Turtle shell diagram

Turtle shell diagram DEFAULT

Tortoises and turtles are the only reptiles with tough, bony shells. The shell is like a suit of armor that protects the body. It has an outer layer of horny shields, called scutes, and an inner layer of bony plates. The domed top of the shell is called the carapace, while the flat layer underneath the animal’s belly is called the plastron. The ribs and backbones of turtles and tortoises are fused to the bones in their shells. This heavy armor weighs the animals down, so they move slowly on land. A few kinds of turtles that live in water do not have horny shields; their shell is covered by rubbery skin instead.

The largest shell ever found was about 8 ft ( m)long, and belonged to a leatherback turtle

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The bottom of the shell, protecting the tortoise’s belly, is flat so it does not scrape against the ground. It is connected to the carapace along the sides.

Hip bones work with the shoulder bones to help the tortoise breathe. They move downward and outward to pull air into the lungs, and upward and inward to push air out.

Bony plates that are fused together form the inner layer of the shell.

The domed upper shell, or carapace, provides a solid defensive wall against any attack.

The flattened ribs are attached to the bony layer of the upper shell. Unlike our own ribs, the ribs of tortoises and turtles cannot move when they breathe.

The shell is covered in scutes (shields) made of a hornlike material called keratin.

A tortoise’s shoulder bones stretch up inside its shell. Turtles and tortoises are the only animals with a backbone whose shoulder blades are inside their rib cage.

The tortoise’s spine is joined to the bony layer inside its shell. This makes the tortoise stronger but means it cannot bend its body.

Most tortoises and turtles have reasonably large eyes and have good color vision.

The tortoise’s brain is protected inside the bones of its skull, which form a box.

The radiated tortoise has a high-domed shell. It lives in dry forests on the island of Madagascar in the Indian Ocean.

A long, flexible neck allows tortoises to pull their heads into their shells if they sense danger. Some freshwater turtles pull their heads in sideways. Sea turtles cannot retract their heads.

Shielded from harm

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The outside of a tortoise’s hard, protective shell is covered by scutes. These horny shields are made of keratin—the same material our fingernails are made of. As the tortoise grows bigger, more keratin is added, so each extra ring corresponds to a spurt of growth.

Sours: https://www.dkfindout.com/us/animals-and-nature/reptiles/inside-tortoise/

Turtle shell

A preserved turtle skeleton showing how the carapace and plastron connect with the rest of the skeleton to form a shell enclosing the body

Scutes (left) and skeletal components (right) of a turtle's carapace.

Scutes (left) and skeletal components (right) of a turtle's plastron Pleurodires have an extra scute known as the intergular. It is absent in cryptodires.

The turtle shell is a shield for the ventral and dorsal parts of turtles (the order Testudines), completely enclosing all the vital organs of the turtle and in some cases even the head.[1] It is constructed of modified bony elements such as the ribs, parts of the pelvis and other bones found in most reptiles. The bone of the shell consists of both skeletal and dermal bone, showing that the complete enclosure of the shell probably evolved by including dermal armor into the rib cage.

The shell of the turtle is an important study, not just because of the obvious protection it provides for the animal, but also as an identification tool, in particular with fossils as the shell is one of the likely parts of a turtle to survive fossilization. Hence understanding the structure of the shell in living species provides comparable material with fossils.

The shell of the hawksbill turtle, among other species, has been used as a material for a wide range of small decorative and practical items since antiquity, but is normally referred to as tortoiseshell.

