page_title: Graptolithina
section_title: Morphology
hierarchical_section_title: Graptolithina / Morphology
caption_attribution_description: English: Rhabdopleura compacta colony with creeping and erect tubes
context_page_description: Graptolithina is a subclass of the class Pterobranchia, the members of which are known as graptolites. These organisms are colonial animals known chiefly as fossils from the Middle Cambrian through the Lower Carboniferous. A possible early graptolite, Chaunograptus, is known from the Middle Cambrian. One analysis suggests that the pterobranch Rhabdopleura represents extant graptolites. Studies on the tubarium of fossil and living graptolites showed similarities in the basic fusellar construction and it is considered that the group most probably evolved from a Rhabdopleura-like ancestor.
  The name graptolite comes from the Greek graptos meaning "written", and lithos meaning "rock", as many graptolite fossils resemble hieroglyphs written on the rock. Linnaeus originally regarded them as 'pictures resembling fossils' rather than true fossils, though later workers supposed them to be related to the hydrozoans; now they are widely recognized as hemichordates.
context_section_description: Each graptolite colony originates from an initial individual, called the sicular zooid, from which the subsequent zooids will develop; they are all interconnected by stolons. These zooids are housed within an organic tubular structure called a theca, rhabsodome, coenoecium or tubarium, which is secreted by the glands on the cephalic shield.
  The composition of the tubarium is not clearly known but different authors suggest it is made out of collagen or chitin. The tubarium has a variable number of branches or stipes and different arrangements of the theca, these features are important in the identification of graptolite fossils. In some colonies, there are two sizes of theca, the authoteca and the bitheca, and it has been suggested that this difference is due to sexual dimorphism.
  A mature zooid has three important regions, the preoral disc or cephalic shield, the collar and the trunk. In the collar, the mouth and anus (U-shaped digestive system) and arms are found; Graptholitina has a single pair of arms with several paired tentacles. As a nervous system, graptolites have a simple layer of fibers between the epidermis and the basal lamina, also have a collar ganglion that gives rise to several nerve branches, similar to the neural tube of chordates.  All this information was inferred by the extant Rhabdopleura, however, it is very likely that fossil zooids had the same morphology. An important feature in the tubarium is the fusellum, which looks like lines of growth along the tube observed as semicircular rings in a zig-zag pattern.
  Most of the dendritic or bushy/fan-shaped organisms are classified as dendroid graptolites (order Dendroidea). They appear earlier in the fossil record during the Cambrian and were generally sessile animals. They lived attached to a hard substrate in the sea-floor, by their own weight as encrusting organisms or by an attachment disc. Graptolites with relatively few branches were derived from the dendroid graptolites at the beginning of the Ordovician period. This latter type (order Graptoloidea) were pelagic and planktonic, drifting freely on the surface of primitive seas. They were a successful and prolific group, being the most important animal members of the plankton until they partially died out in the early part of the Devonian period. The dendroid graptolites survived until the Carboniferous period.
caption: Rhabdopleura compacta colony with creeping and erect tubes.
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