Graptolite


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 through the Lower Carboniferous. 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 which have a basic structure of stacked half-rings. 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, 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.
Graptolite fossils were later referred to a variety of groups, including other branching colonial animals such as bryozoans 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.

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, rhabdosome, and most commonly tubarium. 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 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 and the number of initial stipes per colony. Each thecal tube is mostly made up by two series of stacked semicircular half-rings, known as fuselli. 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 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. 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 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 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, 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 are preserved, it is complicated to distinguish between groups.
Graptolithina includes several minor families as well as two main extinct orders, Dendroidea and Graptoloidea. The latter is the most diverse, including 5 suborders, where the most assorted is Axonophora. 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 :
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 Frech, 1897
  • *Family †Callograptidae Bulman, 1938
  • *Family †Mastigograptidae Bates & Urbanek, 2002
  • Order †Graptoloidea Lapworth, 1875 in Hopkinson & Lapworth, 1875
  • *Suborder †Graptodendroidina Mu & Lin, 1981 in Lin
  • **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
  • **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