The enrichment of living cells with fast-acting substances translates into increased regeneration of battle-damaged tissue. If a creature survives after taking damage, intensive regeneration allows it to restore some of the damaged tissues almost instantly, thus reducing the damage taken. Rapid regeneration is one of the hardest things to evolve, so only 1.5% reduction in armour damage can be achieved with each level.
Regeneration evolution also allows you to build up carapace shields faster, speeding up their regeneration per round by 0.5% each level. Also increases the structure regeneration rate of Dunkleosteus by 10% per level multiplicatively.