Basal vertebrates are the oldest and earliest vertebrates, including stem-group vertebrates. The evolution of basal vertebrates is still a mystery, but some possible theories exist. Tunicates are the closest living relatives of vertebrates, but not true vertebrates. Basal vertebrates quickly became a successful group, dominating the seas and leading to the evolution of large and sophisticated land animals.
A “basal” vertebrate means a vertebrate at the base of the vertebrate family tree, either one of the vertebrates from which all others evolved or a close cousin of the same. Some basal vertebrates are stem-group vertebrates, meaning they are older than the first common ancestor of all living vertebrate species.
Vertebrates, like all other animal phyla, are extremely ancient. The earliest basal vertebrate fossil is probably Yunnanozoon lividum, which lived 530 million years ago in what is now China, although some researchers disagree, calling the species a hemichordate rather than a true chordate (vertebrate). Another basal vertebrate, Pikaia gracilens, resembles a modern lancet. A third basal vertebrate, Myllokunmingia, is a primitive fish and an early vertebrate. Depending on the article, web page, or document, each is sometimes referred to as the oldest vertebrate.
It is a mystery how basal vertebrates initially evolved. There is general agreement that a spine is useful for swimming, giving the muscles something to pull against, producing the elegant ‘S’ shaped locomotion we commonly associate with fish. The first vertebrate may have been a sediment-dwelling animal that evolved a flatter body for swimming, a mobile larva of some type that began to retain its larval characteristics into adulthood, or a stationary filter feeder that evolved in a free-swimming animal. We don’t know for sure, because the fossil record is far from perfect.
According to molecular studies, tunicates, also known as sea squirts, are the closest living relatives of vertebrates. Tunicates are considered chordates, but not true vertebrates, because they have a notochord during their early developmental stage. Other relatives of vertebrates include the hemichordates, which include animals such as the arrow worm, and the echinoderms, which include starfish and relatives.
Whatever the cause of basal vertebrate evolution, they became a very successful group relatively quickly. From the Paleozoic onwards, fish, the most numerous vertebrates, largely dominated the seas. For the evolution of large and sophisticated land animals, a backbone is a must. Invertebrate terrestrial animals, such as snails and insects, do not have the tendency to grow very large or to evolve very complex brains.
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