Acanthodiformes

Acanthodiformes, commonly known as spiny sharks or acanthodians, were a group of extinct armored fish that thrived during the Paleozoic Era, from the Late Silurian to the Late Permian periods, approximately 420 to 250 million years ago. They are notable for their distinctive skeletal structure, which included a combination of bony plates and spines, distinguishing them from both modern sharks and bony fish.

Acanthodians possessed a robust, elongated body covered in overlapping bony scales called cosmoid scales, which provided protection against predators. Their most striking feature was the presence of large, sharp spines located before their dorsal and anal fins, which may have served both as defensive weapons and as hydrodynamic aids. Their jaws were equipped with multiple rows of replaceable teeth, a trait shared with modern sharks, though their teeth were more varied in shape and less specialized.

Fossil evidence suggests that acanthodians were active predators, inhabiting a range of aquatic environments from shallow coastal waters to deeper marine settings. They exhibited diverse body forms, with some species resembling modern sharks in appearance, while others had elongated, eel-like bodies. Their diet likely consisted of smaller fish, invertebrates, and possibly even other acanthodians.

The decline of acanthodians coincided with the rise of more advanced fish groups, such as the placoderms and early bony fish, during the Devonian period. By the Permian, they had largely disappeared, possibly due to competition, environmental changes, or the evolution of more efficient predators. Despite their extinction, acanthodians played a significant role in the evolutionary history of vertebrates, contributing to the development of key adaptations that later influenced the evolution of modern fish and possibly early tetrapods.

Fossil remains of acanthodians have been discovered worldwide, with notable finds in Europe, North America, and Asia. Their study provides valuable insights into the early diversification of jawed vertebrates and the ecological dynamics of Paleozoic marine ecosystems.