Geology: The Science of the Earth's Crust

by William J. Miller

VERTEBRATES comprise the highest subkingdom of all animals with man himself at the very top. They are characterized by the possession of a vertebral column, which, in all but the very simple or primitive forms, is an ossified backbone. Their main subdivisions are given in the classification table near the beginning of the preceding chapter. The oldest known Vertebrates, found in fossil form in middle Ordovician strata, were represented by curious and bizarre creatures called ostracoderms, or more popularly “armor fishes.” They were not true fishes because they were really somewhat lower in the scale of organization than fishes. Some were distinctly fishlike in appearance, and others notably resembled certain of the Arthropods, so that some students consider them to have formed the connecting link between the highest Invertebrates (Arthropods) and low order fishes of the Vertebrates. The vertebral column always consisted of cartilage or gristle and, in most forms, it extended through tail fin. None had true side fins like fishes, but many were provided with a pair of jointed flappers or paddles. The jawlike portions of the heads moved over each other sidewise as, for example, in beetles and not up and down in true Vertebrate fashion. Two eyes were always very close together. One of the most striking features was the protection of the head and fore part of the body by an armor of bony plates, while the rest of the body had scales. They seldom grew to be more than six or seven inches long. Beginning in the Devonian, they remained rare during the Silurian, and then in the Devonian period they reached their climax of development only to become extinct at its close. Many species were abundantly represented in many parts of the world. By some the Ostracoderms are thought to have been a primitive (sharklike) fish development in the wrong direction, and hence they became extinct.

Fig. 57.—Two restored forms of very primitive and ancient (Devonian) types of Vertebrates called “ostracoderms.” They were lower in organization than true fishes. (After Dean-Woodward and British Museum, respectively.)

Fishes, represented only by very primitive sharks, are known to have existed as early as the Silurian period, but the remains are scant. During the Devonian period, however, they showed a marvelous development into many species and countless myriads of individuals. The Devonian is, therefore, commonly called the “Age of Fishes.” These very ancient (Devonian) primitive (fish) types of Vertebrate animal life are of profound significance in organic evolution because they were the direct progenitors of the great groups of still higher Vertebrates which since later Paleozoic time gradually increased in diversity and complexity of structure through amphibians, reptiles, birds, and mammals finally to man himself.

Fig. 58.—Restorations of characteristic Devonian fishes, based upon actual fossils: a, a “lung fish” with leglike fins (after Huasakof); b, a “ganoid.” (After Nicholson.)

In marked contrast to the most typical and highly organized fishes so abundant to-day, all Devonian fishes were of simple types with cartilaginous skeletons and vertebrated tails. Many of them were also generalized types, that is, associated with their clearly defined fish characters were others connecting them with certain higher Vertebrates, as, for example, amphibians and reptiles. Thus all their tail fins were vertebrated as in reptiles; their labyrinthine, internal tooth structure was to be an amphibian feature when those creatures evolved; many had protective armor or bony scales like most early amphibians and many modern reptiles; and many had paired fins which were something like jointed legs. Most abundant and highly organized of the Devonian fishes were “ganoids,” characterized by a covering of small plates or bony scales set together but not overlapping like in typical modern fishes. Their intricate tooth structure and limblike fins strongly suggest the amphibians of later Paleozoic time. The skeleton of cartilage gradually became somewhat ossified during succeeding geologic periods. From their great profusion and diversity in the Devonian period the ganoids have steadily fallen away until they now have very few descendants like the gar pike.

Another important group of remarkable fishes, now totally extinct, but common in Devonian and somewhat later time, had heavy, bony armor plates over the fore part of the body. Those which grew to be fifteen to twenty-five feet long were probably the rulers of the middle Paleozoic seas. Another remarkable Devonian fish was able to breathe in both water and air because, like their few modern descendants, they had both gills and lungs. Because of their leglike fins and lung sac, it is commonly believed that they were progenitors of the later Paleozoic amphibians. The simplest of all fishes, the sharks, began in the Silurian, underwent no important change through the millions of years since, and are now of course well represented. During early Cenozoic time the sharks seem to have reached culmination in size—sixty to eighty feet long, with teeth five or six inches long.