Gould, The Reversal of Hallucigenia

more troublesome question of function, led many paleontologists to dispute Conway Morris's reconstruction (and Conway Morris himself also began to doubt his original conclusions). In my book Wonderful Life, I presented Conway Morris's original version and then opted for a different interpretation proposed by several colleagues before me--that Hallucigenia is a part broken off from a larger (and still unknown) animal. I wrote:

Hallucigenia is so peculiar, so hard to imagine as an efficiently working beast that we must entertain the possibility of a very different solution. Perhaps Hallucigenia is not a complete animal, but a complex appendage of a larger creature, still undiscovered. The "head" end of Hallucigenia is no more than an incoherent blob in all known fossils. Perhaps it is no head at all, but a point of fracture, where an appendage (called Hallucigenia) broke off from a larger main body.

I received several dozen much appreciated letters from readers of my book, suggesting different reconstructions for some of the oddball creatures of the Burgess Shale. Hallucigenia received the lion's share of attention--and one suggestion cropped up again and again, in at least twenty separate letters. These correspondents, nearly all amateurs in natural history pointed out that Hallucigenia would make much more sense turned upside down--for the spines, which never worked in locomotion, could then function far more reasonably for protection!

I responded to these letters with, for me, the decisive rejoinder that a single row of tentacles (Conway Morris's version of the upper surface) would work even more poorly than paired spines as devices of locomotion. How could an animal balance, not to mention hop around, on a single row of tentacles? Yet I couldn't deny that everything else made more sense upside down.

It doesn't happen often, but if Ramsköld and Hou are correct--and I think they are--then the gut feeling of amateurs has triumphed over the weight of professional opinion. For Ramsköld and Hou have, unbeknownst to them of course, followed the advice of my correspondents. They have turned Hallucigenia upside down, but with an added twist (intellectual, not geometric) as well--they have inverted it into an onychophoran!

Ramsköld and Hou present two major arguments for their inversion of Hallucigenia. First, they must tackle the issue that hung me up: how can a single row of tentacles function as legs?. They acknowledge the problem, of course, but suggest that Conway Morris was wrong, and that two rows of paired tentacles are actually present along the surface that he called dorsal, or topmost. If Ramsköld and Hou are correct, then the major objection to reversing Hallucigenia disappears--for two rows of flexible tentacles look like the ordinary legs of a bilaterally symmetrical creature. Moreover, when you turn Hallucigenia upside down on the assumption that two rows of tentacles adorn the topside of Conway Morris's version, then the inverted beast immediately says "onychophoran" to any expert, for the little paired pincers at the end of each tentacle become dead ringers for onychophoran claws. Ramsköld and Hou have not yet developed enough evidence to prove the double row of tentacles conclusively, but our museum at Harvard contains the sample best suited for resolving this issue--a slab of rock with more than a dozen Hallucigenia specimens. I have lent this slab to Ramsköld and Hou, and I suspect that an answer will soon be forthcoming.

Second, they must explain how an onychophoran could possess the several pairs of long, pointed, upward-protruding spines that an inversion of Hallucigenia places along the top edge of the animal--for some fossil onychophorans bear plates (Microdictyon and Luolishania as previously discussed), but none yet described carry spines. Here, Ramsköld and Hou present compelling evidence in a form much favored by natural historians--a sequence or continuum linking a strange and unexpected form to something familiar through a series of intermediates.

Start the series with Microdictyon. This animal, probably an onychophoran, carried pairs of flat plates along the side of its body just above the insertion of lobopods. Go next to a new and as yet unnamed "armored lobopod," again from the prolific Chengjiang fauna. This clear onychophoran also bore paired plates in the same position as in Microdictyon. But each plate now carries a small spine--nothing like the elongation in Hallucigenia, but evidence that onychophoran plates can support spines. For a third step, go to isolated plates collected in lower Cambrian rocks of North Greenland by J. S. Peel and illustrated by Swedish paleontologist Stefan Bengtson in a commentary in Nature written to accompany Ramsköld and Hou's paper. These Greenland plates have the same meshwork structure as those of Microdictyon--and onychophoran affinity seems a reasonable conjecture (although in this case, we have only found preserved plates, not the entire body). But the Greenland plates carry spines verging on the length of Hallucigenia spikes. It is now only a small step to a fourth term in the series--to an onychophoran bearing plates with highly elongated spines: in other words, to Hallucigenia turned upside down!

We are witnessing a veritable explosion of Cambrian onychophorans--Aysheaia and Xenusion with their soft bodies, Microdictyon and Luolishania with plates, the unnamed Chengjiang creature