Mesacanthus body fossils have pectoral, pelvic, anal and dorsal fin spines, as well as a pair of prepelvic spines, which are intermediate between the pectoral and pelvic fin spines. They also have small, unornamented, diamond shaped scales.
The fossil record of Mesacanthus reveals the presence of pectoral, pelvic, anal, and dorsal fin spines, as well as a pair of intermediate prepelvic spines situated between the pectoral and pelvic fins.  They are also characterized by small, plain, diamond-shaped scales.