Shell nomenclature[edit]

Internal anterior carapace of Elseya dentata. Pe=Peripheral, P1=Pleural 1, BCS=Bridge Carapace Suture

The turtle shell is made up of numerous bony elements, generally named after similar bones in other vertebrates, and a series of keratinousscutes which are also uniquely named. Some of those bones that make the top of the shell, carapace, evolved from the scapula rami of the clavicles along with the dorsal and superficial migration of the cleithra.[2] The ventral surface is called the plastron.[3][4] These are joined by an area called the bridge. The actual suture between the bridge and the plastron is called the anterior bridge strut.[5] In Pleurodires the posterior pelvis is also part of the carapace, fully fused with it. This is not the case in Cryptodires which have a floating pelvis.[3][4] The anterior bridge strut and posterior bridge strut are part of the plastron, on the carapace are the sutures into which they insert, known as the Bridge carapace suture.[5]

In the shell there is a turtle's epidermis layer. This layer is important to the strength of the shell surrounding it. In an international study, the layer can be as thick as two to four cells. Even with such a small thickness, the epidermis allows the deformation the shell can experience and provides the shell more support. The epidermis layer is apparent in both sections of the shell, carapace, and plastron, and is thicker in critical areas. A thicker epidermis allows a higher stress force to be experienced without permanent deformation or critical failure of the shell.[6]

The shape of the shell is from its evolutionary process. Causing many microstructures to appear to aid survival and motion. Shell shape allows the animal to escape predatory situations. Microstructures can include the scutes mentioned prior or the ribs found internally of the shell. &#;Many ribs can be found within the shell and throughout the shell. The rib structures provide extra structural support but allows the shells to deform elastically depending on the situation the turtle is in (i.e., predatory escape).[7] Nonstructural mechanisms have also been in the turtle shell that aids the turtle during locomotion. A mucus film like is covering parts of the shell, allowing like friction and drag.

The bones of the shell are named for standard vertebrate elements. As such the carapace is made up of 8 pleurals on each side, these are a combination of the ribs and fused dermal bone. Outside of this at the anterior of the shell is the single nuchal bone, a series of 12 paired periphals then extend along each side. At the posterior of the shell is the pygal bone and in front of this nested behind the eighth pleurals is the suprapygal.[3]

Transverse sections through the first neural of A. Aspideretes hurumshowing the suture between the wide neural bone (N) and the vertebral neural arch (V). B. Chelodina longicollisat pleural IV showing a narrow midline neural bone, lateral pleurals (P) and underlying vertebral neural arch. and C. Emydura subglobosaat pleural IV showing location of a rudimentary neural bone underneath medially contiguous pleurals.

Between each of the pleurals are a series of neural bones,[8] which although always present are not always visible,[9] in many species of Pleurodire they are submerged below the pleurals.[10] Beneath the neural bone is the Neural arch which forms the upper half of the encasement for the spinal chord. Below this the rest of the vertebral column.[4] Some species of turtles have some extra bones called mesoplastra, which are located between the carapace and plastron in the bridge area. They are present in most Pelomedusid turtles.[11]

The skeletal elements of the plastron are also largely in pairs. Anteriorly there are two epiplastra, with the hyoplastra behind them. These enclose the singuar entoplastron. These make up the front half of the plastron and the hyoplastron contains the anterior bridge strut. The posterior half is made up of two hypoplastra (containing the posterior bridge strut) and the rear is a pair of xiphiplastra.[4][5]

Overlying the boney elements are a series of scutes, which are made of keratin and are a lot like horn or nail tissue. In the center of the carapace are 5 vertebral scutes and out from these are 4 pairs of costal scutes. Around the edge of the shell are 12 pairs of marginal scutes. All these scutes are aligned so that for the most part the sutures between the bones are in the middle of the scutes above. At the anterior of the shell there may be a cervical scute (sometimes incorrectly called a nuchal scute) however the presence or absence of this scute is highly variable, even within species.[4][11]

On the plastron there are two gular scutes at the front, followed by a pair of pectorals, then abdominals, femorals and lastly anals. A particular variation is the Pleurodiran turtles have an intergular scute between the gulars at the front, giving them a total of 13 plastral scutes. Compared to the 12 in all Cryptodiran turtles.[4][11]

Carapace[edit]

Exploded view of the carapace of Emys orbicularis.[12]

Legend

The carapace is the dorsal (back), convex part of the shell structure of a turtle, consisting of the animal's ossified ribs fused with the dermal bone. The spine and expanded ribs are fused through ossification to dermal plates beneath the skin to form a hard shell. Exterior to the skin the shell is covered by scutes, which are horny plates made of keratin that protect the shell from scrapes and bruises. A keel, a ridge that runs from front to the back of the animal is present in some species, these may be single, paired or even three rows of them. In most turtles the shell is relatively uniform in structure, species variation in general shape and color being the main differences. However the soft shell turtles, pig-nose turtles and the leatherback sea turtle have lost the scutes and reduced the ossification of the shell. This leaves the shell covered only by skin.[13] These are all highly aquatic forms.

