Sclerite

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The exoskeleton of a spiny lobster is made of a series of sclerites, connected by flexible joints. Monaco.Musee oceanographique089.jpg
The exoskeleton of a spiny lobster is made of a series of sclerites, connected by flexible joints.

A sclerite (Greek σκληρός , sklēros, meaning "hard") is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instead it refers most commonly to the hardened parts of arthropod exoskeletons and the internal spicules of invertebrates such as certain sponges and soft corals. In paleontology, a scleritome is the complete set of sclerites of an organism, often all that is known from fossil invertebrates.

Greek language language spoken in Greece, Cyprus and Southern Albania

Greek is an independent branch of the Indo-European family of languages, native to Greece, Cyprus and other parts of the Eastern Mediterranean and the Black Sea. It has the longest documented history of any living Indo-European language, spanning more than 3000 years of written records. Its writing system has been the Greek alphabet for the major part of its history; other systems, such as Linear B and the Cypriot syllabary, were used previously. The alphabet arose from the Phoenician script and was in turn the basis of the Latin, Cyrillic, Armenian, Coptic, Gothic, and many other writing systems.

Hardness is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. Some materials are harder than others. Macroscopic hardness is generally characterized by strong intermolecular bonds, but the behavior of solid materials under force is complex; therefore, there are different measurements of hardness: scratch hardness, indentation hardness, and rebound hardness.

Arthropod phylum of animals

An arthropod is an invertebrate animal having an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids, myriapods, and crustaceans. The term Arthropoda as originally proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora. Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthopods are bilaterally symmetrical and their body possesses an external skeleton. Some species have wings.

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Sclerites in combination

Sclerites may occur practically isolated in an organism, such as the sting of a cone shell. Also, they can be more or less scattered, such as tufts of defensive sharp, mineralised bristles as in many marine Polychaetes. Or, they can occur as structured, but unconnected or loosely connected arrays, such as the mineral "teeth" in the radula of many Mollusca, the valves of Chitons, the beak of Cephalopod, or the articulated exoskeletons of Arthropoda.

Stinger organ

A stinger, or sting, is a sharp organ found in various animals capable of injecting venom, usually by piercing the epidermis of another animal.

<i>Conus</i> genus of molluscs

Conus is a genus of predatory sea snails, or cone snails, marine gastropod mollusks in the family Conidae. Prior to 2009, cone snail species had all traditionally been grouped into the single genus Conus. However, Conus is now more precisely defined, and there are several other accepted genera of cone snails. For a list of the currently accepted genera, see Conidae.

Polychaete Class of annelids

The Polychaeta, also known as the bristle worms or polychaetes, are a paraphyletic class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. As such, polychaetes are sometimes referred to as bristle worms. More than 10,000 species are described in this class. Common representatives include the lugworm and the sandworm or clam worm Alitta.

When sclerites are organised into an unarticulated structure, that structure may be referred to as a scleritome, a term largely used in paleontology.

Paleontology Scientific study of prehistoric life

Paleontology or palaeontology is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene Epoch. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments. Paleontological observations have been documented as far back as the 5th century BC. The science became established in the 18th century as a result of Georges Cuvier's work on comparative anatomy, and developed rapidly in the 19th century. The term itself originates from Greek παλαιός, palaios, "old, ancient", ὄν, on, "being, creature" and λόγος, logos, "speech, thought, study".

Arthropods

Sclerites of cervix and thorax of Phlebotominae Parasite170028-fig10 Illustrated guide of Phlebotominae (Diptera, Psychodidae).png
Sclerites of cervix and thorax of Phlebotominae

In Arthropoda the hardening that produces sclerites is accomplished either by the cross-linking of protein chains in the exocuticle, a process called sclerotization, or by incorporation of minerals such as calcium carbonate into regions of the exoskeleton, or both. Thus, the arthropod exoskeleton is divided into numerous sclerites, joined by less-sclerotized, membranous regions or sutures.

Sclerotin is a component of the cuticles of various Arthropoda, most familiarly insects. It is formed by cross-linking members of particular classes of protein molecules, a biochemical process called sclerotization. Technically it amounts to a form of tanning. The resulting material increases the rigidity of an insect's chitinous exoskeleton. It is particularly prominent in the thicker, armoured parts of insect and arachnid integument, such as in the biting mouthparts and sclerites of scorpions and beetles.

