Should Crocodilians and Dinosaurs be removed from Reptilia and given their own Class?

[Nikos]

I know what you're probably thinking, "Dinosaurs and Crocodilians have scaly skin, and Crocodilians are cold blooded! They're reptiles!" But apart from these similarities, Crocodilians and the Dinosauria share very little in common with other species in the Reptilia. For example, Snakes and lizards have teeth that are fused to their jaws.

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While Crocodilians and carnivorous Dinosauria have teeth that are in sockets which routinely fall out and are replaced by new teeth that are constantly growing behind the old teeth.

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Other differences include the bio-mechanics of crocodilians, dinosaurs and other reptiles. A lizard, for example, walks with it's four limbs splayed out, like so.

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http://www.waikikitravel.com.sg/Divi...o%20Dragon.jpg

When one looks at the bio-mechanics of Crocodilians and the Dinosauria, they will notice astonishing evolutionary modifications to the rear and front limb anatomy.

Notice, while still slightly splayed outward, the limbs of this crocodile are beginning to show Dinosaur/ bird like modifications. The limbs are now more directly under the animal, rather then splayed out. In the Dinosauria, this is more pronounced as the bio-mechanics have completely changed from lizards and other reptiles. Notice, that the rear limbs are now fully under the body, providing an efficient, upright gait.


It would be too numerous to list ALL the reasons why I think Crocodilia and Dinosauria should be separated from Reptilia and given their own class(or perhaps even be paired in a new class with Aves). But I believe the old system classifying Crocodilians and Dinosaurs in Reptilia is antiquated and should be changed. Thoughts?

[MathiasOfAthens]

[Nikos]

An excellent representation of Avian/ Dinosaurian locomotion.

[Elfdude]

Snakes are also very different, but I fear you've only identified existing boundaries. Key here is that lizards are a part of reptilia, as are crocodilians, dinosaurs etc.

The Reptile class of animal includes: 23 known species of Crocodilia (crocodiles, gavials, caimans, and alligators), 2 known species of sphenodontia (tuataras from New Zealand), 7,900 known species of Squamata (lizards, snakes, and worm lizards), and about 300 known species of Testudines (turtles and tortoises).

Dinosaurs however could fall into the bird category.

[KingOfTheIsles]

As long as you acknowledge that birds are part of Reptilia, the group would be monophyletic. Dinosaurs, crocodiles and birds are already in the Archosaur group so there is no point in taking birds out of that group, given that birds arose from dinosaurs.

[cpdwane]

As long as you acknowledge that birds are part of Reptilia, the group would be monophyletic. Dinosaurs, crocodiles and birds are already in the Archosaur group so there is no point in taking birds out of that group, given that birds arose from dinosaurs.

This ^^^

Reptilia is a paraphyletic group as it is, because of Aves being considered a seperate class (which it shouldn't be according to modern cladistics.)
I think that there may be a better case for the grouping of Aves and the other Archosaurs together to form a seperate class (Archosauria) but at the end of the day it is all completely elementry because the group would be paraphyletic. From reptiles they evolved, so reptiles they should be.

Technically, of course, Mammalia shouldn't be regarded as a class either and should be in the class Synapsida, which would be a sister class to Diapsida, or the reptiles + birds. But in this case it is kind of convenient to just say that mammels are a seperate class, since all of the other Synapsids are dead.

It all comes down to the disparity between the traditional groupings of animals that are alive today, and their actual genetic lineages.

[MathiasOfAthens]

That video captures dinosaur locomotion better than one may think. Avians are the descendent's of dinosaurs but if you watch Pigeons, chickens, etc walk they move the heads back and forth. This eagle still moves his head but unlike them he doesnt show it show much. I find this bit interesting.


There is another video out there thats even better but I could not find it. It shows the same bird or similar running down a hill. Its beautiful.