Life on a Young Planet (29 page)

Most paleontolgists agree that, in general, these disks represent anatomically simple, bottom-dwelling animals related to living Cnidaria, the animal phylum that includes sea anemones and jellyfish. Even those disks shown to be the anchors of colonial animals may find their genealogical home within or near the Cnidaria, as discussed below. Perhaps the most unusual interpretation of Ediacaran disks is that proposed by Dolf Seilacher, the German challenger to Glaessner. Seilacher suggested that least some of these fossils preserve bizarre “sand corals” that ingested sediment as a sandy ballast to hold them in place—“a rock in a sock,” in Dolf’s clever phrase. Others, however, including me, believe these fossils are merely “socks in a rock”—conventional organisms cast in sand.

While the closest living relatives of Ediacaran disks may occur among the Cnidaria, no one suggests that Proterozoic disk formers were identical to species alive today. They are extinct taxa that record an early radiation of anatomically simple animals.

A second group of Ediacaran fossils consists of complex, often leaflike, forms made up of repeating tubular units (
plate 7a
,
c
, and
e
). Called vendobionts, these fossils are represented in Namibia by several species.
Rangea
is an elongate fossil, up to about six inches long, consisting of a
central axis from which two rows of branches emerge; the branches contain numerous interlocking tubes, each a few millimeters wide. This distinctive form reminds many paleontologists of sea pens, modern relatives of jellyfish and sea anemones in which simple individuals form complex, leaflike colonies. If
Rangea
is a colonial animal, then the repeating tubular units may represent its constituent individuals.

Sea pen analogies become harder to sustain, however, when applied to other Nama vendobionts.
Pteridinium
, a second leaflike form found abundantly in a mid-Nama storm bed, superficially resembles a sea pen, but it has three “wings” instead of two, and each wing contains a single rank of tubes arranged perpendicular to the main axis (
plate 7e
). No modern sea pen looks like that!
Swartpuntia
, found by John Grotzinger at the very top of the Proterozoic succession, also has its tubes arranged in three broad wings attached to a stout central axis—like a Chinese fan imagined by Picasso (
plate 7a
). And
Ernietta
is even worse—its many elongate tubes form a complex cup, possibly open at the top.

Vendobionts provide a Rorschach test for paleontologists. Individual fossils have been interpreted as colonial cnidarians, as segmented worms, as primitive arthropods, seaweeds, lichens, and more. At the same time, some paleontologists insist that all vendobionts share a common architecture and so, a common ancestry. It is here that Dolf Seilacher mounted his most audacious challenge. Dolf proposed, not only that vendobionts were all cut from the same cloth, but that the cloth no longer exists. Unimpressed by Glaessner’s interpretations of Ediacaran fossils as early offshoots of phyla seen today, Dolf declared that vendobionts were quiltlike organisms made up of cylindrical tubes filled with “plasmodial fluid” rather than cellular tissue. Interpreted this way, vendobionts seem foreign—more alien than animal—and this was precisely Dolf’s point. In 1992, he formally proposed the Vendobionta as an extinct kingdom separate from the animals, an experiment in macroscopic multicellularity that blossomed for a geological moment but ultimately failed. Earlier interpretations of vendobionts raised only eyebrows; Dolf’s raised hackles, and so inspired a new generation of research.

Nama fossils serve to introduce the odd morphologies of vendobionts, but other localities show their true diversity.
Charniodiscus
is a
spectacular fossil first discovered in Australia. Like
Rangea
, it is leaflike in form, with a large (three-inch) disklike holdfast that anchored an originally erect axis to the seafloor. Thirty to fifty lateral branches depart in rows from either side of the central axis, and each branch bears a flap on one face marked by parallel grooves. The entire fossil can be more than a foot long. In Seilacher’s view,
Charniodiscus
is a typical vendobiontid alien. But Richard Jenkins, a respected paleontologist at Adelaide University, interprets
Charniodiscus
more conventionally as a colonial cnidarian organized much like a living sea pen. (Unlike living sea pens, however, the branches of
Charniodiscus
were fused together to form an unbroken surface.)

