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Which features of Archaeopteryx were similar to those of reptiles?

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Illustration showing various dinosaur species drawn to scale and grouped by geologic era: Late Cretaceous, Early Cretaceous, Jurassic, and Triassic. Dinosaurs range from large species like Brachiosaurus and Tyrannosaurus to smaller ones like Velociraptor and Compsognathus, with a human silhouette included for size comparison. Measurement bars at the bottom indicate 1 meter and 3 feet for scale reference. — dinosaur A selection of dinosaurs grouped by the geologic interval in which they lived.

Archaeopteryx is often cited as a transitional form because it possesses a combination of avian and reptilian characteristics. While it is classified as a feathered dinosaur, it retains several anatomical features typical of small carnivorous theropod dinosaurs, which are ancestral to birds.1

Key features of Archaeopteryx that are similar to those of reptiles (specifically nonavian dinosaurs) include:

Teeth: Unlike modern birds, which have beaks, Archaeopteryx possessed well-developed, narrow-waisted teeth.12
Tail: It had a long, well-developed bony tail, whereas modern birds have a highly reduced tail structure.1
Clawed Fingers: The three fingers on its forelimbs bore claws and were capable of independent movement, contrasting with the fused finger structure found in living birds.1

These traits highlight its evolutionary link to dinosaurs, as these structures were already present in its nonavian coelurosaurian relatives before the evolution of flight.1