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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?

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    $\begingroup$ Once upon a time, taxonomists argued endlessly about things like this. These days, classification is done by sequence, which takes precedence over anatomy or behavior. $\endgroup$ – Superbest Jun 10 '16 at 16:46
  • $\begingroup$ 1 sea turtles have scaly skin 2 amphibians were defined by having to lay eggs in water, which sea turtles most definitely do not. Classifications have changed since then but even using early definitions it doesn't work. $\endgroup$ – John May 7 at 2:29
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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):

enter image description here

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.

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    $\begingroup$ @MasonWheeler The diagram mentions both, which is still unclear. $\endgroup$ – Stop Harming Monica Jun 10 '16 at 14:10
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    $\begingroup$ Given the level at which this question is asked, I think this answer would be much more helpful if you defined the technical terms. At the very least, "amniotic", "synapsid", "diapsid" and "apomorphy" are unlikely to be familiar to somebody at the level of the asker, and there are lots of terms in the quote marks in the last paragraph that are also highly specialized. $\endgroup$ – David Richerby Jun 10 '16 at 14:28
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    $\begingroup$ @DavidRicherby given that 1) the terms are correct - - 2) there's google - - 3) this is a technical site. - - - - - - I think OP and the casual biology enthusiast can overcome this little jargon hurdle. $\endgroup$ – Mindwin Jun 10 '16 at 15:07
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    $\begingroup$ @Mindwin Fine. There is, after all, nothing quite so welcoming as "Go get a dictionary if you want to talk to me." $\endgroup$ – David Richerby Jun 10 '16 at 15:09
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    $\begingroup$ @DavidRicherby I'm not telling OP to get a dictionary. I am telling that in a technical circle it is fine to use technical terms. It is assumed that the enthusiast will not be bothered to learn the jargon. $\endgroup$ – Mindwin Jun 10 '16 at 19:01
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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?

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  • $\begingroup$ Thank you. These sites you mentioned are very helpful and informative. $\endgroup$ – Irena Jun 10 '16 at 16:13
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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.

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