Graptolithina
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 (Miaolingian, Wuliuan) through the Lower Carboniferous (Mississippian).[3] A possible early graptolite, Chaunograptus, is known from the Middle Cambrian.[1] One analysis suggests that the pterobranch Rhabdopleura represents extant graptolites.[2] 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.[4]
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.[4]
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 (a genus of biserial graptolites). The term Graptolithina was established by Bronn in 1849 and later, Graptolithus was officially abandoned in 1954 by the ICZN.[5]
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.[6] 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.[3]
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 a 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. However, in the fossil record where mostly tubaria (tubes) are preserved, it is complicated to distinguish between groups.
Graptolithina includes two main 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 a great development during the Ordovician.[3] 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.
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