Halkieria forms an even better known and more frustrating element of the SSF. Collected from lower Cambrian rocks throughout the world, halkieriids are preserved as sclerites (flattened blades and spines of calcification, just a millimeter or two in length). The sclerites assume several characteristic shapes, named siculate (narrow, crescentric, and asymmetrical), cultrate (elongate and more symmetrical), and palmate (wider and flattened like the palm of a hand). Although some paleontologists have tried to reconstruct halkieriids as tiny creatures, each living within or around a single sclerite, most agree that the halkieriid animal must have been substantially bigger and covered with large numbers of sclerites.
In July 1989, in a most inhospitable spot on Peary Land in northern Greenland, some twenty-one specimens of a halkieriid animal were finally unearthed from another deposit capable of preserving soft parts. (The spectacular results from the Burgess Shale have inspired paleontologists to devote attention to the discovery and exploitation of these rare and precious soft-bodied fossil faunas. Science, at its best, not only answers questions, but provokes new problems and guides fruitful research by posing issues previously unconsidered.)
In July 1990, S. Conway Morris and J. S. Peel published the first report on the halkieriid animal. Again, paleontological suspicions are confirmed, but with an amazing twist and surprise. The halkieriid is, as anticipated, a large animal (up to 7 cm), bearing many sclerites—up to 2,000 or more. The body is elongate, flattened, and wormlike, with the sclerites arranged in zones corresponding to forms previously named (see figure). Siculate sclerites surround the base of the animal (the underlying bottom surface probably carried no hard parts, as the animal crawled on a naked sole). A groove separates the zone of siculate sclerites from a lateral region of cultrates. The top surface of the creature bears more flattened, palmate sclerites.
Figure of a halkieriid showing the fields of sclerites and the two end plates. Reprinted by permission from Nature, June, p. 808; Copyright © 1990 Macmillian Magazines Limited.
So far, so good—and quite in line with predictions. But nature always throws us a surprise or two. Each end carries a prominent and entirely unanticipated shell. These are found in the same position on every specimen, and therefore represent no fluke of juxtaposition or odd preservation. The anterior shell is roughly rectangular, the larger posterior shell (up to 1 cm in length) more oval and flattened. With apparent growth lines and an apex near the margin, this posterior shell, if found separately, would surely have been called a brachiopod or mollusk valve. (I suspect that several named mollusks and brachiopods of the Tommotian will turn out to be halkieriid end plates). Conway Morris and Peel wisely offer no interpretation of these elements, though others have suggested that the terminal shells might have plugged the ends of a U-shaped tube, if halkieriids burrowed as do many modern worm-shaped organisms.
We are, of course, enormously gratified to know, for the first time, some prominent animals of the SSF fauna, the earth’s initial complement of modern creatures with hard parts. But should we be surprised? (I realize that the phrase “SSF fauna” is as redundant as pizza pie and AC current, but abbreviations achieve a life of their own and may then be modified, even by one of their own elements).
One misguided reply might proffer little surprise (and relative indifference to my efforts in this essay). After all, we knew that the scattered SSF elements had to represent some kind of animal or other, and now that we have found two of the creatures, they turn out to be something rather familiar after all. Just a couple of worms—and as Mr. Reagan once said about redwoods, when you’ve seen one, you’ve seen ’em all. But such an attitude would be more than just deplorably Philistine; it would be dead wrong as well. Wormlike is a functional term used to describe flexible, soft-bodied organisms that are basically bilaterally symmetrical, with sensory organs in a head at front, and excretory organs at the rear end. Wormlike is not a genealogical concept uniting a group of organisms related in any evolutionary sense of common parentage. Microdictyon and Halkieria are wormlike only in this functional meaning, and no anatomical plan is more common and more often evolved by radically different creatures. Wormlike bodies are good designs for any mobile creature that must move with efficiency towards food and away from enemies—and no mode of life is more common in nature. Modern wormlike creatures include animals of such genealogical disparity as truly segmented earthworms, slugs of the snail lineage, sea cucumbers of the echinoderm phylum, Amphioxus of our own parentage (or at least cousinship), and a host of phyla that we all once learned in high school—Platyhelminthes (including laboratory planaria and tapeworms in vertebrate intestines), Nematoda, Kinorhynchia, Pogonophora, Chaetognatha, and so forth.
The proper evolutionary perspective is genealogical. Bats may be functionally similar to birds, but they are mammals by descent. Ichthyosaurs may look and work like fishes, but they are reptiles by ancestry. In this more fundamental context of genealogy, both Microdictyon and Halkieria are puzzling. The Microdictyon animal looks like an onychophoran, a small modern group considered by some as transitional between the Annelida (segmented worms) and the Arthropoda (insects, spiders and crustaceans—see next essay). Halkieria has been compared with the later Wiwaxia from the Burgess Shale, but Wiwaxia itself is an enigma, and the two shells at the end of Halkieria are just plain odd. Perhaps better evidence will establish some homologies with known groups, but for now, Halkieria must be viewed as a unique creature of unknown affinity with any other animal.
