Wonderful Life: The Burgess Shale and the Nature of History (24 page)

3.
Dinomischus
. Simon’s third mystery animal carried him another rung up the ladder of evidence. Again, Walcott had set aside and photographed a specimen, but published nothing and left no notes. But this time Conway Morris found himself wallowing in a virtual sea of evidence, for he had three specimens—Walcott’s in Washington, another in our collection at Harvard, and a third discovered on Walcott’s talus slope by the Royal Ontario Museum in 1975.

All animals discussed so far have been mobile and bilaterally symmetrical.
Dinomischus
represents another major functional design: it is a sessile (fixed and immobile) creature with radial symmetry, suited to receiving food from all directions, like many sponges, corals, and stalked crinoids today.
Dinomischus
looks much like a goblet attached to a long thin stem, with a bulbous holdfast at the bottom to anchor the animal to the substrate (figure 3.30). The entire creature scarcely exceeds an inch in length.

The goblet, called a calyx, bears on its outer rim a series of about twenty elongate, parallel-sided blades, called bracts. The upper surface of the calyx contains both a central and a marginal opening, presumably mouth and anus by analogy with modern creatures of similar habits (figure 3.31). A U-shaped gut, with an expanded stomach at the base, runs between the two openings through the interior of the calyx. Strands radiating from the stomach to the inner surface of the calyx may have been suspensory fibers (for the gut) or muscle bands.

A number of superficial similarities may be noted with bits and pieces of various modern animals, but these are probably broad analogies of similar functional design (like the wings of birds and insects), not detailed homologies of genealogical connection. Conway Morris found closest parallels with a small phylum called the Entoprocta (included with bryozoans in older classifications), but
Dinomischus
is basically a bizarre thing unto itself. Conway Morris showed some hesitation in his original paper (1977a, p. 843), but his latest opinion is unequivocal: “
Dinomischus
has no obvious affinity with other metazoans and presumably belongs to an extinct phylum” (Briggs and Conway Morris, 1986, p. 172).

4.
Amiskwia
. With
Amiskwia
, Simon finally tackled a mainstream Burgess organism, though one of the rarest. Five specimens had been discovered, and Walcott had formally described the genus—as a chaetognath, or arrow worm—in 1911.
Amiskwia
had also been a source of some published debate, though none outside the accepted framework of homes within modern phyla. Two articles in the 1960s had suggested a transfer from the chaetognaths to the nemerteans. These phyla are not household names, but both are staples of modern taxonomy.

3.30. Original reconstruction of
Dinomischus
by Conway Morris (1977a). Part of the calyx is broken away to show the interior anatomy of the organism. Note the U-shaped gut going from the mouth (labeled
M
.) to the anus (
An.
), and the muscle bands (
Sus. Fb.
, for “suspensory fibers”) anchoring the gut to the wall of the calyx.

3.31. Three specimens of the stalked animal
Dinomischus
. One bends toward us, showing the openings of the mouth and anus on the top of the calyx. Drawn by Marianne Collins.

Amiskwia
, as a compressed, probably gelatinous animal with no outer carapace, did squash flat on the Burgess rock surfaces. Hence, these fossils are truly preserved in the mode that Walcott incorrectly viewed as normal for all Burgess organisms—as a flat sheet. Without the three-dimensional structure that Whittington found for arthropods, and that Simon confirmed for several other oddballs, little of
Amiskwia
’s anatomy can be well resolved—though enough has been preserved to preclude a place in any modern phylum.

