Aliens Here on Earth

Every important idea is simple.

leo tolstoi

For every complex problem there is a solution that is simple, neat, and wrong.

Before discussing the Ediacaran fauna, a distinction needs to be made between the two major types of animal fossils—body fossils and trace fossils. Body fossils are either actual parts of the organism's body (such as a shell or a bone), or impressions of body parts (even if the parts themselves have been dissolved away or otherwise destroyed). The imprint of a feather or leaf or the external surface of a shell are examples of body fossils. Occasionally, soft body parts (such as the tentacles on a squid) can be preserved. As one might expect, soft-bodied fossils are much rarer than shelly fossils, because soft body parts are easily destroyed by decay and can be fossilized only under exceptional preservation conditions.

Trace fossils are markings in the sediment (usually made while the sediment was still soft) left by the feeding, traveling, or burrowing activities of animals. Familiar examples of trace fossils include tracks and trails made by worms as they plow through sediment looking for food and ingesting sediment. Animals making these traces are usually eating organic matter in the sediment and are called deposit feeders. Footprints are another good example of trace fossils. The distinction between body and trace fossil can be blurred in some cases. Dinosaur footprints are known in which the scaly texture of the bottom of the animal's foot is preserved as part of the footprint. In a sense, this footprint fossil is both a trace fossil and a body fossil; it gives a record of the dinosaur's locomotion, and also gives a clear impression of the dinosaur's sole. Usually, however, it is difficult or impossible to exactly match trace fossils to the body fossils of the tracemaking organisms. Completely unrelated organisms can make trace fossils which are indistinguishable to paleontologists. Trace fossils are part of the fabric of the sediment, and therefore can be very resistant to destruction by metamorphism of the surrounding rock. Body fossils, on the other hand, are often destroyed by chemical reactions with the surrounding sediment. But body fossils are the only fossil type that can consistently give reliable information about the identity of the organism which left the remains.

In the 1920s, a peculiar body fossil was discovered in Germany and was claimed to be of Precambrian age. This fossil caused a minor sensation, first because it was unquestionably of biological origin, and second because it appeared to be a missing link between the Cambrian animals and their dark Precambrian past. Pompeckj (1927] called this fossil Xenusion auerswaldae (figure 2.1), and because it was apparently an incomplete specimen showing a body bearing numerous appendages or limbs, he believed that it was a fossil of a joint-legged animal. This was a satisfying conclusion, because joint-legged animals are an important group in the Cambrian. Trilobites, for instance, are abundant Cambrian joint-legged animals. Most modern joint-legged animals, such as insects and crabs, belong to the arthropod phylum (phylum is a rank of classification just below kingdom).

More difficult to interpret were a group of soft-body fossils found by German scientists in Namibia (formerly South-West Africa) in a sedimentary sequence called the Nama Group. Specimens were collected by P. Range and H. Schneiderhohn as early as 1908, but were not formally described in the scientific literature until the 1930s (Glaessner 1984). G. Gurich named one species Rangea schneider-hohni after its discoverers (Gurich 1930), and named Pteridinium simplex for its simple, fernlike shape (Gurich 1933; the Greek word for fern, genitive case, is pteridos). Both Rangea and Pteridinium (figure 2.2) are fern- or frond-shaped fossils. They both are accepted today as important, genuinely Precambrian fossils, but unfortunately the early studies by Gurich were unable to establish the age of these fossils with certainty, because no undoubtedly Cambrian fossils had been found stratigraphically above the beds with the soft-bodied fossils. (Continued research has yet to uncover convincingly Cambrian body fossils in the Nama sequence.)

Exciting discoveries by R. C. Sprigg in 1946 were used to establish a definitive Precambrian age for frond fossils and other co-occurring soft-bodied fossils. While doing some reconnaisance work as the assistant government geologist of South Australia, Sprigg found abundant soft-body fossils in the Ediacara Hills, a desolate, low range in the desert some 600 km north of Adelaide, Australia (Glaessner 1984). The Ediacara locality subsequently became the most important locality in the world for Precambrian fossils of this type. These soft-bodied organisms, presumed by many to have been metazoans, are now referred to throughout the world as the Ediacaran fauna or

Kimberella
FIGURE 2.1. Reconstruction of Xenusion auerswaldae, an enigmatic Cambrian fossil originally interpreted as a Precambrian arthropod. Length of frond approximately 7.5 cm. (From M. McMenamin 1986; used with permission of the Society of Economic Paleontologists and Mineralogists)

Ediacaran assemblage. The place name "Ediacara" is from the dialect of the 'Kujani, an aboriginal tribe. Loosely translated, it means "veinlike spring of water" (Jenkins 1984), an appropriate word origin considering that the Ediacaran fauna is the wellspring of all subsequent animal history.

