Acanthodii

Acanthodii

Acanthodes, an example of an Acanthodian fish

The Acanthodii, commonly known as "spiny sharks," were a group of extinct jawed fishes that lived during the Silurian and Permian periods, approximately 440 to 250 million years ago. They were among the earliest known jawed vertebrates and played a significant role in the evolution of modern fish.

Taxonomy

The Acanthodii belong to the superclass Gnathostomata, which includes all jawed vertebrates. Within this superclass, they are classified under the class Acanthodii. The group is further divided into several orders, including Climatiiformes, Diplacanthiformes, and Ischnacanthiformes.

Morphology

Acanthodians were characterized by their unique spiny scales, which gave them their common name. These scales were composed of dentine and enamel, similar to the teeth of other vertebrates. The spines provided protection against predators and may have also played a role in intraspecific competition.

Another notable feature of Acanthodii was their paired fins, which were supported by a series of bony rays. These fins were likely used for maneuvering and stabilization in the water. Some Acanthodians also possessed a heterocercal tail, where the upper lobe of the tail fin was larger than the lower lobe, providing additional propulsion.

Ecology and Distribution

Acanthodians were primarily marine fishes, although some freshwater species have also been identified. They inhabited a wide range of environments, from shallow coastal waters to deeper offshore regions. Fossil evidence suggests that they were active predators, feeding on smaller fishes and invertebrates.

The distribution of Acanthodii was global, with fossils found on every continent. They were particularly abundant during the Devonian period, but their diversity declined during the Carboniferous and Permian periods, eventually leading to their extinction.

Importance in Evolution

Acanthodii played a crucial role in the evolution of modern fish. They were among the first jawed vertebrates to develop a more streamlined body shape, which allowed for faster swimming and improved predatory abilities. Their paired fins and heterocercal tail were also important innovations that influenced the development of later fish groups.

Furthermore, Acanthodii shared several characteristics with both cartilaginous and bony fishes, making them a key transitional group in vertebrate evolution. Their dentine and enamel scales resemble those of cartilaginous fishes, while their bony fin rays and other skeletal features are more similar to those of bony fishes.

See Also

• Gnathostomata
• Silurian period
• Permian period
• Devonian period
• Cartilaginous fish
• Bony fish