Where do spiny sharks go?

Hi and welcome to fish fortnight on Dracovenator. You may notice a slight change in style here, as a new year's resolution I'm going to try to write for a more general audience. Does this mean I'm dumbing Dracovenator down? No, I'm just going to try to stop assuming a lot of specialist knowledge on a part of my readers, and will throw in some more basic anatomy for the Form and Function students I'll be teaching later this year.
Anyway by pure coincidence a bunch of fish-related items have come up all at once so I'm running a kind of fish-festival over the course of the next two weeks.
The first of these is a new paper in Nature this week by Martin Brazeau, which desribes the braincase and jaws of an acanthodian (popularly called spiny sharks, although they are not sharks, at least not in the conventional sense). Why is this a big deal? I'll get to that, but first some background.
Most modern vertebrate animals have jaws (only lampreys and hagfish do not). Those that have jaws belong to the great clade Gnathostomata (literally 'jaw-mouths'). Modern gnathostomes can be further divided into two clades: The cartilaginous fish (Chondrichthyes) including sharks and rays and the bony vertebrates (Osteichthyes)including everything else from goldfish to humans.
Both groups are monophyletic, that is they include all descendants from a common ancestor, thus neither group was ancestral to the other, both having split from a common jawed ancestor somewhere back in the past (probably in the Late Ordovician or Early Silurian Period).
Now there are some fossil jawed vertebrates that don't fit into either of these two clades. These are traditionally placed into two groups: The placoderms (armoured fish) and the acanthodians (so-called spiny sharks).



Dunkleosteus, a giant placoderm. Painting by Charles R. Knight. From universe-review.ca

How do these groups relate to the surviving Chondrichthyes and Osteichthyes, or in other words, what is the shape of the first part of the evolutionary tree of jawed vertebrates?
That has been a big mystery indeed. Just about every possible arrangement has been suggested. Placoderms are either regarded as the sister group of all other gnathostomes (that is they branched off first and the others all share a more recent common ancestor) or are thought to share a closer relationship with Chondrichthyes than with Osteichthyes. Nevertheless the plates of dermal bone which encase the front end of these fish has been thought to be an evolutionary novelty (synapomorphy in technical parlance) which marks out the placoderms as a monophyletic group.
The acanthodians have proven even more difficult to place, partly because the internal skeletal anatomy of most species has not been preserved (because like hagfish, lampreys and sharks it was cartilaginous). The favoured hypothesis of more recent years was that acanthodians shared a close relationship with Osteichthyes, that is they split from the line leading to bony vertebrates after the placoderms and chondrichthyans had already split and started their own evolutionary journeys. This was the position taken by Phillippe Janvier in his book 'Early Vertebrates' (1996, Oxford University Press)- an excellent book by the way. Once again the group had been thought to form a clade largely on the characteristic of bony spines supporting the leading edge of their paired fins and some details of their scales. Recent discoveries have shown that both of these features are probably general features for early gnathostomes that were lost in the modern groups, rather than evolutionary novelties unique to acanthodians. It should be noted that even back in 1996, Janvier noted that acanthodian monophyly wasn't all that strong and presented the suggestion that some acanthodians might turn out to be more closely related to chondricthyans (what we would call stem group chondrichthyans) while others might be stem group osteichthyans.



A diversity of acanthodians by Stanton Fink. Image from wikipedia-commons.

Enter Brazeau's recent paper. It includes a cladistic analysis of early jawed vertebrate phylogeny (the branching pattern of evolution) that uses individual genera, rather than large monophyletic groups. Lo and Behold, Janvier was right, or at least on the right track. Indeed Brazeau's analysis finds former acanthodians falling out all over the tree. Some are stem-group gnathostomes that branched away before modern gnathostomes split into Chondrichthyes and Osteichthyes, others are stem-group chondrichthyans others are stem-group osteichthyans. In particular the former acanthodian that forms the centrepiece of the paper, Ptomacanthus, is found in some of his trees to be the closest sister group to the gnathostome crown-group (the crown-group is the clade including the extant species and all descendants of their most recent common ancestor - in this case the common ancestor of Chondrichthyes and Osteichthyes). Ptomacanthus is special because unlike most other acanthodians, including all the early, more relevant species, does preserve some of its internal skeleton, namely the braincase and the jaws. The braincase is remarkably primitive, actually looking more like that of a placoderm than a crown-group gnathostome. This unusual combination of characters (externally crown-group like but internally more primitive) helps to break up the acanthodian group and gives this new tree. There are other odd things in this tree, for instance placoderms are not found to be monophyletic either but that is a topic for a later post (and further investigation methinks).

UPDATE: Check out the ever-informative Catalogue of Organisms for a post that slipped past me before for a little bit more about acanthodians.

Martin D. Brazeau (2009). The braincase and jaws of a Devonian ‘acanthodian’ and modern gnathostome origins Nature, 457 (7227), 305-308 DOI: 10.1038/nature07436