Thousands of specimens have been recovered in the Ediacara Hills from platy quartzite slabs composed primarily of fine to medium sand grains. These rocks compose part of the Rawnsley Quartzite of the Pound Subgroup, a thick sequence of Precambrian marine sediments (Jenkins et al. 1983). The fossils are often large; discoidal and frond-shaped specimens sometimes exceed 1 meter in greatest dimension. Despite their relatively large size (for comparison, most Cambrian fossils are less than a few centimeters in maximum dimension, and no Cambrian animal is known to exceed 50 cm in length), the fossils of the Ediacara Hills are completely soft-bodied,

Cambrian Fossils
FIGURE 2.2. Reconstruction of a three-vaned frond fossil from North Carolina, probably belonging to the genus Pteridinium. Specimen approximately 7 cm long. (After Gibson et al. 1984)

with the possible exception of a few faintly preserved spicules (elongate spines) along the midline of one of the frond fossils.

Most of the Australian fossils are discoid impressions, nearly all of which have been interpreted as fossil jellyfish. This was Sprigg's first impression concerning the nature of the initial specimens collected; he thought he had found fossil "jellyfishes" (Sprigg 1947). Jellyfish belong to a large phylum of animals known as the Cnidaria. Cnidarians include, in addition to jellyfish, corals, sea pens, and sea anemones. All cnidarians share a basic trait —the presence of cnidae (also called nematocysts or stinging cells). These stinging cells are used for stunning and killing prey, and for defense against predators. These stinging cells are the bane of unwary bathers in jellyfish-infested waters.

Cnidae are not actually cells in the sense of having a nucleus with a cell membrane, but rather are greatly enlarged and specialized organelles. All plant and animal cells have organelles, tiny intracel-lular bodies that perform essential tasks such as photosynthesis (in chloroplasts) or respiration (in mitochondria, the "energy factories" of a cell). Most organelles are tiny, only a few fractions of a micron in greatest dimension. Cnidae are giants of the organelle world. They consist of a barbed, tightly coiled filament in a saclike body equipped with a hairlike trigger. When the hair trigger is contacted by a prey or foe, the filament is instantly discharged into the victim. Some filaments are over 1 millimeter (1000 microns) when extended.

The basic cnidarian body plan comes in two varieties: polypoid and medusoid. A sea anemone is a good example of a polypoid cnidarian. Its cylindrical, sac-shaped body is stuck to the substratum on one end and armed with a mouth and contractile tentacles on the other. Jellyfish are examples of the medusoid plan. The gut is similar in overall shape to that of the sea anemone, except that its body is inverted for swimming and is generally free from the substrate. The mouth, at the bottom center of the bell-shaped body, is connected by radial canals to the marginal parts of the bell. At the margin of the bell are fine tentacles loaded with cnidae, plus muscle fibers encircling the margin of the bell. Contraction of these muscles results in the characteristic pulsating swimming stroke of jellyfish.

There are three main shapes of Ediacaran fossils; circular impressions, frond fossils, and wormlike impressions. Circular impressions such as Cyclomedusa (figure 2.3), usually interpreted as medusoids, compose about 70 percent of the body fossil imprints in the Ediacara Hills collections. It is not at all clear, however, that these are all jellyfish impressions. At least one circular fossil, Tribrachidium (figure 2.4), is thought by most paleontologists to have no living relatives; it probably represents an extinct phylum.

Most other circular impressions of the Ediacaran fauna are classed as jellyfish or other cnidarians. Skinnera, for example, is a discshaped fossil with three inner "pouches" that Glaessner (1979) calls a jellyfish. The fossil Mawsonites (shown in color on the August 27, 1982, cover of Science magazine; Cloud and Glaessner 1982), although not considered by Glaessner (1979) to be undoubtedly a jellyfish, is assumed to represent the mouth-end (umbrellar surface) of a medusoid cnidarian.

Kimberella, an elongate bell-shaped fossil has been reconstucted by Jenkins (1984) as a cubozoan or box-jelly. Modern box-jellies are predators, fast and powerful swimmers. Jenkins' (1984) reconstruction of Kimberella has been called into question, however, because the fossil is triradially symmetric rather than having the fourfold radial symmetry required by the cubozoan hypothesis.

A number of different types of frond fossils are known from South Australia, including a few rare specimens of Pteridinium, which was

Martin Gurich Body Fossils

FIGURE 2.3. The discoidal fossil or "medusoid" Cyclomedusa, an Ediacaran body fossil. It has been interpreted by some as a jellyfish-like animal, despite the fact that, unlike in true jellyfish, concentric sculpture dominates the center of the organism, while radial elements dominate the margin. Diameter 6 cm.

