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Why are turtles classified as reptiles and not amphibians?

Why are turtles classified as reptiles and not amphibians?


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I understand that turtles are reptiles because like all reptiles, they have scales on their body. But turtles (specifically sea turtles) live on both land and water, very much like amphibians. Also, don't sea turtles have more of a moist skin unlike reptiles? So is there any anatomical difference which makes turtles different from amphibians?


Amphibians are not defined for having a moist skin, neither are reptiles defined for having scales on their body.

In biology, organisms (elements) are grouped according to their evolutive history in monophyletic groups, also known as clades. Basicaly, a monophyletic group is

A group consisting of an ancestral and all its descendants.

"Tetrapoda" is a monophyletic group, formed by vertebrates with four limbs. Inside Tetrapoda, there are two groups: "Amphibia" and "Amniota". Amphibians are non-amniotes tetrapods, and we have some doubt if they are a monophyletic group. Amniotes, on the other hand, are tetrapods having an amniotic egg, including you and me, and they form a monophyletic group.

The amniotes are divided in two monophyletic group: "Mammals", which are amniotes with a synapsid skull, and "Reptiles", which are amniotes with diapsid skulls (I'm using "reptiles" as synonym of Sauropsida).

Reptiles include turtles, lizards, snakes, alligators and dinosaurs (which include the birds: all birds are dinosaurs). It doesn't matter if a animal has or has not scales, or if it lays eggs or if it is viviparous, or if it has 5 fingers or 3 fingers: All the descendants of a given ancestor are included in the monophyletic group that contains that ancestor.

Turtles, despite being strange or somehow different, are descendants from the same most recent commom ancestor of Reptilia… that's why turtles are reptiles (and that's why birds are reptiles as well).

To make this more clear, have a look at this cladogram (from Hickman, Integrated Principles of Zoology):

The apomorphy that defines the tetrapods is "paired limbs". You have Amphibia to the left and Amniota to the right, whose apomorphy is " egg with extraembrionic membranes". Inside them, you have Reptilia, whose apomorphies are "skull with upper and lower fenestra and beta-keratin in epidermis". Turtles came from an ancestor with these characteristics. So, turtles belong to the monophyletic group of "Reptiles".

Post scriptum: You wrote that "turtles (specifically sea turtles) live on both land and water, very much like amphibians". Just a curiosity: the reason why sea turtles leave the water (sea) from time to time shows exactly that they are not amphibians! Amphibians, being non-amniotes, have eggs that survive under water (actually, with few exceptions, they need to be under water). Turtles, on the other hand, are amniotes, and the amniotic egg cannot be laid under water. That's why the turtles have to leave the water to lay eggs: because, contrary to the amphibians, they cannot lay eggs under water.


In addition to @Gerardo's answer:

Reptiles

The term Reptiles as used in popular language does not represent a monophyletic group. When using the term Reptiles, one is typically thinking of turtles, snakes and lizards but excluding birds and mammals (and a few other things not worth mentioning). There are two clades (monophyletic groups) whose name sounds like Reptiles and whose meanings are related; Reptilia and Reptiliomorpha.

Reptilia

Reptilia is the clade that @Gerardo discusses in his answer. Reptilia includes turtles, crocodilians, snakes, amphisbaenians, lizards, tuatara and birds.

Reptiliomorpha

Reptiliomorpha is a clade that includes Reptilia, Synapsida (mammals and close relatives) and a few extinct lineages.

Visualizing the tree of life by yourself

There are two good online resources: tolweb.org and onezoom.org.

Onezoom.org is better updated and has a nice pleasant look. However, I often find tolweb.org more convenient for investigating the true meaning of a specific clade.

Further reading

You might want to have a look at Understanding Evolution by UC Berkeley. It is a very introductory course on evolutionary biology and it includes a part on the phylogenetic tree.

I also strongly recommend having a look at the related question If dinosaurs could have feathers, would they still be reptiles?


Turtles are not amphibians because they have (largely) non-permeable skin whereas amphibians can absorb oxygen through their skin. There are also differences in their reproductive cycle: turtles are amniotes, so they produce eggs that must be laid on land, whereas amphibians - like fish - are not and must lay their eggs in water.

