Plants survived the event comparatively unscathed, they would not if Earth was unshielded. Marine organisms suffered HUGE losses, but the water would have shielded them, even if the magnetosphere collapsed.
Insects survived ALL other mass extinctions totally unscathed, but they suffered huge losses at the P-T event. Even if the magnetosphere collapsed: a) their exoskeleton should shield them somewhat, and b) many insects are subterreanean and/or build nests/hives, that would shield them.
Most likely, the event was volcanic poisoning.