Most living animal species belong to the infrakingdom Bilateria, a highly proliferative clade whose members have a bilaterally symmetric body plan. Extant bilaterians include the basal group Xenacoelomorpha, but the vast majority belong to two large superphyla: the protostomes, which include phyla such as arthropods, molluscs, flatworms, annelids and nematodes, etc.; and the deuterostomes, which include the three phyla echinoderms, hemichordates and chordates, the latter with the vertebrates being its most successful subphylum. Precambrian life forms interpreted as early complex animals were already present in the Ediacaran biota of the late Proterozoic, but fossils of primitive sponge and other speculative early animals have been dated to as early as the Tonian period. Nearly all modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 539 million years ago (Mya), and most classes during the Ordovician radiation 485.4 Mya. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 Mya during the Cryogenian period.