The evolution of the turtle's shell is unique because of how the carapace represents transformed vertebrae and ribs. While other tetrapods have their scapula, or shoulder blades, found outside of the ribcage, the scapula for turtles is found inside the ribcage.[14][15] The shells of other tetrapods, such as armadillos, are not linked directly to the vertebral column or rib cage allowing the ribs to move freely with the surrounding intercostal muscle.[16] However, analysis of the transitional fossil, Eunotosaurus africanus shows that early ancestors of turtles lost that intercostal muscle usually found between the ribs.[17]

Plastron[edit]

"Plastron" redirects here. For the arthropod structural adaptation, see Gill §&#;Plastrons.

The plastron (plural: plastrons or plastra) is the nearly flat part of the shell structure of a turtle, what one would call the belly or ventral surface of the shell. It also includes within its structure the anterior and posterior bridge struts and the bridge of the shell.[4][5] The plastron is made up of nine bones and the two epiplastra at the anterior border of the plastron are homologous to the clavicles of other tetrapods.[18] The rest of the plastral bones are homologous to the gastralia of other tetrapods. The plastron has been described as an exoskeleton, like osteoderms of other reptilians; but unlike osteoderms, the plastron also possesses osteoblasts, the osteoid, and the periosteum.[19]

The evolution of the plastron has remained more mysterious, though Georges Cuvier, a French naturalist and zoologist in the 19th century, wrote that the plastron developed primarily from the sternum of the turtle.[20] This fits well with the knowledge obtained through embryological studies, showing that changes in the pathways of rib development often result in malformation or loss of the plastron. This phenomenon occurs in turtle development, but instead of experiencing complete loss of the sternum the turtle body plan repurposes the bone into the form of the plastron,[21] although other analyses find that the endochondral sternum is absent and replaced by the exoskeletal plastron. The ventral ribs are effectively not present, replaced by the plastron, unless the gastralia from which the plastron evolved were once floating ventral ribs.[19] During turtle evolution, there was probably a division of labor between the ribs, which specialized to stabilize the trunk, and the abdominal muscles, which specialized for respiration, and these changes took place 50 million years before the shell was fully ossified.[22]

The discovery of an ancestral turtle fossil, Pappochelys rosinae, provides additional clues as to how the plastron formed. Pappochelys serves as an intermediate form between two early stem-turtles, E. africanus and Odontochelys, the latter of which possesses a fully formed plastron. In place of a modern plastron, Pappochelys has paired gastralia, like those found in E. africanus. Pappochelys is different from its ancestor because the gastralia show signs of having once been fused, as indicated by the fossil specimens which show forked ends. This evidence shows a gradual change from paired gastralia, to paired and fused gastralia, and finally to the modern plastron across these three specimens.[23]

In certain families there is a hinge between the pectoral and abdominal scutes allowing the turtle to almost completely enclose itself. In certain species the sex of a testudine can be told by whether the plastron is concave, male or convex, female. This is because of the mating position; the male's concave plastron allows it to more easily mount the female during copulation.

The plastral scutes join along a central seam down the middle of the plastron. The relative lengths of the seam segments can be used to help identify a species of turtle. There are six laterally symmetric pairs of scutes on the plastron: gular, humeral, pectoral, abdominal, femoral, and anal (going from the head to the tail down the seam); the abdominal and gular scute seams are approximately the same length, and the femoral and pectoral seams are approximately the same length.

