Sclerite
Encyclopedia
A sclerite is a hardened body part. The term is used in various branches of biology for various structures including hardened portions of sponges, but it is most commonly used for the hardened portions of arthropod
exoskeleton
s.
, or by incorporation of minerals such as calcium carbonate into regions of the exoskeleton. Thus, the arthropod exoskeleton is divided into numerous sclerites, joined by less-sclerotized, membranous regions or sutures.
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.
As a matter of practical convenience the shapes and arrangement of the different sclerites provide most of the morphological features that are used as characters when reconstructing the phylogenetic relationships among different lineages; they provide the foundation for arthropod systematics
, primarily through the ability of the systematist to accurately assess the homologies
of the different sclerites, as to how they may be lost, gained, fused, divided, or otherwise modified from one lineage to another.
For example, the sclerite called the mandible
in insects varies dramatically in form between orders
, but all the different variations are homologous.
, 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.
At least one modern gastropod
mollusc
living near deep sea hydrothermal vents has structures made of iron sulfides similar to some Cambrian sclerites.
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
s.
Arthropods
In arthropods this hardening is accomplished either by the cross-linking of protein chains in the exocuticle, a process called sclerotizationSclerotin
Sclerotin is a component of the cuticles of various Arthropoda, most particularly insects. It is formed by cross-linking members of particular classes of protein molecules, a biochemical process called sclerotization, but technically. The resulting material adds physical stiffness to the toughness...
, or by incorporation of minerals such as calcium carbonate into regions of the exoskeleton. Thus, the arthropod exoskeleton is divided into numerous sclerites, joined by less-sclerotized, membranous regions or sutures.
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.
As a matter of practical convenience the shapes and arrangement of the different sclerites provide most of the morphological features that are used as characters when reconstructing the phylogenetic relationships among different lineages; they provide the foundation for arthropod systematics
Systematics
Biological systematics is the study of the diversification of terrestrial life, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees...
, primarily through the ability of the systematist to accurately assess the homologies
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
of the different sclerites, as to how they may be lost, gained, fused, divided, or otherwise modified from one lineage to another.
For example, the sclerite called the mandible
Mandible (arthropod)
thumb|250px|The mandibles of a [[Bull ant]]The mandible of an arthropod is either of a pair of mouthparts used for biting, cutting and holding food. Mandibles are often simply referred to as jaws. Mandibles are present in the extant subphyla Myriapoda , Crustacea and Hexapoda...
in insects varies dramatically in form between orders
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
, but all the different variations are homologous.
Prehistory
One specialized usage is to describe hollow calcium carbonateCalcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
, calcium sulfate
Calcium sulfate
Calcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...
or calcium phosphate
Calcium phosphate
Calcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
plates grown as body armor by a number of Early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
animals. Unlike sponge spicules, Early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
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
Chancelloria
Chancelloria is a genus of early animal known from the Middle Cambrian Burgess Shale, the Comley limestone, the Wheeler Shale,, the Bright Angel Shale and elsewhere. It was first described in 1920 by Charles Doolittle Walcott, who regarded them as one of the most primitive groups of sponges...
; an armored slug-like form Wiwaxia
Wiwaxia
Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...
; an armored worm with a pair of brachiopod-like shells Halkieria
Halkieria
Halkieria is a genus of fossil organisms from the Lower to Middle Cambrian. It has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages...
; and another armored worm Microdictyon
Microdictyon
Microdictyon is an extinct "armored worm" coated with dot-likescleritic scales, known from the Early CambrianMaotianshan shale of Yunnan China. Microdictyon is sometimes included...
that is generally considered to be a lobopod
Lobopodia
Lobopodia is a group of poorly understood animals, which mostly fall as a stem group of arthropods. Their fossil range dates back to the Early Cambrian. Lobopods are segmented and typically bear legs with hooked claws on their ends....
/onychophore.
It has been suggested that the sclerites of the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
Wiwaxia
Wiwaxia
Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...
are homologous with the bristles of annelid worms.
At least one modern gastropod
Gastropoda
The Gastropoda or gastropods, more commonly known as snails and slugs, are a large taxonomic class within the phylum Mollusca. The class Gastropoda includes snails and slugs of all kinds and all sizes from microscopic to quite large...
mollusc
Mollusca
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...
living near deep sea hydrothermal vents has structures made of iron sulfides similar to some Cambrian sclerites.