Dorsal sclerites of a body segment, often plate-like, are known as tergites . Similarly the ventral sclerites of a body segment are referred to as sternites . Separate sclerites on the lateral aspects of body segments, the pleura, are called pleurites. [1]

Tergum

A tergum is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the base and posterior edge is called the apex or margin. A given tergum may be divided into hardened plates or sclerites commonly referred to as tergites. For a detailed explanation of the terminology, see Kinorhynchs have tergal and sternal plates too, though seemingly not homologous with those of arthropods.

Sternum (arthropod anatomy)

The sternum is the ventral portion of a segment of an arthropod thorax or abdomen.

The pleuron is a lateral sclerite of thoracic segment of an insect between the tergum and the sternum. The terms pro-, meso- and metapleuron are used respectively for the pleura of the first, second and third thoracic segments.

Invertebrates other than arthropods

Sclerites in clamps in Polyopisthocotylean monogeneans; various sclerites are in grey and black Journal.pone.0079155.g003 Clamps in various genera of Protomicrocotylidae.png
Sclerites in clamps in Polyopisthocotylean monogeneans; various sclerites are in grey and black

Wide ranges of sclerites of various kinds occur in various invertebrate phyla, including Polychaeta and Mollusca. Two taxa that routinely have the term applied however, are the soft corals and the Porifera. In both those groups certain of their structures contain mineralised spicules of silica or calcium carbonate that are of importance structurally and in defense.

In biology, a phylum is a level of classification or taxonomic rank below Kingdom and above Class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia or Metazoa contains approximately 35 phyla, the plant kingdom Plantae contains about 14, and the fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.

Mollusca Large phylum of invertebrate animals

Mollusca is the second largest phylum of invertebrate animals. The members are known as molluscs or mollusks. Around 85,000 extant species of molluscs are recognized. The number of fossil species is estimated between 60,000 and 100,000 additional species.

Taxon Group of one or more populations of an organism or organisms which have distinguishing charachterisics in common

In biology, a taxon is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and given a particular ranking, especially if and when it is accepted or becomes established. It is not uncommon, however, for taxonomists to remain at odds over what belongs to a taxon and the criteria used for inclusion. If a taxon is given a formal scientific name, its use is then governed by one of the nomenclature codes specifying which scientific name is correct for a particular grouping.

Many other invertebrates grow a few hard parts, largely mineralised, as statoliths and similar structures, but those are not generally referred to as sclerites.

Clamps, the main attachment structure of the parasitic Polyopisthocotylean monogenean flatworms, [2] [3] are composed of various sclerites and associated musculature, located on a posterior organ called the haptor. Clamps are specialized structures attached to the host fish, generally to its gill.

Prehistory

The fossilized scleritome of Wiwaxia corrugata Wiwaxia Smithsonian.JPG
The fossilized scleritome of Wiwaxia corrugata

A scleritome is a skeleton made of isolated elements, like the scales of organisms such as the halwaxiids, the teeth in a radula, [4] spicules in a sponge skeleton, or the elements in a conodont apparatus. [5] The term was coined by the palaeontologist Stefan Bengtson. [5]

Although sclerites are of considerable importance in the study of extant animals, in palaeontology they are of far greater relative importance because they often are the only parts of an animal that fossilise at all, let alone well or clearly. Many extinct groups are known only from sclerites, leaving moot the question of what their gross anatomy might have looked like.

An example of the use of the term in paleontology is to describe hollow calcium carbonate, calcium sulfate or calcium phosphate plates grown as body armor by a number of Early Cambrian animals. Unlike sponge spicules, Early Cambrian sclerites appear to be external armor rather than internal structural elements. Sclerites are found on a curious collection of early animals including a common spongelike animal called Chancelloria ; an armored slug-like form Wiwaxia ; an armored worm with a pair of brachiopod-like shells Halkieria ; and another armored worm Microdictyon that is generally considered to be a lobopod/onychophore.

It has been suggested that the sclerites of the Cambrian Wiwaxia are homologous with the bristles of annelid worms. [6] At least one modern gastropod mollusc living near deep sea hydrothermal vents has structures made of iron sulfides similar to some Cambrian sclerites. [7]

Scaly-foot gastropod is the only gastropod with sclerites. Chrysomallon squamiferum 2.png
Scaly-foot gastropod is the only gastropod with sclerites.

