Eli5: why are some prehistoric reptiles not dinosaurs?


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?

In: 103

All dinosaurs belong to a taxonomic group called a clade. They’re all descended from a single ancestor and make up “Dinosauria” clade. But not all large, reptilian animals belong to that group.

It’s not unlike how there are many arthropods, but only some are classified as insects.

It’s the hip shape. Dinosaurs are the ancestors of modern birds and have hips like a bird. Prehistoric reptiles are ancestors of modern reptiles and have hips like a reptile.

You’ve probably heard of the tree of life. Evolution works by starting at the bottom of the tree and groups continually branch off from each other. There are now many groups of every kind of life. Dinosaurs are just one group of reptiles and there were other groups of reptiles that lived at the same time just like there are different groups of reptiles alive today.

As scientists have studied the tree of life, they have discovered a few interesting things about where dinosaurs fit into the tree of life. Birds actually evolved from dinosaurs and survived the extinctions which killed all of the rest of the dinosaurs. They also learned that crocodillians are the next closest living relative to dinosaurs. Dinosaurs are actually not even lizards. Lizards are a different group of reptiles. The group of reptiles that both crocodillians and dinosaurs are a part of is called archosaurs.

Modern taxonomy generally uses phylogenetics which organizes organisms based upon common ancestors. Older taxonomy methods used physical characteristics to organize them. Both methods have some deficiencies. There can be some cross “pollination” between groups which means that some branches can connect to multiple locations. Classifying based solely upon the characteristics can be problematic when unrelated species develop similar adaptations to fill the same general niche (convergent evolution). One example (which didn’t necessarily fool taxonomists) are hyenas. They are quite doglike in appearance, but are actually much more closely related to cats.

One of the major descriptive characteristics of dinosaurs vs other reptiles is the erect nature of the hip/leg joint. Reptiles tend to have a sprawled nature with the legs coming out more or less horizontally before potentially bending down at the elbow/knee–think of an crocodile or turtle. Dinosaurs legs tended to go straight down from the body–think of a tyrannosaur or bird.

Considering how old the fossils of dinosaur are we can’t run DNA tests, and soft tissues don’t fossilize well. Most classification are therefore based upon bone morphologies and other evidence like where they were found. It is strongly suspected that at least some (if not all) dinosaurs were warm blooded as opposed to cold blooded like other reptiles (after all their descendants (birds) are). In 2000 they thought they may have found a fossilized 4-chamber heart in one specimen, however, further analysis has cast doubt on it being a heart, so the mystery remains (birds have a 4-chamber heart while reptiles have only 3).

Dinosaurs are not lizards at all, lizards are a completely different branch of the reptile family tree.

Dinosaurs are a biological *clade,* meaning all of the species descended from a particular common ancestor. Typically, the dinosaur clade is described as everything descending from the most recent common ancestor of Triceratops and modern birds and are distinguished by the structure of their hips, ankles, and some other bones. The first dinosaur, i.e. the most recent common ancestor of all dinosaurs, is estimated to have lived around 230 million years ago. Other than birds (which *are* dinosaurs) the closest living relatives to dinosaurs are the crocodiles; both dinosaurs and crocodiles belong to a group of reptiles called “archosaurs.”

Things that are not dinosaurs:

1. Dimetrodon: Dimetrodon went extinct 40 million years before the first dinosaurs evolved, and are therefore excluded based on the above definition. Moreover, dimetrodon is not even particularly closely related to dinosaurs and it is a stretch to even call it a “reptile;” it’s actually much more closely related to mammals than any living reptile. Their (and our) last common ancestor with dinosaurs would have lived around 320 million years ago.

2. Pterosaurs: Pterosaurs are in fact archosaurs, part of the same group as dinosaurs and crocodiles, but split off shortly before dinosaurs evolved making them one of the dinosaurs’ closest relatives. They lived alongside dinosaurs and went extinct around the same time. Although they could fly, they evolved flight completely independently from birds, and were physiologically more like bats, with membranes of skin stretched over their fingers.

3. Marine reptiles, i.e. the ichthyosaurs, plesiosaurs, mosasaurs, etc: Where some of these groups belong on the reptile family tree is somewhat unclear, but they are not dinosaurs. Ichthyosaurs are very mysterious in their origins, but fossil evidence shows they definitely evolved before dinosaurs. Plesiosaurs are also difficult to classify, but the modern consensus is that they are related to turtles. Mosasaurs are unambiguously lizards, and are closely related to snakes and monitor lizards. Generally speaking, there were no marine dinosaurs (although some, like Spinosaurus, may have been semi-aquatic).