Another Tetrapod Ancestor Claimed

Maybe the Aussies want their share of missing link notoriety; an unusual fish with bony fins has been discovered in western Australia, reported in Nature.¹  The bigger the splash a missing link makes for reporters, the better.  The story on Science Daily said, “A fossil fish discovered in the West Australian Kimberley has been identified as the missing clue in vertebrate evolution, rewriting a century-old theory on how the first land animals evolved.”  The discoverers named it Gogonasus after the Gogo Station near where it was found.  They claim this little fossil fish, claimed to be over 380 million years old, is “the ultimate ‘Mother’ of all tetrapods.”
    OK, so what is special about this fish, compared to other alleged tetrapod ancestors?  Science Daily wrote,

The fossil skeleton shows the fish’s skull had large holes for breathing through the top of the head but importantly also had muscular front fins with a well-formed humerus, ulna and radius – the same bones are found in the human arm.

Actually, no baseball pitcher could operate with a borrowed Gogonasus arm, but this means that the structure and arrangement of the bones (i.e., one upper-arm bone and two lower-arm bones) was established early on in the fossil record.  Moreover, this “proves that features of land-living tetrapods (four-legged vertebrates) evolved much earlier in their evolutionary history than previously thought,” according to team member Erich Fitzgerald.  They think this fish lived at a pivotal time for all subsequent evolution, “from dinosaurs, to kangaroos, and ultimately, us humans.”
    One problem is that, till now, scientists thought tetrapods evolved in the northern hemisphere.  Tiktaalik, you recall, was found in the arctic (04/06/2006).
    The actual paper gets into some messy details that complicate the simple missing-link angle.  “Unexpectedly, Gogonasus shows a mosaic of plesiomorphic and derived tetrapod-like features,” Long et al wrote.  Plesiomorphic invokes the notion of a generalized similarity, where derived hints at an ancestral lineage.  Where do they decide to put it in the tree along with other alleged missing link candidates?

Whereas the braincase and dermal cranial skeleton exhibit generalized morphologies with respect to Eusthenopteron or Panderichthys, taxa that are traditionally considered to be phyletically close to tetrapods, the presence of a deeply invaginated, wide spiracle, advanced internal spiracular architecture and near-horizontal hyomandibula are specialized features that are absent from Eusthenopteron.  Furthermore, the pectoral fin skeleton of Gogonasus shares several features with that of Tiktaalik, the most tetrapod-like fish.  A new phylogenetic analysis places Gogonasus crownward of Eusthenopteron as the sister taxon to the Elpistostegalia.  Aspects of the basic tetrapod limb skeleton and middle ear architecture can now be traced further back within the tetrapodomorph radiation.

Part of the problem is that they want this fish to represent an earlier contender for a tetrapod-wannabee yet it shares some similarities to the later Tiktaalik.  Wherever it fits, there’s going to be some ’splainin’ to do:

The conspicuously large spiracular opening (Fig. 1a-c) is proportionally similar to those recently reconstructed for Panderichthys and Tiktaalik.  The pectoral fin endoskeleton of Gogonasus is described here for the first time (Fig.  2), the new specimen being the only known Devonian fish that shows a complete acid-prepared pectoral limb.  There are some surprising similarities to the recently described pectoral fin in the advanced elpistostegalian Tiktaalik.  As such features could indicate homoplasy between Gogonasus and early tetrapods, we present a revised character analysis to determine whether the new anatomical information supports a more crownward position for Gogonasus in the stem-tetrapod phylogeny.

