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. [Illustration: 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.