Carapace

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Carapace ( plural Carapaces ; of ancient Greek χάραξ Charax "Fortification, Palisade" and πάγιος págios "solid") is a term for a at different groups of animals ( taxa ) independently of one another resulting hard cover of the body top surface. In the case of turtles , the carapace is commonly known as the back shield or back armor , in crustaceans ( crustaceans in the kitchen) it is part of the " shell ".

Crustaceans

Location of the carapace (red) in crustaceans using the example of an Atlantic white shrimp ( Penaeus setiferus )

Many crustaceans (Crustacea) have a fold of skin that extends from the back of the head (segment of the 2nd maxilla ); this can also be absent primarily (e.g. cephalocarida ) or secondary (e.g. woodlice and amphipods ), so it is not part of the basic plan of the crustaceans. In many cases , the chitinous head fold is stiffened like an armor by embedded lime , especially in many decapods . If this structure as a back shield covers some or possibly all trunk segments, it is called a carapace.

The carapace therefore connects to the head, continues beyond its rear edge and extends more or less far over the body of the crab. Depending on the training, it can also cover the head itself (e.g. in the case of mussel crabs ) and be pulled down to the side to a greater or lesser extent. The carapace is a skin duplication of the cuticular exoskeleton , i. That is, it is double-layered and filled with tissue on the inside (at least one double-layer epidermis layer with its supply devices). The trunk below also has its own cuticle. In the decapods (Decapoda), such as crabs, shrimps and lobster-like , the back part of the carapace is as Notum denotes the lateral parts hot Pleuren (same name, but not homologous to the so-called side plates of the insects). The carapace is fundamentally unrelated to the structures of the original body segmentation (which it covers).

The degree of trunk coverage is different for the various cancer groups. The development of the ostracods and clawed tails (water fleas including the mussel shells ) goes furthest, here the carapace envelops the entire body. This extreme form consists of two shells, one on each side of the body, so it is two-lobed like a clam shell, because the shells have been completely split along the center line on the back ( dorsal ). With the claw tails, the shells merge into one another, which is why this design is called bilobed . In the so-called higher crabs, it is fused with a different number of segment tops ( tergites ) and forms a rigid unit with them. There are also cancer groups in which the carapace has not grown together with the covered segments at any point, for example in the "primeval cancer" Triops . The armor is only connected to the body at the rear edge of the head and lies loosely on the torso. The carapace can lengthen forward to form an unpaired, centrally ( median ) protrusion or extension, the rostrum.

Turtles

Back armor of a four-toed tortoise ( Testudo horsfieldii ), the growth strips of the shields can be seen
Scheme of the back armor with horny layer (left) and underlying bone layer (right)

The most striking of the many special features of the turtles (Testudinata) is the bony shell . Many turtles can hide themselves completely in it if necessary - protection from enemies or bad weather. Fossils of turtles from the Triassic show animals with incomplete armor, in one case a skin armor lying loosely on the ribs, in the other a belly armor without a back armor, but the characteristic reshaping of the ribs corresponds to that of today's turtles (see also the origin of the turtle shell ) . The "primeval turtles" of the genera Proterochersis and the only slightly younger Proganochelys from the more than 210 million year old Upper Triassic Stubensandstein in southwest Germany have a modern armor that is very similar to the (more recent ) representative living today .

Structure and shape

The shell of a turtle consists of the more or less strongly arched, dorsal carapace (back armor) and the flat, ventral plastron (belly armor), which are connected on the sides by a bone bridge. Large openings at the front and rear end allow the head and front legs or tail and hind limbs to protrude. Back and abdominal armor have the same basic structure: At the bottom lies a layer of firmly connected plates made of skin bones (dermal bones, osteoderms ). These massive bones form a rigid association and give the armor its shape and stability. A layer of skin stretches over it, which - with the exception of softshell turtles - is covered with thin shields (scales) made of horny substance ( keratin ). These are called "Scuta" ( Latin ; singular Scutum ) because of their shape , after the rectangular, arched wooden shield of the Roman legionaries . In adult specimens, the carapace is completely ossified and firmly fused with the shoulder girdle and the unusually high, broadened ribs as well as with parts of the pelvic girdle and the spinous processes of the spine. The overlying horn shields are slightly larger than the bone elements and arranged slightly offset, the resulting overlap of the spaces increases the stability of the shell.

The softshell turtles living in the water do not have horn shields, but rather flexible, leathery skin over a flat, round back shield, as they have lost the shields that were originally present in the course of their tribal history. The marginal bones are usually also missing. The articulated turtles have also undergone a special development . A hinge-like articulated connection in the rear third of the carapace allows them to close the shell completely.

Training of the back shield

In a simplified way, the bone plates and shields can be divided into groups according to their position on the carapace from the inside to the outside; the individual elements are counted from the front ( cranial ) to the back ( caudal ). The names of the bone plates often agree with those of the horn shields and are therefore not unambiguous, and there are many synonyms .

Bone plates

  • The unpaired, foremost plate in the middle ( median ) is called the nuchal or neck plate (Latin nucha "neck").
  • 8 Neuralia (referring to the neural arch of the vertebral body ) continue the median row. Only the neuralia are fused with the spinous processes of the spine.
  • The series is completed by the pygale (Greek pyge "rump"). A further 1 to 2 pygal plates, the suprapygalia, may appear above this.
  • On both sides ( lateral ) of the neuralia there are 8 costalia (Latin costa "rib"; syn .: pleuralia, Greek pleura "side", "rib") grown together with the ribs.
  • The edge of the bone armor consists of 11 marginalia (lat. Margo "edge") on both sides . Rarely are additional peripheral supramarginalia present, an original feature that most more advanced forms lack.

