Acanthodii

Acanthodii is an extinct group of early jawed fishes, commonly called acanthodians, known from the Silurian to the Permian and often characterized by prominent fin spines.

Gnathostomata: Gnathostomata is the vertebrate clade comprising jawed vertebrates, including fishes and tetrapods. Acanthodii belongs among the earliest known branches of jawed vertebrate evolution.
Fin spine: A fin spine is a stiff, pointed skeletal support associated with a fish fin. Prominent spines are the most conspicuous feature in many acanthodian fossils.
Silurian: The Silurian is a geologic period from about 443.8 to 419.2 million years ago. The earliest acanthodian fossils appear in the Silurian record.
Acanthodes: Acanthodes is an extinct genus of acanthodian fishes known from well-preserved fossils. This familiar genus gives a concrete example of the group’s characteristic fin-spine anatomy.
Placodermi: Placoderms are an extinct group of armored jawed fishes that flourished during the Paleozoic. Placoderms lived alongside acanthodians and represent another major early jawed-fish group.
Paired fins: Paired fins are bilateral appendages, such as pectoral and pelvic fins, used by many fishes for maneuvering. Acanthodian fin arrangements provide evidence about early paired-fin evolution.
Devonian: The Devonian is a geologic period from about 419.2 to 358.9 million years ago, noted for major diversification of fishes. Acanthodians diversified during the period often called the Age of Fishes.
Climatius: Climatius is an extinct genus of small acanthodian fish from the Devonian. Its numerous fin spines exemplify the striking external armature associated with acanthodians.
Chondrichthyes: Chondrichthyes is the vertebrate class of cartilaginous fishes, including sharks, rays, and chimaeras. Acanthodians share evolutionary connections with the lineage that includes living cartilaginous fishes.
Fish scale: A fish scale is a dermal structure that protects the body and can preserve anatomical information in fossils. Acanthodian scales often survive where the rest of the skeleton does not.

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