The gular scute or gular projection on a turtle is the most anterior part of the plastron, the underside of the shell. Some tortoises have paired gular scutes, while others have a single undivided gular scute. The gular scutes may be referred to as a gular projection if they stick out like a trowel.

  • Gular anatomical formations in other species

Plastral formula[edit]

The plastral formula is used to compare the sizes of the individual plastral scutes (measured along the midseam). The following plastral scutes are often distinguished (with their abbreviation):

intergular= intergul
gular= gul
humeral= hum
pectoral= pect
abdominal= abd
femoral= fem
anal= an

Comparison of the plastral formulas provides distinction between the two species. For example, for the eastern box turtle, the plastral formula is: an > abd > gul > pect > hum >< fem[24]

Turtle plastrons were used by the ancient Chinese in a type of divination called plastromancy. See also Oracle bones.

Scutes[edit]

Midland painted turtle showing shedding of scutes

The turtle's shell is covered in scutes that are made of keratin. The individual scutes as shown above have specific names and are generally consistent across the various species of turtles. Terrestrial tortoises do not shed their scutes. New scutes grow by the addition of keratin layers to the base of each scute. Aquatic chelonii shed individual scutes. The scute effectively forms the skin over the underlying bony structures; there is a very thin layer of subcutaneous tissue between the scute and the skeleton. The scutes can be brightly colored in some species, but the basal color is a grey to dark brown color dorsally; the plastral scutes are often white to yellow in base color.[citation needed] Moustakas-Verho and Cherepanov's embryological study reveals that the patterning of the plastral scutes appear independent from the patterning of carapacial scutes, suggesting that the carapace and plastron evolved separately.[25]

The appearance of scutes correlates to the transition from aquatic to terrestrial mode of life in tetrapods during the Carboniferous period ( Ma).[26] In the evolution from amphibians to terrestrial amniotes, transition in a wide variety of skin structures occurred. Ancestors of turtles likely diverged from amphibians to develop a horny cover in their early terrestrial ancestral forms.[27]

Ontogeny[edit]

Developmentof the shell: seen in the egg at stage 16/17, the carapace is developing. In section, the ribs are growing sideways not downwards, into the carapacial ridge, seen here as a bud, to support the carapace.[28]

The carapacial ridge plays an essential role in the development of the turtle shell. Embryological analyses show that the carapacial ridge initiates the formation of the turtle shell.[29] It causes axial arrest which causes the ribs to be dorsalized, the shoulder girdle to be rearranged and encapsulated in the rib cage, and the carapace to develop.[30]Odontochelys semitestacea presents evidence of axial arrest that is observed in embryos but lacks fan-shaped ribs and a carapace. This suggests that the primitive carapacial ridge functioned differently and must have gained the function of mediating the ribs and carapace development later.[31][21] The Pax1 and Sonic hedgehog gene (Shh) serve as key regulators during the development of the vertebral column. Shh expression in the neural tube is essential for the maintenance of Pax1 expression in the ventral sclerotome and thus plays a key role in carapacial rib development. Genetic observations of Pax1 and Shh further provide an understanding in key gene expression that could potentially be responsible for changing turtle morphology.[32]

During the development of the turtle embryo, the ribs grow sideways into the carapacial ridge, unique to turtles, entering the dermis of the back to support the carapace. The development is signalled locally by fibroblast growth factors including FGF[28]

Phylogeny[edit]

Bony dermal plates theory: the "Polka Dot Ancestor"[edit]

Zoologists have sought to explain the evolutionary origin of the turtles, and in particular of their unique carapace. In , J. Versluys proposed that bony plates in the dermis, osteoderms, fused first to each other and then to the ribs beneath them. The theory persisted into the 21st century, when Olivier Rieppel proposed a hypothetical turtle precursor, its back covered by bony armour plates in the dermis, which he called the "Polka Dot Ancestor".[33][34] Michael Lee proposed that the transformation of the carapace began with an unarmoured parareptile and then an armoured pareiasaur, and ended with modern turtles with a fully developed carapace and a relocated rib cage.[35] The theory accounted for the evolution of fossil pareisaurs from Bradysaurus to Anthodon, but not for how the ribs could have become attached to the bony dermal plates.[33]

