This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
The life of the Devonian is, in its larger outlines, very like that of the Silurian, but with many significant differences, which are due, on the one hand, to the dying out of several of the older groups of animals, and on the other, to the great expansion of forms which in the Silurian had played but a subordinate role.
The fossils show that in Devonian times the land was already clothed with a varied, rich, and luxuriant vegetation of the same general type as that whose scanty traces are found in Silurian strata. All the higher Cryptogams are represented, and by large, tree-like forms, as well as by small herbaceous plants. The bulk of the flora is composed of Ferns, Lycopods, or Club-Mosses (especially the great tree-like Lepidodendrids), and Eqai-setales, or Horsetails. The highly important extinct groups, the Sphenophyllales and Cycadqftlices, which were already in existence, will be described from the more complete Carboniferous fossils. Besides these Cryptogams, we find representatives of the lower Helderberg. 10, Tropidoleptus carinatus Conr, x 1/2, Hamilton. 11, Chonetes coronatus Conr , x 1/2, Hamilton. 12, Stropheodonta demissa Conr., x 1/2, Hamilton. 13, Pterinea flabellum Conr., x 1/2, Hamilton. 14, Platyceras dumosnm Conr., x 1/2 Onondaga. 15, Manticoceras oxy Clarke, x 1/6, Portage. 16, Phacops rana Green, x 1/4, Hamilton. 17, Odontocephalusselenurus Eaton, x 1/4, Onondaga. 18, Terataspis grand is Hall, x 1/12, Schoharie. 19, Echinocarispunctatits Hall, x 1/2, Hamilton.
Plate IX. - Devonian Fossils.
Fig. i, Edriocrinus sacculus Hall, x 1/2, Oriskany. 2, Nucleocrintis verneuli Troost, x 1, Onondaga. 3, Sprifer macropleurus Conr , X 1/2, Helderberg. 4, S. disjunctus Sowerby, x 1/2; internal cast of ventral valve, Chemung. 5, S. mucronatus Conr., x 1/2; inner view of dorsal valve, showing arm-supports, Hamilton. 6, Vitulina pustulosa Hall, x 3/2,, Hamilton. 7, Atrypa reticularis Linn., x 1/2, Hamilton. 8, Hypothyris venustula Half, x 5/8, Tully. 8 a, The same, anterior view. 9, Gypidula galeata Dalman, x 1/2 kinds of flowering plants in the Gymnosperms, which presumably grew upon the higher lands. We shall meet this same flora in richer and more varied display in the Carboniferous period.
Sponges are conspicuous elements of the Devonian fauna, as, for example, the Dictyospongida in the Chemung.
The Cystoids have become much rarer than before, and are on the point of extinction; the Blastoids (PL IX, Fig. 2) are still in the background, though locally abundant in a few places, and the Echinoids have not yet become common; but the Crinoids and Star-fishes have greatly increased in number and variety. Important genera of the former group are Cupressocrinus, Platy-crinus, Actinocrinus, Dolatocrinus, etc. Edriocrinus (IX, 1), a free-swimming crinoid, without a stem, occurs in the Helderbergian and Oriskany. The multitude of the crinoids contributed largely to the building up of the calcareous sea-bottom on which they flourished.
The Trilobites had already begun to decline in the Silurian, while in the Devonian the decline becomes much more marked, though they are still far from rare. New species of Silurian genera, like Phacops (IX, 16), Homalonotus, Lichas, Acidaspis, Odontocephalus (IX, 17), etc., are the commonest. Terataspis (IX, 18), is one of the largest and most curious of Trilobites. A characteristic of the Devonian Trilobites is the ornamentation of the margin of the head, as well as the extraordinary development of spines which many display on the head-and tail-shields. (IX, 17, 18).
Fig. 266. - Heliophyllum hal-li E. & H., X 1/2. Hamilton of Michigan. (Rominger).
Fig. 267. - Acervularia davidsoni E. & H., X 1/2- Hamilton of Michigan. (Rominger).
The other Crustacea make notable progress in this period. The Phyllocarida are abundant in the Upper and Middle Devonian. The first of the Isopoda and of the long-tailed Schizopoda make their appearance in the Devonian. The Eurypterids now attain their culmination in size, being actually gigantic for Crustacea, and some of them are as much as six feet long. The genera (Eurypterus, Stylonurus, and Pterygotus) are the same as in the Silurian. Insects, though still rare as fossils, are very much commoner than in the Silurian; they represent both Orthopters and Neuropters, which are among the primitive groups.
As in the Silurian, Brachiopods continue to be the most abundant fossils, both in species and individuals; indeed, the Devonian is the culminating period of brachiopod abundance and variety; in North America at least it has many more genera than any other. Many Silurian genera have died out, but of others, like Orthis, Stropheodonta (IX, 12), Ortho-thetes, Atrypa (IX, 7), Chonetes (IX, 11), and Productella, the species are more numerous. The most characteristic shells are those belonging to the genera Spirifer, especially the very broad "winged" species (IX, 4, 5), Hypothyris (IX, 8), Athyris, Gypi-dula (IX, 9), Tropidoleptus (IX, 10), Vitulina (IX, 6), and those belonging to the still existing family Terebratulidce, of which Renssellaeria and Stringocephalus are Devonian genera.
Bivalves and Gastropods are much as in the Silurian: examples of the former are Pterinea (IX, 13), Conocar-dium, and Grammysia, while larger species of the Gastropod Euomphalus, and spiny Platycerids (IX, 14), are characteristic. A minute Pteropod, Styliolina, is extraordinarily abundant in the Upper Devonian, forming limestone masses. The Cephalo-pods have been revolutionized; the wealth of Nautiloid shells which we found in the Silurian has been much diminished, though Orthoceras, Phragmoceras, Gomphoceras, and Cyrtoceras still persist abundantly in the Lower Devonian, while many other genera have disappeared. More significant is the first appearance of the Ammonoid division of the Tetrabranchiate Cephalopods, a group of shells which was destined to attain extraordinary development in the Mesozoic era. The Ammonoids are distinguished by the complexity of the "sutures," or lines made by the junction of the septa with the outer wall of the shell. In the Devonian Ammonoids, of which the Goniatites (IX, 15) are the common forms, the sutures are much less complex than in the Mesozoic shells. Another member of the group which is far more abundant in Europe than in America is Clymenia, the only Ammonoid in which the siphuncle is on the inner side of the spiral.
Bactrites has a straight shell, like that of Orthoceras, but with the complex sutures which show it to be an Ammonoid.