[greensleeves (OP)]

Although of course dinosaurs evolved from reptiles, so did mammals, and yet mammals are not classified as reptiles. Neither are birds. Evolutionary ancestry is not sufficient reason for inclusion into a recognised order of vertebrates, and yet dinosaurs - despite their obvious differences from modern day reptiles - are still routinely refered to as reptiles. (Many appear closer anatomically and physiologically to birds than to reptiles). So my question is: is there a strong, sound, anatomical or physiological reason for believing that dinosaurs were sufficiently closely allied to lizards, crocodilians and turtles etc, to be classified as such, or is the continued inclusion of dinosaurs into the reptile order purely a case of 'it's always been assumed that they were reptiles since the first bones were studied 200 years ago, so we won't change it without conclusive evidence to the contrary?' Should they be elevated to their own unique 'order' status, or indeed grouped with birds in an avian/dinosaurian order?' I'd appreciate your views.

[Don_1]

The Permian extinction left a great hole in life on Earth. The Apsids, which had been the most dominant species were all but wiped out. The Clade Anapsid was a survivor and probably was the forerunner of modern the Testudines (Tortoises and Turtles). Therapsids were the branch of the reptiles which evolved into mammals, while Archosaurs would become the dinosaurs. Whether these all existed side by side is a matter of contention.

My point is that while the term 'reptile' is pretty narrow today, 250 million years ago, reptiles were quite different and the term covers a number of branches of the Triapsids, Diapsids, Anapsids and ultimately the Apsids.

Well, that's my explanation, anyways. Of course, it could have more holes in it than a Triapsid's skull!

[greensleeves (OP)]

Thanks for your reply Don. I take your point that the reptiles which exist today are but a small sample of the kinds which once existed, notably in the Permian and Triassic, and that reptiles that existed then may well have been markedly different from today's types.

However, most of these reptile groups either became extinct or existed as relatively unimportant sidelines during the heyday of the dinosaurs. So for that matter did early mammals, which one would have thought would be physiologically more advanced than any reptilian order. The fact that dinosaurs were to totally eclipse Mesozoic mammals, as well as all these other groups of land reptiles for more than 100 million years, suggests to me a physiological advantage of a degree which sets them apart from 'other reptiles'.

My feeling as you probably realise, is that just because they evolved from reptiles, doesn't necessarily mean that they should be classified as reptiles. Unless of course there is some compelling anatomical similarity which I am unaware of, which demonstrates an unequivocal closeness of form.

[Geezer]

Isn't it just a hangover from the original assumption that they were "terrible lizards"?

[greensleeves (OP)]

That's exactly my belief - that it dates from early Victorian times when nothing in the way of big animals were known except reptiles and mammals. When dinosaur fossils were discovered, they couldn't be thought of as mammals, and evolution into an entirely different group of animals was not even being imagined (evolution itself was only just emerging as a theory), so giant lizard-like reptiles were the only option.

Of course this is exactly how they were first portrayed by early palaeontologists - as giant crawling lizards.

[CliffordK]

Perhaps Mammal vs Reptile oversimplifies things when considering a species that may have been an ancestor for many species.

I believe people have found dino-eggs.
The Platypus is the only mammal that lays eggs...  Actually, there are a couple of "monotremes" in Australia and New Guinea.

But, Dino eggs would lead one away from mammalian classifications. 

Is the "tail dragging" that is usually depicted with many Dinosaurs reptilian?

Modern birds are descended from some Dinosaurs. 

[greensleeves (OP)]

I think Clifford that as far as tail dragging is concerned, tail impressions may have been found on occaision, but I believe the more common theory today is that the majority if not all dinosaurs carried their tails off the ground most of the time, as counterbalances to the head and neck in quadrupedal species like Brachiosaurus or the fore part of the body in bipedal species like Tyrannosaurus rex. Dragging the tail would seem an unnecessary impediment to agile movement, and unlike 'other reptiles', which have their legs splayed out to the side, dinosaurs had their legs tucked further under the body like mammals, which would naturally raise at least part of the tail up off the ground.

Certainly dinosaurs were not mammalian, but the laying of eggs doesn't necessarily make them reptilian either. After all, birds lay eggs too. So do most other vertebrates including, as you say, two mammals.

[Don_1]

Isn't it just a hangover from the original assumption that they were "terrible lizards"?

Although you are probably right in this assumption, in fact the word 'dinosaur' does not mean 'terrible lizard', the actual meaning is 'fearfully-great lizard'.

On a point of order and perhaps in some way relevant to your question, Greensleeves, I sometimes take objection to my little chums here

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being referred to as surviving dinosaurs, when the tortoise actually predates the dinosaurs, perhaps by anything up to 100 million years.

They'll be coming out of hibernation any time now.

[greensleeves (OP)]

Thankyou Don for a nice message - I do hope your friends do not take offence if I have implied in any way that they may be marginally inferior to dinosaurs which dominated life on land for 120 million years.

It is however difficult to imagine your tortoises rampaging over the countryside in great herds browsing from the tree tops, or leaping on to the backs of prey, terrorising the rest of life on Earth. But maybe their benign good nature and peaceful existence are a sign of greater intelligence??!!!

Incidentally, you've just prompted me to think of a question about tortoises - check it out and see if you can answer it!