A very general visual depiction of the clade Diapsida, which includes such well-known groups as dinosaurs (including birds), pterosaurs, crocodilians, snakes, lizards, plesiosaurs, and mosasaurs, among many others.
My friend and I are always arguing over whether or not dinosaurs could fly and swim. I tell her that the only things that are technically 'dinosaurs' are the land-only ones (brachiosaurs, raptors, etc) but she says that the ones that could fly (pterodactyls, etc) and swim (mosasaurs, pliosaurs, etc) were all dinosaurs too. I keep telling her that although they're related, they're not 'dinos' but she won't believe me :/ can I get your opinion? What's technically a dinosaurs and what's not?
You’re sort of right in terms of land versus air versus ocean. I mean typically when we think of animals from this time period (the Mesozoic), we don’t think of things like crocodiles, lizards, mammals - the other land dwelling creatures that lived alongside dinosaurs. But it’s still not an accurate way of defining dinosaurs because plenty of other things lived on land… and dinosaurs eventually lived in the air (a la birds, which are dinosaurs).
We define organisms nowadays in terms of how they’re related to one another, and not by their physical traits. I know, this is kind of confusing, but it’s really not that hard to understand when you wrap your mind around it.
Of course, you figure out how organisms are related, evolutionarily speaking, using physical traits, but it’s very hard to define groups based on traits alone rather than relationships, because things can often be evolved multiple times in groups not even remotely related to one another - for example, both whales and sharks have the ability to swim… doesn’t make whales sharks, or vice versa.
Dinosaurs are now defined as the group of animals that includes Iguanodon, Megalosaurus, their most recent common ancestor (MRCA,) and all of that ancestor’s descendants. Holy crap, right? How the hell are you supposed to know what those descendants are like?
Well Iguanodon and Megalosaurus aren’t very similar at all - the only thing they have in common is that they are dinosaurs. Iguanodon is an Ornithischian, meaning a group of animals that consist of Eocursor, Triceratops, their most recent common ancestor, and all of that ancestors descendants - okay yeah now I can see you’re saying “HOLD UP, how do you KNOW all these animals are related?”
We can track fossil forms and see how different traits evolved over time in these groups. So dinosaurs typically have their legs directly under the body - they also are at least mesothermic (somewhere between cold-blooded and warm-blooded) if not fully endothermic (warm-blooded); they ancestrally had protofeathers, a structure like fur (though it evolved from a different part of the epidermal layer - the skin - wooo convergent evolution again) that would serve as the starting point for feathers like we know them today; the common ancestor for all dinosaurs was bipedal, meaning it walked on two legs, though many types of dinosaurs re-evolved quadrupedalism; dinosaurs were all archosaurs, a type of Sauropsid (reptile) that is the group that consists of birds, crocodiles, their MRCA, and all of its descendants - archosaurs are typically characterized due to the presence of having their teeth in sockets, unlike other reptiles, as well as some skeletal features that are kind of complicated to explain; and there are a variety of skeletal features that are also complicated that are unique just to dinosaurs.
But here’s the thing - a lot of features we think of as distinctive to dinosaurs were then lost by dinosaurs who didn’t need them anymore - for example, no living bird has teeth, but they’re all dinosaurs, meaning they’re all archosaurs, which is characterized by the tooth socket thing - see the pain of defining groups of organisms based on traits?
Ornithischians and Saurischians are two basic groups of dinosaurs that diverged pretty much at the beginning of dinosaurian history - they had very different hip bones. Ornithischians had the pubis pointing backwards:
And Saurischians, the pubis is pointing forwards:
But that’s just in the common ancestor for both of those groups - the thing is, the pubis pointing backwards like in Ornithischians actually evolved three separate times in dinosaurs - in Ornithischians, in Therizinosaurs (a group of Saurischians,) and in birds (a group of Saurischians) - but the last two aren’t Ornithischians because we can see how they’re related to other dinosaurs based on the general trends of evolution in each dinosaur group and place them in the Saurischian group - do you see why we don’t define groups based on characteristics anymore?
So basically, dinosaurs are what I said above - and every single one we know conclusively is in one of those two groups, though there are some animals that are under debate whether they’re true dinosaurs or animals closely related to the first dinosaur, but not quite the first dino themselves - if that makes sense - like, if the MRCA is the dinosaur “parent,” then these guys could also be “parents”, or they could be “aunts and uncles,” if that makes sense?
The best way to determine if a singular animal was a dinosaur is just to… look it up. Go online, see its classification. For example, these are all dinosaurs:
Some dinosaurs evolved the ability to glide - like Microraptor. From those, some evolved the ability to fly - aka, birds (there’s some debate as to whether certain dinosaurs very closely related to birds could fly, and there’s similar debate on whether some early birds could actually fly or just glided, but the point is some dinosaurs do reside in the air.) Some other dinosaurs have evolved aquatic abilities - Spinosaurus was semi-aquatic, as are animals like penguins. All of these things are dinosaurs.
