Ischnacanthiformes, Ischnacanthus gracilis
GRI #1298

Ischnacanthus had a long, spindle-shaped body, and probably was a fast swimmer. It grew to about 25 cm (Burrow et al., 2018). It had very small teeth on the lower jaw, two spines behind the head, and a spine in each of the fins except the caudal (tail) fin. Along with the shark-like body shape and heterocercal (asymmetrical) caudal fin with a longer upper lobe, the spines explain why acanthodians like Ischnacanthus are informally referred as “spiny sharks.”

Ischnacanthus had a pair of “lateral lines” composed of sensory organs that could detect motion in the water (Burrow, 2018). Hundreds of specimens, including this one from Forfarshire, are known from Lower Devonian sediments in Scotland (Paleobiology Database, n.d.).

Reference:

Burrow, C.J., Newman, M., Den Blaauwen, J., Jones, R. and Davidson, R., (2018) The early Devonian ischnacanthiform acanthodian Ischnacanthus gracilis (Egerton, 1861) from the Midland Valley of Scotland. Acta Geologica Polonica, 68(3), pp. 335-362. doi: 10.1515/agp-2018-0008.

Paleobiology Database (no date) Ischnacanthus, The Paleobiology Database. Available at: https://paleobiodb.org/classic/basicTaxonInfo?taxon_no=34865 (Accessed: August 2024).