Why Pterosaurs aren't Dinosaurs

Which dinosaurs did not? I swore that the Archasaur ankle was a major diagnostic feature, but Crocs also had them (modern ones returned to the classic sprawling pose which is much better for the water.)

Please note that in calling cladistics “old”, you are speaking relatively. Most people’s high-school biology, though it included evolution (except in certain backward and barbarous locales), nevertheless used pre-evolutionary taxonomy, in which a phylogenic line can run sideways between taxonomic siblings.

Where do they come from?

Komodo Dragons are a species of monitor lizard named for the island of Komodo in Indonesia. They used to be far more wide spread, but are now limited to Komodo and one or two other Indonesian islands. They’re endangered but now pretty well protected.

Unlike crocodilians, they’re primarily land animals, thus they expend much more energy. Hence, the must feed more often than crocs, and are more aggressive. The males can grow up to nine feet long. They prey on deer, wild boar, and whatever else they can find. They’ll eat you too, given the chance.

Okay, my terminology is messed up. Let me boil down my point.

Humans are not shrew-sized four-legged fuzzy critters. They evolved from shrew-sized four-legged fuzzy critters, but are no longer so.

Birds evolved from dinosaurs, but are not dinosaurs.

Add a little more info on the Komodo Dragon bit. Where do monitor lizards fit into the overall picture? You’ve taken it one step and assumed the rest of us know how it fits from there. You assume too much.

The only difference is that we are no longer shrew-sized. We certainly are four-legged fuzzy critters.

Irishman, the whole point is that there are two definitions of the word “dinosaur”. The popular definition of “dinosaur” is “one of the many extinct giant reptiles” and does not include birds. This definition also includes things like plesiosaurs and pterosaurs, which popularly are dinosaurs but biologically are not. The cladistic scientific definition of dinosaur does include birds. So your statement **Birds evolved from dinosaurs, but are not dinosaurs. ** is true by the popular definition, and false by the cladistic definition.

Let me ask you this, though: based on what you have read above, do you believe that birds are Archosaurs or not?


It’s certainly true, Irishman, that we can make statements like the following:

(1) Whales descend from land mammals, but they are not land mammals.
(2) Humans descend from small, furry mammals, but we are no longer small (as adults, anyway) or particularly furry (except for some of the hairier ones among us).
(3) All mammals, birds, and reptiles descend from a common ancestor that looked like a lizard–but very few look much like lizards today.

All of the above statements are true, and they convey useful information. But in every case the identifier on the right side is an informal, impressionistic description–nobody would pretend that “land mammal”, “small, furry animal”, or “a thing that looks like a lizard” is a valid zoological taxon.

Now the problem with birds and dinosaurs is that “dinosaur” does have a valid zoological meaning–a member of the clade Dinosauria, as explained by Finch above. And most biologists agree that birds are members are Dinosauria. But because dinosaurs are so well known, the word is also used impressionistically, and it creates a mental picture of a (usually) large, scaly, and long-extinct creature that doesn’t accommodate birds.

One possible compromise is to say that birds are Dinosaurians but not dinosaurs, relegating “dinosaur” to an informal status like “early primate” or “egg-laying animal”. But my personal preference, if the descent from early dinosaurs can be established beyond a reasonable doubt, is to expand our concept of dinosaurs to include birds.

There is a precedent. At one time people had trouble accepting that a whale could be a mammal. It doesn’t fit the impressionistic image of a furry, four-legged land creature. As Melville wrote in Moby-Dick:

Today, who would doubt for two minutes that a whale is a mammal? We learn in school that whales are marine mammals, and we take it for granted. Someday, it may be the same with birds and dinosaurs!

Note that it is not universally accepted that Birds descended from Dinosaurs- as opposed to simply having a common ancestor. We used the “archosauria” term also, which does include crocs, birds & the classic dino.

The problem with using a term like “dinosauria” to include the class Aves (even thought they may be related, or even perhaps the same superclass, or even the same class) is that is confuses the layman & makes old books obsolete. Even though a number of biologists want to CALL birds=Dinos, a new name , which better shows the relationship and does not lead to such confusion is better. Certainly, when Owen coined the name, it was not intended to include “birds”. So- even if birds & dinos are so closely related we need to lump them together- calling the class “dinosauria” is a bad idea- with “political” overtones.

Bakker- who really pushes hard the 'hot blooded dino" theory (which may well be correct) also pushes the “birds= Dinos” concept to strengthen his theory.

Jklann- in the old “birds, beast & fish” classification- a whale fits within “fish” better than “beast”. If you define an animal by outward characteristics & niche (which is a perfectly fine way of doing so, note)- then a whale is more a “fish” than a “beast”. Of course- if you define them cladisticly or phlyogenically- then it is clear a whale is a mammal.

