Gould, The Reversal of Hallucigenia

Left: Conway Morris's Original reconstruction of Hallucigenia, Right, top and bottom; Ramskold and Hou's inversion of Hallucigenia as an onychophoran and their reconstruction of the new Chengjiang onychophoran with side plates and spines.

clearly bears interesting implications. This third way has been supported, often by well-respected taxonomists, but our general preference for shoehorns and straightening rods has given it short shrift. The Onychophora, under this view, might represent a separate group, endowed with sufficient anatomical uniqueness to constitute its own major division of the animal kingdom, despite the low diversity of living representatives. After all, the criterion for separate status should be degree of genealogical distinctness, not current success as measured by number of species. A lineage may need a certain minimal membership just to have enough raw material available so that evolution can craft sufficient difference for high taxonomic rank. But current diversity is no measure of available raw material through geological history. Evolution is ebb and flow, waxing and waning; once great groups can be reduced to a fraction of their past glory. A great man once told us that the last shall be first, but just by the geometry of evolution, and not by moral law, the first can also become last. Perhaps the Onychophora were once a much more diverse group, standing wide and tall in their distinctness, while Peripatus and its allies now form a pitifully reduced remnant.

(By speaking of potential distinctness, I am not making any untenable--indeed it would be nonsensical--claim for total separation without any relationship to other phyla. Very few taxonomists doubt that onychophorans, along with other potentially distinct groups known as tardigrades and pentastomes, have their evolutionary linkages close to annelids and arthropods. But this third view places onychophorans as a separate limb of life's tree--branching off near the limbs of annelids and arthropods and eventually joining them to form a major trunk--whereas the shoehorn would stuff onychophorans into the Arthropoda, and the straightening rod would change life's geometry from a tree to a line and place onychophorans between primitive worms and more advanced insects.)

We can only test this third possibility by searching for onychophorans in the fossil record--a daunting task because they have no preservable hard parts and therefore do not usually fossilize. I write this essay because several striking new discoveries and interpretations, all made in the past year or two, now point to a markedly greater diversity for onychophorans right at the beginning of modern multicellular life, following the Cambrian explosion some 550 million years ago. These discoveries arise from two fortunate and quite different circumstances: first, onychophorans have been found in the rare soft-bodied faunas occasionally preserved by happy geological accidents in the fossil record; second, some ancient onychophorans possessed hard parts and can therefore appear in ordinary fossil deposits.

I fully realize that this expansion in onychophoran diversity at the beginning of multicellular animal life can scarcely rank as the hottest news item of the year. Most readers of this column, after all, have probably never heard the word onychophoran and, lamentably, have no acquaintance with poor, lovely Peripatus. So why get excited about old onychophorans if you never knew that modern ones existed in the first place? Do hear me out