Related Research Articles

Skeleton body part that forms the supporting structure of an organism

The skeleton is the body part that forms the supporting structure of an organism. It can also be seen as the bony frame work of the body which provides support, shape and protection to the soft tissues and delicate organs in animals. There are several different skeletal types: the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside the body, the hydroskeleton, and the cytoskeleton. The term comes from Greek σκελετός (skeletós), meaning 'dried up'.

Sponge Animals of the phylum Porifera

Sponges, the members of the phylum Porifera, are a basal Metazoa (animal) clade as a sister of the Diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. The branch of zoology that studies sponges is known as spongiology.

Spicules are any of various small needle-like anatomical structures occurring in organisms

Exoskeleton External skeleton of an organism

An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton (endoskeleton) of, for example, a human. In usage, some of the larger kinds of exoskeletons are known as "shells". Examples of animals with exoskeletons include insects such as grasshoppers and cockroaches, and crustaceans such as crabs and lobsters. The shells of certain sponges and the various groups of shelled molluscs, including those of snails, clams, tusk shells, chitons and nautilus, are also exoskeletons. Some animals, such as the tortoise, have both an endoskeleton and an exoskeleton.

<i>Microdictyon</i> genus of primitive animal (fossil)

Microdictyon is an extinct "armored worm" coated with net-like scleritic scales, known from the Early Cambrian Maotianshan shale of Yunnan China. Microdictyon is sometimes included in a somewhat ill-defined Phylum – Lobopodia – that includes several other odd worm-like and segmented free-swimming animals that do not appear to be arthropods or worms. The phylum includes Microdictyon, Onychodictyon, Cardiodictyon, Luolishania, Paucipodia, as well as the Anomalocaridids. The isolated sclerites of Microdictyon are known from other Lower Cambrian deposits. Microdictyon sclerites appear to have moulted; one sclerite seems to have been preserved during ecdysis.

<i>Wiwaxia</i>

Wiwaxia is a genus of soft-bodied animals that were covered in carbonaceous scales and spines that protected it from predators. Wiwaxia fossils – mainly isolated scales, but sometimes complete, articulated fossils – are known from early Cambrian and middle Cambrian fossil deposits across the globe. The living animal would have measured up to 5 cm (2 inch) when fully grown, although a range of juvenile specimens are known, the smallest being 2 millimetres (0.079 in) long.

Halkieriid Family of molluscs

The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is Halkieria, which has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages. The best known species is Halkieria evangelista, from the North Greenland Sirius Passet Lagerstätte, in which complete specimens were collected on an expedition in 1989. The fossils were described by Simon Conway Morris and John Peel in a short paper in 1990 in the journal Nature. Later a more thorough description was undertaken in 1995 in the journal Philosophical Transactions of the Royal Society of London and wider evolutionary implications were posed.

<i>Odontogriphus</i>

Odontogriphus is a genus of soft-bodied animals known from middle Cambrian Lagerstätte. Reaching as much as 12.5 centimetres (4.9 in) in length, Odontogriphus is a flat, oval bilaterian which apparently had a single muscular foot, and a "shell" on its back that was moderately rigid but of a material unsuited to fossilization.

<i>Orthrozanclus</i>

Orthrozanclus reburrus is a sea creature known from the Middle Cambrian Burgess shale, about one centimeter long, with long spikes protruding from its armored body

Halwaxiida

Halwaxiida or halwaxiids is a proposed clade equivalent to the older orders Sachitida He 1980 and Thambetolepidea Jell 1981, loosely uniting scale-bearing Cambrian animals, which may lie in the stem group to molluscs or lophotrochozoa. Some palaeontologists question the validity of the Halwaxiida clade.

The Chancelloriids are an extinct family of animal common in sediments from the Early Cambrian to the early Late Cambrian. Many of these fossils consists only of spines and other fragments, and it is not certain that they belong to the same type of organism. Other specimens appear to be more complete and to represent sessile, bag-like organisms with a soft skin armored with star-shaped calcareous sclerites from which radiate sharp spines.

Marine invertebrates

Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some have evolved a shell or a hard exoskeleton. As on land and in the air, marine invertebrates have a large variety of body plans, and have been categorised into over 30 phyla. They make up most of the macroscopic life in the oceans.

