I’ve gone my entire life being told “that’s not a dinosaur that’s a (insert what it is) but why? What’s makes one giant lizard a Dinosaur but a different giant reptile not a dinosaur?
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The term “dinosaur” has been defined by scientist to describe a group of related creatures.
Wether something is or is not a dinosaurs has nothing to do with its size or reptileness or when it lived or wether it is extinct.
People had been coming across fossils of dinosaurs and other for a long time but didn’t really recognize them for what they are until the mid 1800s.
Some of the first few found are today known as Megalosaurus and Iguanodon and when the term dinosaur was coined it was created to basically describe creatures like those two.
The technical definition involves picking two dinosaurs like Iguanodon and Megalosaurs and say that a dinosaur is every creature that descended from the last common ancestor of those two.
You can replace the two examples with more well known ones like T-Rex and Triceratops the result is the same.
So a dinosaur is every creature that is part of the same family as the first few dinosaurs we encountered.
This means that other big extinct creatures that are not part of that family are not dinosaurs.
You have creatures like Dimetrodon which lived long before the dinosaurs and is more closely related to us than to the Dinosaurs.
You have flying reptiles and aquatic reptiles like pteranodon or Plesiosaurs.
If we had encountered one of them among the first giant extinct fossils our definition for dinosaur might have been broader.
It is probably for the best though.
The fact that we define what a dinosaur is based on common descent means that birds are technically dinosaurs, this is more obvious for some birds than for others, but still true for all of the.
If we had included some of the other big extinct reptiles in the original definition a bunch of living reptiles would now also be technically dinosaurs.
It might work for alligators and crocs and snakes and some other group but could get quickly out of hand if we defined it broadly enough
If we had included things like Dimetrodon you and I might qualify as dinosaurs and that would be stupid and useless.
Having to look at a chicken and realize that it is basically a tiny T-Rex cousin is bad enough.
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