They are currently considered to represent a paraphyletic grade of various fish lineages basal to extant Chondrichthyes, which includes living sharks, rays, and chimaeras.

In general body shape, they were similar to modern sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteians (gars, bowfins).

Acanthodians had a cartilaginous skeleton, but their fins had a wide, bony base and were reinforced on their anterior margin with a dentine spine.

The earliest unequivocal acanthodian fossils date from the beginning of the Silurian Period, some 50 million years before the first sharks appeared.

Some scales were greatly enlarged and formed a bony covering on top of the head and over the lower shoulder girdle[citation needed].

[2] A 2017 study of Doliodus morphology points out that it appears to display a mosaic of shark and acanthodian features, making it a transitional fossil and further reinforcing this idea.

[5] Phylogeny after[6] Galeaspida Osteostraci "Placodermi" Osteichthyes Tetanopsyrus Nerepisacanthus Ischnacanthus Poracanthodes Culmacanthus Uraniacanthus Diplacanthus Rhadinacanthus Cassidiceps Mesacanthus Lodeacanthus Triazeugacanthus Promesacanthus Acanthodes Cheiracanthus Homalacanthus Euthacanthus Ptomacanthus Brachyacanthus Climatius Parexus Vernicomacanthus Lupopsyrus Obtusacanthus Kathemacanthus Brochoadmones Gyracanthides Chondrichthyes (conventionally defined) The oldest remains attributed acanthodian-grade chondrichthyans are Fanjingshania and Qianodus from the Early Silurian of China, dating to around 439 million years ago.

[9] The youngest records of the group are isolated scales and fin spines from Middle-Late Permian strata in the Paraná Basin of Brazil.