Complex sentence: The most common plant fossil found is leafy shoots of the dawn redwood, "Metasequoia occidentalis", and the floating fern "Azolla" as mats of plants or as isolated fossils.
Split into simpler sentences: The most common plant fossil found is leafy shoots of the dawn redwood, "Metasequoia occidentalis". The floating fern "Azolla" is found as mats of plants or as isolated fossils.