Charnia
, first discovered in the Charnwood Forest of England and now also known from Newfoundland, Australia, and Russia, superficially resembles
Charniodiscus
, but has no central axis; its branches consist of tubelike units arranged in parallel and tightly joined between rows to form an intricately quilted surface.
Phyllozoon
was also quilted, but apparently spread across the seafloor like a miniature throw rug.

Perhaps the most thoroughly studied—and, therefore, most hotly contested—vendobiont is
Dickinsonia
, known from large populations in Australia and the White Sea.
Dickinsonia
(
plate 7c
) is an elliptical fossil made up of cylindrical tubes joined along their long axes to form a continuous surface; specimens may be as small as a penny or as large as a turkey platter, but they are never more than a few millimeters thick. A narrow but pronounced ridge runs down the middle of the long axis. Mary Wade, a colleague of Glaessner’s at the South Australian Museum, first described
Dickinsonia
as an annelid worm, interpreting the transverse tubes as body segments and the central ridge as a gut cavity. She proposed the living annelid worm
Sphincter
as a modern counterpart, although the flattened shape of this worm is highly unusual (and not at all primitive) within the phylum. In contrast, Misha Fedonkin, the affable Russian who brought the White Sea fossils into focus, proposed that the tubular segments of
Dickinsonia
meet along the central axis but do not cross it. If Fedonkin is correct—and more than one disciple of Glaessner has contested his interpretation—
Dickinsonia
could not possibly be an annelid. To Dolf Seilacher, of course,
Dickinsonia
is just one more extinct vendobiont.

Was
Dickinsonia
a worm or a failed experiment, an early ancestor of
familiar animals or an extinct life-form only distantly related to living invertebrates? Interpretation isn’t easy, and I can easily find someone to disagree with any opinion I might venture. But a few clues shed light on the case. UCLA’s Bruce Runnegar and Jim Gehling of the South Australian Museum discovered folded specimens, which show that
Dickinsonia
had a flexible body. A handful of fossils also provide evidence that tubes could contract and so must have contained muscle cells. And rare specimens in which tubes ripped open but retained their cylindrical shapes suggest that whatever filled these structures, it wasn’t “plasmodial fluid.” That’s one strike against the vendobiont hypothesis. On the other hand,
Dickinsonia
shows no evidence of the organ systems expected in annelid worms: there is no mouth at the end of the medial ridge, no hairlike setae, and no parapodia (stumpy leglike appendages on the body segments of marine annelids). Early in the game, it was possible to blame such absences on preservation, but that won’t work anymore. What we see is pretty much what was there.

Ediacaran rocks contain trackways that could have been plowed by smaller dickinsonids, but few trails that could, even in principle, be associated with larger specimens. Apparently, then,
Dickinsonia
reclined on the sediment surface but did not move across it, very unlike a worm. And one more observation supports the interpretation of
Dickinsonia
as a vendobiont—the arrangement of its tubes closely resembles that seen in the fanlike wings of the three-pronged Nama fossil
Swartpuntia
.

I confess to some uncertainty in interpreting these perplexing fossils. Vendobionts don’t seem to develop like modern algae and they don’t look like living worms. But neither am I convinced that they record, like Shelley’s statue of Ozymandias, a vanished kingdom in an antique land. What is the alternative? Taking a cue from
Rangea
and
Charniodiscus
, I suspect that most vendobionts were colonial animals at least broadly related to the living Cnidaria. Today, coloniality is widespread among cnidarians, from the Portuguese man-of-war that floats on the sea surface (its float, stinging tentacles, and reproductive structures are all anatomically complete individuals) to the massive reef corals and delicate sea fans that proliferate on the ocean floor. In the absence of well-developed organ systems, cnidarians achieved complexity by differentiating
individuals
within colonies, and this may have been the case for vendobionts, as well.