Since the categorisation pre-dates the development of cladistics, and the group normally called reptiles are not monophyletic, I don't think cladistic based answers are a good answer to this question. It is the morphology and physiology that matters.


What is amphibians in biology?

An amphibian is a vertebrate (an animal with a backbone). Amphibians are cold-blooded, which means that their body temperature changes with their surroundings. They spend part of their lives in water (breathing with gills) and part of their lives on land (breathing with lungs).

Secondly, what are 3 examples of amphibians? The three modern orders of amphibians are Anura (the frogs and toads), Urodela (the salamanders), and Apoda (the caecilians). The number of known amphibian species is approximately 8,000, of which nearly 90% are frogs.

Similarly, what are the 7 main characteristics of amphibians?

Many amphibians, such as frogs, undergo a metamorphosis, meaning they begin life in the water and live on land as adults. Defining characteristics of these creatures include moist, scaleless skin and the fact that they are cold-blooded. Amphibians absorb water and undergo gas exchange through their skin.

What are the main features of amphibians?

Characteristics. Amphibians are cold-blooded animals, meaning they do not have a constant body temperature but instead take on the temperature of their environment. They have moist, scaleless skin that absorbs water and oxygen, but that also makes them vulnerable to dehydration (loss of bodily fluids).


Characteristics of Reptiles

Reptiles are tetrapod meaning they walk on four limbs or are descendants of animals that walked on four limbs. They have a backbone just like all animals in the phylum Chor data.In evolution terms, reptiles are the intermediates between the warm-blooded mammals and the cold-blooded amphibians. Reptiles are amniotes, meaning that the females lay eggs although a few such as the squamate lizards give birth to young ones. The main difference between the young ones born by mammals and reptiles is the lack of the placenta among reptiles. All reptiles have scales made of several layers of dead skin. Crocodiles have a tough armor-like outer skin while turtles and tortoises have shells. Scales are not limited to reptiles as fish and birds possess them too. Reptiles are cold-blooded, their body temperature depends on the temperature of the environment they are. Cold-blooded animals bask in the sun to raise their temperature and crawl beneath rocks, shrubs, barks or enter burrows to lower the temperature. A majority of reptiles have lungs for respiratory purposes, but a few utilize other forms or respiration. Lizards breathe through the same muscles they use to run, and that is why they cannot breathe while in motion.


There’s No Such Thing As Reptiles Any More – And Here’s Why

You have likely been to a zoo at some point and visited their reptile house. A building where the climate control dial is stuck on the “wet sauna” setting, and filled with maniacal children competing to be the first to press their ice cream covered face and hands on every available piece of clean glass.

Assuming you managed to find some clean glass, and supposing the animals were not hiding from the incessant banging and requests to perform like circus animals, you would have likely seen turtles, crocodiles, snakes and lizards.

But what if I told you reptiles don’t exist.

I am not suggesting that you imagined seeing scaly creatures, but the group of animals we refer to as “reptiles” does not exist – at least not anymore.

It all has to do with our (humans, that is) penchant for categorisation. We just love putting things into boxes, and those boxes into bigger boxes. Scientists have even turned this activity into an entire field of biological research, called taxonomy.

Several methods of classification have been used throughout the history of taxonomy. The current, most widely accepted method – cladistics – is considered to be the most objective as it takes into account an organism’s evolutionary history.

The picture below maps out our current understanding of the relationships between land vertebrates. First thing to notice, the label tetrapoda on the left is the base of the diagram, indicating that all species to the right are within the group tetrapoda. From here you can go down each path, labelling each group.

Cladogram of tetrapods (land vertebrates). Reptiles as a group exist on different branches rather than all together.

For example, the three groups of animals at the top (caecillians, salamanders and frogs) all belong to the group amphibia, and all the groups of animals from junction [B] onwards are grouped as amniota.

The construction of such a diagram depends on the common ancestors that groups of animals share. For example, junction [A] represents the common ancestor between us and an echidna. Humans are part of eutheria and echidnas are monotremes. All animals sharing this common ancestor are labelled as mammals. Also, in evolutionary terms, we would say that two species that share a common ancestor at junction [A] would be more closely related than those species sharing a common ancestor at junction [B].

All pretty straightforward – but, this is where the reptile label runs into a problem.

