Reptiles, in the traditional sense of the term, are defined as animals that have scales or scutes, lay land-based hard-shelled eggs, and possess ectothermic metabolisms.
So defined, the group is paraphyletic, excluding endothermic animals like birds and mammals that are descended from early reptiles.
A definition in accordance with phylogenetic nomenclature, which rejects paraphyletic groups, includes birds while excluding mammals and their synapsid ancestors. So defined, Reptilia is identical to Sauropsida.
Though lots of reptiles today are apex predators (top of the food chain), many examples of apex reptiles have existed in the past. Reptiles have an extremely diverse evolutionary history that has led to biological successes, such as dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and ichthyosaurs.Reptiles: paraphyletic -- descended from a common evolutionary ancestor or ancestral group, but not including all the descendant groups.
So, now, it would be interesting to see which contemporaries of the various reptilian descendants are extant today but the reptilian contemporaries died out.
For example:
- air pair:
- pterosaurs: reptilian, died out
- birds: dinosaurs, lived on
- land pair:
- dinosaurs: lived on as birds, but others died out
- ocean non-mammalian pair:
- plesiosaurs (Pliosauridae): replaced by mososaurs
- ocean mammalian pair:
- mosasaurs: with the extinction of the ichtyosaurs and pliosaurs, mosasaurs became the dominant marine predators; extinct as the result of the K-Pg event at the end of the Cretaceous period, 66 million years ago; breathed air; birth to live young
- sea mammals such as whales
- sea pair:
- ichthyosaurs: died out
- fish: lived on
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