A shark with a structure on its back, such as an stethacanthid, could not have possibly been a fast swimmer. The first dorsal fin and spine could have produced a considerable amount of drag during fast locomotion. This suggests that Stethacanthids may have been rather sluggish bottom dwellers. The crowns of the dermal denticles on the first dorsal fin point forward and those on the head point backward, however it is unlikely that these were used for biting or tearing food. If the animal was disturbed by a potential predator while resting or feeding near the bottom it may have raised the head and tilted forward the first dorsal fin and spine, simulating a toothed open mouth of a much larger fish, therefore, an effective defence mechanism. However, only males possessed the "armoured" first dorsal fin and spine, and this suggests that the function was merely sexual display.