So for other animals - like the ones your friend is confusing with dinosaurs - let’s, once again, stop with the characteristics, and start with the relationships.
Pterodactylus, or what is typically thought of as a “flying dinosaur” (those things with the membraneous wings)was a type of Pterosaur - a group of animals defined as Anurognathus, Preondactylus, Quetzalcoatlus, their MRCA, and all organisms descended from that MRCA. Pterosaurs in general were actually very closely related to dinosaurs - they’re all part of the group Avemetatarsalia, which is a group of archosaurs. Meaning, pterosaurs weren’t dinosaurs, and birds aren’t a group of pterosaurs, but the most closely related group of animals living today to pterosaurs are birds.
Deinosuchus was a huge type of crocodile - which is a kind of archosaur - but crocodiles aren’t Avemetatarsalians… so they’re not particularly closely related to dinosaurs, much less actual dinosaurs.
As for the aquatic reptiles of the Mesozoic Era, they weren’t even closely related to dinosaurs. Plesiosaurus and Pliosaurus were members of the group Plesiosauria, which is a group of Pantestudines, which is part of the even bigger group Archosauromorpha. This means that plesiosaurs & pliosaurs weren’t archosaurs, but they were more closely related to modern archosaurs on the whole (so equally closely related both to crocodiles and birds) than they are to other reptiles like lizards. But you know what their closest modern relatives are? Turtles. Yup, Pantestudines includes Plesiosaria, but another branch, Testudinata, is turtles! Turtles are also a group of Pantestudines, which means they are also Archosauromorphs. We actually found this out using genetic research - turtles lost a lot of the same characteristics as not only archosaurs, but also lizards and snakes - meaning they seemed to be a very primitive type of reptile, rather than a member of one of these more derived groups.
Lizards and snakes (and the Tuatara) are part of the other major grouping of reptiles other than archosauromorphs - the Lepidosauromorpha. You know what else is a Lepidosauromorph? That’s right, mosasaurs. Mosasaurs are a subgroup of Platynota - which are a group of lizards! Meaning Mosasaurs were lizards!! Not anywhere near closely related to dinosaurs.
Now, Icthyosaurs - you didn’t mention them, but I’m sure your friend will ask - they were also reptiles (Sauropsids - which includes Lepidosauromorphs, Archosauromorphs, and some other closely related groups) - they were one of those closely related groups. Icthyosauromorphs diverged from the organisms that would evolve into both Leps and Archos before those two groups diverged from one another. Meaning, all modern reptiles - birds, crocodiles, snakes, lizards, turtles, and tuatara - are equally closely related to icthyosaurs.
Here is a REALLY HANDY DIAGRAM showing all of this:
Now - some other animals, right? Because people mix up stuff all the time.
Dimetrodon is often mixed up with dinosaurs… but it wasn’t even a Sauropsid! In fact, Dimetrodon lived a long time before dinosaurs and even archosaurs evolved. Dimetrodon was a type of Synapsid - a group of animals that contains every Amniote (organism that lays hard-shelled eggs… or at least, the first organism that did so and all of its descendants - including mammals, yes, even the ones that no longer lay eggs) that is more closely related to mammals than to reptiles (sauropsids). Meaning, Dimetrodon was a type of animal, that looked superficially like a reptile… but wasn’t… that was on the lineage of animals that would eventually evolve into mammals. Not closely related to dinosaurs… at all.
Same goes for Wooly Mammoths and Saber-Tooth Cats (like Smilodon) (they are not tigers) - they’re MAMMALS, which means they are no where near closely related to dinosaurs… meaning they’re not dinosaurs. Same for the Mastodon. Same for the Glyptodont. All mammals.
As for Megalodon… Megalodon was a type of shark. Sharks are Chondrichthyes, or cartilaginous fish. You know what was the last group that sharks and dinosaurs had in common was? It was Gnathostomata - all jawed vertebrates - that is the last group that they’re both a part of, that they have a common ancestor in. After that, sharks go off to Chondrichthyes… The other group alive today after that is the Teleostomi, which is all bony fish. All bony fish include lobe-finned fish… which includes tetrapods (amphibians and all other land-dwelling vertebrates)… which includes amniotes, which includes mammals as well as Sauropsids. Meaning, dinosaurs are Teleostomis (and so are we,) but they’re not sharks, at all, meaning Megalodon is not a dinosaur, nor is any other type of shark.
