If you know anything about dinosaurs, you know that there are a lot of things that people call dinosaurs that are, in fact... not dinosaurs. In fact, I made a very handy T-shirt design just for that reason:

 Amy Atwater wearing the DiNopeASaurus shirt

Amy Atwater makes this DiNopeASaurus T-shirt look Amazing. Amyazing. Amayzing? Whatever, she's fantastic.

None of those things, no matter how many dinosaur packs they show up in, are actually dinosaurs. Which begs the question... what exactly is a dinosaur?

Dinosaurs all have a few things in common, morphologically speaking anyhow. To be considered a dinosaur, you need to have these features, or be descended from a lineage with those features (like birds, which have lost some of them). Those features are:

1. A big ol' extra ridge on your upper thigh bone (aka, a 4th greater trochanter).

This ridge is where a few big ol' fatty tail muscles attached in dinosaurs. So a great ass is part of the whole Dinosaur mystique (don't argue, it's science). In fact, some theorize that the trochanter actually is what allowed dinosaurs to be bipedal - so without it, T. rex never would have been able to shake that thang. Or like... walk upright or whatever.

New mid cretaceous dinosaurs from Winston Queensland. Hocknull S, White M, Tischler T, Cook A, Calleja N, Sloan T, Elliott D (2009)

The 'ft' indicates where this magical dinosaur-defining bump is. Image from Hocknull et al 2009. 


2. Two holes in your head (diapsid skill).

I know, I know, you're thinking "but Meaghan, I have two eye sockets" but haha no, that doesn't count. I mean, good for you and whatever, but that's not the hole of importance. You, dear reader, are a synapsid, meaning you only have a single hole - referring to where your jaw muscles pass through your cheekbones. To feel it, place your fingers on your temples and move your jaw up and down - you'll feel the muscle that runs behind your cheekbone moving. 

Pelycosaurs are just like dinosaurs but with fewer holes in their heads

Pelycosaurs like this Dimetrodon were synapsids - so it's actually more closely related to you and I than dinosaurs. Hence why this mug says "Pelycosaurs: just like dinosaurs, but with fewer holes in their heads."

Dinosaurs had that hole, but they also had a second hole up above it. Go broke or go home if you're a dinosaur I guess.

 Anapsid, synapsid, and diapsid skulls. Image from Wikipedia.

The three most common types of skulls - turtles (anapsid), mammals (synapsid), and dinosaurs (diapsid). Image from OpenStax Biology.


 3. A see-through hip hole (an Open Acetabulum).

No, that doesn't mean they fossilized like some sort of transparent gecko or something. Where your thigh bone meets your hip, you have like a little "cup" shape. That's called the Acetabulum, which means "little vinegar cup" in Latin. I'm sure that the person who named it that was just like, weirdly into vinegar, but it does make me wonder if everyone was serving food out of human bones like some sort of cannibalistic tea set.

Anyways, you couldn't use a dinosaur pelvis for your Donner Tea Party because they have no bottom to the cup. Dino leg bones just sat in a hole-punch in the pelvis, for... evolutionary reasons, I'm sure.

dinosaur pelvis from Admiral Hood.

See? All your vinegar would spill out, embarrassing you in front of your MegaGoth friends. Image from AdmiralHood on Wikipedia.

 Now, several of the members of the DiNopeASaurus club have some of these characteristics - but NONE of them have all of them. That is because while they may be related somewhat to dinosaurs, they really are just big reptiley things that kind of look similar.

Well, except for Basilosaurus. That's a whale, which was named the king of lizards. So mammals rule, even non-dinosaur reptiles drool.



  • "The Anatomical and Functional Evolution of the Femoral Fourth Trochanter in Ornithischian Dinosaurs" by Persons and Currie. https://anatomypubs.onlinelibrary.wiley.com/doi/full/10.1002/ar.24094
  • "New Mid-Cretaceous (Latest Albian) Dinosaurs from Winton, Queensland, Australia". Hocknull et al. PLOS ONE. DOI:10.1371/journal.pone.0006190 
  • "What makes a dinosaur a dinosaur?" Riley Black. https://www.smithsonianmag.com/science-nature/ask-smithsonian-what-is-dinosaur-180967448/

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