These mostly small fish first appeared in the Silurian period. They were very diverse in the Devonian and disappeared at the end of the Permian Period. They can be distinguished from the other fishes by their bony, often ornamented spines supporting the fin webs and their very tiny, bony scales. Nowadays more and more evidence is found that they are very closely related to real sharks.
Some acanthodians have spines that were not supporting a fin web. They may have been there just to make them less attractive for predators.
The acanthodians have a shagreen of many tiny, bony scales, often no more than 0.5 mm in length. The outer surface of the scale (the scale crown) is sometimes flat but more often an ornamented structure which makes species identification possible. Also, due to the unique ornamentation of the scale crowns of these scales, they can be used to correlate between different sedimentary units in Scotland and even elsewhere in Europe. The histology (scales sliced very thin and looked under a microscope) of these scales also shows unique structures to identify individual species. As the individual acanthodian species become extinct quite quickly, they can be used to relatively date the rocks they are in as well.
Most of the acanthodians were probably filter feeders although examples are found having well developed jaws with teeth.
In Scotland recently several new acanthodian species have been published.
Two species been also identified from Orkney namely Cheiracanthus peachi (den Blaauwen et al. 2019) from Westray and Fallodentus davidsoni (Newman et al. 2021) from Rousay (in sediments together with Osteolepis panderi). Since we were only able to collect incomplete specimens from Orkney, we only mention these species here. The published description is mostly based on articulated specimens from Caithness.
New information on Mesacanthus peachi and Mesacanthus pusillus will be published in 2023 by Newman et al. with the new genus name Orcadacanthus replacing Mesacanthus.
Most acanthodians have rows of tiny rhombic scales, sometimes overlapping. The scale crown is often composed of mesodentine or dentine and shows an ornamentation that is species specific. The scales grown on all sides and in sections the growth zones often show well. See for more information Burrow 2021 Handbook of Paleoichthyology 2021.
Below are shown some examples of the morphology and histology of the different species listed above. We first show the scale in close up then on a scanning electron microscope picture and after that a histological thin section