Dunkleosteus, that huge monstrous looking fish with the armored head, was a type of Gnathostomata, but wasn’t a shark OR a bony fish - it was a Placoderm, another group of Gnathostomatans which went extinct a long time ago. Meaning it, just as much as any shark, is not a dinosaur.
As for animals that are made up by our collective imagination - such as Godzilla, Barney, and Indominus rex… well, they aren’t dinosaurs, because they aren’t part of that evolutionary group. I dunno what you would classify them as, as they aren’t part of any evolutionary group technically… but they aren’t dinosaurs.
So… I know this was very long, and confusing, but hopefully this helps.
TL;DR: Dinosaurs are a very specific group of animals. You can use many different definitions, here’s another one: Dinosaurs are the group of animals that includes the House Sparrow, Maiasaura, their most recent common ancestor, and all of that ancestor’s descendants. If an animal isn’t in that group, it’s not a dinosaur. Plain and simple.
Definition: The clade of the most recent common ancestor of Ceratosaurus and Carnotaurus, and all of that most recent common ancestor’s descendants.
Organisms Within: The clades Ceratosauridae and Abelisauroidea
Time Range: Given this is another node-based clade, we can only guess at when the earliest member of this group evolved. Since earliest known Abelisauroids are from the earliest portion of the Middle Jurassic, it stands to reason that the earliest Neoceratosaur had to evolve before this; the best guess at such is shown below.
Characteristics: This group contains all the more derived members of Ceratosauria, and they remained as weird and strangely diverse as their less derived relatives. The bulk of this group included the Abelisauroids, which ranged from the huge and tiny-armed Carnotaurus to the small and fast Noasaurids.
Neoceratosaurs were relatively medium-sized predators or smaller during the Jurassic; however, the later Abelisaurids that would inhabit mostly the Southern Hemisphere got very large and often were at the top of the food chain in their environments.
The beginning members of this group would probably have had some feathers, though as Abelisauroids evolved, the derived Abelisaurids primarily lost their fluffy covering. Furthermore, Ceratosaurids probably had osteoderms along their back, though this says nothing about the Noasaurid group, which were primarily small and thus probably retained their feathers.
Biogeography: It is very uncertain where Neocreatosauria originated, given that Ceratosaurs were very widespread and Neoceratosaurs were also; early members of Abelisauroidea are not helpful, either, as they are fairly widespread. As such, it is unlikely it will ever be determined where this group first evolved.
Posts about Ceratosauridae and Abelisauroidea to come soon.
What is the problem with Linnaean classification? And what other system should be used instead?
Linnaean classification is outdated based on our current knowledge of the evolution of life.
It is a method of classifying organisms based on traits and characteristics of them that we find important; it’s entirely subjective.
Cladistics is the classification of organisms based on their evolutionary realtionships; which no, we don’t know exactly all the time, but we’re constantly learning more and making our picture of the evolution of life on earth more complete, making cladograms and phylogenetics more and more informative every day.
For example, birds are in the class “Aves” and the phylum “Chordata.” We arbitrarily decided that birds are different from reptiles (”Reptilia”) because they were warm blooded and had feathers. But dinosaurs, which are in the class Reptilia, are also warm blooded and also have feathers. Also, birds evolved from dinosaurs - so they’re a group of dinosaurs - which means they’re in the class Reptilia and Aves!
Also, Synapsids are the “ancestors” of mammals. In Linnean classification, they’re put in Reptilia. However, as far as evolution is concerned, they aren’t reptiles.
Also, sponges (Phylum “Porifera”) - some of them might be more closely related to more complex animals than others -
Also, almost all protists are more closely related to fungi, animals, or plants than they are to other protists, making the kingdom “Protista” functionally useless.
So we use cladistics, which yes, retains main of the same names as Linnean classification, but they aren’t ranked anymore, and they’re defined based on evolution rather than arbitrary traits that we have magically decided are more important than others.
For example of how we define groups in cladistics is Dinosauria, which is defined as the most recent common ancestor (MRCA) of Meglosaurus and Iguanodon, and all of that MRCA’s descendants.
Another example that shows a different way we define groups is Carnosauria. Carnosauria is defined as all Tetanurans that are more closely related to Allosaurus than to modern birds. See how this is based on something that happened in the past rather than something that we just chose?
Yeah, parts of it are arbitrary - we don’t define every cladistic grade, but that’s because it would be impossible to do so! We name the groups that are really important, and the ones that aren’t we just show using cladograms.
For an example for how you’d list that out, let’s take the house sparrow. Yes, this is going to be a long list. Yes I’m retaining the genus/species, mostly because there isn’t anything else to use. I apologize for nothing. And you can point out the important groups in a classification list by using, say, bold type, which is what I do.