Several authors have argued for the Urochordata to be the sister taxon for the Vertebrata. Early vertebrates possibly originated from a tunicate-like larva that became sexually mature without metamorphosis to the adult body plan through a process called paedomorphosis . Paedomorphosis is divided into two processes Neoteny , which is the retention of a few larval characteristics into the adult stage Progenesis , which is the development of sexually mature gonads in the larval stage. The structure of the tooth-like fossils and the impressions of conodont fossils indicates that these organisms were early vertebrates. Appearance of Bone Primary component of vertebrate bone is calcium phosphate .
Calcium carbonate is the most common mineral in invertebrates but some do have calcium phosphate.
Extensive deposits of bone first appeared in a group of animals called ostracoderms , jawless and without movable paired fins, the head region was encased in a bony dermal armor. The bony armor of the ostracoderm may have been composed of many odontodes that fused into a solid piece of bone. Wether the very earliest vertebrates had bone or if it arose later in the evolution of vertebrates can not be determined at this time. Bone's protective function is probably the most obvious but in addition, it serves as an important means for the storage of calcium and phosphate and may have served as an insulating mechanism for electroreception. Marine origin All non-vertebrate and deuterostome taxa are marine.