I have circled the group of animals we normally refer to as reptiles. If you trace their paths back, you will arrive at junction [C], the last common ancestor of those groups. So, if we are to consider all animals from junction [C] onwards as reptiles, then we must also label birds as reptiles. We could do this I guess, but it would be redundant. The group of animals from junction [C] onwards are already referred to as sauropsida.

Hence, reptile is dead. (I suspect Nietzsche will still be quoted more.)

The tuatara looks like a lizard, but it isn’t one. Wikimedia Commons, CC BY

An obvious question at this point: if there is no such group of animals called reptiles, then why are there reptile houses at zoos? As you might guess, it has to do with history.

Looks aren’t everything

Classification of animals (and plants for that matter) was formalised in the 18th century by Swedish scientist Carl Linnaeus. Linnaeus built his classification system on the way animals looked, in an age when species were considered to be fixed and unchanging.

Using anatomical keys, Linnaeus divided the animal kingdom into six classes:

Then, as with current taxonomical methods, further divided these broad groups into more refined taxonomic categories.

Species of amphibia were divided into two groups:

Reptiles were considered to have feet, with flat naked ears where serpents did not have feet, laid eggs connected in a chain and “penis double”. That’s right, snakes have a pair of hemipenes. In fact, this last observation is odd since lizards also have a pair. Having feet was clearly the more important diagnostic tool for Linnaeus, otherwise he would have known to put snakes and lizards together.

Linnaeus’ categorisation methods meant that many species were incorrectly grouped together. For instance, reptiles comprised turtles, lizards, crocodiles, salamanders and frogs, while serpents included snakes, legless lizards and caecilians. As you can see from our modern branched image above, Linnaeus may have done better just picking names out of a hat – but I suppose that’s always easier with 20/20 hindsight.

The komodo dragon? Try komodo monitor. Wikimedia Commons, CC BY

As naturalists developed new methods of classification, new boxes were created, some species were taken out of one box and added to another.

Early in the 19th century French zoologist Pierre Latreille divided the tetrapod group into four major groups:

Since then, though taxonomists have routinely shuffled and revised the groups, and though genetic techniques have given us new insights into the evolution of these organisms, the name has stuck.

I don’t expect zoos will change the reptile house to the “non-avian reptile house” or “sauropsida house” anytime soon. It doesn’t really have the same ring to it.

As for biology texts, though the reptile label no longer relates to an evolutionary group as mammals or birds does, biologists will still use it.

The “reptile” label groups together a deeply fascinating group of animals who are persecuted far too much, researched far too little and likely have many great stories yet to reveal.

Dustin Welbourne does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.

This article was originally published on The Conversation. Read the original article.


Classification of Reptiles

There are more than 8,200 living species of reptiles, with the majority being snakes or lizards. They are commonly placed in four different orders. The four orders are described in Table below.

Lizards: most have four legs for running or climbing, and they can also swim many change color when threatened they have a three-chamberedheart.

Snakes: they do not have legs, although they evolved from a tetrapod ancestor they have a very flexible jaw for swallowing large prey whole some inject poison into their prey through fangs they have a three-chambered heart.


Evolution of Reptiles and Amphibians

Evolution of Reptiles

Hylonomus is the oldest known reptile was about 8 to 12 inches long with origins 200 million years ago. The first true "reptiles" (Sauropsids) are categorized as Anapsids, having a solid skull with holes only for nose, eyes, spinal cord, etc. Turtles are believed by some to be surviving Anapsids. Shortly after the first reptiles, two branches split off, one leading to the Anapsids, which did not develop holes in their skulls. The other group, Diapsida, possessed a pair of holes in their skulls behind the eyes, along with a second pair located higher on the skull. The Diapsida split yet again into two lineages, the lepidosaurs (which contain modern snakes, lizards and tuataras, as well as, debatably, the extinct sea reptiles of the Mesozoic) and the archosaurs (today represented by crocodilians and birds only, but also containing pterosaurs and dinosaurs).

The earliest, solid-skulled amniotes also gave rise to a separate line, the Synapsida. Synapsids developed a pair of holes in their skulls behind the eyes (similar to the diapsids), which were used to both lighten the skull and increase the space for jaw muscles. The synapsids eventually evolved into mammals.

