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Descent with modification is when traits that have a higher reproductive advantage become more common over time and generations.  Descent with modification is clearly shown with the evolution of turtle shells in order Testudines.  The shell slowly evolved through generations because it gave individuals with stronger and more complete shell structures an advantage.  The advantage was better protection from predators, so these individuals were able to survive longer and reproduce more.  As these turtle ancestors reproduced more, the traits causing these better shells were passed down to offspring more often.  This descent with modification caused the evolution of turtle shells which is clearly evident in the fossil record.

An example of directional natural selection within order Testudines is shell shape in sea turtles.  Sea turtles have very streamlined shells because this trait has a bigger advantage than shells that are not streamlined.  Streamlined shells allow sea turtles to swim more efficiently and reach higher speeds in order to escape predators and catch prey.  Because of this advantage, sea turtles with more streamlined shells survived longer and reproduced more often, selecting for sea turtles with streamlined shells rather than boxy shells or a median between the two extreme shell shapes.  This selection is seen within the intermediate species Santanachelys gaffneyi as shells started to become more streamlined, and within all modern sea turtles such as the Leatherback Sea Turtle and Green Sea Turtle.

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EVOLUTIONARY MECHANISMS:
An example of geographic isolation is between tortoises and sea turtles.  Tortoises and sea turtles diverged from a common ancestor, but at some point some of these common ancestors were isolated from each other.  Some of the common ancestors lived on land and evolved into modern tortoises and the others lived in the water and evolved into sea turtles.  Before the common ancestor diverged into separate species, the two populations that were isolated could not breed with each other, causing them to evolve into separate species.

An example of non-random mating in order Testudines is sexual size dimorphism.  In some turtle species, the male is larger than the female.  In these species, the males often fight each other for the opportunity to mate.  A larger size helps a turtle do better in combat, and as a result the larger turtles have an advantage and get to mate with females more often.  This causes a higher probability for females choosing to mate with larger males because they are seen as most fit.  In species where the female is larger than the male, there is no fighting between males and it is more advantageous to be smaller because it helps the males with mobility and ability to locate females.  This causes the smaller turtles to be chosen as mates more often.

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TYPE OF EVOLUTION:
An example of convergent evolution is between turtles and mollusks.  They share an analogous structure, their shells.  Even though their shells have physical differences, they still serve the same purpose, protection from predators.  This means that turtles and mollusks faced similar environmental pressures, which caused them to evolve similarly and both developed a shell over time.  Mollusks and some turtles use their shells for protection by retracting into them to protect their body from predators that would otherwise be able to eat them and cause harm.  An example of a turtle that can completely retract into its shell is the exemplary species, Russian Tortoises.

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TYPE OF SPECIATION:
Using the evidence presented in this exhibit, order Testudines likely went through gradual speciation.  Gradual speciation is when a species diverges slowly over time in small steps, and this is evident when looking at the fossil record.  As the fossil record shows, the evolution of the modern turtle shell and beak happened very gradually and there were many intermediate species.  The earliest known ancestor of turtles, Eunotosaurus africanus, has an intermediate shell structure made of modified bones.  It took more intermediate species before this structure evolved into a true shell.  This is shown by Odontochelys semitestaca which had a developed plastron, but lacked a fully developed carapace.  Proganochelys quenstedi is the earliest known intermediate species to have a fully developed shell, 50 million years after Eunotosaurus africanus.

Odontochelys semitestaca also had teeth on its upper and lower jaw, which modern turtles lack.  There were also intermediate species before turtles lost their teeth, like Proganochelys quenstedi which had teeth only on the roof of its mouth.  This shows speciation happening slowly over time and in steps.

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A lipid that the Leatherback Sea Turtle and the Green Sea Turtle have in common is a unique, thick, insulating fat that contains a high percentage of lauric acid in comparison to other turtle species.  This lipid is unique because it traps body heat which allows these two species to maintain high body heat.  Because they have this lipid in common, the Leatherback Sea Turtle and Green Sea Turtle are closely related within the order Testudines.  They are both found in the superfamily Chelonioidea.

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A protein that all testudines with scutes have in common is keratin.  Turtle scutes are made of keratin, which is an extremely strong, fibrous protein.  This shows that turtles with scutes have likely evolved in a more similar way to each other than to species without scutes.

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"An order of reptiles which comprises the turtles, terrapins, and tortoises. They are distinguished by having a shell of bony plates covered with horny scales, and many kinds are aquatic. Also called Chelonia. All turtles are placed within the order Testudines."

