| Fig. 9.—Apus (from the Royal Natural History). Dorsal view. | Fig. 10.—Branchipus stagnalis. (From Claus.) |
We must endeavour to dismiss from our imagination such forms as the salmon and shark as representatives of the fish-tribe, and the lobster and spider of the arthropods, and try to picture the kind of animals living in the seas in the early Devonian and Upper Silurian times, and then we find, to our surprise, that instead of the contrast between fishes and arthropods being so striking as to make any comparison between the two seem an absurdity, the difficulty in the last century, and even now, is to decide in many cases whether a fossil is an arthropod or a fish.
I have shown what kind of animal the palæostracan was like. What information is there of the nature of the earliest vertebrate?
The most ancient fishes hitherto discovered have been classified by Lankester and Smith Woodward into the three orders, Heterostraci, Osteostraci, and Antiarcha. Of these the Heterostraci contain the genera Pteraspis and Cyathaspis, and are the very earliest vertebrates yet discovered, being found in the Lower Silurian. The Osteostraci are divided into the Cephalaspidæ, Tremataspidæ, etc., and are found in the Upper Silurian and Devonian beds. The Antiarcha, comprising Pterichthys and Bothriolepis, belong to the Devonian and are not found in Silurian deposits. This, then, is the order of their appearance—Pteraspis, Cephalaspis, and Pterichthys.
In none of these families is there any resemblance to an ordinary fish. In no case is there any sign of vertebræ or of jaws. They, like the lampreys, were all agnathostomatous. Strange indeed is their appearance, and it is no wonder that there should have been a difficulty in deciding whether they were fish or arthropod. Their great characteristic is their buckler-plated cephalic shield, especially conspicuous on the dorsal side of the head. Figs. 11, 14, 15, 16, give the dorsal shields of Pteraspis, Auchenaspis, Pterichthys, and Bothriolepis.
In 1904, Drevermann discovered a mass of Pteraspis Dunensis embedded in a single stone, showing the same kind of head-shield as P. rostrata, but the rostrum was longer and the spine at the extremity of the head-shield much longer and more conspicuous. The whole shape of the animal as seen in this photograph recalls the shape of a Hemiaspid rather than of a fish. It is, then, natural enough for the earlier observers to have looked upon such a fossil as related to an arthropod rather than a fish.
Fig. 11.—Pteraspis dunensis (from Drevermann). Dorsal view of body and spine on the right side. Head-end, showing long rostrum on the left side.
| Fig. 12.—Bunodes lunula. (From Schmidt.) | Fig. 13.—Auchenaspis (Thyestes) verrucosus, natural size. (From Woodward.) |
In Figs. 12 and 13 I have placed side by side two Silurian fossils which are found in the same geological horizon. They are both life size and possess a general similarity of appearance, yet the one is a Cephalaspidian fish known by the name of Auchenaspis or Thyestes verrucosa, the other a Palæostracan called Bunodes lunula.