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Turtle

For other uses, see Turtle (disambiguation).
Families
Testudines, Chelonia

Turtles are reptiles of the order Testudines (all living turtles belong to the crown group Chelonia), most of whose body is shielded by a special bony or cartilagenous shell developed from their ribs. The term turtle is usually used for the aquatic species, though aquatic fresh water turtles are also called terrapins. The term is sometimes used (esp. in North America) to refer to all members of the order, including tortoises, which are predominantly land-based. The order of Testudines includes both extant (living) and extinct species. About 300 species are alive today. Some species of turtles are highly endangered.

Description

All turtles have a protective shell around their bodies. The top part of the shell is called the carapace, the bottom is called the plastron, and the two are connected by a bridge. Some are known to be able to breathe through their rectums as well. Reference the Rheodytes leukops species.

Sea turtles grow to large sizes and live in the oceans in the temperate and tropical regions of Earth. Pond turtles (terrapins) are usually much smaller, while some land terrapins (tortoises) are as large as sea turtles. The sizes of turtles vary from a few centimetres (forest and jungle species) to two metres (the leatherback turtle and the Galapagos tortoise).

Turtles generally live a long time; some individuals are known to have lived longer than 150 years. The oldest tortoise on record is Tui Malila, known to have lived at least 188 years.

Sea turtles lay their eggs on dry sandy beaches. The eggs of the largest species are spherical, while the eggs of the rest are elongated. Their albumen is white and will not coagulate when cooked because of the protein it contains which is different to that of bird eggs. Turtle eggs prepared to eat consist mainly of yolk. In some species, temperature of the egg during development determines whether an egg develops into a male or a female: a higher temperature causes a female, a lower temperature causes a male.

Although they spend large proportions of their lives underwater, turtles are air-breathing reptiles, and must surface at regular intervals to refill their lungs with fresh air. However, aquatic respiration in Australian freshwater turtles is currently being studied. Some species have large cloacal cavities lined with many finger-like projections. These projections, called "papillae", have a rich blood supply, and increase the surface area of the cloaca. The turtles can take up dissolved oxygen from the water through these papillae, in much the same way that fish use gills.

Turtles have a gelatinous substance in their upper and lower shell, called calipash and calipee respectively, the calipash being of a dull greenish and the calipee of a light yellow color.

Evolution

The first turtles are believed to have existed in the era of the dinosaurs, 200 million years ago. Their exact ancestry is disputed. It was believed that they are the only surviving branch of the ancient clade Anapsida, which includes groups such as procolophonoids, millerettids, protorothyrids and pareiasaurs. All Anapsid skulls lack a temporal opening, while all other extant amniotes have temporal openings (although in mammals the hole has become the zygoid arch). Most anapsids became extinct in the late Permian period, except procolophonoids and possibly the precursors of the testudines (turtles).

However, it was recently suggested that the Anapsid-like turtle skull may be due to convergent evolution rather than to anapsid descent. More recent phylogenetic studies with this in mind placed turtles firmly within diapsids, slightly closer to Squamata than to Archosauria. All molecular studies have strongly upheld this new phylogeny, though some place turtles closer to Archosauria. Re-analysis of prior phylogenies suggests that they classified turtles as anapsids both because they assumed this classification (most of them studying what sort of anapsid turtles are) and because they did not sample fossil and extant taxa were broadly enough for constructing the cladogram. While the issue is far from resolved, most scientists now lean towards a Diapsid origin for turtles.

Order Testudines - Turtles

Gulf Coast Box Turtle, Terrapene carolina major (Emydidae) A slider of genus Trachemys A Leatherback Sea Turtle. Photo credit: NOAA

Suborder Paracryptodira (extinct)

Suborder Cryptodira

Suborder Pleurodira


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Suborder Pleurodira. The incidence of twinning among cattle is about 1-4%, and research is underway to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. Suborder Cryptodira. Multiple births are common in many animal species, such as cats, sheep, and ferrets. Suborder Paracryptodira (extinct)
. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes. While the issue is far from resolved, most scientists now lean towards a Diapsid origin for turtles. When monozygotic twins are born with different genders it is because of chromosomal birth defects.

Re-analysis of prior phylogenies suggests that they classified turtles as anapsids both because they assumed this classification (most of them studying what sort of anapsid turtles are) and because they did not sample fossil and extant taxa were broadly enough for constructing the cladogram. The probability of this is so vanishingly small (only 3 documented cases) that multiples having different genders is universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. All molecular studies have strongly upheld this new phylogeny, though some place turtles closer to Archosauria. Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). More recent phylogenetic studies with this in mind placed turtles firmly within diapsids, slightly closer to Squamata than to Archosauria. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details. However, it was recently suggested that the Anapsid-like turtle skull may be due to convergent evolution rather than to anapsid descent. This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings).

