The Relationship Debate

Today there is a hot debate over placoderm relationships with a number of new viewpoints being published. Some workers agree with Stensio that placoderms should be classified with sharks and rays (in a superclass called the Elasmobranchiomorphi). Others believe placoderms are closely related to the bony fishes or Osteichthyes. Yet others believe they occupy a more primitive position than either of these groups, at the base of the radiation of all jawed fishes, evolving well before sharks, acanthodians or bony fishes. Placoderms therefore sit on the evolutionary tree somewhere between sharks and true bony fishes.

New evidence, however, is emerging that favours the original hypothesis of Stensio that sharks and placoderms evolved from a common stock. Strongest evidence is that both groups have external clasping organs in males for internal fertilizaton. It has been suggested that external claspers were a specialized feature which evolved in sharks. Other features that suggest close affinity between sharks and placoderms is the presence of an eyestalk connecting the eyeball to the brain case, the structure of the pectoral and pelvic fins, and the similar shapes of the brain cases. Stensio compared the snout regions of both groups and determined that both have little cartilage rings around the nasal area called annular cartilages. The general shape of the bodies in predatory placoderms such as coccusteus is similar to sharks suggesting a similar swimming style and internal anatomy.