What caused the mass extinction of placoderms at the end of the Devonian?
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I mean, based on the extinction levels, an easier way to look at it might be to identify which characteristics helped some clades survive. The other organisms by definition will be lacking in enough of those traits to survive the mass extinction.
1. Plants colonize land and forests begin to spread
2. Decomposition and runoff from those forests overfertilizes the ocean
3. Microorganisms eat up all the extra nutrients and die
4. Bacteria decompose those dead microorganisms and reproduce out of control, consuming oxygen and leaving the ocean hypoxic
5. Placoderms die off because there isn’t enough oxygen in the water for them to breathe
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