Horn shields (Scuta)

Generalized arrangement of the horn shields on the carapace
  • The unpaired, foremost median cervical or neck shield (syn .: cervicale, nuchale, precentral; Latin cervix "neck") is located in front of the spinal column shields. According to some authors, the neck shield is one of the vertebrals.
  • Then there are 5 vertebral or vertebral shields (Vertebralia, Syn .: Centralia) in a median row above the spine (Columna vertebralis).
  • On both sides of the vertebral shields there are 4 pleural, lateral or rib shields (Costalia, Syn .: Lateralia).
  • The carapace is framed by 12 marginal or edge shields (marginalia) on either side. The two rearmost, caudally located shields, the postcentralia, together form the supracaudal or tail shield (supracaudal, syn .: caudale; Latin cauda "tail").

The often conspicuously drawn and shaped, only a few millimeters thick scuta show a single growth strip around the respective shield center (areole) in a newly hatched turtle. As they grow, more stripes appear, but this happens at irregular intervals and is almost completely discontinued in older specimens, so the number of growth rings does not allow any conclusions to be drawn about age.
The shape of the horn shields, their number and relative position to one another, for example the number of rib shields that touch the neck shield, vary in the individual species. With the help of these characteristics, the species belonging to the living animal can be clearly determined. But the shield relief can also be very different. In some species the scuta form one longitudinal keels, in others a total of three. In the mute turtles, for example, the central keel (dorsal keel) emerges very clearly on the vertebral shields, other species show two additional lateral keelings along the side shields. Often there are also individual deviations, especially in the number of shields. Such shield anomalies are usually not genetically determined, but can be traced back to different breeding conditions of the eggs in the nest. The species-specific information on the number of horn shields therefore corresponds to statistical mean values.

The turtle shell is a stable, but by no means insensitive structure. Immediately below the horny shield is the periosteum of the dermal bones, which is permeated by numerous blood vessels and nerves and is therefore extremely pain-sensitive . Armor fractures can occur with strong mechanical impact .

Function and meaning

These hatchlings of the black- buttoned mute turtle ( Graptemys nigrinoda ) already show the characteristic three-humped central keel on the carapace

The curvature of the back armor can vary in strength. The turtles that are adapted to life in the water, such as the turkey turtles (Pleurodira) and the aquatic or amphibious species of the Halsberger turtles (Cryptodira), have flatter carapaces than their terrestrial representatives, the tortoises (Testudinidae), whose back armor is a characteristic Has dome-like shape. The stronger the curvature of the back armor, the greater the mechanical resilience and thus the protective effect. In addition, a high-arched carapace increases the ability of the cold-blooded animals to store heat. But there are restrictions: Species that are forced to dig deep caves to protect against extreme temperatures, such as the steppe or four-toed tortoise ( Testudo horsfieldii ), have shallower shells than those that live in regions with more moderate climates. The carapace of the sea ​​turtles also shows clear adaptations to the habitat and the way of life. It is flat and streamlined, and the bone content is also reduced to save weight.

The turtle shell is a unique development that has remained almost unchanged for more than 200 million years and has contributed significantly to the evolutionary success of this very ancient group of reptiles. Few enemies are able to overcome the shell of a full-grown turtle. Birds of prey drop turtles from great heights onto rocks, and they are also preyed on by crocodiles (Crocodilia). However, humans are the greatest threat. Tortoise shell , which was particularly popular in the 19th century, can be obtained from the horny substance of various sea turtles . Although the demand has fallen sharply, in order to minimize the hunting pressure on the animals, which are largely on the verge of extinction for other reasons, more than 148 states (as of 2000) have committed themselves in the CITES species protection convention to prohibit the trade in tortoiseshell since 1994 , including the bulk buyer Japan.

Arachnids

Male tarantula ( Grammostola cf. porteri ) with reddish carapace

With the arachnids (arachnida) the carapace (also Peltidium , to Gr.-Latin. Pelta "small crescent-shaped shield") is a uniform chitinous back shield from the fused segment tops (tergites) of the head ( cephalon ) and chest ( thorax ), the Can cover the front of the abdomen (the prosoma , usually imprecisely referred to as the cephalothorax ("head-chest")). If the peltidium consists of several pieces (in the scourge scorpions , palpebras and roller spiders ), the names are from front to back: Pro-, Meso- and Metapeltidium. In many species, the underside of the front abdomen is covered by a triangular abdominal plate, the sternum .

literature

  • Ulrich Lehmann: Paleontological Dictionary. 3. Edition. Ferdinand Enke Verlag, Stuttgart 1985, ISBN 3-423-03039-9 .
  • Robert L. Carroll: Paleontology and Evolution of the Vertebrates . Georg Thieme Verlag, Stuttgart 1993, ISBN 3-13-774401-6 .
  • Hans-Eckhard Gruner: 19th strain: Arthropoda, 1st class: Crustacea. In: HE Gruner, M. Moritz, W. Dunger (Hrsg.): Textbook of special zoology. (founded by Alfred Kästner). Volume I: Invertebrates. 4th part: Arthropoda (without Insecta). 4th edition. Gustav Fischer Verlag, Jena / Stuttgart / New York 1993, ISBN 3-334-60404-7 .

Web links

Individual evidence

  1. Chun Li, Xiao-Chun Wu, Olivier Rieppel, Li-Ting Wang, Li-Jun Zhao: An ancestral turtle from the Late Triassic of southwestern China. In: Nature. Volume 456, 27, 2008, pp. 497-501.
  2. ^ Walter G. Joyce, Spencer G. Lucas, Torsten M. Scheyer, Andrew B. Heckert, Adrian P. Hunt: A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell. In: Proceedings of the Royal Society. B 276, 2009, pp. 507-513.
This version was added to the list of articles worth reading on August 30, 2005 .