GRAPTOLITES, a group of organisms now extinct, from the fossil remains of which it is impossible to state with cer tainty what they were, though it is usually considered that they were Coelenterata (q.v.). The remains are found, often in great abundance, in Palaeozoic rocks. They begin in the Cambrian strata and die out in the Devonian. They occur for the most part as flattened carbonized films in shales or mudstones which were deposited in the Palaeozoic seas ; but sometimes they occur in limestone, in which case the skeleton may retain its original na ture. This was composed of chitinous (horny) material, and in the case of specimens embedded in limestone the matrix can be dissolved away, leaving it in a suitable condition for examination.
The general appearance of graptolites is that of rods or twigs which may be branched or simple, or of leaf-like or cone-like structures. One or both margins of each twig are regularly toothed or serrate. These remains represent the common skeleton of a colony of small animals which were linked together by a stalk; and each tooth along the notched edge of the skeleton marks the site of a small skeletal cup which during life contained one in dividual of the colony. The general effect of a graptolite skeleton is similar to that of some of the modern hydroids (see HYDROZOA), but the two structures are not fully comparable and there is no conclusive reason for supposing that the graptolites were hydroids.
In a graptolite such as Dimorphograptus (fig. 1, A) the skele ton consists of a small helmet-like structure (the sicula), from the apex of which there extends a long hollow thread (the nema). On either side of this thread, and enclosing it, lies the main part of the skeleton, consisting of a row of cups (thecae) on each side, linked up by a common canal. In the part nearest to the sicula, however, there is only one row of cups. The sicula is the skeleton of the individual which originally founded the colony, and the later individuals inhabiting the thecae were produced from this by budding; it is a hollow cone consisting of a delicate apical portion continuous with the nema, and of a stronger prox imal part which is prolonged into a spine pointing in the opposite direction from the nema. A sicula with the first few thecae of the colony of a similar form is shown enlarged in fig. I, B.
Other graptolites differ from Dimorpho graptus in a variety of ways. The general form of the colony may be quite unlike that of the one illustrated. The degree and mode of branching also varies, as well as the shape and arrangement of the thecae and the manner in which they are related to the sicula. In Didymograptus, for instance (fig. 1, C), there are two branches united at the sicula-end, with a single row of thecae on each ; and the mouth of the sic ula points in the same direction as do those of the thecae. Certain graptolites have been found in which a number of stems bearing siculae and thecae radiate from a central body to which they are attached. A com plex system such as this was founded by a single sicula from which at first grew a single theca-bearing stem ; this was at tached at the end farther from the sicula to a little square plate which after a time became a vesicle and probably served as a float for the colony (fig.
2, A). Round the end of the first stem, where it was attached to the float, grew short stalks, each of which was enclosed in a pro tective capsule and which produced a number of siculae ; some of these became free from the stalk and went away to found new colonies ; others grew in situ into theca-bearing stems and in this manner gave the parent system its radial structure (fig. 2) .
It is impossible to deal in so short an article with the evolution of the grapto lites, but it may be stated that a good deal is known about the main changes which took place during the history of the group. From branched forms there arose by re duction unbranched kinds ; the direction of the outgrowth of thecae from the sicula was originally that of Didymograptus, and became converted by degrees into the state of affairs found in Dimorphograptus; the shape and relation to one another of the thecae showed progressive changes in var ious stocks ; and so on. The knowledge which has been gained of the direction of evolution in graptolites collected from series of strata of known succession has proved extremely useful in determining the succession of deposits which contain a similar series of graptolites whose succes sion was not known.
We do not know what habit of life was adopted by the graptolites ; many of them appear to have been pelagic organisms but others may have been attached to foreign surfaces in one way or another.
Outlines of Palaeontology (London, 1923) ; G. C. Elles, Rep. Brit. Assoc. (1923), Proc. Geol. Assoc. (1921), xxxii., p. 168 ; G. C. Elles, Wood, and Lapworth, A Monograph of British Graptolites (Palaeontographical Society, London, 19o1-18) .