Talk:Dinosaur

Birds are now generally believed to represent descendents of dinosaurs, and so are classed with them by those who believe groups should be monophyletic.

Can someone point to a reputable source that claims birds are a kind of dinosaur? - Tim

Just type "bird dinosaur site:.edu" into Google. Here's one that popped up: http://www.ucmp.berkeley.edu/diapsids/avians. "Kind of dinosaur" is of cause a matter of definition, but there is no questions that birds are the closest living relatives of the dinosaurs, closer still than the crocodiles and other reptiles. --AxelBoldt

The point is more than that. Birds are not just the closest living relatives of the dinosaurs, but are in fact closer to some dinosaurs than they are to others. As there has recently been a big push for monophyletic groups, the Dinosauria would either have to be broadened as a formal category or extended to include birds. In practice the latter approach seems to be the one taken. I can't name any particular source, though UCMP and the tree of life would be some of the first I check, but I have seen it in a few more recent books.

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On the main page there had been a comment that only the most adaptable animals survived the K-T extinction, like mammals. I don't believe this is concensus, and there is a lot of evidence against this. Dionsaurs were highly adaptable and diverse, as were ammonites and other groups that were devastated or destroyed, while crocodiles and other such forms managed to pull through. Indeed the placental mammals had not shown much diversity before the Cretaceous. If anything, then, I would say that the more conservative groups were the ones that faired better. If there is a kernel of truth in the statement, it will need considerable modification before it is true enough to replace.

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Okay, here?s my knowledge/reasoning on that:

In the Permian, the dominant phylogeny of large land animals was the Synapsida, (pelycosaurs[?], in particular) the ancestors of mammals, or in the case of the pelycosaurs[?], great-uncles more like. The end of the Permian was marked by the Permian-Triassic extinction event, largest mass extinction known, in which it is estimated that more than 90% of all animal species went extinct. Certainly at least part of the cause was the formation of Pangaea at that time and the climatic (climactic?;-) ) change that would have been caused. Because a) the land fauna had been devestated and b) there was only one continent, the early Triassic had pretty much only one set of land animals that was found all over the world. In the mid-Triassic, several new groups arose to move in the gaping holes. The foremost among these (again only speaking of terrestrial animals) were the crocodiles[?], the dinosaurs, and the mammals (and their close relatives among the therapsids).

At the end of the Triassic, another mass extinction (the Triassic-Jurassic extinction event), wiped out the major contenders from Crocodilia[?] and Mammalia. This left the dinosaurs to inherit all the Earth. Since Pangaea was still all together they spread across the whole continent, leaving no place a competing group might be able to develop from. So the dinosaurs were extremely succesful and utterly dominant as land animals for the 130 odd million years, while the crocodiles[?] and the mammals hung on on the margins by staying out the way of the dinosaurs, concentrating on semi-aquatic ambushing, and nocturnal insectivory, respectively.

The Dinosaur species were thus able to monopolize abundant resources and grow big, and specialize. The other groups, did not have access to the abundant resources and had to remain flexible enough to utilize anything that came their way, and generally stayed rather smaller and/or slower metabolismed than the dinosaurs.

Thus when the disruption that caused the Cretaceous-Tertiary extinction event, probably significantly including the Chixculub meteor, the dinosaurs fared worse.

That is, the dinosaurs, being succesful, were able to be phylogenetically adaptable, which made them individually more conservative.

The crocodiles and mammals, on the other hand, having lost out in the Triassic-Jurassic lottery, had never had the opportunity that the dinosaurs had to be phylogenetically adaptable and diverse, had retained relatively conservative body plans throughout the Jurassic and Cretaceous, but had to be individually more adaptable in more precarious niches than those enjoyed by many dinosaurs.

Which is all to say, yes there is a kernel of truth to the statement, and yes it does need considerable modification.. But all this should probably be put in the Cretaceous-Tertiary extinction event article, which I have just added links to from Dinosaurs. I hope to put up some of this information in when I actually have some references handy. Or someone else can.

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Old talk, moved here from Talk:Dinosaurs:

Therepoda are a subgroup of Saurischia, so that change is not correct --Arco Scheepen

That seems to be the current best guess, but there is not a huge amount of evidence and it is possible opinions might change. In any case Therepoda are sufficiently different to the other Saurischia that i thought they needed a mention on the front dinosaur page. I am happy with the arrangement there now.

245M years ago is *way* too early.

Permian ended 230M years ago, and there were certainly no dinosaurs at that time. And they didn't appear until middle Triassic.

So correct number is most probably somewhere between 220M and 210M years ago.

What about reptiles relationship to dinosaurs? --rmhermen

Interesting question. Does it belong here or in reptiles? Of the surviving reptiles they should be closest to crocodiles, but of course they are even closer to birds. The statement about thecodonts on the page is interesting. My understanding is that thecodonts are supposed to be the common ancestor of dinosaurs birds and crocodiles. Some experts now regard thecodonts as a very dubious grouping, with some of the supposed thecodonts now believed to be not closely related to the others, and much disagreement about what actually are and are not thecodonts. What is the current concensus about this?

Current concensus is that the thecodonts are simply the basal Archosaurs, and so form a group paraphyletic to the dinosaurs-and-birds, pterosaurs, and crocodylians, in the same way that the reptiles are simply the basal amniotes and form a group paraphyletic to birds and mammals. Whether or not you accept either as a valid grouping depends on whether you restrict groups to monophyly. There is an increasing tendency to do this, but I don't think it's near universal yet.