In a post I had an idea about a giant crab creature. I thought it might be pretty awesome to have a similar creature to the trilobite that preys on Trilobites with a short agro range as to not kill off all the Trilobites, but to synthesize a different food chain. These crabs would primarily drop meat and pearls, semi-opposite to the trilos and will run backwards to get away from players but able to rab at you and deal minor damage if you attempt to use your fist. As you get deeper and deeper into the sea, these crabs get larger, much larger! To the point of being the best mobile platform carrier and highest carry weight in the game,but also the slowest. This would also be an easy way to build a sub-nautical base on its back and move from sea to land and vice versa. Or you could kill these gigantic monsters (as difficult as the Gigantosaurus) for a sizable amount of pearls, chitin, and Prime meat.
P.s. the pearls are due to all the Trilobites the crab ate as it grew up,the bigger it is, the more pearls it has lodged in its innards.