Thus, we may dismiss the “seen one worm, seen ’em all” argument as simply wrong, but a more sophisticated version of “should we be surprised” does have potential merit. Consider any genealogical system that ends up with a few well-differentiated survivors, all rather distant one from the other. Modern life surely displays this cardinal feature. Our modern phyla represent designs of great distinctness, and our diverse world contains nothing in between sponges, corals, insects, snails, sea urchins, and fishes (to choose standard representatives of the most prominent phyla). A distant past must have included many linking forms, now extinct. These links would not resemble fanciful hybrids between living organisms (a cat-dog or a cow-horse), because modern lineages have been separate for so long. They would, instead, be odd animals with veiled hints of several lineages to come and many unique features of their own (as we actually find in mammals like Hyracotherium, the 50-million-year-old ancestor of both horses and rhinoceroses).
Consider a figure and a nonbiological analogy (with thanks to R. T. Simmonds of Nordland, Washington, who wrote to me about this example in another context). The modern Romance languages—French, Spanish, Portuguese, Italian, and Romanian—all derive from Latin and represent clearly separate entities, despite evident similarities. But if we could—as we cannot—trace all the lineages leading from Latin, we would find a forest of village dialects linking all these end-points together. Many would be odd and unique, others smoothly transitional. We would learn that our modern descendants are just a small sample of the total richness, most now lost. We do get some hints of the full tree in survival of a few “minor phyla” (Catalan and Romansh, for example), and in historical records of a few extinct lineages (Provençal and Burgundian). But if we could go back to the beginnings of the spread, Dr. Simmonds conjectures, we would probably encounter a veritable Cambrian Explosion of lost variants.
In this sense, a phenomenon like the Cambrian Explosion must generate a majority of lineages that will seem peculiar in comparison with modern survivors—for these form the web of intermediary links that must die out if we are to emerge (as we have) with a limited set of widely separated designs (see figure). But I would raise two strong arguments against any boredom about Microdictyon and Halkieria on these grounds.
First of all, the forest of extinct lineages includes two categories of differing degrees of strangeness with respect to modern survivors. Unless modern survivors include forms at all the ancient peripheries—and this seems most unlikely, since peripheries are te
nuous places—then many extinct lines will lie outside the range of all modern designs, and will feature more than an amalgam of primitive, but intermediary, characters. The point may sound abstract, but can be easily grasped in the diagram. Only lineages 1, 2, and 3 have emerged from the forest of this “Cambrian Explosion” to yield modern descendants. Now consider the lettered representatives from an early time of maximal diversity. Some of these lineages (e-l of the diagram) do lie within the bounds of modern groups; in our retrospective view, we will regard them as unique, but not fetchingly odd. But other lineages (a-d and m-p of the diagram) lie outside the limits of modern groups, often well beyond (p, for example, lies further from lineage 3 than 3 does from any modern survivor). These creatures will be read in our parochial light (recognizing only 1, 2, and 3 for “standard” animals) as bewitchingly peculiar—and all but the most benighted dolt will take a keen interest. (The Burgess Shale excites our imagination largely because several of its “weird wonders” probably lie in this exterior domain, well outside the boundaries of modern groups).
A hypothetical genealogy of the early history of multicellular life, illustrating the likelihood that many of these early forms had anatomies that would be judged outside the range of modern survivors. Iromie Weeramontry. Courtesy Natural History.
Thus, on my first argument, we cannot exclude Microdictyon and Halkieria from fascination just because we recognize that any genealogical system—like our diagram, like modern life, like the Romance languages—must include a great majority of early lineages deemed unique by modern standards. For Microdictyon and Halkieria may belong to the special group of outsiders (a-d and m-p), truly resident in the world of science fiction, and not to the more comfortable insiders (e-l) that only mix and match the cardinal features of later groups.
But suppose that Microdictyon and Halkieria do turn out to rank among the insiders? Do we then lose interest, shrug our shoulders, put down this essay, and move on to the horoscopes and gossip columns? We now come to the second, and I think more important, argument—an aesthetic or moral claim really, not an empirical proposition. What is fascination? Do we invest our interest only in unknown things beyond the boundaries of current categories? Do we not yearn for more beauty, more diversity, more examples, more wrinkles of novelty, more cases for inspiration, in the things we love and partially know? Do we not grieve for one hundred lost cantatas of Bach even though we may listen to more than two hundred? Would we not give our eye tooth (what’s a canine more or less) for the unknown works of Aristotle? Would we not trade half our GNP for tapes of Socrates in conversation with his students?
Why do intellectuals feel such special pain in the destruction of the library of Alexandria—the greatest repository of ancient texts, begun by Alexander the Great, maintained by the Ptolemaic monarchs of Egypt, and finally destroyed, according to the legend you choose to follow, by the Romans, the early Christians, or the conquering Moslems? In part, we lament the loss of the utterly unknown. But we miss just as much the opportunity to relish a greatly expanded diversity among people and ideas that we already know and love. We miss the joy of making concrete, the pleasure of holding what has disappeared forever. What is history all about if not the exquisite delight of knowing the details, and not only the abstract patterns. Even if Microdictyon and Halkieria are only “inside” animals between surviving phyla, they are still prominent creatures of our earliest multicellular world. They have unique forms and peculiar features—shells on both ends, or lateral dabs in pairs. We want to know as many of these creatures as we can, for they are papyrus rolls in the great and largely lost library of our own past.
