Sclerorhynchus atavus and the convergent evolution of rostrum-bearing chondrichthyans

The description of a partial but well preserved head of the sclerorhynchid batoid Sclerorhynchus atavus Woodward 1889 gave the first clear indication of the presence of a puzzling group of extinct rostrum-bearing rays that resembled both the Pristidae (rays) and the Pristophoridae (sharks). Despite recognizing similarities and differences to these extant groups, Woodward (1889, 1892) suggested that Sclerorhynchus be assigned to the Pristidae, although the rostra are very different. Woodward did note similarities of Sclerorhynchus rostrum saw-teeth to those of the Pristiophoridae, including the location of these along the margin of the rostrum, rather than in deep sockets, as seen along the pristid rostrum. In addition, the type specimen of Sclerorhynchus has very distinct saw-tooth denticles not only along the rostrum, but modified denticles along the sides of the head, as in the Pristiophoridae. The enlarged rostral denticles of Sclerorhynchus also appear to rotate into position, another feature seen in the pristiophorids but not in the pristids, and in other sclerorhynchids such as Libanopristis. Although individual fossil rostral tooth-like denticles had been earlier described, Woodward’s description of a rostrum and associated rostral tooth-like denticles meant that for the first time a fossil rostrum could be compared to living forms, highlighting the extreme variation in rostrum saw-tooth morphology among sharks and rays