why are pterosaurs and aquatic marine animals in the past not considered dinosaurs?
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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.
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