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Swallow Whole (Ex): An eryops can try to swallow a grabbed opponent of up to two sizes smaller by making a successful grapple check. The swallowed creature takes x+x points of bludgeoning damage and x points of acid damage per round from the gizzard. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal x points of damage to the gizzard (AC x). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out.
A Large eryops's gizzard can hold x Small, x Tiny, or x Diminutive or smaller opponents.
Shall we tackle Paracyclotosaurus next? Or should we just mention it in the flavor text of eryops that it is the Large part of the advancement line?
Alternatively, we can cut the advancement past Medium from eryops, and keep paracyclotosaurus separate. It might ease the burden on DMs with the different swallow whole capacity.
I'll defer to the experts, but I wonder if there's any other ability that would be reasonable to add if we did it separately? (Something to distinguish it a bit.)