What is--and isn't--a dinosaur?

Factual Questions
1

In another recent thread, somebody asked about the possibility of the Loch Ness Monster being a Plesiosaur. The general consensus was “no.” But one person weighed in with the statement that the Plesiosaur was not a dinosaur. I have heard the same about Dimetrodons and Pteradactyls. Also, that chickens are dinosaurs. “Dinosaur” is rapidly becoming a non-term that doesn’t usefully describe much.

Is there a definitive list of creatures that are generally considered by the public to be dinosaurs, but fail to meet some specific scientific definition of the term? Most of my knowledge of dinosaurs came from that ubiquitous set of plastic toys every boy had in the 1960s, and of course some of those sets included woolly mammoths and sabertooth tigers, so as learning tools, those were suspect.

2

From the wiki:

Pterosaurs are not dinosaurs, nor are icthyosaurs, plesiosaurs, or mosasaurs.
ETA here is a pretty good primer.

3

This is a rather ignorant thing to say. “Dinosaur” has a perfectly well-understood meaning which, while it doesn’t square with the prior superstition of “Them Big Bad Dead Things What Were Dumb And Big”, is extremely useful to paleontologists and biologists in general.

We’re learning more and more about how animals are related, and, due to that, are learning how to group like animals together, as opposed to previous classifications which boiled down to “Things What Got Wings” and “Things What Swim Real Good” and “Things You Can Eat On Friday” and so on, which hurt understanding more than they helped it. In terms of useful descriptions, going by families and lineage is more useful than going by what kinda happened to look similar to a barely-trained Natural Philosopher back in the era before microscopes or toilet paper.

4

Yeah “rapid” is not the term for stuff that dinosaur-obsessed 80s Kindergartners were on top of.

5

In layman’s terms – during the early Triassic, a few groups of reptiles split off from another group of reptiles. The original group (which is already only a subset of reptiles) was called the Archosaurs. They went down a few lineages. Pteradactyla (flying reptiles from the same era as the dinosaurs) are in this group. So are the true dinosaurs. So are modern crocodiles.

Plesiosaurs and mosasaurs are entirely distinct lineages from the Archosaurs. Mosasaurs are actually monitor lizards that took to the sea and became adapted to an aquatic lifestyle. They aren’t dinosaurs – they are lizards. A lizard can’t evolve into a dinosaur – it can develop similar traits, and it can become so distinct from its ancestors that it is no longer truly a lizard (although generally mosasaurs are considered true lizards) but it can’t become a dinosaur.

Plesiosaurs are a lot murkier. The current theory is that they split off from an earlier ancestor of the Archosaurs – so a group of reptiles split off into what would become the plesiosaurs and what would become the Archosaurs, and the Archosaur lineage split off again, into dinosaurs and crocodiles.

Dimetrodon went extinct millions of years before the dinosaurs first evolved. Not only are they not dinosaurs – they are actually more closely related to mammals. That’s right – Dimetrodon is our very distant cousin. One group of reptiles became what are known as Stem Mammals – aka the first Synapsids. Synapsids are all mammals, and all proto-mammals that are more closely related to mammals than to any group of reptiles, and Dimetrodon is in that group. To paleontologists who know what they’re looking for, a Dimetrodon skull looks far more mammalian than reptilian.

So to sum it up – pteranodons are about as closely related to dinosaurs as crocodiles are. Unless you’re going to call a modern crocodile a dinosaur, you shouldn’t call a pteranodon one either. Plesiosaurs are a little bit more distant, but still in the same group. Mosasaurs are about as far removed from dinosaurs as you can get while still being a reptile, and Dimetrodon are more closely related to us than they are to any dinosaur.

All birds are theoropods dinosaurs – members of the same group as Velociraptor and Tyrannosaurus. The only way you can define a bird as not being a dinosaur is if you arbitrarily define dinosaurs as being extinct members of their group, and then you run into trouble. There were birds earlier than 65 million years ago, who coexisted with more traditional dinosaurs. Where do you draw the line?

6

Despite your excellent explanation, you end with this. If there were “birds” earlier than 65 million that were different from “traditional dinosaurs”, then in fact you must be saying there are specific differences between birds and traditional dinosaurs other than extinction. So why don’t those things provide a way to say this group is dinosaurs and that group is not without relying on extinction.

7

That’s like saying that sauropods aren’t dinosaurs. There were sauropods earlier than 65 million years ago, and yet we can characterize dinosaurs into “sauropods” and “non-sauropods”. That doesn’t make the sauropods not dinosaurs – they are a specific type of dinosaur.

Same goes for birds. They are a type of maniraptoran theoropods dinosaur. There’s no physiological differences so grand between a bird and archaropteryx that suddenly makes a bird no longer fit the definition of “dinosaur”.

A chicken is much, MUCH more similar in every way that counts – and separated by less time, too – to a microraptor or even a tyrannosaurus, than either of those animals are to a stegosaurus.

