1911 Encyclopædia Britannica/Ostracoderms
OSTRACODERMS or Osteacophores, the earliest and most primitive group of fish-like animals, found as fossils in Upper Silurian and Devonian formations both in Europe and in North America. They are so named (Gr. shell-skins or shell-bearers) in allusion to the nacreous shell-like appearance of the inner face of the plates of armour which cover the more common members of the group. The Ostracoderms are, indeed, known only by the hard armature of the skin, but this sometimes bears impressions of certain internal soft parts which have perished during fossilization. They agree with fishes in the possession of median fins, and resemble the large majority of early fishes in their unequal-lobed (heterocercal) tail, but they have no ordinary paired fins. They must also have been provided with the usual gill-apparatus, but there is reason to believe that their lower jaw was not on the fish plan. They are, therefore, at least as low in the zoological scale as the existing lampreys, with which Cope, Smith, Woodward and others have associated them. They are all small animals, many of them only a few centimetres in length.
From the Trans. Roy. Soc. Edinburgh.
Fig. 1.—Thelodus scoticus, from the Upper Silurian of Lanarkshire, restored by Dr R. H. Traquair; about one-half nat. size.
From the Proc. Geol. Assoc.
Fig. 2.—Cephalaspis murchisoni, from the Lower Old Red Sandstone of Herefordshire, restored by Dr A. S. Woodward; about one half nat. size.
From British Museum, Catalogue of Fossil Fishes, by permission of the Trustees.
Fig. 3. — Pteraspis rostrata, from the Lower Old Red Sandstone of Herefordshire, restored by Dr A. S. Woodward; about one-third nat. size.
The oldest and lowest family of Ostracoderms, that of Coelolepidae, is known by nearly complete skeletons of Thelodus (fig. 1) and Lanarkia from the Upper Silurian mud stones of Lanarkshire, Scotland. The Ixjdy is completely and uniformly covered with minute granules which resemble the shagreen of sharks, and were erroneously ascribed to sharks when they were first discovered in the Upper Silurian bone-bed at Ludlow, Shropshire. The head and anterior part of the trunk are depressed and shown from above or below in the fossils, and this region sharply contracts behind into the slender tail, which is generally seen in side view, with one small dorsal fin and a forked heterocercal tail. The eyes are far forwards and wide apart. In another family, that of the Cephalaspidae (fig. 2), the animals resemble the Coelolepids in shape, but their skin granules are fused into small plates, which are polygonal where there must have been much flexibility, and in rings round the tail where the underlying successive plates of muscle necessitated this arrangement. The eyes are close together. At the opening of the gill-cavity on each side at the back of the head, there is a flexible flap, which is sometimes interpreted as a paired limb. Part of the armour of the Cephalaspidians contains bone-cells, but the dermal plates of two other families, the Pteraspidae (fig. 3) and Drepanaspidae, consist merely of fused shagreen granules without any advance towards bone. The Pteraspidae are interesting as showing on the inner side of the dorsal shield impressions which suggest that the gill-cavities extended unusually far forwards to the front of the head. Another family, known only by nearly complete skeletons from the Upper Silurian mud stones of Lanarkshire, is that of the Birkeniidae, comprising small fusiform species which are covered with granules disposed in curiously-arranged rows. The highest Ostracoderms are the Asterolepidae, which occur only in Devonian rocks and include the familiar Pterichthys (fig. 4) from the Middle Old Red Sandstone of Scotland. In this family the primitive skin-tubercles seem to have fused, not into polygonal plates, but along the lines of the slime-canals. The Asterolepid armour consists of symmetrically arranged, overlapping plates on the top of the head and round the body, with a pair of flippers similarly armoured and appended to the latter. The tail resembles that of other Ostracoderms and is sometimes covered with scales.
See E. Ray Lankester, The Cephalaspidae (Monogr. Palaeont. Soc. 1868, 1870); R. H. Traquair, The Asterolepidae (Monogr. Palaeont. Soc. 1894, 1904, 1906) and papers in Trans. Roy. Soc. Edinb. vol. xxxix. No. 32 (1899), vol. xl. Nos. 30, 33 (1903, 1905); A. S. Woodward, Catal. Foss. Fishes, B.M. pt. ii. (1891); W. H. Gaskell, Origin of Vertebrates (London, 1908). (A. S. Wo.)