why are pterosaurs and aquatic marine animals in the past not considered dinosaurs?
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Because we define dinosaurs to be a specific kind of animal, not just “animals that are old.”
Dinosaurs are animals of the clade *Dinosauria.* This is a much more specific classification of animals that generally applies to land-based reptiles with the same body structure. The Wikipedia page on dinosaurs does a pretty good job of describing that structure, so I won’t go into detail here.
Pterosaurs are part of a distinct lineage of animals.
Because they are not in the animal group that we call Dinosaurs. We classify biological organisms in groups where every gin that is decent from X is in group X. What we called Dinosaurs are descendent of a specific form of life and those decedents do not include the large marine animals of that time.
A distinguishing feature of dinosaurs is the placement of the legs. Dinosaurs have legs beneath the body as mammals do. Compare to crocodiles that have their legs sticking out on the sides of the body. Look at [this drawing](https://en.wikipedia.org/wiki/Terrestrial_locomotion#/media/File:Sprawling_and_erect_hip_joints_-_horizontal.svg) where crocodiles are the sprawling types.
It is not different then Crocodiles are not dinosaurs but both are reptiles. Crocodiles are in fact closer related to Dinosaurs compared to the Plesiosaur and Ichthyosaur that lived at the sea at that time. You can look at how are related in [this image](https://qph.fs.quoracdn.net/main-qimg-13271ae705ddc6d3dd5f9a46d178a575). The flying Pterosaur is close to dinosaurs the dinosaurs are to Crocodiles.
So if you would like Plesiosaur and Ichthyosaur to be included in what we call Dinosaur you have to include Crocodiles too. But that group’s od already have a name, you could pick for example pick Diapsids or even Reptiles
We classify life based on the evolutionary history of the clade (group) they belong to.
Dinosauria is defined as the most inclusive clade containing *Passer domesticus* and *Triceratops horridus*.
Sereno, P. C. The logical basis of phylogenetic taxonomy. Systematic Biology. 54:595-619. (53)
This means that all of the lineages that split where birds and *Triceratops* have a common ancestor are apart of the clade Dinosauria, and are considered dinosaurs.
The two main divisions of Reptilia are Archosauria and Lepidosauria. Lepidosaurs contains Squamata, which lizards and snakes, as well as the Mosasaurs, belong. The giant Mosasaurs of the Cretaceous are the largest lizards to ever live.
Archosauria contains pterosaurs, crocodylians, and dinosaurs. Archosauria is further broken down into Avemetatarsalia and Psuedosuchia. Avemetatarsalia is where pterosaurs and dinosaurs are united.
Most other aquatic reptiles, such as plesiosaurs, belong to Sauropterygia. Where exactly this group fits in is up to debate.
To *extremely* “ELI5” it, it’s kind of like asking why dogs are not considered Great Apes. Yeah they’re both 4-limbed mammals, but “Great Apes” refers to a specific genetically-related family of animals that dogs just aren’t in.
“Dinosaurs” doesn’t mean “any animal from ___ era”, it means “animals of the clade *dinosauria*”, which pterosaurs and aquatic marine animals just aren’t. They have different evolutionary ancestors and successors than the members of *dinosauria* do.
I’ve literally explained this to <5 year olds before (I used to work at a museum). Basically, pterosaurs/marine reptiles aren’t dinosaurs for a few reasons, but an easy one to remember is that their hips are different. Dinosaur hips point their legs straight down (like ours!) and pterosaurs/marine reptiles have their hips point their legs to the side (like a crocodile).
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