time, and the outstanding fact of interest concerning them is that they furnish one of the finest illustrations of an important class of animals which has undergone practically no conspicuous change or evolution during all those millions of years of time.

Fig. 53.—Sketches of chambered cephalopods showing the main steps in the evolution of the shell forms and compartment partitions: a, b, the only kinds in Cambrian time; c, d, forms added in the Ordovician; e, added in the Devonian; f, added in the late Paleozoic; g, h, characteristic of the Mesozoic era; and i, a living form (pearly nautilus) cut through. (Drawn by the author.)

We shall now turn our attention to the highest order of Mollusks—the cephalopods. These creatures, whose heads are armed with powerful tentacles and supplied with complex eyes, propel themselves by forcible ejection of water. One general type—the chambered cephalopod—has a shell divided into compartments (e.g., modern pearly nautilus) which are successively built up and abandoned by the animal as it grows larger. These chamber-shelled cephalopods constitute one of the most remarkable and instructive illustrations of evolutionary change within any important subgroup of invertebrate animals, ranging from early Paleozoic to the present. Both because of the abundance of fossil forms in rocks of all these periods of geological times, and because certain of the evolutionary changes are so clearly expressed in the well preserved shell portions, they are specially adapted for study. In the late Cambrian only straight and slightly curved forms with smooth, nearly straight chamber partitions existed. Notable advance took place during the next (Ordovician) period when there were straight, curved, open-coiled, and even close-coiled forms. All had simple partitions, and the straighter forms predominated. "The size attained by the Ordovician cephalopods was probably never surpassed by representatives of the class. Some of the (straight) shells were twelve to fifteen feet in length, and a foot in diameter. From this great size they ranged down to or below the size of a pipe stem." (Chamberlin and Salisbury.) They were more than likely the undisputed masters of the Ordovician seas. Silurian time marked no important change in their structures, but the coiled forms predominated for the first time. During the second half of the Paleozoic era all preceding types with simple partitions persisted, but in some forms the simple partitions gradually became angled and finally rather complexly curved. During the Mesozoic era the partition lines of the close-coiled forms evolved until a most remarkable degree of complexity was attained, comparable, indeed, to the sutures of the human skull plates. These remarkable forms called ammonites, of which more than 2,000 species are known, began with the Mesozoic, reached their climax, and passed out of existence toward the close of the same era. Certain strata of Jurassic age are literally filled with ammonites, some shells being several feet in diameter. Various eccentric changes took place in the ammonites shortly before their extinction. Some shells became uncoiled and even straight, thus outwardly at least showing reversion to the original early Paleozoic ancestors, but with retention of the complex partitions. Others assumed spiral shapes and still others became curved or coiled at each end. While these extraordinary evolutionary changes were going on among the chambers of cephalopods during Mesozoic time, some of the ancient close-coiled forms with very simple partitions managed to persist. In fact this simple type, almost exactly like its early Paleozoic ancestor, has been the only one out of this whole remarkable class of animals to persist to the present time, being now barely represented by the well-known pearly nautilus of the Indian Ocean.