Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian (Miaolingian, Wuliuan) through the Lower Carboniferous (Mississippian). A possible early graptolite, Chaunograptus, is known from the Middle Cambrian. Recent analyses have favored the idea that the living pterobranch Rhabdopleura represents an extant graptolite which diverged from the rest of the group in the Cambrian.
Fossil graptolites and Rhabdopleura share a colony structure of interconnected zooids housed in organic tubes (theca) which have a basic structure of stacked half-rings (fuselli). Most extinct graptolites belong to two major orders: the bush-like sessile Dendroidea and the planktonic, free-floating Graptoloidea. These orders most likely evolved from encrusting pterobranchs similar to Rhabdopleura. Due to their widespread abundance, planktonic lifestyle, and well-traced evolutionary trends, graptoloids in particular are useful index fossils for the Ordovician and Silurian periods.
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.
History The name "graptolite" originates from the genus Graptolithus ("writing on the rocks"), which was used by Linnaeus in 1735 for inorganic mineralizations and incrustations which resembled actual fossils. In 1768, in the 12th volume of Systema Naturae, he included G. sagittarius and G. scalaris, respectively a possible plant fossil and a possible graptolite. In his 1751 Skånska Resa, he included a figure of a "fossil or graptolite of a strange kind" currently thought to be a type of Climacograptus (a genus of biserial graptolites).
Graptolite fossils were later referred to a variety of groups, including other branching colonial animals such as bryozoans ("moss animals") and hydrozoans. The term Graptolithina was established by Bronn in 1849, who considered them to represent orthoconic cephalopods. By the mid-20th century, graptolites were recognized as a unique group closely related to living pterobranchs in the genera Rhabdopleura and Cephalodiscus, which had been described in the late 19th century. Graptolithus, as a genus, was officially abandoned in 1954 by the ICZN.Bulman, M. (1970) In Teichert, C. (ed.). Treatise on Invertebrate Paleontology. Part V. Graptolithina, with sections on Enteropneusta and Pterobranchia. (2nd Edition). Geological Society of America and University of Kansas Press, Boulder, Colorado and Lawrence, Kansas, XXXII + 163 pp.
Morphology
Colony structure 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. 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.
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.
The earliest graptolites appeared in the fossil record during the Cambrian, and were generally 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 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. 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,Fundamentals of Invertebrate Palaeontology: Macrofossils but based on extant Rhabdopleura, it is likely that the grapotlite zooids had the same morphology.
Taxonomy
Since the 1970s, as a result of advances in electron microscopy, graptolites have generally been thought to be most closely allied to the pterobranchs, a rare group of modern marine animals belonging to the phylum Hemichordata. Comparisons are drawn with the modern hemichordates Cephalodiscus and Rhabdopleura. According to recent phylogenetic studies, rhabdopleurids are placed within the Graptolithina. Nonetheless, they are considered an incertae sedis family.
On the other hand, Cephalodiscida is considered to be a sister subclass of Graptolithina. One of the main differences between these two groups is that Cephalodiscida species are not colonial organisms. In Cephalodiscida organisms, there is no common canal connecting all zooids. Cephalodiscida zooids have several arms, while Graptolithina zooids have only one pair of arms. Other differences include the type of early development, the gonads, the presence or absence of gill slits, and the size of the zooids. In the fossil record, where mostly tubaria (tubes) are preserved, it is complicated to distinguish between groups.
Graptolithina includes several minor families as well as two main extinct orders, Dendroidea (benthic graptolites) and Graptoloidea (planktic graptolites). The latter is the most diverse, including 5 suborders, where the most assorted is Axonophora (biserial graptolites, etc.). This group includes Diplograptids and Neograptids, groups that had great development during the Ordovician. Old taxonomic classifications consider the orders Dendroidea, Tuboidea, Camaroidea, Crustoidea, Stolonoidea, Graptoloidea, and Dithecoidea but new classifications embedded them into Graptoloidea at different taxonomic levels.
Taxonomy of Graptolithina by Maletz (2014):
Subclass Graptolithina Bronn, 1849 Incertae sedis Family Rhabdopleuridae Harmer, 1905 Family †Cysticamaridae Bulman, 1955 Family †Wimanicrustidae Bulman, 1970 Family †Dithecodendridae Obut, 1964 Family †Cyclograptidae Bulman, 1938 Order †Dendroidea Nicholson, 1872 Family †Dendrograptidae Roemer, 1897 in French, 1897 Family †Callograptidae Bulman, 1938 Family †Mastigograptidae Bates & Urbanek, 2002 Order †Graptoloidea Lapworth, 1875 in Hopkinson & Lapworth, 1875 (planktic graptolites) Suborder †Graptodendroidina Mu & Lin, 1981 in Lin (1981) Family †Anisograptidae Bulman, 1950 Suborder †Sinograpta Maletz et al., 2009 Family †Sigmagraptidae Cooper & Fortey, 1982 Family †Sinograptidae Mu, 1957 Family †Abrograptidae Mu, 1958 Suborder †Dichograptina Lapworth, 1873 Family †Dichograptidae Lapworth, 1873 Family †Didymograptidae Mu, 1950 Family †Pterograptidae Mu, 1950 Family †Tetragraptidae Frech, 1897 Suborder †Glossograptina Jaanusson, 1960 Family †Isograptidae Harris, 1933 Family †Glossograptidae Lapworth, 1873 Suborder †Axonophora Frech, 1897 (biserial graptolites, and also retiolitids and monograptids) Infraorder †Diplograptina Lapworth, 1880 Family †Diplograptidae Lapworth, 1873 Subfamily †Diplograptinae Lapworth, 1873 Subfamily †Orthograptinae Mitchell, 1987 Family †Lasiograptidae Lapworth, 1880e Family †Climacograptidae Frech, 1897 Family †Dicranograptidae Lapworth, 1873 Subfamily †Dicranograptinae Lapworth, 1873 Subfamily †Nemagraptinae Lapworth, 1873 Infraorder †Neograptina Štorch et al., 2011 Family †Normalograptidae Štorch & Serpagli, 1993 Family †Neodiplograptidae Melchin et al., 2011 Subfamily †Neodiplograptinae Melchin et al., 2011 Subfamily †Petalolithinae Bulman, 1955 Superfamily †Retiolitoidea Lapworth, 1873 Family †Retiolitidae Lapworth, 1873 Subfamily †Retiolitinae Lapworth, 1873 Subfamily †Plectograptinae Bouček & Münch, 1952 Superfamily †Monograptoidea Lapworth, 1873 Family †Dimorphograptidae Elles & Wood, 1908 Family †Monograptidae Lapworth, 1873
Ecology Graptolites were a major component of the early Paleozoic ecosystems, especially for the zooplankton because the most abundant and diverse species were planktonic. Graptolites were most likely suspension feeders and strained the water for food such as plankton.
Inferring by analogy with modern pterobranchs, they were able to migrate vertically through the water column for feeding efficiency and to avoid predators. With ecological models and studies of the facies, it was observed that, at least for Ordovician species, some groups of species are largely confined to the epipelagic and mesopelagic zone, from inshore to open ocean.Cooper, R., Rigby, S., Loydell, D. & Bates, D. (2012) Palaeoecology of the Graptoloidea. Earth-Science Reviews, 112(1):23-41. Living rhabdopleura have been found in deep waters in several regions of Europe and America but the distribution might be biased by sampling efforts; colonies are usually found as epibionts of shells.