Graptolite

Source: Wikipedia, the free encyclopedia.
Not to be confused with Graptolitha, a genus of moths.

Graptolites
Temporal range: Mid
Ma
 Survive to the present via the living genus Rhabdopleura.[2]
Cryptograptus from the Silurian of South America. Specimen at the Royal Ontario Museum
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Hemichordata
Class: Pterobranchia
Subclass: Graptolithina
Bronn, 1849
Subgroups

Graptolites are a group of

Lower Carboniferous (Mississippian).[3] A possible early graptolite, Chaunograptus, is known from the Middle Cambrian.[1] 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.[2]

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

index fossils for the Ordovician and Silurian periods.[4]

The name graptolite comes from the

hemichordates.[4]

History

The name "graptolite" originates from the genus Graptolithus ("writing on the rocks"), which was used by

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.[5]

Morphology

Colony structure

Rhabdopleura compacta colony with creeping and erect tubes, showing the zigzag suture where the fuselli meet

Each graptolite colony originates from an initial individual, called the sicular zooid, from which the subsequent

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.[4] 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.[4]

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.

Diversity of graptolite colony forms

The earliest graptolites appeared in the fossil record during the Cambrian, and were generally

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 hypothetical graptolite zooid inside its theca, reconstructed based on living pterobranchs

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.[6] 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,[7] but based on extant Rhabdopleura, it is likely that the grapotlite zooids had the same morphology.[4]

Taxonomy

Further information: List of graptolite genera

Since the 1970s, as a result of advances in

Hemichordata.[8] 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,

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.

Phylogeny of Pterobranchia[3]
Graptolithina

Rhabdopleurida

Eugraptolithina

Dendroidea

Graptoloidea

Graptolithina includes several minor families as well as two main extinct orders, Dendroidea (

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.

Taxonomy of Graptolithina by Maletz (2014):[3][4]

Subclass Graptolithina Bronn, 1849

Ecology

Hypothetical zooid inspired by modern pteropods, with swimming appendages developed from the cephalic shield.

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.[9]

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

mesopelagic zone, from inshore to open ocean.[10] 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,

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.[10]

There are still many questions regarding graptolite locomotion but all these mechanisms are possible alternatives depending on the species and its habitat. For benthic species, that lived attached to the sediment or any other organism, this was not a problem; the zooids were able to move but restricted within the tubarium. Although this zooid movement is possible in both planktic and benthic species, it is limited by the stolon but is particularly useful for feeding. Using their arms and tentacles, which are close to the mouth, they filter the water to catch any particles of food.[10]

Life cycle

The study of the developmental biology of Graptholitina has been possible by the discovery of the species R. compacta and R. normani in shallow waters; it is assumed that graptolite fossils had a similar development as their extant representatives. The life cycle comprises two events, the ontogeny and the astogeny, where the main difference is whether the development is happening in the individual organism or in the modular growth of the colony.

The life cycle begins with a planktonic planula-like larva produced by sexual reproduction, which later becomes the sicular zooid who starts a colony. In Rhabdopleura, the colonies bear male and female zooids but fertilized eggs are incubated in the female tubarium, and stay there until they become larvae able to swim (after 4–7 days) to settle away to start a new colony. Each larva surrounds itself in a protective cocoon where the metamorphosis to the zooid takes place (7–10 days) and attaches with the posterior part of the body, where the stalk will eventually develop.[4]

The development is indirect and

sympodial budding. These new organisms break a hole in the tubarium wall and start secreting their own tube.[4]

Graptolites in evolutionary development

Left and right gonads (g) in Rhabdopleura compacta.

In recent years, living graptolites have been used as a hemichordate model for

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.[12]

Saccoglossus kowalevskii. An important conserved glycine–cysteine–phenylalanine (GCF) motif at the site of autocatalytic cleavage in hh genes, is altered in R. compacta by an insertion of the amino acid threonine (T) in the N-terminal, and in S. kowalesvskii there is a replacement of serine (S) for glycine (G). This mutation decreases the efficiency of the autoproteolytic cleavage and therefore, the signalling function of the protein. It is not clear how this unique mechanism occurred in evolution and the effects it has in the group, but, if it has persisted over millions of years, it implies a functional and genetic advantage.[13]

Geological relevance

Preservation

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.

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

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

Pyritized
graptolite fossils are also found.

A well-known locality for graptolite fossils in Britain is

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

Palaeozoic rocks as they evolved rapidly with time and formed many different distinctive species. Geologists can divide the rocks of the Ordovician and Silurian periods into graptolite biozones; these are generally less than one million years in duration. A worldwide ice age at the end of the Ordovician eliminated most graptolites except the neograptines. Diversification from the neograptines that survived the Ordovician glaciation began around 2 million years later.[14]

The

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).[4][15]

Graptolite diversity was greatly reduced during the Sedgwickii Event in the Aeronian.[16] 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).[16]

EdiacaranCambrianCambrianOrdovicianOrdovicianSilurianSilurianDevonianDevonianCarboniferousCarboniferousPermianPermianTriassicTriassicJurassicCretaceousTertiaryPrecambrianPaleozoicMesozoicCenozoicplanktonicmya (unit)
Ranges of Graptolite taxa.

Researchers

The following is a selection of graptolite and pterobranch researchers:[4]

See also

References

  1. ^ a b Maletz, J. (2014). Hemichordata (Pterobranchia, Enteropneusta) and the fossil record. Palaeogeography, Palaeoclimatology, Palaeoecology, 398:16-27.
  2. ^ a b Mitchell, C.E., Melchin, M.J., Cameron, C.B. & Maletz, J. (2013) Phylogenetic analysis reveals that Rhabdopleura is an extant graptolite. Lethaia, 46:34–56.
  3. ^
    ISSN 1214-1119
    .
  4. ^
    ISBN 9781118515617
    .
  5. ^ 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.
  6. ^ a b Sato, A., Bishop, J. & Holland, P. (2008). Developmental Biology of Pterobranch Hemichordates: History and Perspectives. Genesis, 46:587-591.
  7. ^ Fundamentals of Invertebrate Palaeontology: Macrofossils
  8. ^ a b Fortey, Richard A. (1998). Life: A Natural History of the First Four Billion Years of Life on Earth. New York: Alfred A. Knopf. p. 129.
  9. ^ "Graptolites". samnoblemuseum.ou.edu. Retrieved 2018-12-28.
  10. ^ a b c Cooper, R., Rigby, S., Loydell, D. & Bates, D. (2012) Palaeoecology of the Graptoloidea. Earth-Science Reviews, 112(1):23-41.
  11. ^ Röttinger, E. & Lowe, C. (2012) Evolutionary crossroads in developmental biology: hemichordates. Development, 139:2463-2475.
  12. ^ a b Sato, A. & Holland, P. (2008). Asymmetry in a Pterobranch Hemichordate and the Evolution of Left-Right Patterning. Developmental Dynamics, 237:3634 –3639)
  13. ^ Sato, A., White-Cooper, H., Doggett, K. & Holland, P. 2009. Degenerate evolution of the hedgehog gene in a hemichordate lineage. Proceedings of the National Academy of Sciences, 106(18):7491-7494.
  14. ^ Bapst, D., Bullock, P., Melchin, M., Sheets, D. & Mitchell, C. (2012) Graptoloid diversity and disparity became decoupled during the Ordovician mass extinction. Proceedings of the National Academy of Sciences, 109(9):3428-3433.
  15. doi:10.1080/10292389309380442
    .
  16. ^
    doi:10.5027/andgeov50n2-3617
    .

External links

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