8

As with many such terms, there are the scientific uses, and there are the public uses. It’s like the issue with “fruits” and “vegetables”, a dispute that results in confusion over things like tomatoes (biological fruits, culinary vegetables). cf. “Berry”

It’s not shocking that the public image of what a “dinosaur” is was formed by an inaccurate understanding of what the term was being used to describe many decades ago. The unscientific public tends to be very indiscriminate in mis-using technical terms. It’s not helped any by the fact that scientific usage tends to sharpen with time, as more information is known. Most people on the street still think there are nine “planets” in the Solar System, because that’s what we learned as kids, and the fact that the term “planet” had to undergo some more rigorous definition to be useful to the scientific community doesn’t phase them.

When I learned about “dinosaurs” back as a child in the 60s, there was very little distinction made in the juvenile books I was reading between different types of reptilian creatures of the Mesozoic Era. So we cheerfully called pteranodons, pterodactyls, plesiosaurs, etc. “dinosaurs”, because that’s how they were presented (it’s quite possible that there were distinctions being made in the books I read, but a seven-year-old often doesn’t bother with such things). And, of course, the whole idea that modern birds are just “dinosaurs” that survived extinction wasn’t really being discussed at the time. They may well be “dinosaurs” in a biological classification sense, but you’ll never convince the public that they are “dinosaurs” in an every-day usage sense.

9

I dunno; you just have to have the right incentives in place for people to believe it. When the food cart outside of the best dinosaur museum in the world is selling “dino nuggets”, and the most famous dinosaur paleontologist in the world (who’s also at that museum) is assuring you that, yes, those nuggets are in fact genuinely made from dinosaur meat, the kid who’s eating his lunch at the museum is going to believe that, yes, he is eating dinosaurs, because that’s just so cool!!!.

10

Sorry, they already have those. :smiley:

11

I don’t think a definitive list exists, all I find are pop-culture listicles like this one, and it’s not very good, I doubt anyone is really calling Dunkleosteus a dinosaur.

Faze, not phase.

12

DSYoungEsq, I know they exist. The scenario in my post wasn’t hypothetical: They really do serve dino nuggets at the Museum of the Rockies, home of Jack Horner, champion of calling birds dinosaurs.

13

Dinosaurs aren’t. Dinosaurs were.

14

We’ve been over this, there are about 10,000 species of dinosaur alive today!

15

Do you think Jack Horner is more famous than Robert Bakker? Because those are the top two names I was speculating about from your earlier post.

16

Well, Dr. Grant from Jurassic Park was (at least loosely) based on Horner, a point that’s sometimes mentioned in articles about the book or movie, which probably gives him more widespread exposure than Bakker. Mind you, I have no idea which one of them is more deserving of fame.

17

Ok, back in 1841 paleontologist Sir Richard Owen, used “Dinosaur” or “Dinosauria” to refer to the “distinct tribe or sub-order of Saurian Reptiles” . He lumped in Theropoda (which include modern birds) and Ornithischia (which oddly means “bird hipped” but are not that closely related to birds). Owen had no idea that Theropoda were related to birds, he was sure they were reptiles. He also didn’t know that Ornithischia were quite separate from Theropoda. Thus, he made a error in lumping the two together, ad thinking they were reptiles.
The Ornithischia are completely extinct.

Not every scientist agrees that Aves should be part of Theropoda, and thus, technically “dinosaurs” It’s a cladistic definition and not all accept it.

However, since many scientist do, you have the awkward term “Non-avian dinosaur” and if you dont include “non-avian” some pedantic smart ass will be sure to correct you. “*No- dinosaurs are not extinct, i saw some in my backyard this morning, and I will have fried dino today for dinner!”.
*
While somewhat correct, and sometimes humorous, they can also be called wrong.

Why wrong? Because the non-scientific term “dinosaur” is different than that actually correct term Dinosauria. Yes, I know.

So, yes, *Dinosauria *includes modern birds and does NOT include pterosaurs, mosasaurs, ichthyosaurs, plesiosaurs, and Dimetrodon, etc.

But “Dinosaur” as a layman’s term means “big extinct reptile like creature” and can certainly include pterosaurs, mosasaurs, ichthyosaurs, plesiosaurs, and Dimetrodon, but does *not *include Birds.

Yes, it’s not scientific, but everyone knows what they are talking about.

So, do be kind when correcting usage and make sure that you’re not hijacking or getting off track. If someone posts “Humans never lived along dinosaurs, that movie is wrong” he is obviously using the layman’s term.

18

Well, I recall that years and years ago (10+ years, IIRC) I was Pitted for citing Bakker in some dinosaur thread that I don’t even remember what it was all about at the moment. Apparently he wasn’t considered a “real” paleontologist by other scientists, so maybe Horner is the more famous of the two.

Back on topic, I was pleased when we got some dinosaur-themed breakfast cereal recently that proclaimed it had “8 dinosaur shapes” on the box, and my 8-year-old son said that it was wrong because pterodactyls and dimetrodons weren’t dinosaurs.

19

No. No “error” was made–he correctly grouped them together because all three groups shared the derived characteristic (the term modern, not his) of five fused vertebrae in the sacrum. See this, this, and for an illustration this.

Also, no credible paleontoligist denies the dinosaur origin of birds–that is nutbag flat-earther territory.

20

Found it. And this is what the guy said to you here in GQ.