The earliest unequivocal fossils of these creatures date from the beginning of the Silurian Period, some 50 million years before the first sharks appeared. For decades, paleontologists have marveled at the strange mosaic of features found in Acanthodii, an extinct class of jawed fishes that defied simple classification. They possessed the streamlined, shark-like body shape with a strongly upturned tail, yet their skin was covered in tiny rhomboid platelets similar to the scales of modern gars and bowfins. This unique combination of traits earned them the popular name spiny sharks, though they were not true sharks and their relationship to modern cartilaginous fish remains a subject of intense debate. Their skeleton was made of cartilage, but their fins were reinforced on the anterior margin with a dentine spine, creating a sturdy bony base that often preserved better than the soft tissue. As a result, the fossil record of these ancient swimmers is often reduced to isolated spines and scales found in ancient sedimentary rocks, leaving scientists to reconstruct their lives from fragments.
A Mosaic of Evolutionary Traits
Despite being called spiny sharks, acanthodians predate sharks and represent a critical link in the evolutionary history of jawed vertebrates. Many paleontologists originally considered the acanthodians close to the ancestors of the bony fishes, noting that a bonelike material had developed in their skins in the form of closely fitting scales. Some scales were greatly enlarged and formed a bony covering on top of the head and over the lower shoulder girdle, while others developed a bony flap over the gill openings analogous to the operculum in later bony fishes. However, most of these characteristics are now considered homologous characteristics derived from common placoderm ancestors, and present also in basal cartilaginous fish. Their jaws are presumed to have evolved from the first gill arch of some ancestral jawless fishes that had a gill skeleton made of pieces of jointed cartilage. This complex evolutionary history suggests that the split between bony fish and cartilaginous fish was not a simple divergence but a complex interplay of shared traits and independent adaptations.The Four Orders of Ancient Swimmers
Acanthodians have been divided into four distinct orders, each with its own unique ecological niche and morphological adaptations. The Climatiiformes were an early group characterized by robust bony shoulder girdles and many small sharp spines, known as intermediate or prepelvic spines, located between the pectoral and pelvic fins. The Diplacanthida subgroup was later elevated to its own order, Diplacanthiformes, and included species like Rhadinacanthus longispinus, whose impression fossils can be seen at the Museum für Naturkunde in Berlin. Ischnacanthiforms were formidable predators with tooth plates fused to their jaws, while Acanthodiforms were filter feeders with a single dorsal fin, toothless jaws, and long gill rakers. These filter feeders were the last and most specialized off the traditional acanthodians, as they survived up until the Permian period. The diversity of the group was consistently low but stable during the Carboniferous, slightly decreasing going into the Permian, with the youngest records of the group being isolated scales and fin spines from Middle-Late Permian strata in the Paraná Basin of Brazil.