>>35389337The entire line shoots water out their shells like jets. Squirtle and Wartortle's tails are stylized to look like water is always jetting out. This ever present detail is why Wartortle's dex entries bring up how it is unable to tuck in its tail into its shell.
By extension, since Wartortle's ears are stylized the same way its tail is, it foreshadows how Blastoise will be capable of shooting water from the front of its shell two ways using actual jets. Blastoise using jets also forgoes it needing to tuck in to its shell to shoot water.
There's also the aspect of the frills on Wartortle pointing it towards it being a minogame, or a turtle that is thousands of years old. Considering how Wartortle and Blastoise build up to Gen 1's kaiju aesthetic, and how the water jet idea is based on Gamera, it it could be said that the line progresses from a turtle that is thousands of years old to a turtle that is prehistoric.
