Oh boy, here is one that causes confusion
Dinosaurs are a group of animals so famous and set into the mainstream media that they have become synonymous with all things prehistoric. While this has been the singular thing that made paleontology such a popular science, it does mean that most forget that the term 'dinosaur' is an official scientific name for a specific group of animals with very specific features. Therefore a lot of animals get the title of 'dinosaur', just because they're a reptile that is extinct (or maybe even just the latter).
While dinosaurs are one the most diverse clades to have existed, you'd be surprised as to how many animals are not dinosaurs. They may also be surprised as to what DOES count as a dinosaur. Case in point: birds are officially classed as dinosaurs...no, don't throw things at me, they really are!
In fact, the ONLY dinosaurs to have achieved flight are birds (pterodactyl is not a dinosaur!) and the closest they ever got to swimming (that we currently know of) were semi-aquatic animals such as spinosaurids, halszkaraptorines or even ducks and penguins. So all those marine reptiles you're thinking aren't dinosaurs either.
While it does seem like we're splitting hairs here, distinctions are important to help group these animals and track their evolutionary pathways. Small differences in skeletal structures lead to massive distinctions!
So, what makes a dinosaur a dinosaur? Is it being scaly or feathery? Is it a certain skull shape? Is it just being a reptile? Are they even reptiles if birds are included? Let's answer those nagging questions.
First of all, any animal is classed and grouped purely based on their skeleton, nothing more. Yes, it's true that most animals of certain groups all share features, but these are inherited and not always necessarily seen. All mammals have hair and give live birth and all reptiles have scales and lay eggs, right? Well how do you explain a platypus, who's a hairy mammal who lays eggs? Or how about 14 whole families of snakes that give live birth? We can leave them out because of that, however, the one thing that never changes is that all important skeleton.
When it comes to determining what is and isn't a dinosaur, we must know what groups they belong in and what skeletal features are diagnostic.
First up, dinosaurs are reptiles (yes even birds are technically classed as reptiles!), so let's imagine you've just found a full skeleton fossil which you want to find out whether it is a dinosaur or not and start there. You have two main categories of reptiles: synapsids and diapsids. The distinction is fairly simple: both groups have what is known as a temporal fenestra, which basically refers to a hole in the skull just behind the eye ('fenestra' meaning 'window'). The difference between the two is that synapsids had one temporal fenestra and diapsids had two.
I know this seems like being picky, but this distinction is what separates reptiles from all other amniotes (a land-dwelling vertebrate that doesn't need water to lay its offspring in...basically anything that isn't amphibian) after the Triassic period. The diapsids went on to become the only true reptiles after the PT mass extinction right up until today. The synapsids? Well, they first started off as reptiles such as Dimetrodon:
But after the PT mass extinction, the few survivors of this group became the mammals. That's right, that guy is our great, great, great (insert a near infinite amount of 'greats' here) uncle!
But back to diapsids. This group encompasses all reptiles, which is a VERY large group indeed. Does this fossil have two temporal fenestrae on either side of the skull? Tick. So the next thing to do is look for defining characteristics of groups within diapsids. The next group we need to look for that dinosaurs exist within is called archosauria. Archosaurs were the dominant form during the Mesozoic as this group includes all crocodilians, dinosaurs, pterosaurs and a few others, all of which look superficially dinosaur or crocodile-like. This group is defined by a few things, it must be a diapsid with: an antorbital fenestra, a mandibular fenestra, teeth set in very deep sockets and a ridge on each femur known as the fourth trochanter. Here's an image that may help:
Does your fossil have those? Yes? Good, then we can move on to the next step.
Archosaurs are split into two main groups: Pseudosuchia (crocodilians and their relatives) and Avemetatarsalia (archosaurs more closely related to birds, such as dinosaurs and pterosaurs). The main characteristic that differs the two is that Avemetatarsals have ankles that operate on a single hinge, meaning they have an upright posture with their legs underneath them rather than sprawled out to the side. This group also started off as bipeds, which means all dinosaurs that walk on four legs originally started off walking on two!
So, let's say our fossil has this type of ankle. We can now narrow this animal down to one of two groups: pterosaurs or dinosauromorphs (which include dinosaurs and some groups which are very dinosaur-like). Pterosaurs are fairly easy to distinguish, with their small flat-footed hind limbs, highly fused skeletons and elongated 'little fingers' for wings. If the fossil doesn't have those, it's likely not a pterosaur. But not all members of the dinosauromorph group were actually dinosaurs (remember this group means dinosaur or very dinosaur-like), so how do we know which is which? Well, there is one distinguishing feature that makes it a dinosaur...just one...drum roll please...
Yup, that's it! What you can see here is a side view of two dinosaur pelvises (both saurischian and ornithischian, which I'll explain in just a second) with something called an acetabulum. The acetabulum is essentially the hip socket where the thigh bone inserts into the hip, except with dinosaurs, you can peek straight through to the other side of this socket (most animals don't have a straight-up hole here). This allowed dinosaurs to stand upright with a much more efficient way of distributing stress during movement.
That's it! That tiny hole is what makes a dinosaur a dinosaur! Seems a bit anticlimactic, I know, but like I said earlier, these tiny differences are often the ones that carry over to all members of the group no matter what their variations are.
So a dinosaur is simply an archosaurian reptile with a hole going through its acetabulum. You now have a definitive dinosaur fact to tell at social gatherings... just don't try those big words when you're drunk!
So, you now know you have a dinosaur, what next? Well, this is one that's caused some confusion in recent years. Traditionally you would have established it as either saurischian or ornithischian (either lizard hipped or bird hipped), if the pubis is tilted forwards it is saurischian (lizard hipped) and if it is tilted backwards it is ornithischian (bird hipped, which, strangely doesn't include birds since they started off as saurischian then reverted convergantly to a backwards tilted pubis... things were named before this was found out, just try to keep up).
This tidied things up neatly into something that looked like this:
However, in 2017, Mathew Baron et al. decided to shake things up by proposing this structure:
Which one to believe? Well you can decide after for yourself by either reading the literature or watching this space for a new post discussing this issue. Just know that if all the boxes are ticked up until two paragraphs ago, it's a dinosaur... which probably doesn't include what you might think....