Evolution of Amphibians

The first major groups of amphibians developed in the Devonian Period( A period of geological time around 350 million years ago) from fishes similar to the modern coelacanth where the fins had evolved into legs. These amphibians were around five meters long in length, which is rare now. In the Carboniferous Period, the amphibians moved up in the food chain and began to occupy the ecological position where we now find crocodiles. These amphibians were notable for eating the mega-insects on land and many types of fishes in the water. Towards the end of the Permian Period and the Triassic Period, the amphibians started having competition with proto-crocodiles which led to their drop in size in the temperate zones or leaving for the poles. (Amphibians were able to hibernate during the winter whereas crocodiles could not, allowing the amphibians in higher latitudes protection from the reptiles.)


Why Are There No Limbless Mammals?

There exist limbless reptiles, amphibians, fish and arthropods, but why are there no limbless mammals? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.

Answer by Jelle Zijlstra, Bachelor of Arts in Organismic and Evolutionary Biology, on Quora:

There exist limbless reptiles, amphibians, fish and arthropods, but why are there no limbless mammals?

We can extend the pattern a bit: there are numerous groups of limbless lizards (including snakes Jelle Zijlstra's answer to What is the difference between a legless lizard and a snake? ) and amphibians (two salamander families, caecilians, and the extinct aistopods and lysorophians), but no limbless mammals, birds, turtles, or crocodiles. The major living subgroup of amphibians, the frogs, also lacks limbless species.

Why is that? According to the review by Wake (2001), limblessness in vertebrates requires a form of locomotion that emphasizes flexible lateral movements among the vertebrae. Therefore, it is hard for animals with short, rigid vertebral columns like frogs, turtles, and birds to evolve limblessness. Mammals also don’t tend to move their vertebrae laterally, which makes it hard for them to become limbless.

However, limblessness has been suggested (Bjork, 1975) for at least one fossil mammal, the extinct hedgehog Proterix. No complete skeleton is known, so we cannot be certain, but Proterix has very peculiar vertebrae, with lengthened lumbars flanked by lateral plates. Bjork saw resemblances to whales and hypothesized that Proterix may have been fossorial and used a similar mode of locomotion as amphisbaenians (a group of legless lizards).

Wake also cited treadmill studies that found that snakes use less energy for movement than lizards do. Also, snakes can be fairly fast, although not as fast as e.g. humans ( Can a snake run faster than a man? )

  • Bjork, P.R. 1975. Observations on the morphology of the hedgehog genus Proterix (Insectivora: Erinaceidae). Papers on Paleontology 12:81-88.
  • Wake, M.H. 2001. Tetrapod limbless locomotion. Encyclopedia of Life Sciences, 4 pp. http://onlinelibrary.wiley.com/d.

This question originally appeared on Quora - the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google+. More questions:


Evolution of Reptiles

Reptiles originated approximately 300 million years ago during the Carboniferous period. One of the oldest known amniotes is Casineria, which had both amphibian and reptilian characteristics. One of the earliest undisputed reptile fossils was Hylonomus, a lizardlike animal about 20 cm long. Soon after the first amniotes appeared, they diverged into three groups—synapsids, anapsids, and diapsids—during the Permian period. The Permian period also saw a second major divergence of diapsid reptiles into stem archosaurs (predecessors of thecodonts, crocodilians, dinosaurs, and birds) and lepidosaurs (predecessors of snakes and lizards). These groups remained inconspicuous until the Triassic period, when the archosaurs became the dominant terrestrial group possibly due to the extinction of large-bodied anapsids and synapsids during the Permian-Triassic extinction. About 250 million years ago, archosaurs radiated into the pterosaurs and both saurischian “lizard hip” and ornithischian “bird-hip” dinosaurs (see below).

Although they are sometimes mistakenly called dinosaurs, the pterosaurs were distinct from true dinosaurs (Figure). Pterosaurs had a number of adaptations that allowed for flight, including hollow bones (birds also exhibit hollow bones, a case of convergent evolution). Their wings were formed by membranes of skin that attached to the long, fourth finger of each arm and extended along the body to the legs.

