Shenacanthus is a genus of chondrichthyan from the Huixingshao Formation of China. It contains one species, Shenacanthus vermiformis. It is one of the earliest known Gnathostomata, at roughly 435 million years old.
Description
Shenacanthus is roughly long, with a body shape like most other jawed fish. It shares many characteristics with chondrichthyans, specifically the posteriorly retracted gills, a lack of dermal bones around the jaw and small diamond-shaped scales. In addition, the lack of teeth and jaw plates, alongside the spineless paired fins (alongside a spineless putative anal fin), are similar to Early Devonian stem-chondrichthyans like
Paucicanthus. Its pectoral region is unique for having relatively
placoderm-like armour, with both lateral and ventral plates, alongside a large teardrop-shaped dorsal plate with worm-like ornaments. This large dorsal plate is preceded by a smaller plate towards the head, and a series of linearly ornamented scutes down the body interrupted by a dorsal spine. In phylogeny,
Shenacanthus is consistently placed next to
Paucicanthus and
Doliodus near the top of the chondrichthyan stem. However, this placement is suspect due to the presence of placoderm-like dorsal armour.
Etymology
Shenacanthus's genus name honours
Congwen Shen, a distinguished writer who located their most famous story
Border Town near the fossil site. The genus name also includes the Greek word
acanthus, a common suffix for stem-chondrichthyan names meaning "spine". The species name
vermiformis means "worm-like" in reference to the shape of the ornament on its dorsal plates.
