Wonderful Life: The Burgess Shale and the Nature of History (15 page)

Modern taxonomies recognize seven basic levels of increasing inclusion
—
from species (considered as the fundamental and irreducible units of evolution) to kingdoms (the broadest groupings of all): species, genera, families, orders, classes, phyla, and kingdoms
.

At the highest level
—
the kingdom
—
the old folk division into plants and animals, and the old schoolboy system of plants, animals, and single-celled protists, have been largely superseded by a more convenient and accurate five-kingdom system: Plantae, Animalia, and Fungi for multicellular organisms; Protista (or Protoctista) for single-celled organisms with complex cells; and Monera for single-celled organisms (bacteria and cyanophytes) with simple cells devoid of nuclei, mitochondria, and other organelles
.

The next level
—
the phylum
—
is the basic unit of differentiation within kingdoms. Phyla represent the fundamental ground plans of anatomy. Among animals, for example, the broadest of basic groups are designated as phyla
—
sponges, “corals” (including hydras and jellyfish), annelids (earthworms, leeches, and marine polychaetes), arthropods (insects, spiders, lobsters, and the like), mollusks (clams, snails, squid), echinoderms (starfishes, sea urchins, and sand dollars), and chordates (vertebrates and their kin). In other words, phyla represent the major trunks of life’s tree
.

This book treats the early history of the animal kingdom. In focusing on the origin of phyla and their early number and degree of differentiation, we ask the most basic of all questions about the organization of our animal kingdom
.

How many phyla of animals does our modern earth contain? Answers vary, since this question involves some subjective elements (a terminal twig is an objective thing, and species are real units in nature, but when is a branch large enough to be called a bough?). Still, we note some measure of agreement; phyla tend to be big and distinct. Most textbooks recognize between twenty and thirty animal phyla. Our best modern compendium, a book explicitly dedicated to the designation and description of phyla (Margulis and Schwartz, 1982) lists thirty-two animal phyla
—
a generous estimate in comparison with most. In addition to the seven familiar groups already mentioned, the animal phyla include, among others, the Ctenophora (comb jellies), Platyhelminthes (flatworms, including the familiar laboratory
Planaria
), Brachiopoda (bivalved invertebrates common as Paleozoic fossils, but rarer today), and Nematoda (unsegmented roundworms, usually tiny and fantastically abundant in soil and as parasites
).

After such a long disquisition, the point of this exegesis with respect to the Burgess Shale may be quickly stated: the Burgess Shale, one small quarry in British Columbia, contains the remains of some fifteen to twenty organisms so different one from the other, and so unlike anything now living, that each ought to rank as a separate phylum. We hesitate to give such a “high” designation to single species because our traditions dictate that phyla achieve their distinctness through hundreds of speciation events, each building a bit of the total difference, piece by piece. Hence, the anatomy of a group should not become sufficiently distinct to rank as a separate phylum until a great deal of diversity has been accumulated by repeated speciation. According to this conventional view
—
obviously incorrect or incomplete by evidence from the Burgess
—
lineages of one or a few species cannot diverge far enough to rank as phyla. But
que faire?
The fifteen to twenty unique Burgess designs are phyla by virtue of anatomical uniqueness. This remarkable fact must be acknowledged with all its implications, whatever decision we ultimately make about the formalities of naming
.

Genius has as many components as the mind itself. The reconstruction of a Burgess organism is about as far from “simple” or “mere” description as Caruso from Joe Blow in the shower, or Wade Boggs from Marvelous Marv Throneberry. You can’t just look at a dark blob on a slab of Burgess shale and then by mindless copying render it as a complex, working arthropod, as one might transcribe a list of figures from a cash-register tape into an account book. I can’t imagine an activity further from simple description than the reanimation of a Burgess organism. You start with a squashed and horribly distorted mess and finish with a composite figure of a plausible living organism.

This activity requires visual, or spatial, genius of an uncommon and particular sort. I can understand how this work proceeds, but I could never do it myself—and I am therefore relegated to
writing
about the Burgess Shale. The ability to reconstruct three-dimensional form from flattened squashes, to integrate a score of specimens in differing orientations into a single entity, to marry disparate pieces on parts and counterparts into a functional whole—these are rare and precious skills. Why do we downgrade such integrative and qualitative ability, while we exalt analytical and quantitative achievement? Is one better, harder, more important than the other?

Scientists learn their limitations and know when they need to collaborate. We do not all have the ability to assemble wholes from pieces. I once spent a week in the field with Richard Leakey, and I could sense both his frustration and his pride that his wife Meave and their coworker Alan Walker could take tiny fragments of bone and, like a three-dimensional jigsaw puzzle, put together a skull, while he could do the work only imperfectly (and I saw nothing at all but fragments in a box). Both Meave and Alan showed these skills from an early age, largely through a passion for jigsaw puzzles (curiously, both, as children, liked to do puzzles upside down, working by shapes alone, with no help from the picture).