The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian Period. They are very diverse, and there is no formal definition of "small shelly fauna" or "small shelly fossils". Almost all are from earlier rocks than more familiar fossils such as trilobites. Since most SSFs were preserved by being covered quickly with phosphate and this method of preservation is mainly limited to the Late Ediacaran and Early Cambrian periods, the animals that made them may actually have arisen earlier and persisted after this time span.

Since 1990 there has been intense debate among paleontologists about the evolution in the Early Cambrian period of the "super-phylum" Lophotrochozoa, which is thought to include the modern molluscs, annelid worms and brachiopods, as well as their evolutionary "aunts" and "cousins".

<i>Canadia</i> (annelid) species of annelid (fossil)

Canadia is a genus of extinct annelid worm present in Burgess Shale type Konservat-Lagerstätte. It is found in strata dating back to the Delamaran stage of the Middle Cambrian around 505 million years ago, during the time of the Cambrian explosion. It was about 3 centimeters in length. Charles Doolittle Walcott named Canadia in 1911 after Canada, the country from which its remains have been found. 28 specimens of Canadia (annelid) are known from the Greater Phyllopod bed, where they comprise 0.05% of the community.

<i>Volborthella</i> genus of animal (fossil)

Volborthella is an animal of uncertain classification, whose fossils pre-date 530 million years ago. It has been considered for a period a cephalopod. However discoveries of more detailed fossils showed that Volborthella’s small, conical shell was not secreted but built from grains of the mineral silicon dioxide (silica), and that it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. This illusion was a result of the laminated texture of the organisms' tests. Therefore, Volborthella’s classification is now uncertain. It has been speculated that it may in fact represent a sclerite of a larger organism, on the basis of one specimen; however, it may be premature to accept this hypothesis, as the arrangement of sclerites producing this impression may have occurred by chance. The Ordovician scleritome-bearing Curviconophorus, as well as the Halwaxiids, lobopods and echinoderms, demonstrate the diversity of organisms which may produce a scleritome of this nature. The related Campitius was originally suggested to be part of a radula rather than a scleritome, but is now considered a synonym of Volborthella.

Tommotiids are Cambrian (Terreneuvian) shelly fossils thought to belong to the Brachiopod + Phoronid lineage (Brachiozoa).

The fossils of the Burgess Shale, like the Burgess Shale itself, formed around 505 million years ago in the Mid Cambrian period. They were discovered in Canada in 1886, and Charles Doolittle Walcott collected over 60,000 specimens in a series of field trips up from 1909 to 1924. After a period of neglect from the 1930s to the early 1960s, new excavations and re-examinations of Walcott's collection continue to discover new species, and statistical analysis suggests discoveries will continue for the foreseeable future. Stephen Jay Gould's book Wonderful Life describes the history of discovery up to the early 1980s, although his analysis of the implications for evolution has been contested.

Arthropod cuticle

The cuticle forms the major part of the integument of the Arthropoda. It includes most of the material of the exoskeleton of the insects, Crustacea, Arachnida, and Myriapoda.

References

  1. Richards, O. W.; Davies, R.G. (1977). Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology. Berlin: Springer. ISBN   0-412-61390-5.
  2. Bychowsky, B. E. (1961). Monogenetic Trematodes their systematics and phylogeny. English translation edited by W. J. Hargis Jr. Washington: American Institute of Biological Sciences.
  3. Kearn, G. C. (2004). Leeches, Lice and Lampreys. A natural history of skin and gill parasites of fishes. Dordrecht: Springer.
  4. Butterfield, N. J. (2008). "An Early Cambrian Radula". Journal of Paleontology. 82 (3): 543–554. doi:10.1666/07-066.1.
  5. 1 2 Bengtson, S. (1985). "Taxonomy of Disarticulated Fossils". Journal of Paleontology. 59 (6): 1350–1358. JSTOR   1304949.
  6. Butterfield, N. J. (1990). "A Reassessment of the Enigmatic Burgess Shale Fossil Wiwaxia corrugata (Matthew) and Its Relationship to the Polychaete Canadia spinosa Walcott". Paleobiology . 16 (3): 287–303. JSTOR   2400789.
  7. Anders Warén, Stefan Bengtson, Shana K. Goffredi & Cindy L. Van Dover (2003). "A hot-vent gastropod with iron sulfide dermal sclerites" (PDF). Science . 302 (5647): 1007. doi:10.1126/science.1087696. PMID   14605361.