Tubelike individuals in vendobiont colonies must have had a simple anatomy, but, like living cnidarians, they could have had both nerve nets and functionally coordinated muscle cells. Moreover, the structurally competent material within tubes could have been an inert substance like the “jelly” in jellyfish. And, as Mark McMennamin of Mount Holyoke College first speculated, vendobionts might have gained nutrition from symbiotic algae or bacteria, as (once again) do many living cnidarians. “Could have,” “might have”—much remains tantalizingly beyond our grasp. However, by viewing the complex shapes of vendobionts as colonies built by individually simple animals, we can interpret most if not all of these fossils as members of a single clade, and one that vanished long ago. I suspect that the vendobionts do not comprise an extinct parallel to the animal kingdom, but rather record early animals that possessed some but not all of the features found in living cnidarians. (Cnidarian features
not
found in vendobionts include a mouth fringed by tentacles.) Indeed, in limited retreat from the extinct kingdom hypothesis, Dolf Seilacher and Yale’s Leo Buss have suggested such a possibility.

Sherlock Holmes famously recognized the importance of absence—of missing clues and things that did not happen:

“Is there any other point to which you would wish to draw my attention?”
“To the curious incident of the dog in the night-time.”
“The dog did nothing in the night-time.”
“That was the curious incident,” remarked Sherlock Holmes.

Absence is also worth noting in Nama and other latest Proterozoic rocks, and the fossils most obviously missing are those found so conspicuously in Cambrian rocks deposited only 10–20 million years later. Ediacaran disks and vendobionts suggest that cnidarian-like animals were common in latest Proterozoic ecosystems, but where were the ancestors of trilobites, of mollusks, of brachiopods, … of
us
?

Absence of evidence is suggestive, but when are we justified in interpreting it as evidence of absence? The answer, as always, lies in sampling. Cambrian rocks tell us where and under what conditions complex animals became fossilized, and only when we have searched thoroughly in all the right Proterozoic rocks can we have confidence in an evolutionary interpretation of missing fossils.

Cambrian rocks contain the tracks, trails, and burrows of anatomically and behaviorally complex animals that inhabited shallow seaways; in the cliffs along the Kotuikan River, their abundance and diversity increased dramatically as we climbed higher into the Cambrian succession. Nama sandstones also contain tracks and trails made by early animals as they moved through bacteria-rich sediments just below the surface of the seafloor. The trackways are small and simple—about the size and shape of a spaghetti strand dropped carelessly onto the floor (
figure 10.2
). The tracks run parallel to the sediment surface and rarely penetrate to even modest depths. But, based on observations of living trail-formers, it appears that most of these traces were made by creatures more complicated than sea anemones and jellyfish. Animals that make tracks like these have one distinctive feature in common—bilaterally symmetric bodies in which a single plane of symmetry runs from head to hind end.
Bilaterian
animals (discussed further in
chapter 11
) include all anatomically complex metazoans from trilobites to vertebrates. Trace fossils document their presence in latest Proterozoic oceans. Might bilaterian animals also lurk among Ediacaran impressions?

Figure 10.2.
Simple track fossils made by bilaterian animals in a latest Proterozoic seaway. The specimen comes from South Australia. Each trail is a bit over 1 millimeter wide.

Some fossils from Australia and the White Sea are neither disks nor vendobionts, and these provide a hunting ground for more complex animals.
Tribrachidium
, for example, is a circular cast that might easily be lumped with Ediacaran disks, except that it displays three large grooves that branch repeatedly as they spiral outward from its center. Along with a handful of related fossils from the White Sea,
Tribrachidium
has variously been allied with the sponges, cnidarians, or echinoderms. Its affinities, however, remain problematic—three-part symmetry is rare among living animals. Functionally, the canal-like internal structure suggests an animal that flushed large volumes of seawater through its body, as sponges do today.