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Class Graptolithina

Abstract

This page presents a brief overview of the Graptolithina, from their origins until their extinction in the mid Devonian (Graptoloidea) and Upper Carboniferous (Dendroidea).

Keywords: Graptolithina, graptolite, Hemichordata, Pterobranchia, pterobranch, Graptoloidea, graptoloid, Dendroidea, dendroid

Introduction

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Related Topics

Further Reading

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Morphology

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Phylogeny and Evolution

Affinities

In the literature, graptolites are most commonly classed within the "phylum" Hemichordata. However, there is some doubt that the Hemichordata is a monophyletic group (e.g. Nielsen 2001, p. 416, who regards the Hemichordata as an artificial grouping of at least two "real" phyla: the Pterobranchia and the Enteropneusta). In any event, graptolites are thought to be related to the extant pterobranchs, most closely to the rhabdopleurids (Armstrong et al. 1984).

The Pterobranchia is a very small phylum, comprising only two or three genera: Rhabdopleura, Cephalodiscus, and possibly Atubaria, which has only been recorded once, and resembles Cephalodiscus but is supposedly not tube-building (Nielsen 2001, p. 411).

??? are these the only two genera ???

Fossil Record

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The pterobranchs also first appear in the Middle Cambrian. Rare occurrences are reported from throughout the Phanerozoic.

Origins

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Evolution

One of the most compelling facts about graptolite evolution is the interaction between grade and clade: A similar grade of organisation (e.g. four stipes) was reached by more than one clade (line of descent) of graptolites, at more or less the same time. The details of this noteably parallel evolution is an area of active research today. (After Palmer & Rickards 1991, pp. 71-72.)

Extinction

After a number of marked extinction events which repeated reduced graptoloid diversity [can i get a figure?], the diverse graptoloid assemblage finally disappeared entirely, fairly suddenly, for reasons that remain obscure but which coincided with the spread of fish into the planktonic realm. However, no plausible cause and effect relationship between these two events has been proposed; at this time simple coincidence seems the most likely conclusion. Koren' & Rickards 1979 suggests a link with changing land plant developments and global paleogeographical changes. The last surviving graptoloids occur in the early to mid Devonian. Even these forms exhibited considerable complexity and evolutionary change was evident right to the end.

 obtain a figure
However, the dendroids persisted, more or less unchanged, as a very minor benthic component into the Upper Carboniferous before they, too, became extinct.
The presumed relatives of graptolites, the pterobranchs, remain extant today.

(After Palmer & Rickards 1991, p. 22.)

Systematics

Early attempts to address the systematics of graptolites date back at least to Charles Lapworth's 1873 review, in which he laid the foundations of the modern graptolite families.
For many years, however, graptolite taxa were simply binned into form genera [® sidebar] based on the gross morphology of the colony – the number of stipes and their arrangement, whether or not the stipes were scandent, the closeness of the thecae – rather than the characteristics which, today, are believed to be good indicators of phylogeny. As a result, though many of the early-defined genera were extremely useful stratigraphically, especially in the field, they probably were not natural units.

Subsequently, the "good old days" of instant field identification have been sacrificed and the catch-all taxa have been split into units which more meaningfully indicate evolution. Similar developments are paralleled in the study of many other fossil groups.
Form genera (or form species, etc.) are groupings of organisms formulated for "convenience" rather than because they are believed to be good indications of true phylogenetic relationships. Usually they are defined on the basis of gross morphological characteristics which are easy to identify. For example, most trace fossils are form taxa: it is rare to know what kind of creature made them, and it is considered likely that quite unrelated organisms made similar traces. Similarly, fossil spores and pollen, especially the more ancient examples, are mostly form taxa, although they may be given names suggesting a relationship with some group of plants (e.g. Proteacidites).

Phylum Pterobranchia

Class Graptolithina [Author?]

<synonymy>

Type: [Taxon] [Authority]

Original Diagnosis: xxx

Description: xxx

Occurrence: Graptolites are found all over the world. They sometimes co-occur with acritarchs, brachiopods, sponges and trilobites, but more often occur alone.
Discussion: xxx

Summary of Orders etc. ...
The orders Archaeodendrida, Cameroidea, Crustoidea, Dithecoidea, Stolonoidea and Tuboidea are quite rare; lack of information prevents me from further describing these taxa here.

Order Dendroidea _Authority

<synonymy>

Type: [Taxon] [Authority]

Original Diagnosis: xxx

Description: xxx

Occurrence: xxx

Discussion: xxx

Order Graptoloidea _Authority

<synonymy>

Type: [Taxon] [Authority]

Original Diagnosis: xxx

Description: xxx

Occurrence: xxx

Discussion: xxx

Suborder Dichograptina _Authority

<synonymy>

Type: [Taxon] [Authority]

Original Diagnosis: xxx

Description: xxx

Occurrence: xxx

Discussion: xxx

Suborder Virgellina _Authority

<synonymy>

Type: [Taxon] [Authority]

Original Diagnosis: xxx

Description: xxx

Occurrence: xxx

Discussion: xxx

Beginning a Collection

Unless you are fortunate enough to live in an area where graptolites are found, you will either have to travel or else perhaps buy some specimens from a scientific supplies outlet or over the Internet. Graptolites are seldom seen advertised at the usual Internet fossil sites - they lack the popular appeal of trilobites and dinosaur bones - but companies such as Wards which have an educational emphasis will usually stock some.

Conclusion

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Further Information

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References

Armstrong, W.G.; Dilly, P.N.; Urbanek, A. 1984: Collagen in the Pterobranch Coenecium and the Problem of Graptolite Affinities. Lethaia 17: 145-152.

Koren', T.N.; Rickards, R.B. 1979: Extinction of the Graptolites. In Harris, A.L.; Holland, C.H.; Leake, B.E. (eds.) 1979: The Caledonides of the British Isles - Reviewed. Geological Society Special Publication 8: 457-466.

Lapworth, C. 1873:

Neilsen, Claus 2001: Animal Evolution. Oxford.

Palmer, Douglas; Rickards, Barrie (eds.) 1991: Graptolites - Writing in the Rocks. Boydell Press, 182 pp.
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