acanthodians

Acanthodians, commonly known as spiny sharks, are an extinct group of early jawed vertebrates (gnathostomes) that lived during the Paleozoic era. They are among the earliest gnathostomes in the fossil record, appearing in the Silurian and reaching their greatest diversity in the Devonian before declining and disappearing by the end of the Paleozoic.

Morphology includes typically small to medium-sized bodies with elongate shapes, covered by rhomboid scales and a row of stout fin spines, especially on the anterior dorsal and paired fins. The skeleton was primarily cartilaginous, with some ossified elements, and their jaws bore denticles or small teeth that indicate an early mechanism for grasping prey.

Taxonomy and phylogeny: acanthodians were traditionally classified as a separate class, Acanthodii, but their exact evolutionary position is debated. Many studies regard them as early gnathostomes, possibly forming a stem group to the lineages leading to both cartilaginous and bony fishes; as a result, acanthodian relationships are often treated as paraphyletic in modern classifications.

Fossil record and ecology: fossils are found in diverse regions and represent both marine and freshwater environments. The group includes genera such as Acanthodes, Diplacanthus, and Mesacanthus, illustrating considerable diversity in form and ecology. Acanthodians are important for understanding the early evolution of jawed vertebrates and the development of paired fins and jaws.