Phylum. The classification system goes: Kingdom, Phylum, Class, Order, Family, Genus, Species. Anything that shares a class also share anything above it.
I believe its is because the fossils of these animals looked like living species and they had a connection to the modern. A fossil is any preserved evidence of life from a past geological age, such as the impressions and remains of organisms embedded in stratified rocks. Fossils are created when organisms die, are in cased in dirt and rock, and are slowly replaced by minerals over time.
