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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6Once upon a time, taxonomists argued endlessly about things like this. These days, classification is done by sequence, which takes precedence over anatomy or behavior. – Superbest Jun 10 '16 at 16:46
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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. – John May 07 '19 at 02:29
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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):
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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1Wait, mammals are classified by their skulls? I had always heard that they were classified by mammary glands, the anatomical feature that is the root of the word mammal... – Mason Wheeler Jun 10 '16 at 14:08
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@MasonWheeler Although compare "vertebrata" - where "having a spine" isn't actually sufficient (assuming "axial skeleton" means "some kind of spine") – OrangeDog Jun 10 '16 at 14:11
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The reptiles, as usually defined, are not a monophyletic group and the distinction between amphibians and reptiles pre-dates modern cladistic forms of classification. – Jack Aidley Jun 10 '16 at 14:14
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What I wrote is that "Mammals are amniotes with a synapsid skull", which is correct. They are not the only amniotes with synapsid skull, but the only extant group with that characteristic. Regarding the apomorphies that define "Mammalia", the presence of mammary glands are not the only one, but simply the one that gives the name of the clade. There are a lot: 1. One single dentary, 2. three bones in middle ear, 3. diphyodontism, 4. sweat glands, 5. mammary glands, 6. rbc without nucleous, 7. hair, among others. Btw, mammals that have lost any of these characteristics are still mammals. – Jun 10 '16 at 14:27
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8Given 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. – David Richerby Jun 10 '16 at 14:28
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3@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. – Mindwin Remember Monica Jun 10 '16 at 15:07
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9@Mindwin Fine. There is, after all, nothing quite so welcoming as "Go get a dictionary if you want to talk to me." – David Richerby Jun 10 '16 at 15:09
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2Assuming that the diagram is not your original creation, please cite a source and note its usage terms. – 200_success Jun 10 '16 at 17:10
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1Wait just a minute. You say "Turtles came from an ancestor with these characteristics" but present no proof for this. Isn't it really the case that a turtle's apomorphy is "skull with upper and lower fenestra and beta-keratin in epidermis", and this is why it is considered a reptile? The belief that turtles came from ancestors with this trait is because turtles have this trait, right? Why state a derivational statement for which you provide no proof, instead of stating an objective statement for which you do? – ErikE Jun 10 '16 at 18:19
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5@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. – Mindwin Remember Monica Jun 10 '16 at 19:01
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Perhaps you would want to clarify that both "reptiles" and "amphibians" are kind of an obsolete classification. These classifications were done when there was not much data. Moreover, reptilia is paraphyletic because it does not include birds. – WYSIWYG Jun 10 '16 at 20:06
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1The currently accepted answer contains an error. Reptiles are not a monophyletic group. Of the related species from which one evolved into all the mammals we know today, several others evolved into still extant reptile lineages. – Jun 10 '16 at 23:05
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@Mindwin The jargon doesn't bother me. And the answer's pretty clear, but of course I had to make a bit of an effort to understand specific terms. – Irena Jun 11 '16 at 15:30
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@Imi was it worth? that "bit of effort"? Did your horizons expanded? I also had first contact with some terms in this Q&A. – Mindwin Remember Monica Jun 13 '16 at 12:57
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1I'm going to answer this (hoping it's not a rhetorical question) -- I've got a reply up here which emphasises on the phylogenetic and ancestral relationship, and this is exactly what I did not know and wanted to. So everything right from that effort to googling the professional jargon becomes worth the time. – Irena Jun 15 '16 at 15:44
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@GerardoFurtado So, hypothetically: Say there were another species identical to humans in every way; except they just happened to only ever develop 2 bones in the middle ear, as opposed to 3.. Would they be considered something other than mammals? – voices Nov 03 '16 at 07:52
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@tjt263 it depends on the ancestor. If that specie's ancestor is a mammal, it doesn't matter if it has two or twenty bones, they are mammals. In that case, its particular feature (2 bones instead of 3 in the middle ear) is an apomorphy, or an autapomorphy. – Nov 03 '16 at 10:25
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then what about Gymnophiona or Apoda like caecilians?! "they live underground in burrows in damp soil, in rotten wood and under plant debris" and "most species lay their eggs underground and when the larvae hatch, they make their way to adjacent bodies of water" and yet they are amphibians. and you said: "contrary to the amphibians, they cannot lay eggs under water" and neither can caecilians. but yet they are amphibians. – M D P Jan 25 '18 at 11:24
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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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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.
Jack Aidley
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