Complex sentence: Then, around 520 mya, the archaeocyathids went into a sharp decline, with almost all species becoming extinct by the middle Cambrian, with the final-known species disappearing just prior to the end of the Cambrian period.
Split into simpler sentences: Then, around 520 mya, the archaeocyaths went into a sharp decline. Almost all species became extinct by the Middle Cambrian, with the final-known species disappearing just prior to the end of the Cambrian period.