Spiny sharks were not sharks, and came first
For generations textbooks called acanthodians spiny sharks: sleek bodies, paired fins, and stiff spines ahead of every fin but the tail. Yet they are not true sharks. Unequivocal fossils appear in the early Silurian, tens of millions of years before sharks, and modern work treats them as a grade basal to living cartilaginous fishes.
Acanthodii were jawed fishes whose bodies mixed traits of bony and cartilaginous lineages. Skin carried tiny rhomboid platelets reminiscent of gar and bowfin scales, while the skeleton stayed largely cartilaginous and fins rode wide bony bases edged with dentine spines. Those spines and scales often outlast soft anatomy in the rock record. Four ordinal names—Acanthodiformes, Climatiiformes, Diplacanthiformes, Ischnacanthiformes—sort habits from filter feeding to jaw tooth plates, though Climatiiformes is itself a messy early assemblage. Scale ornament differs by order enough that micropaleontologists use it to age sedimentary layers.
Despite the nickname, acanthodians predate sharks. Ambiguous Ordovician scraps are debated; clear acanthodian fossils begin near the start of the Silurian, roughly fifty million years before the first sharks. Later they entered fresh water and thrived in Devonian rivers and Carboniferous coal swamps. Older textbooks often parked them near the ancestry of bony fishes because skin bone, enlarged head plates, and gill covers looked osteichthyan. Shared deep vertebrate features now look like inheritance from early placoderm-grade stock rather than a direct bony-fish pedigree.
Phylogeny keeps shifting. Davis and colleagues in 2012 split acanthodians across Osteichthyes and Chondrichthyes, even placing famous Acanthodes among bony fishes despite a shark-like braincase. The Silurian placoderm Entelognathus, with jaws resembling bony fish and tetrapods, helped reframe acanthodians as a paraphyletic grade leading toward cartilaginous fishes. Burrow and coauthors in 2016 nested living chondrichthyans among acanthodians, near Doliodus and Tamiobatis; Doliodus itself looks transitional. The oldest named acanthodian-grade chondrichthyans, Fanjingshania and Qianodus from Early Silurian China, date to about four hundred thirty-nine million years ago, with diversity peaking in the Early Devonian Lochkovian.
Source: Acanthodii
