Graptolites (Graptolitoidea).—
Value of Graptolites to Stratigraphist.—
The Graptolites were so named by Linnaeus from their resemblances to writing on the slates in which their compressed remains are found. They form a very important group of Palaeozoic fossils in all parts of the world where these rocks occur, and are well represented in Australasia. The species of the various Graptolite genera are often restricted to particular beds, and hence they are of great value as indicators of certain horizons or layers in the black, grey or variously coloured slates and shales of Lower Ordovician to Silurian times. By their aid a stratum or set of strata can be traced across country for long distances, and the typical species can be correlated even with those in the older slates and shales of Great Britain and North America.
Nature of Graptolites.—
The Graptolites were compound animals, consisting of a number of polyps inserted in cups or thecae which budded out in a line from the primary sicula or conical chamber, which chamber was probably attached to floating sea-weed, either by a fine thread (nema), or a disc-like expansion. This budding of the polyp-bearing thecae gives to the polypary or colony the appearance of a fret-saw, with the teeth directed away from the sicula.
The habit of the earlier graptolites was to branch repeatedly, as in Clonograptus, or to show a compound leaf-like structure as in Phyllograptus. Later on the many-branched forms had their branches reduced until, as in Didymograptus, there were only two branches. Sometimes the branches opened out to direct the thecae upwards, the better to procure their food supply. In Diplograptus the thecae turned upwards and acquired a support by the formation of a medium rod (virgula), often ending in a disc or float. In Silurian times Monograptus prevailed, a genus having only a single row of thecae supported by a straight or curved virgula. In Retiolites the polypary opened out by means of a net-work of fine strands, rendering it better able to float, at the same time retaining its original strength.
Lower Ordovician Graptolites, Victoria.—
The Lower Ordovician slates and shales of Victoria have been successfully divided into several distinct series by means of the Graptolites. These, commencing at the oldest, are:—
(1) Lancefield Series. Characterised by Bryograptus clarki, B. victoriae, Didymograptus pritchardi, D. taylori and Tetragraptus decipiens. Other forms less restricted are, Clonograptus magnificus (measuring over a yard in breadth), C. flexilis, C. rigidus, Leptograptus antiquus and Tetragraptus approximatus ([Fig. 73]).
(2) Bendigo Series. Characterised by Tetragraptus fruticosus, T. pendens, Trichograptus fergusoni and Goniograptus thureaui. This series also contains Tetragraptus serra (ranging into Darriwill Series), T. bryonoides, T. quadribrachiatus, T. approximatus (base of the series), Phyllograptus typus, Dichograptus octobrachiatus, Goniograptus macer and many Didymograpti, including D. bifidus ([Fig. 74]).