Pterosaurs. Pterosaurs, such as this Quetzalcoatlus, which existed from the late Triassic to the Cretaceous period (230 to 65.5 million years ago), possessed wings but are not believed to have been capable of powered flight. Instead, they may have been able to soar after launching from cliffs. (credit: Mark Witton, Darren Naish)


Difference Between Lizard and Reptile

Lizards and reptiles shouldn’t be confused with each other. If you’re confused, just think of the lizard as Pepsi Max and a reptile as a soda. A lizard is a kind of reptile that also has several other subfamilies.

Under the taxonomic classification system, reptiles are part of the class Reptilia. Below it are different subclasses like Anapsida and Euriptilia. Further below Euriptilia are the orders of Crocodilia, Sphenodontia, molecular Testudines, and Squamata. Now, the lizard is just one of the reptiles classified under the order Squamata. In this group, there are about 7,900 different species making it the biggest order in the reptilian class. This would also imply that not all reptiles are lizards but all lizards are considered reptiles.

Perhaps the confusion is more of the common perception of associating reptiles to snakes. However, snakes are the same as lizards because they both belong to the order Squamata. Reptiles are creatures of the Reptilia class and are placed in the Linnaean system in-between birds and amphibians in the Kingdom of Animals. Previously, giant reptiles were said to have dominated the planet. Since they have already died out, all that’s left are their smaller descendants.

Reptiles are cold-blooded creatures often having backbones. All throughout their lifetime, reptiles breathe using their lungs. Unlike amphibians, reptiles do not possess slime glands making their skin scaly and dry. This is one of the reasons why they are able to thrive primarily on land. Other reptiles include: crocodiles, turtles, snakes, and many others. Nonetheless, the turtle class of reptiles is different from most reptiles in the sense that they possess a soft body with an outer protective shell that serves as protection or a normal covering.

As mentioned, the order Squamata has thousands of species comprising both snakes and lizards (about 3,800 species) of very diverse animals. They are characterized by their limbs and outer ears contrary to their absence among snakes. One of the most intriguing abilities of lizards is autotomy – the automatic detachment of their tails for them to move away from their predators. It is a survival capability that is, unfortunately, not present in all lizard species. Lizards are also able to communicate using colors with the aid of their color vision sense. Although belonging to different families, some of the most common lizards are the gecko, chameleon, varanid lizard and, of course, the enormous Komodo dragons growing more than nine feet in length.

1.Lizards are part of the order Squamata together with snakes.
2.Class Reptilia’s four main suborders: Crocodilia, Sphenodontia, molecular/traditional Testudines, and Squamata
3.“Reptile” is a more general classification or description of lizards.
4.There are about 3,800 different species of lizards.
5.Lizards are best known for their ability of autotomy.


What’s the point of the term “non-avian reptiles” when referring to reptiles?

It’s seems kind of redundant because we already have a name for birds. I understand that birds evolved from reptiles but reptiles also evolved from amphibians, so why is the term “non-reptilian amphibian” not used? Or “non-mammalian birds”?

The classifications such as amphibian, reptile, bird, mammal and so on were made in the pre-genetics world where this was all being done based on physical characteristics. As genomes began being sequenced and scientists were able to more accurately define the tree of life using genetics, it turned out that while the separation of amphibians from reptiles and birds was accurate, there were some issues with the way birds were grouped separately from reptiles.

It's true what you say that reptiles evolved from amphibians. Although it's probably more technically accurate to say they evolved from amphibian-like creatures. But all the living amphibians today share a most recent common ancestor that is entirely separate from the most recent common ancestor of all reptiles. So they cleanly separate into two groups. However, there are some reptile species, such as crocodiles and alligators, where the most recent ancestor of them and birds is actually more recent than their common ancestor with other reptiles such as lizards and snakes. So they're actually more closely related to birds. This means that the term "reptile" either must include birds, or must not include crocodiles and alligators. So if birds are reptiles but you don't want to include them in your discussion, then youɽ use the term "non-avian reptiles". Another term you might see is "archosaur" which refers to crocodilians, dinosaurs, and by extension birds, which all share a most recent common ancestor apart from the separate branch containing all the other species classified as reptiles.

The tree looks something like this:

So you can see that while amphibians and mammals are there own branch of the tree, birds are mixed in with the reptiles branch, and can't be separated without also removing some other reptiles.