Sorry, I stand corrected. I checked my own references and it turns out that I was just remembering that information incorrectly. Yes, all dinosaurs did stand upright. Crocs, by the way, were always sprawled to the side,and are intermediates between traditional lizards and dinosaurs. The Croc hip-joint is actually referred to as “pre-dino” in my book, since Crocs are more upright than lizards, but their legs are still out to the side, unlike dinosaurs.

I’m not referring to high school biology, I’m talking about what I’ve been learning in my university studies. Cladograms were the old way of showing evolutionary trends, but that is changing over to phylogenies, because phylogenies are now being accepted as “more correct” though cladograms are often still used as supplements. Most modern biological texts show phylogenies, not cladistics.

Actually, that statement is false either way, because birds are NOT descended from dinosaurs, they’re descended from archaeopteryx. Archaeopteryx was NOT a dinosaur, but is related to them to the degree that all are considered part of archosauria.

Do they eat other animals, these Komodo dragons?

That’s your opinion. Many evolutionary biologists hold a different opinion. See, for example, http://www.ucmp.berkeley.edu/diapsids/avians.html. Or, for that matter, see the OP.

Yes, and you’re talking, in large part, to a bunch of confused laymen who are lucky if they remember more of their high-school biology than they do of their fourth-grade “general science”. This is “The Straight Dope”, not “The Journal of Evolutionary Biology”.

What I originally said still goes; the essential problem here is a failure to “get” the new classificatory principle, as first made popular by cladistics, that evolutionary descendants ought not to be regarded as taxonomic siblings.

(Actually, I find myself rather confused by your account of phylogeny-not-cladistics. It sounds as though this new system replaces deductions based on concrete fossil evidence with mere guesswork.)

Yeah, I was wondering about that too. My understanding was the cladistics was the primary methodology by which we determine phylogenetic relationships, especially within the fossil record where DNA comparison isn’t an option.

Actually, phylogeny is more based on DNA, rather than strictly physical characteristics. A lot of taxonomy has changed since DNA was found to be the source of genetic material in the 50’s.

It’s not my opinion. It’s what I’ve seen in several different Herpetological texts.

(note: herpetology is the study of reptiles and amphibians)

Golly! I did, actually, go on vacation for a week, and only now got back. I didn’t expect to see this thread resurface!

Most of the points that have been raised in my absence have been addressed, but there are a couple issues that I’d like to clarify:

Essentially, the whole point of phylogenetic systematics (the methods of systematics which uses cladistics to produce phylogenetic trees) is to reconstruct evolutionary lineages. Linnaean classification fails to do this, as it implies no necessary relationships between, say, birds (Class Aves), and reptiles (Class Reptilia), and essentially only groups “like with like”.

Cladistics (which is, again, a method; the resulting cladogram is not a phylogenetic tree, but rather one hypothesis for an evolutionary relationship between organisms) relies on nested groupings of unique features to arrive at proposed relationships. The hypotheses thus created can then be tested against other organisms/groups to further refine the relationships. For example, if I wish to claim that birds are directly descended from dinosaurs, then I should be able to point to specific features which both dinosaurs (as popularly understood) and birds possess. Further, I should be able to produce features which are unique to birds (or that I can establish as being analogous rather than homologous, if a similar character is found elsewhere). And so on. The same applies for establishing relationships between pterosaurs and dinosaurs. Thus far, the weight of the evidence is that a) birds are descended from dinosaurs (though there are still a few detractors, e.g., ornithologist Alan Feduccia), and b) pterosaurs aren’t.

As far as terminology of clades, personally, I don’t have a problem with birds being dinosaurs, as opposed to simply stating they are descended from dinosaurs. Ultimately, both mean the same thing: birds are birds, and dinosaurs are ancestral to birds, just as earlier archosaurian reptiles were ancestral to dinosaurs, and so on.

I’m still kinda confused. Is it true that cladistics is “not generally being done any more”? What other methods besides cladistics are used to create phylogenetic trees? (DNA analysis?) And where do “dendrograms” (referenced by mightyaprhrodite enter the picture?

It most certainly is not true that cladistics is a thing of the past – all things considered, it’s relatively new; as you can see from the OP, dinosaurs didn’t start getting a cladistic workover until the '70s. Cladistic methodology itself was developed by Willi Hennig in 1950. Cladistics is, in fact, the current dominant paradigm in systematics.

A dendrogram is simply an alternate form of branching diagram. An example (for dinosaurs) can be seen here. Compare this to the cladogram version here.