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GRAPTOLITES

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Originally appearing in Volume V12, Page 367 of the 1911 Encyclopedia Britannica.
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GRAPTOLITES , an assemblage of See also:

extinct zoophytes whose skeletal remains are found in the Palaeozoic rocks, occasionally in See also:great abundance. They are usually preserved as branching or unbranching carbonized bodies, See also:tree-like, See also:leaf-like or See also:rod-like in shape, their edges regularly toothed or denticulated. Most frequently they occur lying on the bedding planes of See also:black shales; less commonly they are met with in many other kinds of sediment, and when in See also:limestone they may retain much of their See also:original See also:relief and admit of a detailed microscopic study. Each Graptolite represents the See also:common horny or chitinous investment or supporting structure of a See also:colony of zooids, each tooth-like See also:projection marking the position of the sheath or theca of an individual zooid. Some of the branching forms have a distinct outward resemblance to the polyparies of Sertularia and Plumularia among the See also:recent Hydroida (Calyptoblastea); in none of the unbranching forms, however, is the similarity by any means See also:close. The Graptolite polyparies vary considerably in See also:size: the See also:majority range from r in. to about 6 in. in length; few examples have been met with having a length of more than 30 in. Very different views have been r. sld as to the systematic 365 See also:place and See also:rank of the Graptolites. See also:Linnaeus included them in his See also:group .of false fossils (Graptolithus=written See also:stone). At one See also:time they were referred by some to the See also:Polyzoa (Bryozoa), and later, by almost See also:general consent, to the Hydroida (Calyptoblastea) among the See also:Hydrozoa (See also:Hydromedusae). Of See also:late years an See also:opinion is gaining ground that they may be regarded as constituting collectively an See also:independent phylum of their own (Graptolithina). There are two See also:main See also:groups, or sub-phyla: the Graptoloidea or Graptolites proper, and the Dendroidea or tree-like Graptolites; the former is typified by the unbranched genus Monograptus and the latter by the many-branched genus Dendrograptus. A Monograptus makes its first See also:appearance as a See also:minute See also:dagger-like See also:body (the sicula), which represents the flattened covering of the See also:primary or embryonic zooid of the colony.

This sicula, which had originally the shape of a hollow See also:

cone, is formed of two portions or regions—an upper and smaller (apical or embryonic) portion, marked by delicate See also:longitudinal lines, and having a See also:fine See also:tabular See also:thread (the nema) proceeding from its See also:apex; and a See also:lower (thecal or apertural) portion, marked by transverse lines of growth and widening in the direction of the mouth, the See also:lip or apertural margin of which forms the broad end of the sicula. This margin is normally furnished with a perpendicular spine (virgella) and occasionally with two shorter lateral spines or lobes. A bud is given off from the sicula at a variable distance along its length. From this bud is See also:developed the first zooid and first serial theca of the colony. This theca grows in the direction of the apex of the sicula, to which it adheres by its dorsal See also:wall. Thus while the mouth of the sicula is directed downwards, that of the first serial theca is pointed upwards, making a theoretical See also:angle of about 18o° with the direction of that of the sicula. From this first theca originates a second, opening in the same direction, and from the second a third, and so on, in a continuous linear See also:series until the polypary is See also:complete. Each zooid buds from the one immediately preceding it in the series, and intercommunication is effected by all the budding orifices (including that in the wall of the sicula) remaining permanently open. The sicula itself ceases to grow soon after the earliest theca have been developed; it remains permanently attached to the dorsal wall of the polypary, of which it forms the proximal end, its apex rarely reaching beyond the third or See also:fourth theca. A fine cylindrical rod or fibre (the so-called solid See also:axis or virgula) becomes developed in a median groove in the dorsal wall of the polypary, and is sometimes continued distally as a naked rod. It was formerly supposed that a virgula was See also:present in all the Graptoloidea; hence the See also:term Rhabdophora sometimes employed for the Graptoloidea in general, and rhabdosome for the individual polypary; but while the virgula is present in many (Axonophora) it is absent as such in others (Axonolipa). The GRAPTOLOIDEA are arranged in eight families, each named after a characteristic genus: (I) Dichograptidae; (2) Leptograptidae; (3) Dicranograptidae; (4) Diplograptidae; (5) Glossograptidae (sub-See also:family, Lasiograptidae); (6) Retiolitidae; (7) Dimorphograptidae; (8) Monograptidae.

