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
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.
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.
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).
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.
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).
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.
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.
It’s more “ELI9-12”, but if you want to take an accessible and comprehensive deep dive into the evolutionary paths of prehistoric animals, I highly recommend “Dinosaur Empire!” by Abby Howard. It’s the first comic book in a series all about the concepts of the evolutionary tree, it’s amazingly detailed and very fun.
It’s more “ELI9-12”, but if you want to take an accessible and comprehensive deep dive into the evolutionary paths of prehistoric animals, I highly recommend “Dinosaur Empire!” by Abby Howard. It’s the first comic book in a series all about the concepts of the evolutionary tree, it’s amazingly detailed and very fun.
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