In other words, they invoke the old Darwinian explanation of convergent evolution (homoplasy) to explain why this early fish would have similar structures to a later one.  For example, in the spiracle, “No previously described tetrapodomorph fish shows such a large spiracular opening, or a downward facing dermal lamina forming a posterior wall to the spiracular chamber, so the condition in Gogonasus is highly unusual,” they wrote.  How to explain it?  “This indicates that spiracular breathing might have evolved independently in some stem tetrapodomorphs.”  Yet spiracular breathing is no simple single-mutation change.  It would have involved multiple adaptations involving soft parts as well as bone—making independent convergence on the same pattern highly improbable.  Another problem is that if this specimen is a perfect intermediate between two other candidates in terms of the angle of the spiracle,² what’s it doing down under when the other fossils are up yonder?
    Getting into the fin bones, the authors state that various interpretations are possible.  “Such features can be interpreted as either generalized (plesiomorphic) for Gogonasus and elpistostegalians, or shared apomorphies that unite them, and as such would exclude the rhizodontids and tristichopterids from the higher clade.”  Indeed, their phylogenetic diagram (figure 3) shows two very different possible trees.  Nothing in the paper suggests that there is any certainty to their favorite solution.  There are plenty of “may have” and “might have” qualifiers in the text, and even their proposal overturns previous beliefs and raises new questions.³  They can only speculate about what environment any of the creatures lived in, and how different forms arrived at different parts of the globe.
    One other thing.  Whatever happened, happened quickly.  Based on the assumed dates of these bones, and the scatter of different specimens from China to Europe, from the arctic to Australia, “indicates that the initial radiation of tetrapods from elpistostegalian fishes, with evidence currently confined to the northern hemisphere landmass of Euramerica, was probably an extremely rapid global event.”

¹Long et al, “An exceptional Devonian fish from Australia sheds light on tetrapod origins,” Nature advance online publication 18 October 2006 | doi:10.1038/nature05243; Received 4 June 2006; Accepted 11 September 2006; Published online 18 October 2006.
²Ibid, “The shallower angle of the spiracular chamber margin in Gogonasus (Fig. 1g) is a perfect intermediate morphology between the deeper spiracular chamber of Eusthenopteron (Fig. 1h) and the almost horizontal chamber of Panderichthys (Fig. 1f).  However, in having the entopterygoid located lateral to the ventral opening of the spiracular tract, the condition in Panderichthys is more derived than either Eusthenopteron or Gogonasus.
³e.g., “Our new phylogeny replaces the tristichopterid Eusthenopteron as the typical fish model for the fish-tetrapod transition.  It also raises the question of what environment the immediate stem group of the elpistostegalians inhabited.  The marine environment inhabited by Gogonasus is in accord with the marginal marine environments of some elpistostegalians (Panderichthys, Elpistostege, Tiktaalik) and the tetrapod Tulerpeton.  Such observations support a model in which the first tetrapods, like their immediate piscine sister taxa, were capable of marine dispersal, thus explaining the widespread global distribution achieved shortly after their first appearance in the late Frasnian.

When you read scientific papers, with all their unknowns, all their qualifiers and disclaimers and uncertainties and admissions of doubt and lack of evidence, then read the popular news reports gleaming with confidence and glittering generalities glibly stating how some new fossil proves evolution, it gets really disgusting.  Any Darwin Party advocate holding up a stack of science journals at a school board meeting and claiming they represent mounds of evidence backing up Charlie’s wacko story about humans coming from bacteria is either a charlatan or a dupe of the popular press.
    When you hear a wild, reckless claim like “This is the mother of all tetrapods!” don’t be a sucker.  Read the original source paper like we do.  It has the fine print.  It suggests a revised claim that, unfortunately, makes for a very poor sound bite for reporters: something like:

If we could figure out how these shallow-water inhabitants got from Australia to the arctic, and if we had the soft parts, and if we understood how morphological features could appear and disappear and re-appear within a Darwinian mechanism, and if we could unscramble these mosaics and redistribute them into lineages, and if we had the vaguest idea of what kind of environments the creatures actually lived in, and if we could rule out the possibility (as in Coelacanth) that the observed bones were used for other purposes other than what we expect, and if we could wiggle out of the Lamarckian charge of orthogenesis, and if we could somehow connect these morphological differences to beneficial mutations that natural selection could act on (with no purpose or goal in mind that they might prove advantageous on land, if both the breathing apparatus and the fin bones were to get lucky at the same time), then we might be able to make the claim that our particular fossil fits somewhere in an ancestral relationship to tetrapods, however controversial, that could be a contender in scientific conferences, and could get us some powerpoint slides that won’t be criticized, and might get us some brief popularity at the closing dinner, and maybe even a question from a reporter, or at least avoidance of ridicule, until our rivals find something else the following spring that blows our entire scheme out of the water.

That, discerning students, is how real Darwinism is done.