In all these families the polypary originates as in Monograptus from a nema-bearing sicula, which invariably opens downwards and gives off only a single bud, such branching as may take place occurring at subsequent stages in the growth of the polypary. In some See also:

species See also:young examples have been met with in which the nema ends above in a small membranous disk, which has been interpreted as an See also:organ of See also:attachment to the underside of floating bodies, probably See also:sea weeds, from which the young polyparyhung suspended. Broadly speaking, these families make their first appearance in time in the See also:order given above, and show a progressive morphological See also:evolution along certain See also:special lines. There is a tendency for the branches to become reduced in number, and for the serial thecae to become directed more and more upwards towards the See also:line of the nema. In the See also:oldest family—Dichograptidae—in "which the branching polypary is bilaterally symmetrical and the thecae uniserial (monoprionidian)—there is a gradation from earlier groups with many branches to later groups with only two; and from species in which all the branches and their thecae are directed downwards, through species in which the branches become See also:bent back more and more outwards and upwards, until in some the terminal thecae open almost vertically. In the genus Phyllograptus the branches have become reduced polypary. In the family of the Diplograptidae the branches are reduced to two; these also coalesce similarly by their dorsal walls, and the polypary thus becomes biserial (diprionidian), and the line of the nema is taken by a See also:long axial See also:tube-like structure, the nemacaulus or virgular tube. Finally, in the latest family, the Monograptidae, the branches are theoretically reduced to one, the polypary is uniserial throughout, and all the thecae are directed outwards and upwards. The thecae in the earliest family—Dichograptidae—are so similar in See also:form to the sicula itself that the polypary has been compared to a colony of siculae; there is the greatest variation in shape in those of the latest family—Monograptidae—in some species of which the terminal portion of each theca becomes isolated (Rastrites) and in some coiled into a rounded See also:lobe. The thecae in several of the families are occasionally provided with spines or lateral processes: the spines are especially conspicuous at the See also:base in some biserial forms: in the Lasiograptidae the lateral processes originate a marginal meshwork surrounding the polypary. Histologically, the perisarc or test in the Graptoloidea appears to be composed of three layers, a See also:middle layer of variable structure, and an overlying and an underlying layer of remarkable tenuity. The central layer is usually thick and marked by lines of growth; but in Glossograptus and Lasiograptus it is thinned down to a fine membrane stretched upon a See also:skeleton framework of lists and See also:fibres, and in Retiolites this membrane is reduced to a delicate network.

The groups typified by these three genera are sometimes referred to, collectively, as the Retioloidea, and the structure as retioloid. It is the general practice of palaeontologists to regard each graptolite polypary (rhabdosome) developed from a single sicula as an individual of the highest order. Certain See also:

American forms, however, which are preserved as stellate groups, have been interpreted as complex See also:umbrella-shaped colonial See also:stocks, individuals of a still higher order (synrizabdosomes), composed of a number of biserial polyparies (each having a sicula at its See also:outer extremity) attached by their nemacauli to a common centre of origin, which is provided with two disks, a See also:swimming See also:bladder and a See also:ring of capsules. In the DENDROIDEA, as a See also:rule, the polypary is non-symmetrical in shape and tree-like or See also:shrub-like in See also:habit, with numerous branches irregularly disposed, and with a distinct See also:stem-like or See also:short basal portion ending below in See also:root-like fibres or in a membranous disk or See also:sheet of attachment. An exception, however, is constituted by the comprehensive genus Dictyonema, which embraces species composed of a large number of divergent and sub-parallel branches, See also:united by transverse dissepiments into a symmetrical cone-like or See also:funnel-shaped polypary, and includes some forms (Dictyograptus) which originate from a nema-bearing sicula and have been claimed as belonging to the Graptoloidea. Of the See also:early development of the polypary in the Dendroidea little is known, but the more mature stages have been fully worked out. In Dictyonema the branches show thecae of two kinds: (r) the See also:ordinary tubular thecae answering to those of the Graptoloidea and occupied by the nourishing zooids; and (2) the so-called bithecae, birdnest-like cups (regarded by their discoverers as gonothecae) opening alternately right and See also:left of the ordinary thecae. Internally, there existed a third set of thecae, held to have been inhabited by the budding individuals. In the genus Dendrograptus the gonothecae open within the walls of the ordinary thecae, and the branches present an outward resemblance to those of the uniserial Graptoloidea. But in striking contrast to what obtains among the Graptoloidea in general, the budding orifices in the Dendroidea become closed, and all the various cells shut off from each other. The See also:classification of the Dendroidea is as yet unsatisfactory: the families most conspicuous are those typified by the genera Dendrograptus, Dictyonema, Inocaulis and Thamnograptus. As regards the modes of See also:reproduction among the Graptolites little is known.

