Life on a Young Planet (30 page)

More promising are other small fossils that resemble arthropods to a greater or lesser degree.
Parvancorina
consists of shieldlike molds, mostly less than half an inch long (
figure 10.3
). A pronounced rim runs along the outer margin, while the interior displays a T-shaped ridge whose top curves along the rounded (front?) edge; faint lines sometimes interpreted as legs can be seen beneath the shield.
Parvancorina
certainly exhibits bilateral symmetry, and looking at it, one can hardly avoid thoughts of trilobites. But, of course,
Parvancorina
is not a trilobite,
and though it has certain features reminiscent of this group, it lacks a host of additional characters evident in trilobites, lobsters, crabs, and all other arthropods.

Figure 10.3.
Parvancorina
, a problematic fossil that superficially (and, I think, only superficially) resembles trilobites.

The same can be said about an assortment of other Ediacaran fossils, including
Praecambridium
and
Vendia
.
Spriggina
, named for Ediacara pioneer Reg Sprigg, is particularly intriguing in this regard; its two-inch body has both segments and what appears to be a rounded head shield—very like an arthropod. Dolf Seilacher rather hopefully interpreted it as a vendobiont made up of interlocking tubes, but many other paleontologists see
Spriggina
as a segmented, bilaterally symmetric animal, “arthropoid” if not truly arthropod.

Finally, there is
Kimberella
, a small organism that looks like a smoked mussel preserved in rock. According to Misha Fedonkin and Ben Waggoner, of the University of Central Arkansas, this resemblance is more than coincidental. They interpret
Kimberella
as a bilaterally symmetric organism that possessed a muscular foot for locomotion, a baglike body packed with visceral organs, and a tough organic mantle over its back. These features all occur in mollusks, the phylum that includes mussels and clams, along with snails and squid. Like
Spriggina
, however,
Kimberella
lacks other features found in the living animals to which it is compared.

Such fossils are both exciting and frustrating—exciting because they show flashes of a familiar biology and frustrating because individually familiar characters occur in decidedly unfamiliar combinations. If, however, we can lay aside the urge to see modern animals in Proterozoic fossils and accept Ediacaran morphologies for what they are, the balance tips strongly toward excitement. The unmistakable sense I get from Ediacaran fossils like
Kimberella
and
Spriggina
is that they preserve early stages in the assembly of features that collectively mark Cambrian animals as “modern.” Ediacaran species weren’t there yet, but at least some were on their way.

Contrasting the simple trace fossils below the Proterozoic-Cambrian boundary with the more abundant, diverse, and complicated tracks and burrows found above supports the idea that something big really did happen at the start of the Cambrian Period. But much of our sense of Cambrian diversity comes from skeletons preserved in carbonate rocks. The Nama Group contains plenty of limestones; thus, it offers one
more test of evolutionary pattern. Do Nama carbonates contain ancient skeletons? If so, do these fossils establish biological continuity between Proterozoic and Cambrian animals, or do they reinforce the differences between the two?

Generations of paleontologists regarded the origin of mineralized skeletons as synonymous with the Cambrian Explosion. In 1972, however, Gerard Germs showed this view to be wrong. In the course of his field research in Namibia, Germs discovered small tubes made of calcium carbonate in Nama limestones (
Figure 10.4a
). Germs christened these fossils
Cloudina
, in honor of Preston Cloud, and recognized two species, one of which he named for his mother.
Cloudina hartmannae
and
C. reimkeae
differ in size—the first an inch or two long and a quarter inch wide; the second smaller by a factor of two—but they share a common organization. The fossils are gently curving cylinders ornamented by closely but irregularly spaced flanges that protrude outward—the whole resembling a series of tiny funnels stacked one inside the next. Pogonophoran worms (distant relatives of earthworms) live in tubes built like this, but simpler animals can form mineralized tubes, as well. In fact, rare specimens found in China are branched, suggesting that, like Ediacaran disks,
Cloudina
might be related to sea anemones and jellyfish. Skeleton walls are thin and appear to have been flexible in life—
Cloudina
probably sported only a light coat of calcium carbonate.

Cloudina
is important because it shows that animals learned how to build mineralized skeletons well before the Cambrian Period began. But how much do we wish to make of this? Is
Cloudina
a singularity—the exception that proves the rule of Cambrian biomineralization? Or, was it part of a larger assembly of latest Proterozoic animals that presaged the skeletal diversity of Cambrian faunas?

