If the evolution of that species could reach a dead end way back then, and still survive, why couldn't another?
7 seconds sound clip from the Creature from the Black Lagoon (1954) movie soundboard.
You can hear this line at 00:40:55.336 in the Blu-ray version of the movie.
Quote context
[...]
- This is no legend, as you'll soon see for yourself.
- Interesting that the fossil I found was out of the same period.
- The kamongo was a fish that breathed air.
- That doesn't prove the possibility of a gill-man.
- I was sure I had him in my finder.
- For a merman, he takes a fine picture.
- This wasn't imagination, Doctor.
- What proof do we have if we never find it again?
[...]