Editing Acanthodii (section)

==Characteristics==
[[File:Mesacanthus Parexus Ischnacanthus.JPG|thumb|left|Three acanthodians from the [[Early Devonian]] of [[Great Britain]]: ''[[Mesacanthus]]'' (an acanthodiform), ''[[Parexus]]'' (a "climatiiform"), and ''[[Ischnacanthus]]'' (an ischnacanthiform) ]]
[[File:Diplacanthus.jpg|thumb|Impression fossil of the [[Diplacanthidae|diplacanthid]] ''[[Rhadinacanthus|Rhadinacanthus longispinus]],'' at the [[Museum für Naturkunde]], [[Berlin]]]]
[[File:Acanthodi NT.jpg|thumb|Various acanthodians, from top left clockwise: ''[[Cheiracanthus]]'', ''[[Acanthodes]]'', ''[[Climatius]]'', ''[[Ischnacanthus]]'', ''[[Parexus]]'', ''[[Gyracanthus]]''. center: ''[[Diplacanthus]]''.]]

The [[scale (zoology)|scale]]s of Acanthodii have distinctive ornamentation peculiar to each order. Because of this, the scales are often used in determining relative age of sedimentary rock. The scales are tiny, with a bulbous base, a neck, and a flat or slightly curved diamond-shaped crown.

Despite being called "spiny sharks", acanthodians predate sharks. Scales that have been tentatively identified as belonging to acanthodians, or "shark-like fishes" have been found in various Ordovician strata, though, they are ambiguous, and may actually belong to jawless fishes such as [[thelodont]]s. The earliest unequivocal acanthodian fossils date from the beginning of the [[Silurian]] Period, some 50 million years before the first sharks appeared. Later, the acanthodians colonized fresh waters, and thrived in the rivers and lakes during the [[Devonian]] and in the [[coal swamp]]s of [[Carboniferous]]. By this time [[bony fish]]es were already showing their potential to dominate the waters of the world, and their competition proved too much for the spiny sharks, which died out in [[Permian]] times (approximately 250 million years ago).

Many palaeontologists originally considered the acanthodians close to the ancestors of the bony fishes.{{citation needed|date=November 2022}} Although their interior [[skeleton]]s were made of [[cartilage]], a bonelike material had developed in the skins of these fishes, in the form of closely fitting scales (see above){{clarify|date=November 2022}}. Some scales were greatly enlarged and formed a bony covering on top of the head and over the lower [[shoulder girdle]]{{citation needed|date=November 2022}}. Others developed a bony flap over the gill openings  analogous to the [[Operculum (fish)|operculum]] in later bony fishes{{citation needed|date=November 2022}}. However, most of these characteristics are considered homologous characteristics derived from common [[placoderm]] ancestors{{clarify|date=November 2022}}, and present also in basal [[cartilaginous fish]]{{citation needed|date=November 2022}}. Overall, the acanthodians' [[Fish jaw|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.{{citation needed|date=November 2022}}