The head region bears a pair of tentacles, inserted on the front ventral surface (figure 3.32). The trunk sports two fins, unsupported by rays or any other stiffening device, in the plane of body flattening—lateral (at the sides) and caudal (forming a tail). (The chaetognaths often have fins in roughly similar positions, hence Walcott’s designation. But a true chaetognath also has a head with teeth, hooks, and a prominent hood—and no tentacles. Nothing else about
Amiskwia
even vaguely suggests chaetognath affinities, and the rough similarity of fins represents separate evolution for similar function in swimming.)
Amiskwia
is probably one of the few Burgess animals that did not live in the bottom community engulfed by the mudslide. It was presumably a pelagic (or swimming) organism, living in open waters above the stagnant basin that received the Burgess mudslide. This different mode of life would explain the great rarity of
Amiskwia, Odontogriphus
, and a few other creatures that may have lived in open waters above the grave, but away from the original home, of the main Burgess community. Only a few animals of the water column above would have died and settled into the sediments below during the short time when the mudslide was coalescing into a layer of sediment in the stagnant basin.

3.32. The flattened swimming animal
Amiskwia
, with a pair of tentacles on the head, and side and tail fins behind. Drawn by Marianne Collins.

Within the head, a bilobed organ may represent cerebral ganglia, while the gut can be traced as a straight tube from an enlarged region at the head to an anus at the other end of the body, just in front of the caudal fin (figure 3.33). The head, lacking the characteristic proboscis with a prominent fluid-filled cavity and muscular walls, looks nothing like that of a nemertean—the other candidate for a conventional taxonomic home; while the caudal fin exhibits only superficial similarity (in nemerteans, the fin is bilobed, and the anus opens at the very tip of the body). Conway Morris, now becoming quite comfortable with the idea of taxonomic uniqueness at high anatomical levels, concluded:

5.
Hallucigenia
. We need symbols to represent a diversity that we cannot fully carry in our heads. If one creature must be selected to bear the message of the Burgess Shale—the stunning disparity and uniqueness of anatomy generated so early and so quickly in the history of modern multicellular life—the overwhelming choice among aficionados would surely be
Hallucigenia
(though I might hold out for
Opabinia
or
Anomalocaris
). This genus would win the vote for two reasons. First, to borrow today’s vernacular, it is really weird. Second, since names matter so much when we are talking about symbols, Simon chose a most unusual and truly lovely designation for his strangest discovery. He called this creature
Hallucigenia
to honor “the bizarre and dream-like appearance of the animal” (1977c, p. 624), and also, perhaps, as a memorial to an unlamented age of social experiment.

3.33. Reconstruction
of Amiskwia
by Conway Morris (1977b). (A) Bottom view: note the insertion of the tentacles (labeled
Tt
.), the position of the mouth (
Mo.
), the path of the gut (
Int.
) to the anus (
An.
), and the structure interpreted as possible cerebral ganglia (
Ce. Ga.
). (B) Side view.

3.34.
Hallucigenia
, supported by its seven pairs of struts, stands on the sea floor. Drawn by Marianne Collins.

Walcott had assigned seven Burgess species to
Canadia
, his principal genus of polychaetes. (Polychaetes, members of the phylum Annelida, the segmented worms, are the marine equivalent of terrestrial earthworms, and are among the most varied and successful of all animal groups.) Conway Morris later showed (1979) that Walcott’s single genus was hiding remarkable disparity under one vastly overextended umbrella—for he eventually recognized, among Walcott’s seven “species,” three separate genera of true polychaetes, a worm of an entirely different phylum (a priapulid that he renamed
Lecythioscopa
), and
Hallucigenia
. Walcott, mistaking the strangest of all Burgess creatures for an ordinary worm, referred to this oddball as
Canadia sparsa
.

How can you describe an animal when you don’t even know which side is up, which end front and which back?
Hallucigenia
is bilaterally symmetrical, like most mobile animals, and carries sets of repeated structures in common with the standard design of many phyla. The largest specimens are about an inch long. Beyond these vaguest of familiar signposts, we are forced to enter a truly lost world (figure 3.34). In broad outline,
Hallucigenia
has a bulbous “head” on one end, poorly preserved in all available specimens (about thirty), and therefore not well resolved. We cannot even be certain that this structure represents the front of the animal; it is a “head” by convention only. This “head” (figure 3.35) attaches to a long, narrow, basically cylindrical trunk.