Editing Graptolite (section)

==Morphology==

=== Colony structure ===
[[File:Rhabdopleuratubes.png|thumb|150px|''[[Rhabdopleura compacta]]'' colony with creeping and erect tubes, showing the zigzag suture where the fuselli meet|left]]
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]], a true colonial system shared by ''Rhabdopleura'' but not ''Cephalodiscus''. These zooids are housed within an organic structure comprising a series of tubes secreted by the glands on the [[cephalic shield]]. The colony structure has been known from several different names, including coenecium (for living pterobranchs), rhabdosome (for fossil graptolites), and most commonly '''tubarium''' (for both). The individual tubes, each occupied by a single zooid, are known as '''theca'''.<ref name="Maletzgp" /> The composition of the tubarium is not clearly known, but different authors suggest it is made out of [[collagen]] or [[chitin]]. In some colonies, there are two sizes of theca, the larger autotheca and smaller bitheca, and it has been suggested that this difference is due to [[sexual dimorphism]] of zooids within a colony.<ref name="Maletzgp" />

Early in the development of a colony, the tubarium splits into a variable number of branches (known as '''stipes''') and different arrangements of the theca, features which are important in the identification of graptolite fossils. Colonies can be classified by their total number of theca rows (biserial colonies have two rows, uniserial have one) and the number of initial stipes per colony (multiramous colonies have many stipes, pauciramous colonies have two or one). Each thecal tube is mostly made up by two series of stacked semicircular half-rings, known as '''fuselli''' (sing: fusellum). The fuselli resemble growth lines when preserved in fossils, and the two stacks meet along a suture with a zig-zag pattern. Fuselli are the major reinforcing component of a tubarium, though they are assisted by one or more additional layers of looser tissue, the cortex.[[Image:Graptolites-EncBrit.jpg|thumb|295x295px|Diversity of graptolite colony forms]]

The earliest graptolites appeared in the fossil record during the Cambrian, and were generally [[Sessility (motility)|sessile]] animals, with a colony attached to the sea floor. Several early-diverging families were encrusting organisms, with the colony developing horizontally along a substrate. Extant ''Rhabdopleura'' fall into this category, with an overall encrusting colony form combined with erect, vertical theca. Most of the erect, dendritic or bushy/fan-shaped graptolites are classified as dendroids (order Dendroidea). Their colonies were attached to a hard substrate by their own weight via an attachment disc. Graptolites with relatively few branches were derived from the dendroid graptolites at the beginning of the [[Ordovician]] period. This latter major group, the graptoloids (order Graptoloidea) were [[pelagic]] and [[planktonic]], drifting freely through the water column. They were a successful and prolific group, being the most important and widespread macroplanktonic animals until they died out in the early part of the [[Devonian]] period. The dendroid graptolites survived until the [[Carboniferous]] period.

=== Zooids ===
[[File:Zooidgrap.jpg|thumb|A hypothetical graptolite zooid inside its theca, reconstructed based on living pterobranchs|left]]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.<ref name="Satodevbio" /> Proper fossils of the soft parts of graptolites have yet to be found, and it is not known if they had pharyngeal gill slits or not,<ref>[https://books.google.com/books?id=pTygDQAAQBAJ&dq=graptolite+zooid+soft+pharyngeal+gill&pg=PA218 Fundamentals of Invertebrate Palaeontology: Macrofossils]</ref> but based on extant ''[[Rhabdopleura]]'', it is likely that the grapotlite zooids had the same morphology.<ref name="Maletzgp" />