The Tiny Spiny Shark
Mesacanthus pusillus is a small fish with spines located on the leading edge of all of its fins, with exception of the tail. It has a single dorsal fin, an anal fin, and pectoral and pelvic fins, and located between the latter two it has a pre-pelvic fin. The name Mesacanthus derives from the spine of the pre-pelvic fin, meaning “middle spine”. These spines served as a deterrent against larger predatory fishes. Most organisms found are between two and five centimeters in lenght.
Age and Distribution
Mesacanthus pusillus is from the Middle Devonian of Scotland. This fish lived approximately 385 million years ago. At the time this area of Scotland was part of the Orcadian Basin, which was a huge complex of lakes that stretched for hundreds of miles. During this period plants started to rapidly colonize the land and fishes dominated nearly every niche of seas and lakes, hence why this period is also known as the “Age of Fishes”. Mesacanthus pusillus co-inhabited the lake with other fishes like the acanthodian Diplacanthus, the osteolepids Gyroptychius, Osteolepis, and Thursius, the lungfish Dipterus, the porolepid Glyptolepis, the enigmatic Paleospondylus gunni, and the placoderms Dickosteus, Coccosteus, Pterichtyodes, Millerosteus, Rhamphodopsis and Homostius.
The Acanthodii are one of the earliest known jawed vertebrates. The oldest acanthodians found are from the Lower-Silurian. This group disappears from the known fossil record at the Permian-Triassic extinction event, long after they had already dwindled in diversity.
Mesacanthus pusillus belongs to the Mesacanthidae which is a subclass of the Acanthodii (‘spiny sharks’, which means that, though they also had a skin cover of denticles, it is uncertain that they are closely related to sharks). There are no other known Mesacanthidae from Middle Devonian Scotland, but the genus is represented in the Lower Devonian of Scotland with Mesacanthus mitchelli.