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 |