Sphenocephalus was about 20 centimetres (8 in) long, with a rather large head, and may have resembled a modern black bass in appearance. It was one of the earliest fish to have the pelvic fins placed beneath the pectoral fins, a common feature in modern fish that improves swimming manoeuvrability. Like the modern trout-perches, it possessed a mixture of modern and primitive features, and it was probably one of the earliest perciform fish.