GRAPTOLITE, grap'to-lit (from Gk. yparrec, graptos, written, marked 4- 2.10oc, lithos, stone).
A class of extinct animals, allied to the hydrozoa, fossil remains of which are found in the lower Paleozoic rocks, especially- in those of Upper Cambrian, Ordovician, and Silurian age. The group derives its name from the likeness of its members to pencil markings on the rocks. They are found most abundantly in black shale forma tions, and they are of great importance to the geologists because of their numerous species, that follow each other in rapid evolutional series through successively higher formations. Because of these lines of descent and the wide distribution of the species, graptolites are ideal horizon-mark ers or index fossils in the lower Paleozoic rocks.
The graptolites are delicate hydroid-like col onies growing from a minute embryonal polyp that was incased in a chitinous theca or cell, called the `sicula' From this sicula grew, by budding, a series of zooids or individuals, the form of which is still recognizable in that of their fossilized chitinous coverings, the theca:. Refined methods of isolation and decoloration of the graptolite colonies have been developed by Holm and Wiman, and have enabled these ob servers to remove the colonies bodily from the inclosing rock, and section them in a microtome after much the same methods employed in his tology. In this way the structure of the grapto lite colonies has been shown to be much more complicated than had been suspected. This is especially true of the first group, the Dendroidea, to which belong sessile colonies with the forms of miniature trees (Dendrograptus) or of woven baskets (Dictyonema) (q.v.). In these Den droidea three different forms of them have been found, viz.: Theca; proper, or nourishing indi viduals; alternating canals, considered by Wiman as gonangia or reproductive individuals; and gemmating or budding individuals. Other au thors, as Frech, held the alternating canals to have been nematophores. See HYDROIDS.
The second group, Graptoloidea, consists of the older Dichograptidm and the younger Diplograp tidre. In the former there grow, by successive forkings, from the sicula or embryonal cell, which is centrally situated, more or less numerous branches, each of which grows distally (at its extremity) and consists of a single series of them proper. This family, which begins in the upper Cambrian and does not extend beyond the Ordo vician, exhibits a most remarkable evolution from irregular many-branched forms (Clonograp tus) to symmetrical forms with eight branches (Dichograptus), then to four-branched forms (Tetragraptus), and finally to birnmous species (Didymograptus), with which last the family becomes extinct. An aberrant side line from this family is the genus Phyllograptus, in which the four branches of Tetragraptus become united along their dorsal sides to form colonies with cross-shaped sections.
The younger Diplograptidte, of which the type genus is Diplog,raptus, possess a central disk which apparently was a 'float,' and which bore a circle of reproductive sacs (gonangia), containing akar and numerous stipes (hydrorhabds). The latter are biserial in Diplograptus, each being borne on a long stem (hydrocaule), with a sicula at its farther end, from which the cells grow backward along the stem. The latter forms hence an axis within the stipe. wherefore Diplo graptus and its allies have been separated, as Axonophora, from the preceding forms (Dicho graptidx) without axes, as Axonolipa. Diplo graptus begins in the Middle Ordovician, and ex tends a short distance into the Silurian. It is succeeded by forms possessing the same general structure of the colonies, but with only uniserial arrangement of the cells, the Monograptidx. These flourished in the Silurian, where their colonies attained a great variety of form. Only the genus Dictyonema passes with a few species into the Devonian system.