Today this class is represented by sharks and rays. Fins arrangement is sophisticated and varies widely within class. Appear in the Devonian. At the beginning of the Paleozoic stromatolites are still the dominant plant form. They were widespread in the Cambrian becoming increasingly restricted after the Ordovician.
Some modern members exist today.
The earliest land plants were primitive, resembled ferns and remained close to the water. Plants reproduced by shedding gametes into the water and the earliest plants did not have well developed vascular tissues. They possessed more efficient vascular tissue, a stronger bark and support system. Leaves emerged directly from the trunk of the tree and they still reproduced by spores in the water. By Mississippian and Pennsylvanian time vast swampy lowlands were covered with these trees.