One of the most characteristic features of Devonian life is the great development of the aquatic Vertebrates, which is so striking that the period is often called the " Age of Fishes." So numerous and so finely preserved are these fossils that a satisfactory account may be given of the structure and systematic position of many of the genera. This great assemblage of fishes and fish-like forms, it should be remembered, was not somethng entirely new in the earth's history, but was rather the wondenul expansion of types which during the Ordovician and Silurian had remained rare and obscure.
Of the Devonian Vertebrates none are more peculiar and characteristic than the Ostracoderms, which, though generally called fishes, really belong to a type much below the true fishes, being devoid of true jaws and of paired fins. The head and more or less of the body are sheathed in heavy plates of bone, and the remainder of the body and the tail are covered with scales. No trace of the internal skeleton is preserved, and it evidently was not ossified. The genus Cephalaspis of this group is curiously like a Trilobite in appearance, though, of course, the resemblance is entirely superficial. The head-shield is formed of a single great plate of bone, shaped like a saddler's knife, with rounded front edge and with the hinder angles drawn out into spines; the eyes are on the top of the head and very close together. The body is covered with large, angular plates of bone, arranged in rows; a small median dorsal fin and a larger triangular tail-fin make up the locomotor apparatus.
Fig. 268. - Pterichthys testudlnarius. Restored. Old Red Sandstone. (Dean after Smith Woodward).
Pteraspis has a bony plate over the snout, a large shield on the back, and another on the belly, with rhomboidal scales covering the rest of the body.
A most extraordinary-looking creature is Pterichthys (Fig. 268), in which the head and most of the body are encased in heavy plates, the remainder in overlapping scale-like bones; the eyes are even closer together than in Cephalaspis. Dorsal and tail-fins are present and what appear to be pectoral fins. The pair of appendages referred to doubtless acted as fins, but they are not comparable to the paired fins of the true fishes, being merely jointed extensions of the head-shield. These three genera, CephaU aspis, Pteraspis, and Pterichthys, have been selected as types of the Ostracoderms, each one of which has several allies, differing from it in one or other particular.
Of the true Fishes there is great variety in the Devonian. The Selachians are well represented, one of which is Cladoselache (Fig. 269), a small shark, from two to six feet in length, and the most primitive known member of the group. The Dipnoi, or Lung-fishes,, were important elements of the Devonian fish fauna. Dipterus (Fig. 270), an example of this group, is very like the modern lung-fishes, which have dwindled to three genera, one in South America, one in Africa, and one in Australia. A remarkable series of fishes, the Arthrodira, is very characteristicall) Devonian. One of the best-known members of this group is the European genus Coccosteus (Fig. 271), in which the head, back, and belly are covered with bony plates, but the rest of the body is naked. This bony armour gives the fish something of the appearance of the Ostracoderms, with which group it is often, though erroneously, classified. The backbone is represented by an unsegmented rod (the notochord, N, Fig. 271), to which arches of bone are attached (N, H, Fig. 271). Paired ventral fins were present, but pectorals have not been found. The jaws were provided with teeth, which fuse into plates. In the uppermost Devonian of Ohio are found some huge fishes allied to Coccosteus, but much larger and more formidable.
The most important of these are Dinichthys and Titanichthys, the latter attaining a length of 25 feet.
Fig. 269. - Cladoselache newberryi. Restored, x 1/5. Ohio shale. (Dean).
Fig. 270. - Dipterus valenciennesi Sedg. and Murch. Restored. Old Red Sandstone. (Smith Woodward after Traquair).
Fig. 271. - Coccosteus decipiens Ag. Restored. Old Red Sandstone. (Dean, after Smith Woodward).
Fig. 272. - Holoptychius nobilissimus Ag. Restored. Old Red. (Smith Wood' ward after Traquair.) The ornamentation of the scales is not shown.
A higher type of Devonian fish is that of the Crossopterygii, an ancient group of which but two representatives remain at present, both of them African. These fishes, like the Dipnoans, have " lobate " paired fins (see Fig. 272), i.e. the part of the fin belonging to the internal skeleton is large and covered with scales, forming a lobe to which the fin-rays are attached. Most of the Devonian members of the group have massive rhomboidal scales, but in others, like Holoptychius, the scales are thinner, rounded, and overlapping.
The most advanced fishes of the period are the Ganoid members of the Actinopteri, which from the Devonian until nearly the end of the Mesozoic era continue to be the dominant type of fishes. Nearly all of them have thick, shining scales of rhomboidal shape.
The Devonian fish fauna (using that term in a very comprehensive sense) is thus seen to be a rich and varied one, including Ostracoderms, Sharks, Lung-fishes, Arthrodirans, Crossop-terygians and Actinopteri, each with many representatives and mostly of very curious and bizarre forms. While thus varied and plentiful, this assemblage differs from the modern fish fauna in the primitive character of the groups which are represented, and in the entire absence of the Bony Fishes (Teleosts), which now make up the vast majority of fishes, both fresh-water and marine.
Certain footprints which have been reported from the Upper Devonian of Pennsylvania, show that the Amphibia, the lowest of air-breathing vertebrates, had already begun their career; that is, if the correlation of the rocks in which these footprints occur has been correctly made.