Broadened ribs theory[edit]

Diagram of origins of turtle body plan through the Triassic: isolated bony plates evolved to form a complete shell.[33]

Permian: first stem-turtles[edit]

Recent stem-turtle fossil discoveries provide a "comprehensive scenario" of the evolution of the turtle's shell. A fossil that may be a stem-turtle from the Permian of South Africa, Eunotosaurus, some million years ago, had a short broad trunk, and a body-case of broadened and somewhat overlapping ribs, suggesting an early stage in the acquisition of a shell.[33] The fossil has been called "a diapsid reptile in the process of becoming secondarily anapsid".[36] Olivier Rieppel summarizes the phylogenetic origins of the ancestral turtles: "Eunotosaurus is placed at the bottom of the stem section of the turtle tree, followed by Pappochelys and Odontochelys along the turtle stem and on to more crown-ward turtles".[37]

Tyler Lyson and colleagues suggest that Eunotosaurus might imply a fossorial origin for the turtles. During the Permian, the broadened ribs may have provided great stability in burrowing, giving a body shape resembling the extant fossorial gopher tortoise, with strong shoulders and forelimbs, and increased muscle attachment structures such as their tubercle on the posterior coracoid and their large and wide terminal phalanges creating shovel-like "hands". Fossoriality may have helped Eunotosaurus survive the global mass extinction at the end of the Permian period, and could have played an essential role in the early evolution of shelled turtles.[38][39]

Triassic: evolution of complete shell[edit]

A stem-turtle from the Middle Triassic of Germany, some million years ago, Pappochelys, has more distinctly broadened ribs, T-shaped in cross-section.[33] They vary in shape along the spine.[40]

A Late Triassic stem-turtle from Guizhou, China, Eorhynchochelys, is a much larger animal, up to metres (&#;ft) long, with a long tail, and broadened but not overlapping ribs; like the earlier fossils, it has small teeth.[33]

Also in the Late Triassic, some million years ago, the freshwater Odontochelys semitestacea of Guangling in southwest China has a partial shell, consisting of a complete bony plastron and an incomplete carapace.[41][31] The fossil showed that the plastron evolved before the carapace.[42] Like crown turtles, it lacked intercostal muscles, so rib mobility was limited. The ribs were laterally expanded and broadened without ossification, like the embryos of modern turtles.[43]

The development of a shell reaches completion with the late Triassic Proganochelys of Germany and Thailand.[43][44] It lacked the ability to pull its head into its shell, and had a long neck and a long, spiked tail ending in a club, somewhat like an ankylosaur.[45]

Diseases[edit]

Shell rot[edit]

Septicemic cutaneous ulcerative disease (SCUD) or "shell rot" causes ulceration of the shell.[46] This is caused by bacteria or fungi entering through an abrasion, and poor animal husbandry. The disease progresses to a septicemic infection causing the degradation of the liver and other organs.[47]

Pyramiding[edit]

Pyramiding is a shell deformity of captive tortoises, in which the shell grows unevenly resulting in a pyramid shape underlying each scute. Factors which may contribute to pyramiding include inadequate water supply; the consumption of excessive animal or vegetable protein; inadequate calcium, UVB and/or vitamin D3; poor nutrition.[48][49][50]

  • Plastron of wild male Hermann's tortoise with ongoing shell rot (circled in red) and scars from previous shell rot (circled in black)

See also[edit]

References[edit]

