# What is a dinosaur? **URL:** \ **Category:** Factual Questions\ **Created:** [June 5, 2008, 12:13pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868 "2008-06-05T12:13:00Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Alive\_At\_Both\_Ends](https://avatars.discourse-cdn.com/v4/letter/a/73ab20/32.png) [@Alive\_At\_Both\_Ends](https://boards.straightdope.com/u/Alive_At_Both_Ends)\ **Post date:** [June 5, 2008, 12:13pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/1 "2008-06-05T12:13:00Z")
As I understand it, not all prehistoric reptiles are dinosaurs. Also, birds are now classified as dinosaurs. So what actually makes an animal a dinosaur? Has the definition of a dinosaur changed over time (since birds were certainly not classified as dinosaurs when I was at school)? ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [June 5, 2008, 12:23pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/2 "2008-06-05T12:23:00Z")
I think that dinosaurs are all the creatures grouped as Saurischians or Ornithschians. This includes your classic dinos (T. Rex, Stegosaurus, Triceratops, Diplodocus, etc.), but does not include Mesozoic flying saurians (Pterodactyl, Rhamphorynchus, Quetzalcoatlus) or seagoing reptiles (Icthyosaurus, Plesiosaurus, etc.) I don’t think the latter two groups ever were considered dinosaurs, although they’ve been lumped together in the popular mind because they co-existed with the dinosaurs, and were reptilian. And movies like **When Dinosaurs Ruled the Earth** (which features a plesiosaur-type and some flying reptiles, and one imaginary creature, but no identifiable dinosaurs hat I can recall) don’t help. Birds are birds, not dinosaurs. Even if they’re legitimate descendants of dinosaurs, there are enough differences to set them aside as a separate class. The declaration “Birds are dinosaurs” is hyperbole. ---
**Author:** ![Alive\_At\_Both\_Ends](https://avatars.discourse-cdn.com/v4/letter/a/73ab20/32.png) [@Alive\_At\_Both\_Ends](https://boards.straightdope.com/u/Alive_At_Both_Ends)\ **Post date:** [June 5, 2008, 12:31pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/3 "2008-06-05T12:31:31Z")
[QUOTE=CalMeacham] I think that dinosaurs are all the creatures grouped as Saurischians or Ornithschians. This includes your classic dinos (T. Rex, Stegosaurus, Triceratops, Diplodocus, etc.), but does not include Mesozoic flying saurians (Pterodactyl, Rhamphorynchus, Quetzalcoatlus) or seagoing reptiles (Icthyosaurus, Plesiosaurus, etc.) I don’t think the latter two groups ever were considered dinosaurs, although they’ve been lumped together in the popular mind because they co-existed with the dinosaurs, and were reptilian. [/QUOTE] So are you saying that all the non-flying and non-aquatic extinct reptiles \*were \*dinosaurs? If, say, Komodo dragons had become extinct and were only known by fossils, would they be considered dinosaurs as well? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [June 5, 2008, 1:30pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/4 "2008-06-05T13:30:39Z")
[QUOTE=Alive At Both Ends] If, say, Komodo dragons had become extinct and were only known by fossils, would they be considered dinosaurs as well? [/QUOTE] IIRC, the Komodo dragon is a fairly recent species only a few million years old. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [June 5, 2008, 1:39pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/5 "2008-06-05T13:39:52Z")
[QUOTE=Alive At Both Ends] So are you saying that all the non-flying and non-aquatic extinct reptiles \*were \*dinosaurs? If, say, Komodo dragons had become extinct and were only known by fossils, would they be considered dinosaurs as well? [/QUOTE] I don’t believe he’s saying that at all. Saying “X are not classified as Y” does not imply “All not-X are classified as Y”. And of course, lizard-like and crocodile-like reptilians are not Dinosaurs. ---
**Author:** ![Alive\_At\_Both\_Ends](https://avatars.discourse-cdn.com/v4/letter/a/73ab20/32.png) [@Alive\_At\_Both\_Ends](https://boards.straightdope.com/u/Alive_At_Both_Ends)\ **Post date:** [June 5, 2008, 2:09pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/6 "2008-06-05T14:09:50Z")
[QUOTE=Quercus] I don’t believe he’s saying that at all. Saying “X are not classified as Y” does not imply “All not-X are classified as Y”. [/QUOTE] I realise that, which is why I asked for clarification. Look at it this way. I can define a bird as a creature which has feathers (at least, I don’t know of any non-birds with feathers or any birds without them). I can define a mammal as a creature which has mammary glands. What I’m after is a similar definition for “dinosaur”. What do dinosaurs have that other reptiles do not? ---
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [June 5, 2008, 2:19pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/7 "2008-06-05T14:19:35Z")
