I want this world to have two things, typically on the complete opposite end of geological timescales:
- Land fauna is incredibly "Young" - Pretty much nothing larger than a "large" insect (Some liberties allowed), and even the flora isn't that complex. Still need some tree-analog, though.
- Radioactive isotopes are old. Or, rather, the solar system the planet is in is old, at least relative to Earth. I'm not going to nail down details (Since I'd get it wrong!), but effectively U-235 is a lot rarer than it is on our planet, simply by aging away. This means nuclear power, while not impossible, isn't discovered as easily.
So, how do I get these two things co-existing in my world? By massive death, of course. An impactor of sufficient size to effectively reset the surface would play the part nicely.
Looking up on the mass extinctions, the K-T Extinction Event is a little smaller than what I'd need - but to be fair, I'm not particularly looking at that currently.
What I'm looking for: Given an impactor of sufficient size to reduce surface life to no animal life larger than "Insect," what sea creatures would survive?
Granted, this is alien biology, but using terrestrial equivalents is fine. I'm mostly looking in to what sort of sea creatures I can expect my characters to encounter.