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  42. ^Li, Chun; Wu, Xiao-Chun; Rieppel, Olivier; Wang, Li-Ting; Zhao, Li-Jun (). "An ancestral turtle from the Late Triassic of southwestern China". Nature. (): – BibcodeNaturL. doi/nature ISSN&#; PMID&#; S2CID&#;
  43. ^ abLi, C.; Wu, X.-C.; Rieppel, O.; Wang, L.-T.; Zhao, L.-J. (). "An ancestral turtle from the Late Triassic of southwestern China". Nature. (): – BibcodeNaturL. doi/nature PMID&#; S2CID&#;
  44. ^Gaffney, Eugene S. (). The comparative osteology of the Triassic turtle Proganochelys. OCLC&#;
  45. ^Asher, J. Lichtig; Spencer G., Lucas; Klein, Hendrik; Lovelace, David M. (). "Triassic turtle tracks and the origin of turtles". Historical Biology. 30 (8): – doi/ S2CID&#;
  46. ^Kaplan, H. M. (). "Septicemic, cutaneous ulcerative disease of turtles". Proc. Animal Care Panel. 7: –
  47. ^Mader, D. () Reptile Medicine and Surgery, 2nd ed., Saunders, ISBN&#;X.
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  49. ^Innis, Charles. "BASIC HUSBANDRY AND NUTRITION OF CHELONIANS"(PDF). Cabi.Org.
  50. ^"Pyramiding in Tortoises". www.reptilesmagazine.com. 23 January

External links[edit]

Sours: https://en.wikipedia.org/wiki/Turtle_shell
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What's in a Shell.




Bodyplan



The bodies of chelonians are evolutionary wonders. In order to deal with living in a shell, turtles had to go through some major changes to accomodate this new and improved lifestyle. The shell of turtles is the backbone of their success (pun intended; Sorry.). These amazing little marvels are what have kept chelonians in business for over million years. But what is in a shell? What is it made up of?

Let's find out :)

The turtle shell is broken up into three parts. We have:
  1. The carapace. This is the upper part of the shell.
  2. The Plastron. This is the lower part of the shell.
  3. The Bridge. A line of bone inbetween the fore and hind limbs that connects the plastron to the carapace.
These pieces of shell are all made up of the same component parts. First a layer of cartilaginous bones which form the shape of broadened ribs. This is then covered by a layer of membrane bones or thecals. Then this is followed by the tuff stuff. The osteoderms or epithecals. Also known as dermal armor, for that's exactly what it is. The osteoderms are fused plates of bone that make up the general shape of the shell and give it it's rigidity. The last covering in chelonians is the scutes. These are the tuff outer scales covering the shell and giving it it's color and texture.

The bones of the shell (the osteoderms) can be broken down into even further parts.

The Carapace

Diagram of turtle shell
The top of the carapace in the middle is made up of many small pieces of bones called neurals. These end in the front at a large piece of bone known as the proneural and at the back in two pieces called the suprapygals.

The midsections of bones on the carapace are called pleurals and on the front and rear of the pleurals are two pieces of bones. In the front there is the axillary buttress, while in the back we have the inguinal buttress. Both pieces are used to strengthen the shell more.

Finally we come to the end of the shell. All around the side are many little pieces of bones called peripherals. In the front the peripherals meet up with the proneural and in the back the peripherals meet with the pygal.
The plastron also has a different array of bony plates.

Plastron

again
At the very front of the plastron there is two pieces of bone known as the epiplastron. These two pieces then meet up with a third piece near the bottom of them known as the entoplastron.

After the entoplastron there are two large plates of bone known as the hyoplastron.

Behind the hyoplastron is two more large plates called the hypoplastron.

Finally at the end of the hypoplastron are the last two pieces of bones. They are called the xiphiplastron.

In the middle on the sides is the bridge which composes two pieces only with a notch on both the front and hind ends called the axillary notch.
The osteoderms of the shell are then covered by the horny keratinous scutes. These scutes can also be broken into different parts.

Carapace

Diagram of scutes
The piece of scute directly behind the head is known as the nuchal. Then all the scutes directly behind the nuchal are known as vertebrals.
Radiating along the sides of the shell are scutes called marginals. Named of course because they are at the margins or fringes of the shell.

Finally all the scutes inbetween the vertebrals and the marginals are called costals, thus making up the outer surface of the carapace.

The Plastron

and again
The first pair of scutes directly behind the head are known as the gulars.