Here, at last, we return to the Nama reefs mapped by John Grotzinger. The reefs are astonishing to see in the field—great lumps as much as 200 feet high that rise out of the desert floor, excavated in recent millennia by erosion that peeled away encompassing shales (
figure 10.1
). The architects of the reefs were microorganisms, likely including algae as well as cyanobacteria, but skeleton-forming animals also found happy homes in small niches set high above the flat seafloor. Animal fossils are abundant in Nama reefs, and, judging from cross sections seen on weathered rock surfaces, they came in a variety of shapes and sizes (
figure 10.4b
).
Tubes are common, although only a few display the flanges that identify them as
Cloudina
. More abundant are rounded cups up to an inch or so wide. Then there are goblet-shaped fossils with cup above and tube below, and fossils with clear hexagonal symmetry.

Figure 10.4.
Calcified fossils from microbial reefs of the Nama Group. (a)
Cloudina
, tubular fossils lightly mineralized by calcium carbonate. (b)
Namacalathus
population showing the variety of shapes seen on rock surfaces in Nama limestones. Note centimeter scale bar in upper photo.

John and I spent hours crawling along these reefs, trying to figure out what kinds of animals were present and how many different species were represented. Those are difficult questions to address in the field, because the fossils can’t be freed from the rocks that contain them. To find an answer, we had to cart large slabs of rock back to Cambridge, where John designed a system to prepare a smooth surface of each, and then slowly grind away the surface 25 microns at a time, taking a carefully registered digital photographic image after each shave. Using software originally developed for medical research, the library of digitized cross sections was assembled into three-dimensional virtual fossils (
figure 10.5
). Wes Watters, a bright MIT student with a head for physics but paleontology in his heart, did most of the work.

The computer models are eerily lifelike—they seem to bob and sway in some virtual current. The reconstructed fossils look like flexible wine glasses, with a cylindrical stem that opened upward into a round cup as much as an inch wide. Six (rarely, seven) regularly spaced holes impose a hexagonal pattern on the cups. As in
Cloudina
, the walls of these cups were thin and flexible, and so could have been mineralized only lightly. Evidently, early animals had little need for the robust mineral skeletons that protected their descendants from predators.

Given virtual fossils, we can simulate cross sections in any plane we wish. And when allowance is made for crushing and bending, nearly all of the tubes, cups, and goblets seen on reef surfaces can be interpreted as slices through a single form. Scyphopolyps, goblet-shaped relatives of (once again) jellyfish that attach to seaweeds in the present-day ocean, provide at least a general guide to the Nama fossils.

Clearly,
Cloudina
was not the only skeleton former in the Nama seaway.
Namacalathus
, the “goblet of Nama,” thrived wherever microbial communities paved the seafloor, and continuing studies show that other mineralizing species were present as well, including coral-like animals that colonized cracks in the reefs. But just as clearly, diversity was limited. We may have discovered abundant new fossils in the reefs of Nama, but there are no bivalves or arthropods, no brachiopods or echinoderms. When we have searched everywhere we can, latest Proterozoic life still looks very different from the Cambrian.

Figure 10.5.
Virtual fossils of
Namacalathus
, reconstructed from digitized images, as described in the text. (Image generated by Wes Watters)

On a sun-baked afternoon in Namibia, John Grotzinger and I stroll along the crest of an isolated hill, taking one more look at some of the youngest Proterozoic rocks exposed here or anywhere. The rocks are full of fossils: the vendobionts
Swartpuntia
and
Pteridinium
, calcareous
Cloudina
and
Namacalathus
, and trace fossils of modest diversity. These preserve a record of animals—but animals of distinctly Proterozoic aspect, far different from the diverse and complex invertebrates found in Kotuikan cliffs. The most conspicuous Nama fossils are to Cambrian animals what dinosaurs were to the mammals that graze on the plain below us—ecological antecedents but not direct ancestors.

Nama fossils would provide cold comfort to Darwin, who believed
that Cambrian complexity took shape gradually over long stretches of Proterozoic time. Now it seems that the familiar biology of Cambrian seas emerged only as the Cambrian Period began. What was wrong with Darwin’s solution, first considered along the Kotuikan River? How far
had
animal diversification progressed by the end of the Proterozoic Eon, and what ushered in the Cambrian world?

The complex forms of modern animals emerged only during the Cambrian Period, taking shape over a time span of at least 10 to 30 million years. Emerging insights into the genetics of development help us to understand the tempo and mode of Cambrian evolution, but we also need to factor in ecology—both the permissive ecology that enabled early variants to gain a foothold in the oceans and the ecological interactions among species that guided subsequent diversification.

We shall not cease from exploration
And the end of all our exploring
Will be to arrive where we started
And know the place for the first time.