Paraphyly Watch 1: Fossil Fish & Missing Links

There are many ways to say "Oh #!@*! My group is paraphyletic! The following is perhaps the most eloquent:

"These recent fossils [Palaeozoic ‘acanthodians’] started to make us question: are these a natural group or are we looking at a bunch of organisms closely related to the common ancestor of all jawed vertebrates?" says Brazeau. "It's tempting to put them all into one group; however, they might come from different groups but all look very similar." This, Brazeau adds, is a common problem" (Nature 2009, 457:234).

Paraphyly sure is a 'common problem'. But what has lead to this devastating discovery?

"Ptomacanthus is placed as a basal stem chondrichthyan, but this result should be viewed with caution. A large part of the acanthodians, including Acanthodes, form a cohesive monophyletic group on the osteichthyan stem. However, the position of Ptomacanthus is problematical" (Brazeau 2009:307).

It certainly is! According to cladistic and phenetic (Bayesian) analysis Palaeozoic ‘acanthodians’ are paraphyletic. Why is this problematic?

"Current conceptions of gnathostome phylogeny depict a rather simplistic arrangement of nominally monophyletic and, apparently, morphologically disparate groups. The emerging picture of acanthodian (and perhaps placoderm) paraphyly does not overturn a general consensus about gnathostome interrelationships. Instead, it populates the long, naked internal branches, revealing a much richer picture of character evolution in early gnathostomes" (Brazeau 2009:307).

What Brazeau had found is analogous to the platypus - an organism that has characteristics of two different groups, in this case, osteichthyans and chondrichthyans. Like the platypus, Brazeau (or at least the media) are tempted to state that a 'missing link' has been found, but to their surprise, this missing link (read 'ancestor') turns out to be nested within the chondrichthyans. Whoops! Not only is the missing link gone, but so too is the assumed monophyly of acanthodians. What to do?

What any systemtist should do - re-classify the osteichthyans and chondrichthyans in light of this new evidence. Brazeau is naive to suggest that this discovery will "...not overturn a general consensus about gnathostome interrelationships" If Ptomacanthus is more closely related to chondrichthyans then bang goes the acanthodians. They need to be reclassified along with the chondrichthyans. But rather than saying the obvious, Brazeau descends into evolutionary explanation "... populates the long, naked internal branches, revealing a much richer picture of character evolution in early gnathostomes". No it does not reveal anything other than that Ptomacanthus is a chondrichthyan and that acanthodians are paraphyletic! And this is exactly what the media has picked on:

"The study also suggests that some acanthodians are ancestors to all modern jawed vertebrates" (BBC Online, 19 January 2009).

This is false and misleading - the study shows quite the opposite.

I place Brazeau (2009) as the first Paraphyly Watch entry for 2009 (and the first in the race for the Pewter Leprechaun) for mis-using paraphyly. Rather than reclassifying the gnathasomes, Brazeau (2009) as alluded to a missing link (which admittedly could have been done without the cladistic and phenetic analyses).

References

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