But I digress.
At the proper scale, all vertebrates are fishes, and what a varied bunch they are. There are the jawless fishes or
A bony fish specialized for living in water of negative depth. Recently.
There are, however, two other major groups of fishes that are now completely extinct, but in whose understanding might allow the unlocking of many enigmas - many, indeed, conundramatic riddles. These are the Acanthodians, or 'spiny sharks', mainly small, scaly, spiny fishes whose internal skeletons are rarely preserved, but are known mainly from smears of scales and spiny bits that look like the parts of a fish that the cat would have thrown up as indigestible. Acanthodians are known from the Silurian right up to the end of the Permian. Acanthodes, one of the best known acanthodians, is a Permian form, one of the last of its kind, and not necessarily representative of the group's much earlier heyday. The wider relationships of acanthodians has been a matter of controversy, with opinion shifting between allegiances with sharks or bony fishes.
Then there are the Placoderms, another large and very diverse group that emerged in the Silurian but disappeared at the end of the Devonian. These were often heavily armoured, with robust head- and trunk armour, and look superficially like some of the various jawless ostracoderms, for all that they have jaws. But the pattern and styling of the armour of placoderms is so unique that it has been hard to relate them to either bony fishes or sharks. Some placoderms were tiddlers. Others were, as we scientists say, mean meat-eatin' muthas.
Underlying all this is the vast morphological gulf that separates jawless vertebrates from the various kinds with jaws, or Gnathostomes. It's not just a question of the evolution of jaws (not a trivial restructuring in any case). The evolution of jaws was accompanied by wholesale changes in many other parts of the anatomy, from the wiring of the brain to the appearance of paired limbs. What one should like to do is get some idea of how the various features that make up a gnathostome were acquired, and which order. Without that, we are left with a vast evolutionary jump, from
time, only we'd failed to recognise them. They are -- ba boom, tish -- the acanthodians and the placoderms.
Most people have become accustomed to thinking of placoderms and acanthodians as 'natural' groups. This is why there has been so much debate about whether acanthodians are closer to sharks or bony fishes. The question is unanswerable, unless one makes an intellectual leap and challenges the assumption that acanthodians are all one, natural group, like - say - sharks, or bananas. However, there is an alternative, in which acanthodians are less a natural group than a grade of evolution in which some acanthodians are closer to sharks, others are closer to bony fishes, and still others are offshoots from a time when these two lineages were as yet indistinct. This revelation has come about thanks to recent work on new and extremely rare material of acanthodian braincases. This insight is crucial as for the first time we can get an idea of what the latest common ancestor of sharks and bony fishes looked like - and it would have looked very much like an acanthodian.
But what of placoderms? Once one breaks this group up into its many constituent parts, the group starts to arrange itself as a series of grades between agnathans and gnathostomes. Of the two major placoderm groups, for example, the antiarchs look much more primitive and ostracoderm-like, whereas the arthrodires seem much more gnathostome-like.
This is, however, only the beginning of what promises to be an exciting if somewhat tricky path of research. For once one breaks up the monophyly (natural-group-ness) of acanthodians and placoderms, many things become possible - and many new problems surface. Crucially, placoderms of any kind look very different from acanthodians, so which group of placoderm is most closely related to which group of acanthodian? Did placoderms evolve into acanthodians? At present this seems as likely as girrafes evolving from unicycles. This is a circle whose squaring will come in the next few years of research into an old problem dramatically renewed, simply by breaking down a couple of cherished assumptions.