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Why are birds and crocodiles classified more closely together than either is with lizards?

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A diagram showing the hierarchy of animal classification in taxonomy, with colored boxes arranged from broadest to most specific: domain, kingdom, phylum, class, order, family, genus, and species. — taxonomy Animals and other organisms are classified within a succession of nested groups that ranges from the general to the particular.

In modern biological classification, birds and crocodiles are grouped together as members of the archosaur lineage, a relationship defined by cladistics. Unlike older methods that grouped animals based on overall physical similarity, cladistics classifies organisms based on their most recent common ancestor and the presence of shared derived characters, known as synapomorphies.1

Birds and crocodiles share a common archosaurian ancestor that is more recent than the ancestor they share with lizards. While lizards belong to the lineage of lepidosaurs, archosaurs are distinguished by specific anatomical traits, such as an opening in the snout in front of the eye (antorbital fenestra) and teeth set in sockets. Because birds evolved from within the archosaur group—specifically from theropod dinosaurs—they are phylogenetically closer to crocodiles than to lizards. Consequently, cladistic analysis places birds and crocodiles within the same clade, reflecting their shared evolutionary history rather than superficial physical resemblances.23415