[QUOTE=Alive At Both Ends] ILook at it this way. I can define a bird as a creature which has feathers (at least, I don’t know of any non-birds with feathers or any birds without them). I can define a mammal as a creature which has mammary glands. [/QUOTE] That’s OK for your definition. . . But I think both of those statements are factually wrong. Some dinosaurs had feathers, and monotremes don’t have mammary glands. ---
**Author:** ![cmkeller](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmkeller/32/118_2.png) [@cmkeller](https://boards.straightdope.com/u/cmkeller)\ **Post date:** [June 5, 2008, 2:26pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/8 "2008-06-05T14:26:40Z")
Earl Snake-Hips Tucker: > [@](#): > > and monotremes don’t have mammary glands. Yes they do. They don’t suckle their young - the milk leaks out to be lapped up by the puggles rather than sucked from a nipple - but the glands that produce the milk are there, and active, and providing nourishment. ---
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [June 5, 2008, 2:29pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/9 "2008-06-05T14:29:01Z")
Cool! Consider ignorance fought. ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [June 5, 2008, 2:36pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/10 "2008-06-05T14:36:38Z")
[QUOTE=CalMeacham] Birds are birds, not dinosaurs. Even if they’re legitimate descendants of dinosaurs, there are enough differences to set them aside as a separate class. The declaration “Birds are dinosaurs” is hyperbole. [/QUOTE] No, it’s not hyperbole. Birds are dinosaurs in exactly the same way whales are mammals. For a while, birds were not classified as dinosaurs because there was not strong evidence about their relationship. That is, one reasonable theory was that birds and dinosaurs descended separately from a common ancestor. However, modern evidence now confirms that birds are descended from one branch of dinosaurs. Tyrannosaurs are more closely related to birds then they are to triceratops. So if you want to call both tyrannosaurs and triceratops dinosaurs, you must also call birds dinosaurs. [Explore the Tree of Life for more information.](http://www.tolweb.org/Dinosauria) ---
**Author:** ![puppygod](https://avatars.discourse-cdn.com/v4/letter/p/ce7236/32.png) [@puppygod](https://boards.straightdope.com/u/puppygod)\ **Post date:** [June 5, 2008, 2:37pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/11 "2008-06-05T14:37:48Z")
[QUOTE=Johnny L.A.] IIRC, the Komodo dragon is a fairly recent species only a few million years old. [/QUOTE] And they branched from line leading to dinosaurs even earlier than crocodiles. It’s rather hard to explain any question related to modern taxonomy without explaining [cladistic](http://en.wikipedia.org/wiki/Cladistic), and that’s rather complex idea. In brief, we classify living things rather by how close they are related to each other, not by their superficial characteristics (I know it’s not the whole truth, just trying to keep it simple). So by birds we don’t understand “all things feathered”, but rather “all living things that branched from other dinosaurs at certain (defined) point in the past” - they all share common ancestor, as well as some characteristic. With defining dinosaurs there are more problems, because into their clade falls also birds. So, depending upon whether you want to include birds into classification or exclude, you have two options: a) define dinosaurs as every descendant of some proto-dinosaur that spawned tyranosaurus and diplodocus (but not crocodiles - they branched earlier); and then birds ARE dinosaurs; b) define dinosaurs as every descendant of some proto-dinosaur that spawned tyranosaurus and diplodocus - EXCLUDING birds. Thus forming not-all including clade, so called paraphyletic clade. Note, that none of this options is “better”. Both are quite valid and status of birds as dinosaurs is a matter of definition rather than objective facts. We know for sure that they are direct descendants of dinosaurs and related to them closer than to any living creature, but whether we define them as distinct or not - is just matter of convention. Also note, that this is extremely layman explanation and verging on untrue - but whole cladistic is more complex concept. Maybe \*\*Darwin Finch \*\*or other competent doper will explain it in more detailed manner. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [June 5, 2008, 3:02pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/12 "2008-06-05T15:02:47Z")
