acanthodiform

Acanthodiform is an extinct order of bony fish that lived from the Silurian to the Permian periods. Acanthodians, often referred to as "spiny sharks," are characterized by the presence of prominent spines in front of their dorsal, pectoral, and pelvic fins, as well as often a caudal fin spine. These spines were typically covered in dentine, and their exact function is debated, with theories including support for fins, defense, or even steering.

The body shape of acanthodiforms varied, with some being elongate and streamlined, while others were more robust. Their jaws were equipped with small, sharp teeth, or in some cases, toothless with bony plates. The earliest acanthodiforms were relatively small, but some later forms grew to considerable sizes. They inhabited both freshwater and marine environments, and their fossil record is widespread across the globe.

Acanthodiforms are considered an early and diverse group of jawed vertebrates. Their phylogenetic position has been a subject of much discussion, but they are now generally accepted as a paraphyletic group within the gnathostomes, meaning they represent an ancestral lineage that gave rise to other groups, rather than a distinct monophyletic clade that includes all their descendants. They are important for understanding the early evolution of fish, particularly the development of jaws, paired fins, and skeletal structures. The extinction of acanthodiforms coincided with the end-Permian mass extinction event, though their decline likely began earlier as they faced competition from the increasingly diverse ray-finned and lobe-finned fish.