FROM: http://www.oxforddictionaries.com/us/definition/american_english/testudines

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Evidence of Evolution - Chromosomal Analysis

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Evidence of Evolution - Chromosomal Analysis

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Testudines have diverse chromosome numbers, with different species having anywhere from 28 to 66 diploid chromosomes.  Many chromosomal rearrangements have occurred during their evolution, causing such diversity.  They also have varied sex-determining mechanisms, such as genotypic (like in humans) or temperature-dependent.  There is a strong coevolution between these traits (chromosome number and sex-determining mechanisms) which is evident because the rates of evolution in turtle chromosome number are much higher when transitions between sex-determining mechanisms also occur.  Data shows that the evolution of sex determination is closely linked to genomic changes related to diploid number evolution.

Spiny Softshell Turtles have 66 diploid chromosomes, whereas Leatherback Sea Turtles only have 56 diploid chromosomes.  This shows the diversity of chromosome number within order Testudines.  Spiny Softshell Turtles have genetic sex determination, meaning they have normal sex chromosomes, and Leatherback Sea Turtles have temperature-dependent sex determination.  These two turtles are not very closely related within order Testudines.

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Relatedness Between Softshell Turtles and Tortoises

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Relatedness Between Softshell Turtles and Tortoises

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The embryonic development of the tortoise shows some of the earlier stages of development and the embryonic development of the softshell turtles shows some later stages of development.  In picture ts16 of the tortoise and picture A of the softshell turtle, the embryos look extremely similar, which shows that they are closely related, being in the same order.  They then later develop differently, the tortoise being in the family Testudinidae and having several differences from the softshell turtle, being in the family Trionychidae.

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Embryology of Tortoises Compared to Other Animals

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Embryology of Tortoises Compared to Other Animals

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As can be seen from these embryonic drawings of different organisms, many organisms look very similar in early embryonic development.  As the embryos develop however, they begin to change more.  A tortoise embryo in it's halfway point of development looks closest to the bird embryo.  This may show that tortoises share a common ancestor more recently with birds, but they then evolved separately.  The most prominent characteristic of a turtle embryo is the shell development.

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Embryonic Development of a Softshelled Turtle

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Embryonic Development of a Softshelled Turtle

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All testudines develop within eggs and are usually incubated between 45 and 75 days.  The rate of development in the egg is temperature-dependent, with warmer temperatures speeding development and cooler temperatures slowing it.  Sex determination is also temperature-dependent, with warmer temperatures making the hatchlings more likely to turn out female, and cooler temperatures male.  This is caused by a gene that, based on temperature, tells other genes what to do.

The eggshells can be leathery, like in sea turtles, or they can be brittle, like in many tortoise shells.  Calcium carbonate is a constituent in both kinds of eggshells, but the leathery eggshells have less.

Embryology provides evidence for evolution because embryos go through similar processes when developing, which shows that they probably had a common ancestor.  The more similar the processes, the more closely related they likely are.

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(210 mya, Triassic Period)

Proterocheris is an ancient fossil turtle.  There is evidence that it is a side-necked turtle, which is very important in the evolution of turtles.  Two major suborders of testudines is Pleurodira (side-necks) and Cryptodira (hidden necks), so Proterocheris is the oldest known fossil to be side-necked.  This is the beginning evolution of the order Testudines diverging into these two taxonomic suborders.

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http://www.britannica.com/animal/turtle-reptile/Origin-and-evolution

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(110 mya, Cretaceous Period)

Santanachelys gaffneyi is the oldest known sea turtle.  It is an important link in the evolution of turtles because it shows the evolution of ancient turtles into modern sea turtles.  It had a streamlined shell that was about five feet long and had forelimbs that were evolving into the flippers we see on sea turtles today.

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(210 mya, Triassic Period)

The fossil species Proganochelys quenstedi shows further evolution of the teeth and shell.  This intermediate species had teeth, but rather than being on their upper and lower jaw, they were located on the roof of the mouth.  The shells of these ancient turtles closely resemble the shells of modern turtles, more so than the older fossils.  Their shells completely encase the pelvic and shoulder girdles.

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(220 mya, Triassic Period)

Odontochelys semitestaca is an intermediate species.  This fossil finding is extremely important because it gives evidence that turtles may have originated from a water environment instead of land.  It also gives more evidence on the evolution of turtle shells.  This fossil species has a complete plastron, broad dorsal ribs, and a series of neural plates, but it lacks a fully developed carapace.  This evidence suggests that the carapace we see on modern turtles today developed after the plastron.  This species also had teeth on its upper and lower jaw, which shows that at some point during the evolution of turtles, they lost their teeth.

