Ostracoderm

Ostracoderms are extinct armored jawless vertebrates, chiefly known from Paleozoic fossils. The name groups several early vertebrate lineages rather than a single natural evolutionary group.

Vertebrate: An animal with a backbone or an embryonic structure that develops into one. Ostracoderms belong among the earliest known vertebrates.
Pteraspidomorphi: An extinct group of armored jawless vertebrates that includes pteraspid and heterostracan forms. Its members exemplify the large head shields found in many ostracoderms.
Cambrian period: A geologic period from about 539 to 485 million years ago, marked by major diversification of animal life. The earliest vertebrate fossils predate the main Paleozoic record of ostracoderms.
Conodont: An extinct group of eel-shaped animals known chiefly from microscopic tooth-like elements. Conodonts offer a different fossil record of early vertebrates and their feeding structures.
Jawless fish: A broad grouping of fishes that lack hinged jaws. Ostracoderms are fossil members of this broad body-plan grouping.
Thelodonti: An extinct group of jawless vertebrates covered with small tooth-like scales called denticles. Their scale-covered bodies contrast with heavily plated ostracoderm forms.
Ordovician period: A geologic period from about 485 to 444 million years ago, when marine life diversified extensively. Ostracoderm-like vertebrates appear in the fossil record during this period.
Placoderm: An extinct group of armored jawed fishes that flourished chiefly during the Devonian Period. Placoderms combined armor with jaws, unlike ostracoderms.
Dermal bone: Bone that forms within the skin rather than replacing a cartilage or connective-tissue precursor. Many ostracoderms are recognized by extensive dermal bony armor.
Osteostraci: An extinct group of jawless vertebrates with a bony head shield and paired pectoral fins. Their fins and sensory structures provide clues to ostracoderm locomotion and anatomy.

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