These are then followed by the humerals, then the pectorals, followed by the abdominals and femorals. Note how these scutes are basically named after the part of the body they are located at or near.

The final and most rearward scutes would be, of course, the anals.

On the front of the bridge there is the axillary and on the back of the bridge there is the inguinal.
Together these pieces make up the shell, one of the most amazing pieces of defensive machinery ever evolved. The above was a basic view of the many different parts to chelonian shells. But, not all chelonian shells are alike. Some major modifications have gone into these things.

For instance in some types of chelonians the shell has evolved a cartilaginous hinge inbetween certain bones. This hinge allows the chelonian to actually close up that part of the shell making it that much safer from predators.

Different joint arrangements


The hinges themselves are different as well. Some types such as the genus Kinosternon (mud turtles) there is a hinge inbetween the pectoral and abdominal scutes which allows the turtle to close up the front half of it's shell. In the genus Kinixys (Hingebacks) the hinge has evolved on the carapace instead of the plastron and lies inbetween the second and third costals. It allows the tortoises to close the hind quarters of their shells. Finally we have the genus Terrapene (box turtles) which decided on having the best of both worlds. They developed two hinges. One inbetween the pectorals and the abdominals and another inbetween the abdominals and the femorals. Thus allowing the turtles to close up both halves of their shells making an impenetrable fortress or Box as their common names imply.

There are also species of chelonian who have decided to mess with their protective covering and make it more maneuvarable and less protective. In the tortoise Malacochersus tornieri (Pancake tortoise) the bony casing has been reduced so as to allow the animal to move more freely. The shell has also been widened and flattened so as to accomodate it's new lifestyle. These tortoises live in rocky areas where when danger threatens, they run to the nearest rocky crevice and cram themselves in. The new shape of the shell allows the tortoise to both move quickly and fit in very narrow spaces.

Then there are the Trionychids. They are more commonly refered to as the soft shelled turtles. There shells have undergone a major tranformation. Not only has the bony layer been radically reduced, but the horny keratinous covering of scutes has been dumped in favor of a tough leathery skin. The plastron has been modified into a set of strut like supports instead of a solid casing. This relieves a lot of weight of the turtle's backs making them very fast and maneuverable. This change in shell is not doubt due to the turtle's need for speed when catching their prey and avoiding predators.

Another species that has undergone some radical change in shell structure is that of Dermochelys coriacea (The Leatherback sea turtle). It is the only species of turtle to have basically dumped it shell completely. Instead of bony plates there are many small interwoven bones that provide it's support. It's entire outer covering is cartilaginous making it very streamlined in the water but very susceptible to cuts and bruises when hauling itself out on land to lay eggs.

Among other changes in chelonian shell structure there is plastron reductions. These reductions can be nearly to the point of nonexistence and are probably made to allow the turtle to move with greater ease as it's chases down frogs and insects.

So in the beginning there was the shell and it was good. But then some chelonians found that they could do better with a softer, lighter shell and one species even decided to basically remove the shell entirely. As I said before, a remarkable evolutionary achievement.

Although the shell is made from horny scutes and plates of bone, it can still be damaged and even bleeds when damaged severely. If the shell is damaged, it will regenerate, but depending on the severity of the damage and the health of the animal, regeneration could take a long or relatively short time. Along with an amazing protective shell, chelonians had to also evolve new ways for them to move.

Vertebrae

In most vertebrates, the vertebrae are a flexible series of bones that allow for fairly unrestricted movement on land and in the water. In chelonians though this would be a bad thing. The carapace has to attach from somewhere and the best place to be would be the vertebrae. So a flexible vertebrae would quickly put an end to their marvelous shells. The dorsal and sacral (the vertebrae that make up the middle of the backbone) vertebrae have been fused together and join with the neural bones of the shell. The verts here been shortened to ten to add strength. The cervical (neck) and caudal (tail) vertebrae are the most flexible parts. In chelonians there are eight cervical vertebrae that are extremely flexible. These verts allow the chelonians to perform the maneuvers needed to get in their shells also. Chelonian necks are also usually more flexible that that of mammalian necks and can be compared with birds in terms of maneuverability. The caudals are made up of up to 33 bones and are also highly flexible. While most chelonians have characteristicallly short tails, there are plenty of others with very long tails.