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(260 mya, Permian Period)

The Eunotosaurus africanus is recognized to be the earliest known ancestor of modern turtles.  It is an intermediate species between earlier reptiles and turtles, as it has some lizard-like characteristics and the beginning development of turtle characteristics.  This species does not have a plastron or carapace like testudines have today, but they did have modified bones that may have served as an intermediate shell structure.  This fossil shows the early evolution of turtles and the beginnings of having shells.

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Using the taxonomy of these exemplary species, it can be seen that the lowest level of taxonomy that they all have in common is order Testudines.  They do not belong to any of the same families, so past them all being testudines, they are not as closely related to each other as they would be to species from their own families.  Looking at the phylogenetic tree, it can be seen that Russian Tortoises and Leatherback Sea Turtles are likely to be closer related to each other than they are to Spiny Softshell Turtles because Russian Tortoises and Leatherback Sea Turtles belong to the group Cryptodira, meaning they have hidden necks, while Spiny Softshell Turtles belong to the group Pleurodira, meaning they are side-necked.

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Spiny Softshell Turtles are an exemplary species from order Testudines because they are a good example of other softshell turtles that have rubbery skin.

LOCATION FOUND & HABITAT INFO:
Spiny Softshell Turtles are found throughout parts of the United States, Canada, and Mexico.  They live in a temperate climate in freshwater.
PHYSICAL CHARACTERISTICS:
Spiny Softshells have soft, flat, rounded shells with small spines on the edge of their carapace.  Their nose is long and upturned and they have webbed claws and feet.  Females can grow up to 18.9 inches.
BEHAVIORAL CHARACTERISTICS:
Spiny Softshell Turtles mate between 8-10 years old and are diurnal, meaning they spend most of the day basking in the sun and looking for food.  They are carnivores and insectivores.

TAXONOMY:
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Testudines
Family: Trionychidae
Genus: Apalone
Species: Spinifera

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Leatherback Sea Turtles are an exemplary species from order Testudines because of their large size and unique shells.

LOCATION FOUND & HABITAT INFO:
Leatherback Sea Turtles are found throughout the Atlantic and Pacific oceans.  They inhabit the open ocean and require nesting beaches on the coast.
PHYSICAL CHARACTERISTICS:
Leatherback Sea Turtles are the largest of all living turtles and can grow up to 7.2 feet.  They have a hydrodynamic body design and thick, leathery skin instead of scutes on its shell.  They are colored dark gray to black with white spots.
BEHAVIORAL CHARACTERISTICS:
Leatherback Sea Turtles feed almost entirely on jellyfish.  They are also one of the deepest diving marine animals and one of the fastest-moving reptiles.  They are able to maintain high body temperatures using endothermy.

TAXONOMY:
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Testudines
Family: Dermochelyidae
Genus: Dermochelys
Species: Coriacea

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This is a phylogenic tree of the order Testudines, showing family and genus.  Order Testudines is in the class Reptilia, which is in the phylum Chordata, which is within kingdom Animalia.

Phylogenetic trees show the many taxonomic groups an organism is a part of.  The taxonomy of different species can show you how they are related because the more taxonomic groups they have in common, the closer they are related.  Testudines are all closer related to each other than they are to other orders in the class Reptilia.  A specific family within the order Testudines are even more closely related to each other than testudines from other families within the order.

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Russian Tortoises are an exemplary species from order Testudines because of their unique survival tactic and their hardiness.

LOCATION FOUND & HABITAT INFO:
Russian Tortoises are found throughout much of central Asia.  They live in a harsh and variable climate with hot, dry summers and cold winters.
PHYSICAL CHARACTERISTICS:
Russian Tortoises are tortoises, meaning they live only on land.  Russian Tortoises grow to 5-10 inches and have four toes.  They have scutes covering their shells and are yellow, brown, and black colored.
BEHAVIORAL CHARACTERISTICS:
Russian Tortoises are herbivores and eat mostly weeds and wild flowers.  Because of the climate they live in, they may remain burrowed underground for up to nine months.

TAXONOMY:
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Testudines
Family: Testudinidae
Genus: Agrionemys
Species: Horsefieldii

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Testudines have adapted to a large variety of different habitats.

