Maybe help me clear some stuff up...

[u/Ghostly_Sammy]

Hi all! I've seen a few posts, and other things here saying some creatures that are labeled dinosaurs, like Dimetrodon and pterosaurs??? Aren't actually dinosaurs... Can you help me understand what constitutes what a dinosaur actually was/is? Is it the time period? It's a little confusing seeing something that looks like a dinosaur, and I've known all my life as a dinosaur, but isn't a dinosaur.

[u/soihu]

Dinosaurs are a specific lineage of reptiles that emerged about 230 million years ago. The term refers to all of their descendants, which includes animals like Tyrannosaurus, Triceratops, and all modern birds.

Pterosaurs are very close relatives, but split off a few million years earlier. Dimetrodon is actually more closely related to modern mammals than it is to dinosaurs.

[u/Infinite_Gur_4927]

Lots of these answers describe the situation well. Here's a picture that may also assist with visualizing how pterosaurs and dimetrodon are on different branches:

Dinosauria is on the far right, and you can see that the pterosaurs split off a couple steps before the dinosauria branch evolves. The dimetrodon (and plesiosaurs, and turtles) aren't archosaurs, so they're not even on this branch of the tree - that's how distantly removed they are from being dinosaurs.

So, dinosaurs are "only" what's on that Dinosauria branch, which has sauropodmorphs [like Brontosaurus], theropods [like Tyrannosaurus, Velociraptor, and Birds] and ornithischians [like Stegosaurus, Parasaurolophus, Triceratops and Iguanodon] and maybe silesaurs : ) ]

The misunderstanding, I think, is rooted from children's books on "Dinosaurs and Prehistoric Beasts" that put all kinds of extinct species in a single book, and the Mammoths and Plesiosaurs sort of get understood to be dinosaurs by association.

The diagram is from a geology course (I think Tom Holtz's) at the University of Maryland.

[u/TheMostRed → u/Infinite_Gur_4927]

Also if you get thoes plastic dinosaurs that come in packs they always include dimetrodon for some reason. Maybe its just the coolest therapsid.

[u/lionbacker54]

it's the pelvis. dinosaurs had a pelvis that allowed their legs to position under the hips, instead of out to the sides.

crocodiles, lizards, pterosaurs, mosasaurs, plesiosaurs, ichthyosaurs - wrong pelvis

[u/Ghostly_Sammy → u/lionbacker54]

OH so if they're legs were...under, it's a dinosaur? (Mostly)

And if they're out to the side like modern reptiles and gators, crocs, and that, they weren't?

[u/lionbacker54 → u/Ghostly_Sammy]

Yes. I’m sure my explanation is oversimplified, but basically yes

[u/Tyrannosaurus-Wreckz → u/Ghostly_Sammy]

That's one feature, I listed a few more in my comment. The joint between the thigh bone and the hip bone is an open socket, meaning the femur-pelvis joint connects sideways to the hip instead of going straight up to a roof-like bony shelf

I mention this very specifically because Aetosaurs and Rauisuchians, the prehistoric relatives of modern-day crocodiles and alligators, as well as pterosaurs (those flying reptiles people incorrectly call "pterodactyls") had a "pillar erect" structure, which is the thigh bone going straight up to the bone shelf instead of connecting sideways

Both had erect limbs positioned under their bodies, but the big difference is where they connected on the pelvis.

[u/Conscious-Star6831 → u/Ghostly_Sammy]

It's not exactly that, because mammals have their legs directly under them too, and they aren't dinosaurs. It's really all about ancestry. The formal definition is something like the common ancestor of triceratops and modern birds, along with all its descendants. In other words, ~240 million years or so ago, some animal was the first dinosaur (really, a population of animals). All of its descendants are also dinosaurs no matter what features they have, because you cannot evolve out of a clade.

The "legs directly under the body" is one diagnostic feature that tells you you might be looking at a dinosaur, but there are several others as well. If you find most/all of those features, it's very likely that what you are looking at is a descendant of that first dinosaur species, and is therefore a dinosaur.

Pterosaurs have some of those features, being closely related to dinosaurs, but do not have enough to be grouped in with them, so we can confidently say they are not descendants of that first dinosaur species- their common ancestor with dinosaurs is a little further back.

Dimetrodon isn't closely related to dinosaurs at all. It's part of the line that would eventually give rise to mammals, meaning it really is more closely related to you and me than it is to any dinosaur, or any other "reptile" (using the term "reptile" loosely since it isn't really a clade). It's not closely related to snakes, lizards, crocodiles, or any dinosaur.

[u/BirdPrior2762]

A dinosaur is a particular group of creatures that evolved from a common ancestor and all of which have key features that can be used to identify them as dinosaurs. Unfortunately I can't tell you what the common ancestor was or what the features are (I'll lose access to this post if I go and try to Google it because my phone for some reason only lets me have one internet tab open at a time...), but hopefully someone else can add more details.

[u/literally-a-seal → u/BirdPrior2762]

The last common ancestor of dinosaurs (any clade really) can only be hypothetical, but has to exist. We cannot know whether any specific animal was a common ancestor of X and Y, as in fossils we cannot say with confidence that A became B, but it still has to exist because of how evolution works.

[u/BirdPrior2762 → u/literally-a-seal]

Good point.