One legend of Alexandria, almost surely false, states that the library was still intact when Moslems captured the city in the seventh century. The emir Amrou Ibn el-Ass, having conquered Alexandria in 640, wrote to the caliph Omar asking (in part) what should be done with the library (and hoping against hope that the caliph would spare this great treasure). But Omar replied with the most stunning statement of “heads I win, tails you lose” in all human history. The books, he proclaimed, are either contrary to the Koran, in which case they are heretical and must be destroyed—or they are consonant with the Koran, in which case they are superfluous and must also be destroyed. The contents of the library were therefore burned to heat water in the public baths of Alexandria. The books and scrolls kept the fires going for six months.
The Omar of this legend will never win any praise from intellectuals, but I do grasp his point in an entirely reversed way. Microdictyon and Halkieria are, in a sense, either heretical (if lying outside the range of modern forms) or superfluous (if lying inside). But they are equally wonderful, and worthy of our most cherished interest and protection, in either case—and in this judgment lies the difference between most of us and the enemies of the light. In this lies the turf that we must defend at all costs.
24 | The Reversal of Hallucigenia
YOU CAN GENERATE a lot of mischief just by strolling. When God asked Satan what he’d been doing, the foremost of the fallen angels responded: “…going to and fro in the earth and…walking up and down in it” (Job 1:7). But you can also do a lot of good. Aristotle preferred to teach while ambling along the covered walk, or peripatos, of his Lyceum in Athens. His followers were therefore called peripatetics. In Greek, a patos is a path, and peri means “about.” The name for Aristotle’s philosophical school therefore reflects the master’s favorite activity.
The same etymology lies behind my all-time favorite technical name for an animal—the genus Peripatus. I just love the sound, especially when pronounced by my Scottish friends who really know how to roll their r’s. I can hardly ever bring myself to write about this animal without expressing delight in its name. The only reference in my book Wonderful Life speaks of the “genus with the lovely name Peripatus.”
Peripatus is an elongated invertebrate with many pairs of stout, fleshy legs—hence the chosen name for this obligate walker. The Reverend Lansdown Guilding—quite a name itself, especially given the old stereotype of English clergymen as amateur natural historians—discovered and designated Peripatus in 1826. He falsely placed his new creature into the mollusk phylum (with clams, snails, and squids) because he mistook the antennae of Peripatus for the tentacles of a slug. Since true mollusks don’t have legs, Guilding named his new beast for a supposed peculiarity.
Peripatus is the most prominent member of a small group known as Onychophora. Modern onychophorans are terrestrial invertebrates of the Southern Hemisphere (with limited extension into a few regions of the Northern Hemisphere tropics)—hence little known and never observed in natural settings by residents of northern temperate zones.
About eighty species of living onychophorans have been described. They live exclusively in moist habitats, usually amid wet leaves or rotting wood. Most species are one to three inches in length, although the size champion from Trinidad, appropriately named Macroperipatus, reaches half a foot. They resemble caterpillars in outward appearance (although not in close evolutionary relationship). They are elongated, soft bodied, and unsegmented (the ringlike “annulations” on antennae, legs, and sometimes on the trunk are superficial and do not indicate the presence of segments, or true divisions of the body). The onychophoran head bears three paired appendages: antennae, jaws, and just adjacent to the jaws, the so-called slime papillae. Onychophorans are carnivores and can shoot a sticky substance from these papillae, thus ensnaring their prey or their enemies. Behind the head, and all along the body, onychophorans carry fourteen to forty-three pairs (depending on the species) of simple walking legs, called lobopods. The legs terminate in a claw with several spines—the source of their name, for Onychophora means “talon bearer.”
The Onychophora present the primary case for a classical dilemma in taxonomy: How do we classify small groups of odd anatomy? (Oddness, remember, is largely a function of rarity. If the world contained a million species of onychophorans and only fifty of beetles, we would consider the ins
ects as bizarre.) The chief fault and foible of classical taxonomy lies in its passion for clean order—an imposition bound to distort a messy world of continuity and complexity. A small group of distinctive anatomy sticks out like the proverbial sore thumb, and taxonomists yearn to heal the conceptual challenge by enforcing an alliance with something more familiar. Two related traditions have generally been followed in this attempt, both misleading and restrictive: the shoehorn (“cram ’em in”) and the straightening rod (“push ’em between”).
The shoehorn works by cramming odd groups into large and well-established categories, usually by forced and fanciful comparison of one or two features with characteristic forms of the larger group. For example, the Onychophora have sometimes been allied with the Uniramia, the dominant arthropod group that includes insects and myriapods (millipedes and centipedes), because both have single-branched legs (never mind that arthropod legs are truly segmented and that onychophoran lobopods are constructed on an entirely different pattern).
Eight Little Piggies Page 33