The first organisms that reproduced sexually were free-floating plankton.

最早的有性繁殖生物是自由漂浮的浮游生物。

《柯林斯英汉双解大词典》

But the analysis by Kruta and her colleagues indicates their ammonites would have dined on small organisms floating in the water, such as zooplankton, tiny crustaceans, and even other petite ammonites.

BBC: Ammonite diet revealed in X-rays