To the best of my knowledge, the common ancestor of the Saurischia (+birds) and Ornithischia is not yet known paleontologically, but cladistically, there is some pretty strong evidence that there was one – i.e., a “first dinosaur” that included those two groups (along with the birds, almost certainly an offshoot of the Saurischia) but not the other Archosaurs: the “thecodonts” of the Triassic, the Pterosaurs, the Phytosaurs, and the Crocodylia. Possibly **Darwin’s Finch** may have some information more current or more detailed than what I do on this. The Saurischia include: [ol][li]the Coelurosaurs, (mostly) lightly built smaller carnivorous dinosaurs (plus the Tyrannosaurs, giant heavily built offshoots of coelurosaur stock)[/li][li]the Carnosaurs, the heavier large carnivores such as Allosaurus[/li][li]the “Prosauropods,” a melange of Triassic and Early Jurassic plant-eaters[/li][li]the Sauropods, the small-headed, long-necked large quadrupedal plant-eaters like Brontosaurus, Brachiosaurus, and Diplodocus[/li][li]the Ornithomimids, a group resembling ostriches and emus with forelimbs instead of vestigial wings[/li][li]the birds[/ol][/li] The Ornithischia included: [ol][li]the Ornithopods, bipedal planteaters including iguanodonts, heterodontosaurs, anatosaurs (duckbills), and similar forms[/li][li]the Pachycephalosaurs, the “bonehead” dinosaurs, largely bipedal planteaters[/li][li]the Scelidosaurs and Stegosaurs, planteaters with bony protective plates in their hides[/li][li]the Ankylosaurs and their close relatives the Nodosaurs, short-legged robust planteaters with body armor[/li][li]the Ceratopsians and Psittacosaurs, planteaters with head/neck shields, beaks, and generally horns, Triceratops being a good example[/ol][/li] I believe there’s some support for a usage that includes the larger, Mesozoic forms omitting the surviving “crown group”, the birds. Such a classification is not cladistically valid but useful in avoiding having to repeatedly draw the distinction between birds and the other dinosaurs. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [June 5, 2008, 3:25pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/13 "2008-06-05T15:25:29Z")
[QUOTE=Polycarp] The Saurischia include: [list=1][li]the Coelurosaurs, (mostly) lightly built smaller carnivorous dinosaurs (plus the Tyrannosaurs, giant heavily built offshoots of coelurosaur stock)[/li][li]the Carnosaurs, the heavier large carnivores such as Allosaurus[/li][li]the “Prosauropods,” a melange of Triassic and Early Jurassic plant-eaters[/li][li]the Sauropods, the small-headed, long-necked large quadrupedal plant-eaters like Brontosaurus, Brachiosaurus, and Diplodocus[/li][li]the Ornithomimids, a group resembling ostriches and emus with forelimbs instead of vestigial wings.[/li][/QUOTE] Wait, what? I thought the Tyrannosauridae were the prototypical Coelurosaurians. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [June 5, 2008, 3:38pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/14 "2008-06-05T15:38:02Z")
I forget which book of his this is in, but the chapter Gould wrote called “What, if anything, is a Zebra” would be a good reference. There’s a strong case to call birds dinosaurs, but there are lots of paraphyletic groups out there. To take a strictly cladistic view, birds are dinosaurs-- but one needn’t take a strictly cladistic view, and some biologists don’t (or at least don’t always take one). Apes are similarly problematic since old world monkeys are more closely related to apes than they are to new world monkeys. Same thing with “fish”. The lobe finned fish are more closely related to humans than they are to the ray finned fish. ---
**Author:** ![Darwins\_Finch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darwins_finch/32/20304_2.png) [@Darwins\_Finch](https://boards.straightdope.com/u/Darwins_Finch)\ **Post date:** [June 5, 2008, 4:07pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/15 "2008-06-05T16:07:46Z")
[QUOTE=Alive At Both Ends] As I understand it, not all prehistoric reptiles are dinosaurs. Also, birds are now classified as dinosaurs. So what actually makes an animal a dinosaur? Has the definition of a dinosaur changed over time (since birds were certainly not classified as dinosaurs when I was at school)? [/QUOTE] See the thread [Why Pterosaurs aren’t Dinosaurs](http://boards.straightdope.com/sdmb/showthread.php?t=132981) for an explanation of what, based on current taxonomies, constitutes a dinosaur. The “TLDR” version of my OP in that thread is this: The cladistic _definition_ of a dinosaur today is “the most recent common ancestor of sparrows and _Triceratops_es, and any of its descendants.” The _diagnosis_ of a dinosaur (i.e, how you actually _identify_ one) is based on several unique characters, such as the structures of the hip and ankle. > [@Really Not All That Bright](#): > > thought the Tyrannosauridae were the prototypical Coelurosaurians. Well, technically, the prototypical Coelosaurian would be _Coelosaurus_. _Tyrannosaurus_ was a heavily derived coelosaur. _Compsagnathus_ is perhaps most representative of the early coelosaurs. ---