Most anapsids became extinct in the late Permian period, except procolophonoids and possibly the precursors of the testudines (turtles). Among fraternal twins, in rare cases, the eggs are fertilised at different times with two or more acts of sexual intercourse, either within one menstrual cycle (superfecundation) or, even more rarely, later on in the pregnancy (superfetation). All Anapsid skulls lack a temporal opening, while all other extant amniotes have temporal openings (although in mammals the hole has become the zygoid arch). There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case. It was believed that they are the only surviving branch of the ancient clade Anapsida, which includes groups such as procolophonoids, millerettids, protorothyrids and pareiasaurs. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been invaluable in the exploration of human nature. Their exact ancestry is disputed. Twin studies are studies that assess identical (monozygotic) twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from environmental influence.

The first turtles are believed to have existed in the era of the dinosaurs, 200 million years ago. Many fertility treatments have no effect on the likelihood of multiple births. Turtles have a gelatinous substance in their upper and lower shell, called calipash and calipee respectively, the calipash being of a dull greenish and the calipee of a light yellow color. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. The turtles can take up dissolved oxygen from the water through these papillae, in much the same way that fish use gills. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. These projections, called "papillae", have a rich blood supply, and increase the surface area of the cloaca. This can vary depending on what types of fertility treatments are used.

Some species have large cloacal cavities lined with many finger-like projections. Women undergoing certain fertility treatments may have a greater chance of multiple births. However, aquatic respiration in Australian freshwater turtles is currently being studied. Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:. Although they spend large proportions of their lives underwater, turtles are air-breathing reptiles, and must surface at regular intervals to refill their lungs with fresh air. Some evidence suggests that the environment of the womb causes the zygote to split in most cases. In some species, temperature of the egg during development determines whether an egg develops into a male or a female: a higher temperature causes a female, a lower temperature causes a male. Fewer than 20 families have been described with an inherited tendency towards monozygotic twinning (people in these families have nearly a 50% chance of delivering monozygotic twins).

Turtle eggs prepared to eat consist mainly of yolk. The cause of monozygotic twinning is unknown. Their albumen is white and will not coagulate when cooked because of the protein it contains which is different to that of bird eggs. Multiple pregnancies are usually delivered before the full term of 40 weeks gestation: the average length of pregnancy is around 36 weeks for twins, 34 weeks for triplets and 32 weeks for quadruplets. The eggs of the largest species are spherical, while the eggs of the rest are elongated. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher order births so rare as to be almost unheard of. Sea turtles lay their eggs on dry sandy beaches. If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, and so on.

The oldest tortoise on record is Tui Malila, known to have lived at least 188 years. Sometimes multiple births may involve more than two fetuses. Turtles generally live a long time; some individuals are known to have lived longer than 150 years. Nevertheless, the rate of identical twins remains at about 1 in 250 across the globe, further suggesting that pregnancies resulting in identical twins occur randomly. The sizes of turtles vary from a few centimetres (forest and jungle species) to two metres (the leatherback turtle and the Galapagos tortoise). Thus, approximately 6% of children born in the US in 2001 were twins. Pond turtles (terrapins) are usually much smaller, while some land terrapins (tortoises) are as large as sea turtles. In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births.

Sea turtles grow to large sizes and live in the oceans in the temperate and tropical regions of Earth. The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of multiple births". Reference the Rheodytes leukops species. The rate of twinning varies greatly among ethnic groups, ranging as high as about 6% for the Yoruba or 10% for a tiny Brazilian village (see [1]). Some are known to be able to breathe through their rectums as well. Historically, about 1 in 80 human births (1.2%) has been the result of a twin pregnancy. The top part of the shell is called the carapace, the bottom is called the plastron, and the two are connected by a bridge. Similar to vanishing twin.

All turtles have a protective shell around their bodies. Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. . A chimera may arise either from identical twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which could be identified by chromosomal comparisons from various parts of the body. Some species of turtles are highly endangered. A chimera is a person who is a completely normal human with no extra parts, but some of the parts actually came from his or her twin. About 300 species are alive today. Sometimes the parasitic twin just becomes an almost indistinguishable part of the other.

The order of Testudines includes both extant (living) and extinct species. One fetus acts as a parasite towards the other. in North America) to refer to all members of the order, including tortoises, which are predominantly land-based. Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. The term is sometimes used (esp. This condition occurs in about 1 in 100,000 pregnancies. The term turtle is usually used for the aquatic species, though aquatic fresh water turtles are also called terrapins. This occurs where the single zygote of identical twins fails to separate completely.

