Had it been too warm, would fish have survived? ""
The "fish" of that era were there. Everything adapts or dies.
I recall seeing a nature special that featured a minnow-sized fish called a Pupfish which live in hot springs in some desert or other. It looked very similar to a brackish/salt water minnow common in our area, called a Killie. Killies get landlocked in fresh water around here and thrive there too.