The Geological Distribution of Fishes.—The oldest unquestioned remains of fishes have been very recently made known by Mr. Charles D. Walcott, from rocks of the Trenton period in the Ordovician or Lower Silurian. These are from Cañon City in Colorado. Among these is certainly a small Ostracophore (Asteraspis desideratus). With it are fragments (Dictyorhabdus) thought to be the back-bone of a Chimæra, but more likely, in Dean's view, the axis of a cephalopod, besides bony, wrinkled scales, referred with doubt to a supposed Crossopterygian genus called Eriptychius. This renders certain the existence of Ostracophores at this early period, but their association with Chimæras and Crossopterygians is questionable. Primitive sharks may have existed in Ordovician times, but thus far no trace of them has been found.
Fig. 246.—Fragment of Sandstone from Ordovician deposits, Cañon City, Colo., showing fragments of scales, etc., the earliest known traces of vertebrates. (From nature.)
The fish-remains next in age in America are from the Bloomfield sandstone in Pennsylvania of the Onondaga period in the upper Silurian. The earliest in Europe are found in the Ludlow shales, both of these localities being in or near the horizon of the Niagara rocks, in the Upper Silurian Age.
It is, however, certain that these Lower Silurian remains do not represent the beginning of fish-life. Probably Ostracophores, and Arthrodires, with perhaps Crossopterygians and Dipnoans, existed at an earlier period, together perhaps with unarmed, limbless forms without jaws, of which no trace whatever has been left.
Fig. 247.—Fossil fish remains from Ordovician rocks, Cañon City, Colo. (After Walcott.) a. Scale of Eriptychius americanus Walcott. Family Holoptychiidæ? b. Dermal plate of Asteraspis desideratus Walcott. Family Asterolepidæ. c. Dictyorhabdus priscus Walcott, a fragment of uncertain nature, thought to be a chordal sheath of a Chimæra, but probably part of a Cephalopod (Dean). Chimæridæ?
The Earliest Sharks.—The first actual trace of sharks is found in the Upper Silurian in the form of fin-spines (Onchus), thought to belong to primitive sharks, perhaps Acanthodeans possibly to Ostracophores. With these are numerous bony shields of the mailed Ostracophores, and somewhat later those of the more highly specialized Arthrodires. Later appear the teeth of Cochliodontidæ, with Chimæras, a few Dipnoans, and Crossopterygians.
Devonian Fishes.—In the Devonian Age the Ostracophores increase in size and abundance, disappearing with the beginning of the Carboniferous. The Arthrodires also increase greatly in variety and in size, reaching their culmination in the Devonian, but not disappearing entirely until well in the Carboniferous. These two groups (often united by geologists under the older name Placoderms) together with sharks and a few Chimæras made up almost exclusively the rich fish-fauna of Devonian times. The sharks were chiefly Acanthodean and Psammodont, as far as our records show. The supposed more primitive type of Cladoselache is not known to appear before the latter part of the Devonian Age, while Pleuracanthus and Cladodus, sometimes regarded as still more primitive, are as yet found only in the Carboniferous. It is clear that the records of early shark life are still incomplete, whatever view we may adopt as to the relative rank of the different forms. Chimæroids occur in the Devonian, and with them a considerable variety of Crossopterygians and Dipnoans. The true fishes appear also in the Devonian in the guise of the Ganoid ancestors and relatives of Palæoniscum, all with diamond-shaped enameled scales. In the Devonian, too, we find the minute creature Palæospondylus, our ignorance of which is concealed under the name Cycliæ.
Carboniferous Fishes.—In the Carboniferous Age the sharks increase in number and variety, the Ostracophores disappear, and the Arthrodires follow them soon after, the last being recorded from the Permian. Other forms of Dipnoans, Crossopterygians, and some Ganoids now appear giving the fauna a somewhat more modern aspect. The Acanthodei and the Ichthyotomi pass away with the Permian, the latest period of the Carboniferous Age.