Ribs

In order for the carapace to form it needs to drape itself over some kind of supporting structure. It's already connected to the backbone so what else can it use? The answer is ribs. The ribs are peculiar in chelonians in that they actually envelope the limb girdles. The reason for this strange occurence can be found in the embryonic stage. Here the anlage (early carapace) is actually growing faster than the ribs forcing them to grow with it.

Limbs

As you can imagine the limbs have undergone some major changes themselves. The clavicle (collar bone) has become a part of the plastron known as the epiplastron. The scapula (shoulder blade) has joined to the carapace at it's outer surface where it can anchor the legs more effectively. The sternum which is normally used to protect the body has been totally lost since a nice bony plastron has taken it's place. The humerus and femur have been shortened considerably except near the end which has become considerably enlarged. The reason for this is that it has to support the weight bearing joints of the forelimb and shank. Further down the carpal and tarsal elements have fused thus strengthening the distal parts of the legs.

Turtles and tortoises are mostly digitigrade animals. That is they walk on their toes. All chelonians (except fully aquatic of course) are digitigrade in their forelimbs, while most tortoises are digitgrade in the hind limbs also. For the most part their forelimb terminates in five digits while the hind limb terminates in four. There are exceptions with some species have only four digits on the front leg or only three in the hind.

Aquatic chelonians have either fully webbed feet or as is the case with marine turtles, flippers. All chelonians have beaks instead of toothed mouths. This is a feature they share with birds and certain deinosaurs as well.

The beak is a horny piece of keratin that is self sharpening and continually grows. In some species this beak is covered over by a layer of skin which, combined with their strange noses, makes them look like their lips are in a perpetual pucker.

One of the biggest problems with growing an immovable and impenetrable shell is that breathing can be a big problem. Since their is no movable flesh to allow for expansion of the lungs nor a diaphragm to expand them. So chelonians had to find a new way of handling this problem.

When this was first being considered, researchers thought that, like amphibians, chelonians achieved respiration by gular pumping. That is they thought that the constant throat pumping movements seen in these animals was used to force air into the lungs. This has turned out to be false and gular pumping in chelonians is now known to be of olfactory (smelling) signifigance.

So how do they breathe?

Breathing is accomplished by the creation of a negative pressure differential (i.e. the air outside has a higher pressure than the air inside, so through the process of diffusion, the air will enter this "negative air space" and fill the lungs). Mammals and crocodylians accomplish this through the use of their diaphragms and intercostal (between the ribs) muscles, while squamates use their intercostal muscles and, in some, gular pumping (throat breathing). It has already been established that they don't use gular pumping for the purpose of inhalation, they don't have diaphragms and the ribs now form a part of the shell, so intercostal breathing is out as well. Chelonians had to find another way.

This negative pressure differential (NPD) is partly achieved via the shell. In tortoises breathing is accomplished by the use of the rigid shell and the toroise's musculature. The muscles used for breathing expand into the limb pockets at the borders of the shell and serve to modify the internal pressure within a chelonian's body. So when the tortoise moves it is expelling air in one movement and taking in air with another. This would also explain why resting turtle's and tortoise's forelimbs move in and out.

Now in species like Chelydra serpentina (Common Snapper) the plastron has been severely reduced, which actually makes breathing on land easy since the weight of the animal's internal organs force air into the lungs in a kinda cantelevering process, which makes the expulsion of air the only muscular activity (imagine that, having to work to keep air out of your system.)

When in the water though, things change and inspiration is the newly required muscular activity, while expiration happens virtually spontaneously due to the water pressure. Of course marine species which dive to great depths have a better handle on this.

So in order to deal with a life enclosed within a shell, chelonians had to not only find a way to make such a thing, but also had to modify the ways in which they moved and breathed.

Absolutely amazing.



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