The support for the impact theory comes from the fact that shallow marine organisms were hardest hit, with land plants not suffering to any great extent.
This would suggest that maybe a large meteor hit the ocean and caused a global tsunami that would disrupt marine life and destroy coastal habitats.
This theory would also explain why an impact crater has not been found for such an impact. Sea floor is generated and subducted at a fairly rapid pace in geologic terms. If such an impact did exist in the Devonian, then evidence of its existence would be long subducted into the mantle at the present time.
This extinction event would further increase the selective
advantage for organisms with the ability to remain on land for any substantial
period of time.