Complex sentence: The "Paleodictyon nodosum" is the creature thought to produce a certain form of Paleodictyon burrows, which are found around mid-ocean ridge systems in the Pacific and Atlantic Oceans.
Split into simpler sentences: The "Paleodictyon nodosum" is the creature which is thought to produce a certain form of Paleodictyon burrows. They are found around mid-ocean ridge systems in both the Pacific and Atlantic Oceans.