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I want this world to have two things, typically on the complete opposite end of geological timescales:

  1. 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.
  2. 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.

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  • $\begingroup$ Your post needs a few more details like, which theory on the KT mass extinction are applying, and if it is the one the giant space rocks well did it hit the ocean or land(both?). But the main organisms that will be surviving a land animal extinction, the deep sea dwellers are your best bet(especially the ones that can preform chemosynthesis) $\endgroup$ – Amoeba Mar 5 '18 at 1:58
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    $\begingroup$ Two comments: one, we don’t really know how life started and there’s no guarantee it’s always going to be an early event in geologic time, so it really doesn’t stretch suspension of disbelief to say that life simply didn’t start on your planet for X number of years. Two, making U235 rare requires not only an old Earth but also an old solar system, so you’ll want to be careful about the age and type of your sun as well. $\endgroup$ – Dubukay Mar 5 '18 at 3:08
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    $\begingroup$ @Dubukay exactly. There was a time you could get away with just pulling random ideas out of thin air, just because they sound cool, but not anymore. We know too much. $\endgroup$ – RonJohn Mar 5 '18 at 3:19
  • $\begingroup$ Have you looked into sharks? They've existed since the time of the dinosaurs (eg: pre extinction event) although I'm not sure exactly how long they've been around. Crocodiles as well. $\endgroup$ – Aify Mar 5 '18 at 6:08
  • $\begingroup$ @Dubukay Yep, It's something I'm aware of. I don't particularly know the implications of this, but that's outside of the scope of this question. Definitely something to ask later. $\endgroup$ – Andon Mar 5 '18 at 22:23
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K-T too puny, eh? Maybe you could lift principles from the Permian Extinction, or "Great Dying". https://en.wikipedia.org/wiki/Permian-Triassic_extinction_event#Changes_in_marine_ecosystems

It is the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It is the only known mass extinction of insects. Some 57% of all families and 83% of all genera became extinct. Because so much biodiversity was lost, the recovery of life on Earth took significantly longer than after any other extinction event, possibly up to 10 million years.

In this event, complex, mobile marine life did better - examples given are crabs and crustaceans. Bivalves made it through and did well though that smells like luck to me.

You can tweak the disaster according to your desired outcome.
Probably if the land creatures all go, air breathers in the water would go too - as was the case with the K-T. If the event causes patchy destruction in the water, mobile creatures would do better - when there is something like a red tide the big things can move fast and clear the area and the small things all die. If the event is some global stressor like anoxia or heat, size would not matter but durability might - perhaps worms or echinoderms could hunker down and ride things out at depth.

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  • $\begingroup$ "Studies in Bear Lake County near Paris, Idaho showed a quick and dynamic rebound in a marine ecosystem, illustrating the remarkable resilience of life" Oh, I like the sound of this, too! $\endgroup$ – Andon Mar 5 '18 at 22:21
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Of course, all complex of organisms that live around underwater volcanoes are the answer - they simply don't depend on our world, they have another one, with different source of energy.

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