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Description
The fossil records of jawless vertebrates began when they evolved the capacity to form a bony skeleton. The early agnathans are commonly termed ostracoderms(shell skinned) because of their extensive exoskeleton, in the form of a solid carapace, large bony plates, or scales. The presence of a bony covering complicated growth in early vertebrates. One of the most important changes we see among the ostracoderms is the evolution of different ways in which an exoskeleton forms early in development to allow for continued growth. We may conclude from the distinctive nature of the armor that bone probably evolved separately in each of the major groups of ostracoderms. Some ostracoderms with a light flexible covering of small scales and a fusiform body may have lived in the open water. Others, with heavier armor and a dorsoventrally flattened body, were presumably benthonic. Bone was not present in the earliest vertebrates, but it evolved separately in many distinct lineages. Bony tissue may have evolved initially as a metabolic reserve for phosphate and calcium. It may have served to insulate the electrosensory organs in early vertebrates and later became elaborated to form a protective covering over the entire body. Early jawless fish, such as Astraspis desiderata(1), Sacabambaspis janvieri(2) and Arandaspis prionotolepis(3) most likely swam like tadpoles. They didn't have fins, which usually serve fish as a kind of stabilizers in water. The lower part of the body of jawless fish was covered with rows of tuberculate bone plates, and the head was protected by a heavy "shield" - perhaps it saved it from attacks by giant sea scorpions. They didn't have jaws or teeth, and they simply scooped or sucked out food particles from the thickness of the silt.

Description
The fossil records of jawless vertebrates began when they evolved the capacity to form a bony skeleton. The early agnathans are commonly termed ostracoderms(shell skinned) because of their extensive exoskeleton, in the form of a solid carapace, large bony plates, or scales. The presence of a bony covering complicated growth in early vertebrates. One of the most important changes we see among the ostracoderms is the evolution of different ways in which an exoskeleton forms early in development to allow for continued growth. We may conclude from the distinctive nature of the armor that bone probably evolved separately in each of the major groups of ostracoderms. Some ostracoderms with a light flexible covering of small scales and a fusiform body may have lived in the open water. Others, with heavier armor and a dorsoventrally flattened body, were presumably benthonic. Bone was not present in the earliest vertebrates, but it evolved separately in many distinct lineages. Bony tissue may have evolved initially as a metabolic reserve for phosphate and calcium. It may have served to insulate the electrosensory organs in early vertebrates and later became elaborated to form a protective covering over the entire body. Early jawless fish, such as Astraspis desiderata(1), Sacabambaspis janvieri(2) and Arandaspis prionotolepis(3) most likely swam like tadpoles. They didn't have fins, which usually serve fish as a kind of stabilizers in water. The lower part of the body of jawless fish was covered with rows of tuberculate bone plates, and the head was protected by a heavy "shield" - perhaps it saved it from attacks by giant sea scorpions. They didn't have jaws or teeth, and they simply scooped or sucked out food particles from the thickness of the silt.