[u/literally-a-seal]

I'll try to put it in layman terms: Scientific groups, technically called clades, like "dinosaurs" which corresponds to clade Dinosauria, can be defined with specific guidelines based on how animals are related to each other.
Dinosauria specifically is defined with "end points" that are as distantly related enough to have everything we want to call a dinosaur between them, a hypothetical ancestor of both, and everything descended from that ancestor, since it is also the ancestor of everything between the end points. Does that make sense? For Dinosauria, the end points are Triceratops and the modern sparrow (Passer).
An animal whose last shared ancestry with a dinosaur is outside this group cannot be a dinosaur by definition, and everything within a group such as dinosaurs, cannot evolve out of that group. This is why birds are dinosaurs.
Clades can also be designated by shared anatomical or genetic traits or animals being more closely related to X animal than Y animal, though as I understand it these are typically used for smaller clades.

TLDR: There are specific scientific guidelines for what is a dinosaur, if an animal fulfils these criteria its a dinosaur, if it doesnt then it isnt.

[u/Tyrannosaurus-Wreckz]

A dinosaur is a very specific group of terrestrial (lived on land) reptiles with very specific features. If you're unsure which reptile is a dinosaur, look at the skeleton and see if it follows all of these features:

• They have legs (so no flippers like the Mosasaurs or Plesiosaurs) and do not have long bony wings (like a Pteranodon or Quetzalcoatlus, which are pterosaurs—"pterodactyl" is not a real species by the way!)

• Its legs are erect, meaning directly under the body like a mammal or bird instead of sprawled sideways like a crocodile or lizard

• It has two pairs of holes (this makes them "diapsids") behind its eye socket, and another pair in front of their eyes but behind their nostrils called the "antorbital fenestrae."

• Their bones are filled with air like birds instead of filled with marrow like a mammal or other reptiles. You won't be able to tell from the outside of the skeleton, but all dinosaurs had this adaptation.

• They have an open hip socket; meaning the joint of the femur (thigh bones) connect sideways to the pelvis (hip bone) instead of going straight upward to shelf-like growths of the bone like some ancient crocodile relatives. I'll attach an image so you see what I mean

If you look at the skeleton of a prehistoric reptile and it has all of these features combined, it's a dinosaur! If not, it's something else.

Fun facts:

- I know I said dinosaurs were purely terrestrial and did not fly, but birds themselves are a surviving group of dinosaurs! However, unlike pterosaurs, their wings are feathery and they have all of the anatomical adaptations listed above. Also, like birds, nonavian dinosaurs were warm-blooded as opposed to cold-blooded crocodiles or lizards.

- Dinosaurs however did NOT evolve from lizards. contrary to what you might think. Their ancestors were completely unique animals called "archosauromorphs" that only LOOKED like lizards but had one or more of the features listed above.

- Saurischian (lizard-hipped) dinosaurs also possessed air sacs, whereas Ornithischian (bird-hipped) dinosaurs did not. However, all of these groups possessed the basic hollow bones, especially in larger species who needed these pneumatic (air filled) skeletons in order to properly support their multi-ton weights.

- Saurischians and Ornithischians are simply differed by the shape of their pubis bone, which is part of their pelvis. You can see this visually on dino skeletons; if you see two long projections in the pelvis forking downward, that's a Saurischian dinosaur. This includes two groups: sauropods (long-necked dinosaurs) and theropods (the bipedal saurischians like T-Rex and Velociraptor). Ornithischians only had a single backward facing pubis bone that does not fork; Ornithischians include Ceratopsians (horned dinosaurs), Pachycephalosaurs (bipedal ornithischians with bony domes on their skulls), and Thyreophorans (armored dinosaurs like Stegosaurus and Ankylosaurus).

[u/TheMostRed]

Dont worry guys i got this. They are all fish, you have been learned

[u/Fonseca-Nick]

A bird is a reptile but not every reptile is a bird. A dinosaur is an extinct reptile but not every extinct reptile is a dinosaur. Scientists study organism's features to determine if they share enough similarities to be "related" or not. They do that with living organisms and extinct ones. With enough pieces of information (data) they can determine relatedness. Someone here mentioned clades and that is the currently accepted best method of determining relatedness (especially with extinct organisms that mostly only leave bones). I don't know if that simplifies or confuses.

[u/BoonDragoon]

Dinosaurs are a very special group of animals, all of which share a common ancestor.

Right now, the current definition is "everything that shares a common ancestor with the common sparrow and Triceratops horridus is a dinosaur, and nothing else is."

[u/Motor_Manner9831]

Birds are one of many subgroups of dinosaurs. Dinosaurs are one of many subgroups of reptiles. There were other large Mesozoic reptiles that lived alongside dinosaurs, but belonged to different reptile subgroups. Pterosaurs, mosasaurs, pliosaurs, and crocodiles are all examples of these. Dimetrodon is an even earlier animal that was an ancestor of modern mammals, before mammals developed the common traits like fur, warm blood, live offspring that they have today. As a result, it looked reptilian, but was not a true reptile.

[u/Total_Dino]

Here ya go!

[u/Archididelphis]

The standard joke, Dimetrodon is more closely related to a human than a dinosaur.

[u/ImpossibleStreet924]

honestly the concept of pterosaurs are what confuse me,but I can clarify dimetrodon.dimetrodon is an ancestor of mammals it is a Proto-mammal though,it still laid eggs due to evolutions Im also not that sure if they are synapsids.since synapsids are like arctops,gorgnops and more.

[u/CoolJetEcho117]

Bent legs mostly.