Turtles are reptiles of the order Testudines (all living turtles belong to the crown group Chelonia), most of whose body is shielded by a special bony or cartilagenous shell developed from their ribs. Conjoined twins are monozygotic twins, whose bodies are joined together at birth. Superfamily Pelomedusoidea. Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. Superfamily Chelonioidea. Researchers suspect that more pregnancies start out as multiples than come to term that way. Superfamily Kinosternoidea. Such conditions are usually associated with a higher incidence of other birth defects.

Superfamily Trionychoidea. This is where some or all of the organs will be on the opposite side of the body, such as the heart being on the right(Dextrocardia). Superfamily Testudinoidea. One mirror may or may not have situs inversus. They result from a late split of the fertilized egg at around 9-12 days. The incidence of mirror twinning is comparatively rare.

These are identical twins with opposite features, that is one may be right handed and the other may be left handed; hair will whorl in the opposite direction, and so on. Some percentage of monozygotic twins are called "mirror twins" or mirror image twins. (Fraga, et al., 2005). Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference.

50-year-old twins had over 3 times the epigenetic difference that the 3-year-old twins had. The number of differences between identical twins increases with age. A study of 80 pairs of twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. This is called epigenetic modification.

Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. There are usually obvious signs of differences when the identical twins are observed separately or together. Twins are unique individuals that establish their own individual likes and dislikes. Many identical twins spend most of their time together (especially as children), so people often assume that they will behave alike just as they look alike; however, this is not the case.

They develop their own individual personalities to enable themselves to be identified as individual persons. Identical twins can behave as differently as any other siblings (a matter of much interest to psychologists). The exact cause for the splitting of a zygote or embryo is unknown. While it was originally thought that identical twins do not run in families, but occur more or less randomly, some recent research has suggested that a genetic predisposition may exist.

As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars. Examination of details such as fingerprints can tell them apart. (On extremely rare occasions, an original XXY zygote may form monozygotic boy/girl twins by dropping the Y chromosome for one twin and the extra X chromosome for the other.) Monozygotic twins generally look alike, although sometimes they appear as mirror images of each other. Monozygotic twins are genetically identical unless there has been a mutation in development, and they are almost always the same gender.

About 50% of mono-mono twins die from umbilical cord entanglement. These twins may develop such that blood passes disproportionately from one twin to the other through connecting blood vessels within their shared placenta, leading to twin-to-twin transfusion syndrome. Monochorionic twins, sharing one placenta, usually also share the placental blood supply. For example, the umbilical cords of monoamniotic twins can become entangled, reducing or interrupting the blood supply to the developing fetus.

Sharing the same amnion (or the same amnion and placenta) can cause complications in pregnancy. Twinning after 12 days post-fertilization will typically result in conjoined twins. Twinning between 8 to 12 days after fertilization will usually result in monochorionic-monoamniotic ("mono-mono") twins. Twinning between 4 to 8 days after fertilization typically results in monochorionic-diamniotic ("mono-di") twins.

Zygotes that twin at the earliest stages will be diamniotic and dichorionic ("di-di"). The later in pregnancy that twinning occurs, the more structures will be shared. This condition does not occur for fraternal twins. Also note that any monochorionic or monoamniotic twins are identical twins.

All monoamniotic twins are monochorionic. Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic). Depending on the stage at which the zygote divides, identical twins may share the same amnion (in which case they are known as monoamniotic) or not (diamniotic). The two embryos develop into fetuses sharing the same womb.

Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) but the zygote then divides into two separate embryos. However, it is only the female that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum. Studies show that there is a genetic basis for fraternal twinning—that is, non-identical twins do run in families. Dizygotic twins may be a different sex or the same sex, just as with any other siblings.

Dizygotic twins, like any siblings, have a very small chance of having the exact same chromosome profile, but most likely have a number of different chromosomes that distinguish them. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins. Fraternal twins (commonly known as "non-identical twins") usually occur when two fertilized eggs are implanted in the uterine wall at the same time. .

Since some premature births often have health consequence to the babies, twins birth are often handled with special procedures. Due to the limited size of the mother's womb, multiple pregnancy is much less likely to carry to full term than singleton birth (twins usually around 34 to 36 weeks). A fetus alone in the womb is called a singleton. The term twin most notably refers to two individuals (or one of two individuals) who have shared the same uterus (womb) and are usually, but not necessarily, born on the same day.

Several previous pregnancies. Greater than average height and weight. Between the age of 30 and 40 years. She is of African descent.

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