Tortoises are terrestrial and live only on land, while turtles live mostly in water, and terrapins live both on land and in water.

There are testudines that inhabit deserts, rainforests, oceans, woodlands, rivers and ponds, and in many other habitats.

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Testudines are not social animals, and although many individuals may inhabit the same area, there is little interaction between them.

Most testudines eat a variety of different foods, and some are herbivores while others are not.

All testudines lay their eggs on land, and none show parental care.  This means that they leave their eggs and do not care for their young.

The age that an individual will start reproducing varies, but reproductive activity is generally seasonal.  Most testudines dig their nests and lay eggs annually.

There is a wide range of courtship patterns between the different species of testudines.

Some species are also migratory.

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http://www.oxforddictionaries.com/us/definition/american_english/testudines

http://animals.jrank.org/pages/3536/Turtles-Tortoises-Testudines-PHYSICAL-CHARACTERISTICS.html

http://www.britannica.com/animal/turtle-reptile

http://www.untamedscience.com/order/testudines/

http://faircloth-lab.org/assets/pdf/crawford-et-al-2014-mpe.pdf

http://www.lllreptile.com/articles/51-russian-tortoise

https://en.wikipedia.org/wiki/Russian_tortoise

https://en.wikipedia.org/wiki/Leatherback_sea_turtle

http://www.biokids.umich.edu/critters/Apalone_spinifera/

http://www.hngn.com/articles/125919/20150903/eunotosaurus-bloated-lizard-reptile-recognized-world-s-earliest-turtle.htm

http://www.nature.com/news/2008/081126/full/news.2008.1260.html

http://www.cdb.riken.jp/eng/labtour/downloader/archive.html

http://www.cdb.riken.jp/emo/old-japanese/pubj/pdfj02/tokita_0118.pdf

https://newsela.com/articles/turtle-temperature/id/17880/

https://en.wikipedia.org/wiki/Turtle#Genomics

http://www.nature.com/ng/journal/v45/n6/full/ng.2615.html

http://wbd.etibioinformatics.nl/bis/turtles.php?menuentry=soorten&id=216

http://wbd.etibioinformatics.nl/bis/turtles.php?menuentry=soorten&id=196

https://www.ncbi.nlm.nih.gov/pubmed/21644965

http://www.earthrangers.com/wildwire/top-10/top-ten-facts-about-turtle-shells/

https://books.google.com/books?id=nNOQghYEXZMC&pg=PA156&lpg=PA156&dq=lipids+found+in+turtles&source=bl&ots=ufBZQn3Kxn&sig=bc-WkfZoZ7rgMP6JP3AOZ21UlmE&hl=en&sa=X&ved=0ahUKEwjphqXdmZDNAhXHVD4KHV5fBFwQ6AEIKTAC#v=onepage&q=lipids%20found%20in%20turtles&f=false

http://palaeos.com/evolution/glossary.html

https://www.boundless.com/biology/textbooks/boundless-biology-textbook/invertebrates-28/superphylum-lophotrochozoa-168/phylum-mollusca-652-11873/

http://www.jstor.org/stable/4216009?seq=1#page_scan_tab_contents

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Internal and External Physical Characteristics 

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Internal and External Physical Characteristics 

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Testudines have bony upper and lower shells.  The upper shell is called a carapace and the lower shell is called the plastron and they are connected by either a bony bridge or ligament.  Species with hard shells have large scales called scutes covering their shells, and softshells have leathery or rubbery skin covering their bony shells.

Testudines have a backbone, meaning that they are vertebrates, but their hip and shoulder bones are inside the rib cage.  This is because if they were outside the rib cage, they would be outside the shell.

Testudines do not have any teeth.  Instead of teeth, they have hard, flat surfaces on their jaws, called beaks, which they use to grip and tear off bits of plants or animals that they eat.

Testudines are ectotherms, meaning that they are dependent on external sources of body heat.

All testudines have good vision and can smell and hear.

Testudines have eight cervical vertebrae, and can fold their neck.  Some testudines can fold their neck vertically into their shell, while others can only fold their necks to the side.

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Examples of Organisms Found in Order Testudines

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Examples of Organisms Found in Order Testudines

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1. Galápagos Tortoise

2. Green Sea Turtle

3. Wood Turtle

4. Apalone

5. Striped Mud Turtle

6. Alligator Snapping Turtle

7. Flattened Musk Turtle

8. Red Eared Slider

9. Loggerhead Sea Turtle

10. Painted Turtle