In the Dendroidea, as already pointed out, the bithecae were possibly gonothecae, but they have been interpreted by some as nematophores. In the Graptoloidea certain lateral and vesicular appendages of the polypary in the Lasiograptidae have been looked upon as connected with the reproductive See also:

system; and in the umbrella-shaped synrhabdosomes already referred to, the common centre is surrounded by a ring of what have been regarded as ovarian capsules. The theory of the gonangial nature of the vesicular bodies in the Graptoloidea is, however, disputed by some authorities, and it has been suggested that the zooid of the sieula itself is not the 21 28 r, Diptograpius, young sicula. 2, Monograptus dubius, sicula and first serial theca (partly restored). Young form (allaboveafter Wiman). Older form. Showing virgula(after Holm). Rastrites distans. ) Base of Diptograpius (after Wiman). D. calcaratus. Dimorphogra plus. Base of Didymograplus minulus (after Holm).

to, Young Dictyograptus, with primary disk. 11, Ibid. Diptograpius (after Ruedemann). 12 a-b, Base and transverse See also:

section, Retiolites Geinitzianus (after Holm). Bryograptus Kjerulfi. Dichograptus octobrachialus, with central disk. 15, Didymograptus Murchisoni. 16, D. gibberulus. 17 a-b, Phyllograplus and trans-See also:verse section. 18, Nemagraptus gracilis. 19, Dicranograptus ramosus. 23 24 20, Climacograptus Scharenbergi.

21, Glossograptus Hincksii. 22, Lasiograptus costatus (after Elles and See also:

Wood). Dictyonema (-graptus) flabelti- forme (-is). 24, Dictyonema (-dendron) Pei-'See also:alum with base of attachment. 25, D. cervicorne, branches (after Holm). 26, D. rarum (section after Wiman). 27, Dendrograptus Hallianus. 28, Synrhabdosome of Dipto- graptus (after Ruedemann). S, Sicula. u, Upper or apical portion. 1, Lower or apertural. as, Mouth.

N, Nema. nn, Nemacaulus or virgular tube. V, Virgula: vv, Virgella. zz, Septal strands. T, Theca. C, Common See also:

canal (in Retiolites). G, Gonangium. g, Gonotheca. b, Budding theca. 13, 14, 23, to four and these coalesce by their dorsal walls along the line of the nema, and the sicula becomes embedded in the base of the 3, 4a, 4b, 5, 6, 7, 8, 9, product of the normal or sexual mode of See also:propagation in the group, but owes its origin to a See also:peculiar type of budding or non-sexual reproduction, in which, as temporary resting or protecting structures, the vesicular bodies may have had a See also:share. As respects the mode of See also:life of the Graptolites there can be little doubt that the Dendroidea were, with some exceptions, sessile or benthonic animals, their polyparies, like those of the recent Calyptoblastea, growing upwards, their bases remaining attached to the sea See also:floor or to See also:foreign bodies, usually fixed. The Graptoloidea have also been regarded by some as benthonic organisms.

A more prevalent view, however, is that the majority were pseudo-planktonic or drifting colonies, See also:

hanging from the underside of floating seaweeds; their polyparies being each suspended by the nema in the earliest stages of growth, and, in later stages, some by the nemacaulus, while others became adherent above by means of a central disk or by parts of their dorsal walls. Some of these See also:ancient seaweeds may have remained permanently rooted in the littoral regions, while others may have become broken off and drifted, like the recent Sargassum, at the See also:mercy of the winds and currents, carrying the attached Graptolites into all latitudes. The more complex umbrella-shaped colonies of colonies (synrhabdosomes) described as provided with a common swimming bladder (pneumatopllore?) may have attained a holo-planktonic or See also:free-swimming mode of existence. The range of the Graptolites in time extends from the See also:Cambrian to the Carboniferous. The Dendroidea alone, however, have this extended range, the Graptoloidea becoming extinct at the close of See also:Silurian time. Both groups make their first appearance together near the end of the Cambrian; but while in the succeeding Ordovician and Silurian the Dendroidea are comparatively rare, the Graptoloidea become the most characteristic and, locally, the most abundant fossils of these systems. The species of the Graptoloidea have individually a remarkably short range in See also:geological time; but the See also:geographical See also:distribution of the group as a whole, and that of many of its species, is almost See also:world-wide. This See also:combination of circumstances has given the Graptoloidea a See also:paramount stratigraphical importance as palaeontological indices of the detailed sequence and correlation of the Lower Palaeozoic rocks in general. Many Graptolite zones, showing a See also:constant uniformity of See also:succession, paralleled in this respect only by the longer known Ammonite zones of the See also:Jurassic, have been distinguished in See also:Britain and See also:northern See also:Europe, each marked by a characteristic species. Many See also:British species and associations of genera and species, occurring on corresponding horizons to those on which they are found in Britain, have been met with in the graptolite-bearing Lower Palaeozoic formations of other parts of Europe, in See also:America, See also:Australia, New See also:Zealand and elsewhere.

End of Article: GRAPTOLITES

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