Harry Whittington, who shares this rare visual genius, also expressed his gift at an early age. Harry began with no particular advantages of class or culture. He grew up in Birmingham, the son of a gunsmith (who died when Harry was only two) and grandson of a tailor (who then raised him). His interests wandered toward geology, thanks largely to the inspiration of a sixth-form (just pre-university) geography teacher. Yet Harry had always recognized and exploited his skill in three-dimensional visualization. As a child, he loved to build models, mostly of cars and airplanes, and his favorite toy was his Meccano set (the British version of an Erector set, providing strips of steel that can be bolted together into a variety of structures). In beginning geology courses, he excelled in map interpretation and, especially, in drawing block diagrams. The consistent theme is unmistakable: a knack for making three-dimensional structures from two-dimensional components, and inversely, for depicting solid objects in plane view. This ability to move from two to three dimensions, and back again, provided the key for reconstructing the fauna of the Burgess Shale.

Harry Whittington was clearly the best possible person for the Burgess project. He was not only the world’s leading expert on fossil trilobites (the most conspicuous arthropods of the fossil record), but he had done his most elegant work (Whittington and Evitt, 1953, for example) on rare three-dimensional specimens preserved in silica. The original calcium carbonate of these fossils had been replaced by silica, while the surrounding limestone retained its carbonate base. Since carbonates are dissolved by hydrochloric acid, while silicates are unaffected, the matrix could be dissolved away, providing the rare advantage of three-dimensional preservation completely separable from the surrounding rock. Whittington had therefore been blessed with an ideal, if unwitting, preparation for the Burgess Shale many years later. He had studied three-dimensional structure within rock and then been able to judge his hunches and hypotheses by dissolving the matrix and recovering the fossils intact. These studies “preadapted” Whittington, to use a favorite word in the jargon of evolutionary biology, for his discovery and exploitation of three-dimensional structure in the Burgess Shale fossils.

Don’t accept the chauvinistic tradition that labels our era the age of mammals. This is the age of arthropods. They outnumber us by any criterion
—
by species, by individuals, by prospects for evolutionary continuation. Some 80 percent of all named animal species are arthropods, the vast majority insects
.

The higher-level taxonomy of arthropods therefore becomes a subject of much concern and importance. Many schemes have been proposed, and their differences continue to inspire debate. But general agreement can be wrested from most quarters concerning the number and composition of basic subgroups within the phylum. (The evolutionary relationships among subgroups are more problematical, but this subject will not be a major concern of this book
).

1. Representative fossil specimens of the four great groups of arthropods, taken from the most widely used textbook in the history of paleontology, the late-nineteenth-century work of Zittel. (A) A giant dragonfly from the Carboniferous, representing the Uniramia. (B) A fossil eurypterid, representing the Chelicerata. The first pair of head appendages is small and hidden under the carapace; the other five pairs are visible in this figure. (C) A fossil crab, representing the Crustacea. (D) A trilobite.

The scheme that I follow here is conservative and traditional, the closest to consensus that can be achieved. I recognize four major groups, three still living, one exclusively fossil (figure 1), and I make no proposal about evolutionary connections among them
.

1.
Uniramia
, including insects, millipedes, centipedes, and perhaps also the onychophores (a small and unusual, but particularly fascinating group, of which a good deal more later on, for the Burgess Shale contains a probable member
).

2.
Chelicerata
, including spiders, mites, scorpions, horseshoe crabs, and the extinct eurypterids
.

3.
Crustacea
, primarily marine (the terrestrial pillbug, an isopod, ranks as an exception), and including several groups of small bivalved forms, little known to nonprofessionals, but fantastically diverse and common in the oceans (copepods and ostracodes, for example), the barnacles, and the decapods (crabs, lobsters, and shrimp), whom we eat with relish while regarding their insect cousins as disgusting and unpalatable
.

4.
Trilobita
, everybody’s favorite invertebrate fossil, extinct for 225 million years, but common in Paleozoic rocks
.

Since the resolution of the Burgess Shale fauna depends so centrally upon an understanding of the amazingly diverse and disparate arthropods, we must enter into some details of arthropod anatomy. Lest this prospect sound daunting, let me assure you that I shall keep the jargon to an absolute and fully comprehensible minimum
—
only about twenty terms from among more than a thousand available. (I shall not list these terms, but rather define them in the course of discussion. All key terms are
underlined
at their first use.
)

The basic principle of arthropod design is
metamerism
, the construction of the body from an extended series of repeated segments. The key to arthropod diversification lies in recognizing that an initial form composed of numerous nearly identical segments can evolve by reduction and fusion of segments, and by specialization of initially similar parts on different segments, into the vast array of divergent anatomies seen in advanced arthropods. Fortunately, we can grasp the complexities of this central theme in arthropod evolution by considering just two matters: the fusion and differentiation of segments themselves, and the specialization of appendages
.

The numerous separate and similar segments of ancestral arthropods (figure 2) tended to coalesce into fewer specialized groups. The most common arrangement is a three-part division, into head, middle, and rear (called by various names, such as
cephalon
,
thorax
, and
pygidium
in trilobites, or head, thorax, and abdomen in insects and crustaceans). Most chelicerates have a two-part division, with a
prosoma
followed by an
opisthosoma
. The fused tailpiece of many crustaceans is called a
telson
.

Arthropods have external skeletons, or
exoskeletons
(stiff, but unmineralized in most groups, thus explaining the rarity of many arthropods as fossils). As segments fused, their exoskeletal parts joined to form discrete skeletal units called
tagma
. This process of fusion is called
tagmosis
. Different patterns of skeletal tagmosis provide a primary criterion for identifying fossil arthropods
.