Just to add to this, it has been found that turtles are the closest relatives to the archosaurs (birds+crocodilians). This group as a whole is called the archelosaurs (turtles+(birds+crocodilians)

When naming groups of organisms, we always try to create groupings that include a particular common ancestor and all of its descendants. Such groups are called clades, and are described as monophyletic, in contrast to groups which do not meet these criteria, which are para- or polyphyletic (here's a good figure to show the difference). So, as you already know, a monophyletic definition for the group Reptilia consists of the common ancestor of all reptiles and all of its descendants, including birds. The term "non-avian reptiles" is thus a way to refer to the paraphyletic group represented by other reptiles but not counting birds, which under taxonomic convention should not have an official name. There are plenty of situations where this is a useful term to have even though it doesn't describe a monophyletic group (e.g., "birds are warm-blooded, in contrast to all living non-avian reptiles").

The situation with amphibians is actually a little different. Though there isn't a complete consensus on this, it's generally agreed that all living amphibians do in fact form a monophyletic group called Lissamphibia (also sometimes called crown Amphibia), which is the sister group to Amniota (reptiles and mammals). There are several extinct groups of organisms that are of somewhat less certain affiliation, so these do complicate things a bit. Some of them are more closely related to modern amphibians than to amniotes, and so we would call these "stem amphibians" (see this figure for good depiction of stem vs. crown terminology), while others may have branched off before the split between the lineages that would become Lissamphibia and Amniota. In this latter case, even though these organisms probably looked and acted in ways that we associate with amphibians, they do not belong to the clade Amphibia, and are more properly described as "stem tetrapods". This includes several well-known species like Tiktaalik and Ichthyostega. So basically, even though reptiles (and all amniotes) did evolve from ancestors that may have superficially shared some characteristics with amphibians, reptiles are not part of the clade Amphibia, and so cannot be excluded from it with a term like "non-reptilian amphibian".

Your final example of "non-mammalian birds" is much simpler. Birds and mammals are just independent lineages with no overlap (though they are both members of Amniota) so there's no reason to use this term. To be a little more technical, terms like "non-X Y's" can describe paraphyletic groups (as in non-avian reptiles), but they aren't useful for describing polyphyletic groups, like "mammals + birds" (see first figure again if that helps).


Critter Catalog

Reptiles are vertebrates that have scales on at least some part of their body, leathery or hard-shelled eggs, and share a number of other features. Snakes, lizards, turtles, crocodilians, and birds are reptiles. Like all vertebrates, reptiles have bony skeletons that support their bodies.

Scales help prevent reptiles from losing water through their skin. The leathery or hard shells on their eggs protect the young from drying out while they grow inside the egg. Most reptiles can live their entire lives on land and reproduce in dry habitats. Some types of reptiles (such as sea turtles and penguins) are adapted to living in water, but even these species come onto land to lay their eggs. All reptiles also have lungs, so even those living in water must come to the surface to breath air. Reptiles are found living in all habitats but are more common in warm, tropical places.

So, why are birds considered reptiles? Birds look very different from other reptiles, they have feathers, fly, sing complex songs, and have many other unique features.

Animal groupings are most useful when they reflect the evolutionary history of the animals in that group. So, for example, "mammals" is the name of a group of animals that are all more closely related to each other than to other kinds of animals. It's not hard to imagine that a skunk is more closely related to you than it is to a turtle, because you and a skunk both have fur (yours is mostly on your head!), you both have mothers that fed you milk, and other features. Even though you and the skunk look very different, you share features that show you share an ancestor. The "mammals" group name reflects that shared history.

Snakes, lizards, turtles, crocodilians, and birds also all share an evolutionary history. Many years of research has proven that the ancestors of birds were bird-like dinosaurs (visit the Dinobuzz page for more on this). Even though birds look very different from other living reptiles, they’re most closely related to alligators and crocodiles and clearly belong in the reptile group.

. "Reptilia" (On-line), Animal Diversity Web. Accessed June 24, 2021 at http://www.biokids.umich.edu/accounts/Reptilia/

BioKIDS is sponsored in part by the Interagency Education Research Initiative. It is a partnership of the University of Michigan School of Education, University of Michigan Museum of Zoology, and the Detroit Public Schools. This material is based upon work supported by the National Science Foundation under Grant DRL-0628151.
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