Graptolite - WikiHQ

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.

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.

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. but based on extant Rhabdopleura, it is likely that the grapotlite zooids had the same morphology. 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.

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. 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.

Their locomotion was relative to the water mass in which they lived but the exact mechanisms (such as turbulence, buoyancy, active swimming, and so forth) are not clear yet. One proposal, put forward by Melchin and DeMont (1995), suggested that graptolite movement was analogous to modern free-swimming animals with heavy housing structures. In particular, they compared graptolites to "sea butterflies" (Thecostomata), small swimming pteropod snails. Under this suggestion, graptolites moved through rowing or swimming via an undulatory movement of paired muscular appendages developed from the cephalic shield or feeding tentacles. In some species, the thecal aperture was probably so restricted that the appendages hypothesis is not feasible. On the other hand, buoyancy is not supported by any extra thecal tissue or gas build-up control mechanism, and active swimming requires a lot of energetic waste, which would rather be used for the tubarium construction. Astogeny happens when the colony grows through asexual reproduction from the tip of a permanent terminal zooid, behind which the new zooids are budded from the stalk, a type of budding called monopodial. It is possible that in graptolite fossils the terminal zooid was not permanent because the new zooids formed from the tip of latest one, in other words, sympodial budding. These new organisms break a hole in the tubarium wall and start secreting their own tube. The significance of these discoveries is to understand the early vertebrate left-right asymmetry due to chordates being a sister group of hemichordates, and therefore, the asymmetry might be a feature that developed early in deuterostomes. Since the location of the structures is not strictly established, also in some enteropneusts, it is likely that asymmetrical states in hemichordates are not under a strong developmental or evolutionary constraint. The origin of this asymmetry, at least for the gonads, is possibly influenced by the direction of the basal coiling in the tubarium, by some intrinsic biological mechanisms in pterobranchs, or solely by environmental factors.

Geological relevance

Preservation

thumb|Pendeograptus fruticosus from the Bendigonian Australian Stage (Lower Ordovician; 477–474 mya) near [[Bendigo, Victoria, Australia. There are two overlapping, three-stiped rhabdosomes.]]

Graptolites are common fossils and have a worldwide distribution. They are most commonly found in shales and mudrocks where sea-bed fossils are rare, this type of rock having formed from sediment deposited in relatively deep water that had poor bottom circulation, was deficient in oxygen, and had no scavengers. The dead planktic graptolites, having sunk to the sea floor, would eventually become entombed in the sediment and were thus well preserved.

These colonial animals are also found in limestones and cherts, but generally these rocks were deposited in conditions which were more favorable for bottom-dwelling life, including scavengers, and undoubtedly most graptolite remains deposited here were generally eaten by other animals.

thumb|[[Cyrtograptus|Cyrtograptus canadensis from the middle Silurian of the Cape Phillips Formation, Nunavut. ]]

Fossils are often found flattened along the bedding plane of the rocks in which they occur, though may be found in three dimensions when they are infilled by iron pyrite or some other minerals. They vary in shape, but are most commonly dendritic or branching (such as Dictyonema), sawblade-like, or "tuning fork"-shaped (such as Didymograptus murchisoni). Their remains may be mistaken for fossil plants by the casual observer, as it has been the case for the first graptolite descriptions.

Graptolites are normally preserved as a black carbon film on the rock's surface or as light grey clay films in tectonically distorted rocks. The fossil can also appear stretched or distorted. This is due to the strata that the graptolite is within, being folded and compacted. They may be sometimes difficult to see, but by slanting the specimen to the light they reveal themselves as a shiny marking. Pyritized graptolite fossils are also found.

A well-known locality for graptolite fossils in Britain is Abereiddy Bay, Dyfed, Wales, where they occur in rocks from the Ordovician Period. Sites in the Southern Uplands of Scotland, the Lake District and Welsh Borders also yield rich and well-preserved graptolite faunas. A famous graptolite location in Scotland is Dob's Linn with species from the boundary Ordovician-Silurian. Since the group had a wide distribution, fossils are also abundant in several parts of the United States, Canada, Australia, Germany and China, among others.

Stratigraphy

Graptolite fossils have predictable preservation, widespread distribution, and gradual change over a geologic time scale. This allows them to be used to date strata of rocks throughout the world.

The Great Ordovician Biodiversification Event (GOBE) influenced changes in the morphology of the colonies and thecae, giving rise to new groups like the planktic Graptoloidea. Later, some of the greatest extinctions that affected the group were the Hirnantian in the Ordovician and the Lundgreni in the Silurian, where graptolite populations were dramatically reduced (see also Lilliput effect).

Graptolite diversity was greatly reduced during the Sedgwickii Event in the Aeronian. This event has been attested in locations such as today's Canada, Libya as well as in La Chilca Formation of Argentina (then part of Gondwana).