**Author:** ![Darwins\_Finch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darwins_finch/32/20304_2.png) [@Darwins\_Finch](https://boards.straightdope.com/u/Darwins_Finch)\ **Post date:** [June 5, 2008, 4:36pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/16 "2008-06-05T16:36:10Z")
[QUOTE=Alive At Both Ends] I can define a bird as a creature which has feathers (at least, I don’t know of any non-birds with feathers or any birds without them). [/QUOTE] “Has feathers” has, in recent years, become invalid as a concrete diagnosis for birds, what with the discovery of dinosaurs (of the non-avian variety) that possessed feathers, feather-like structures. These include _Microraptor_, _Sinosauropteryx_, _Caudipteryx_, _Beipiaosaurus_, _Sinornithosaurus_, _Shuuvia_, and _Protoarchaeopteryx_. None of these are currently classified as birds, and with the likely exception of _Microraptor_, none were capable of flight. Then, there’s also the early tyrannosaurid _Dilong_, a juvenile specimen of which has been found sporting what appear to be proto-feathers, indicating that feathers or their precursors evolved well before birds proper used them flying. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [June 5, 2008, 6:15pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/17 "2008-06-05T18:15:45Z")
[QUOTE=John Mace] To take a strictly cladistic view, birds are dinosaurs-- but one needn’t take a strictly cladistic view, and some biologists don’t (or at least don’t always take one). Apes are similarly problematic since old world monkeys are more closely related to apes than they are to new world monkeys. [/QUOTE] I think you meant to say that both “monkeys” and “apes” aren’t coherent groups, if we define monkeys as excluding apes, or apes as excluding humans. “Monkeys” meaning old world monkeys and new world monkeys but not apes doesn’t work, because old world monkeys and apes are more closely related to each other than either are to new world monkeys. “Apes” meaning gibbons, chimps, gorillas, and orangutans but not humans doesn’t work because orangutans, chimps and gorillas are more closely related to humans than they are to gibbons, chimps and gorillas are more closely related to humans than they are to orangutans, and chimps are more closely related to humans than they are to gorillas. The problem disappears if we take “monkeys” to include apes, and “apes” to include humans. Then we can finally stop correctly people who call chimpanzees monkeys, which will be a relief. The only way we declare that birds aren’t dinosaurs is to declare that the term “dinosaur” has no scientific meaning, and is just a descriptive term meaning “extinct critter”. The advantage of that is that all those packs of plastic dinosaurs that include woolly mammoths, pterodactyls, pleisosaurs, and cave men will finally be accurately labeled. Just like Apatasaurus is one of the few dinosaurs that have a common name: brontosaurus. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [June 5, 2008, 6:37pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/18 "2008-06-05T18:37:27Z")
[QUOTE=Lemur866] I think you meant to say that both “monkeys” and “apes” aren’t coherent groups, if we define monkeys as excluding apes, or apes as excluding humans. “Monkeys” meaning old world monkeys and new world monkeys but not apes doesn’t work, because old world monkeys and apes are more closely related to each other than either are to new world monkeys. “Apes” meaning gibbons, chimps, gorillas, and orangutans but not humans doesn’t work because orangutans, chimps and gorillas are more closely related to humans than they are to gibbons, chimps and gorillas are more closely related to humans than they are to orangutans, and chimps are more closely related to humans than they are to gorillas. [/QUOTE] I picked “apes” because of the way the analogy works with birds. Ape is to monkey as bird is to dinosaur. That’s all. Sorry if my post wasn’t clear. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [June 5, 2008, 7:02pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/19 "2008-06-05T19:02:18Z")
Sure, but then “apes” is still a perfectly good scientific term (as long as we include humans), and so is “birds”. The problematic terms would be “monkeys” and “dinosaurs” if we decide that apes aren’t monkeys and birds aren’t dinosaurs. Obviously we’re busy agreeing with each other. I guess “apes are problematic” just seemed to say it backwards…I guess you mean it in the sense that “apes” make a problem for term “monkey”. ---
**Author:** ![Astroboy14](https://avatars.discourse-cdn.com/v4/letter/a/df705f/32.png) [@Astroboy14](https://boards.straightdope.com/u/Astroboy14)\ **Post date:** [June 5, 2008, 8:01pm UTC](https://boards.straightdope.com/t/what-is-a-dinosaur/451868/20 "2008-06-05T20:01:10Z")
I can’t help with all of them, but as far as brontosaurs go, it’s fairly straightforward: _[Anne Elk]My theory by A. Elk. Brackets Miss, brackets. This theory goes as follows and begins now. All brontosauruses are thin at one end, much thicker in the middle and then thin again at the far end. That is my theory, it is mine, and belongs to me and I own it, and what it is too.[AE]_ [Next page](https://boards.straightdope.com/t/what-is-a-dinosaur/451868.md?page=2)