Gould, The Reversal of Hallucigenia

if you harbor these doubts. Much more than Peripatus lies at stake, for validation of the third position--that onychophorans represent a separate branch of life's tree--has broad and interesting implications for our entire concept of evolution and organic order. I also think that you will marvel at the details of these early onychophorans for their own sake--and their weirdness.

We have actually known a bit about ancient onychophorans for most of this century, thanks once again to that greatest of treasure troves for soft-bodied fossils, the Burgess Shale. In 1911, two years after discovering the Burgess Shale, C. D. Walcott gave the unpronounceable name Aysheaia (we generally call it a-shy-a in the trade) to an animal that he described as an annelid worm. Many taxonomists, just viewing Walcott's illustrations, immediately saw that the creature looked much more like an onychophoran. In 1931, G. Evelyn Hutchinson, who became the world's greatest ecologist and was, perhaps, the finest person I have ever had the privilege of knowing (he died just a few months ago in his early nineties), published a definitive account on the onychophoran affinities of Aysheaia. Hutchinson had studied Peripatus in South Africa and he knew onychophoran anatomy intimately. As an ecologist, he was powerfully intrigued by the issue of how an ordinary marine invertebrate like Aysheaia could evolve into a terrestrial creature like Peripatus with such minimal change in outward anatomy. (Aysheaia had fewer pairs of legs and fewer claws per leg than do modern onychophorans. It also bore a terminal mouth at the body's end, while living onychophorans have a ventral mouth on the underside. In addition, Aysheaia had no slime papillae and could not use such a device to shoot sticky stuff through ocean waters in any case. But, all told, the differences are astonishingly slight for more than 500 million years of time and a maximal ecological shift from ocean to land.)

One other ancient onychophoran was recognized before last year--a European form named Xenusion, found during the 1920s. But Aysheaia and Xenusion did not shake the shoehorn or the straightening rod. Only two fossils, both so similar to modern forms, do not make an impressive show of diversity. Onychophorans remained a tiny and uniform group, ripe for stuffing in or between larger phyla and not meriting a status of its own.

In a recent column (October 1990) I described the beginning of the onychophoran coming of age (I was going to say "renaissance," but a renaissance is a rebirth, and onychophorans never had an earlier period of flowering in our consciousness). In that essay, and in another context, I described the discovery in China of the animal that bore the small, circular, meshwork plates known for many years from lowermost Cambrian rocks as Microdictyon. This fossil comes from the remarkable Chengjiang fauna of south-central China, a Burgess Shale equivalent (although slightly older), with beautiful soft-bodied preservation of many animals already known from the more famous Canadian site (and several novelties as well, including the Microdictyon animal). The plates called Microdictyon are attached in pairs to the side of the animal just above the junction of paired lobopods with the trunk of the body. The animal itself looks like an onychophoran. If this interpretation holds, then some ancient onychophorans had hard parts. The Chengjiang fauna also contains a second probable onychophoran with plates, named Luolishania.

Thus, the early fossil record of onychophorans had begun to expand in numbers and anatomical variety, including both fully soft-bodied forms like Aysheaia and creatures with small pairs of plates like Microdictyon and Luolishania. But the big boost, the event that might finally push onychophorans over the border of distinct respectability, finally occurred on May 16,1991, when the Swedish paleontologist L. Ramskold and his Chinese colleague Hou Xianguang published an article in the British journal Nature (science, at its best, is truly international): "New Early Cambrian Animal and Onychophoran Affinities of Enigmatic Metazoans" (vol. 351, pp. 225-28).

Ramsköld and Hou dropped a bomb-shell that makes elegant sense of a major paleontological puzzle of recent years. In 1977, Simon Conway Morris described the weirdest of all Burgess Shale organisms with the oddest of all monikers: Hallucigenia, named, as Conway Morris wrote, for "the bizarre and dream-like appearance of the animal." He described Hallucigenia as a tubular body supported by seven pairs of long, pointed spines--not jointed arthropod appendages or fleshy lobopods, but rigid spikes. In Conway Morris's reconstruction, a single row of seven fleshy tubes, each ending in a pair of little pincers, runs along the back, with a tuft of six smaller tubes, perhaps in three pairs, behind the larger seven. The head, not well preserved on any specimen, was depicted as a bulbous extension and the tail as a straight, upward-curving tube.

Hallucigenia was bizarre enough in appearance, but even more puzzlement attended the issue of how such a creature could function. In particular, how could a tiny animal, no more than an inch in length, be stable on seven pairs of rigid spikes for "legs"? And if stable, how could it possibly move? Some of our best functional morphologists, including Mike Labarbera of the University of Chicago, struggled with this issue and could not resolve it.

The unlikely morphology, and the even