FIGURE 2.3. The discoidal fossil or "medusoid" Cyclomedusa, an Ediacaran body fossil. It has been interpreted by some as a jellyfish-like animal, despite the fact that, unlike in true jellyfish, concentric sculpture dominates the center of the organism, while radial elements dominate the margin. Diameter 6 cm.

first discovered in the Nama Group of Africa. Jenkins and Gehling (1978) consider the frond fossils of the Ediacara assemblage to be representatives of the modern pennatulaceans or sea pens. Pennatu-laceans are colonial cnidarians with plumose colony outline that gives them their common name. The sea pen colony is fixed to the sea floor by a bulbous base; when disturbed, the plume part of the colony contracts and the creature seems to disappear. In a few specimens of the frond fossil Charniodiscus, known from England and Australia, the frond is associated with a circular, concentrically-ringed impression at the base of the frond. The frond seems to emanate from the center of the disk, leading to the reasonable interpretation that the disk was an attachment or holdfast for the frond. Glaessner (1959) first pointed out the striking similarities between these frond fossils and the body architecture of modern sea pens. Incidentally, the holdfast hypothesis may explain the nature of a large number of circular "jellyfish" impressions in the Ediacara assemblage; they could be holdfasts of which the upper frondose part of the colony was not preserved (Narbonne and Hofmann 1987).

The final major component of the Ediacaran fauna consists of ovate and leaf-shaped fossils that have been interpreted as fossil annelids (segmented worms) or flatworms. The organisms Dickin-sonia and Sphggina (figures 2.5 and 2.6) are bilaterally symmetric and composed of numerous, elongate segments that seem to widen outward. No mouth or eyes are known from these fossils (despite an intensive search, using digital image analysis, by Kirschvink et al.

FIGURE 2.4. The enigmatic Precambrian fossil Tribrachidium from the Edi-acaran fauna. Width approximately 2.5 cm.

Cambrian Explosion

FIGURE 2.4. The enigmatic Precambrian fossil Tribrachidium from the Edi-acaran fauna. Width approximately 2.5 cm.

Cambrian Fauna

FIGURE 2.5. A: Dickinsonia costata, a distinctive form from the Ediacaran fauna of Australia and northern Russia that reached up to one meter in length. This specimen was 4.6 cm in length. (After Runnegar 1982); B: another specimen of the same species from late Precambrian strata of the Flinders Range, South Australia. Greatest length of specimen 13.4 cm.

FIGURE 2.5. A: Dickinsonia costata, a distinctive form from the Ediacaran fauna of Australia and northern Russia that reached up to one meter in length. This specimen was 4.6 cm in length. (After Runnegar 1982); B: another specimen of the same species from late Precambrian strata of the Flinders Range, South Australia. Greatest length of specimen 13.4 cm.

Ediacaran Fauna
FIGURE 2.6. Spriggina, a wormlike form from the Ediacaran fauna. Maximum width of specimen 1.5 cm. (Copyright © 1989 by J. G. Gehling. All rights reserved)

1982), but one end of the elongate specimens of Spriggina is fused into a horseshoe shaped, headlike structure, and Runnegar (1982) has interpreted an elongate ridge running along the axis of Dickinsonia as a sediment-filled gut.

Nearly all current historical geology textbooks accept the above interpretations of the affinities of the Ediacaran fauna as essentially correct, and include a discussion of these fossils and their classification into familiar phyla such as Cnidaria and Annelida. There are, however, difficulties with the comfortable ensconcement of these fossils in modern phyla.

The first difficulty concerns the preservation of the soft bodies of the Ediacaran organisms. Jellyfish are uncommon but not unknown as fossils. Spectacular Jurassic jellyfish fossils are known from Germany, but these Jurassic jellyfish remains are in fine-grained lime-mud sediment, not the relatively coarse-grained, sandy sediments that preserve the Ediacara fossils. It seems strange that the delicate soft tissues of medusoids and worms were capable of being preserved in Precambrian sandstone in large numbers (thousands of Australian specimens are known). Such fossils are virtually unknown in sandstone deposits lain down later in earth history; modern jellyfish are more than 95 percent water by weight, and decay readily. Why didn't the Ediacaran animals decay away like their modern counterparts in similar (i.e., sandy nearshore) depositional environments? Glaessner (1984:48) addresses the problem of cnidarian impressions in sandstone with a brief account of his encounter with a stranded jellyfish. Glaessner stood on the jellyfish, and observed that the cnidarian was so tough that it could support his entire weight. The downward force of Glaessner's 80 kg body mass resulted in a "clear impression of its oral surface on the sand," but according to Glaessner there was "no damage to the medusa" (the jellyfish might disagree). Glaessner's experiment does attest to the strength of jellyfish tissue, but sheds little light on the burial conditions that entombed the Ediacaran creatures. No one was there to stand on the beach when a Dickin-sonia was stranded.

An alternative to the "modern phyla" model for Ediacaran fauna affinites was offered by Adolf Seilacher in 1983. Seilacher is a West German paleontologist, well known for his innovative and sometimes controversial observations and studies of invertebrate paleontology.

The central point in Seilacher's critique of the conventional classification of Ediacaran fossils is this —Ediacaran organisms that pre serve as soft-bodied fossils have very flat, almost ribbonlike bodies. This is not a body plan that is in accordance with classifying them as cnidarians and annelids, because in many cases it leaves no room for a gut, saclike or otherwise. Cnidarians, from large free-living individuals such as a jellyfish to branching colonies such as corals, are three dimensional rather than flat. Seilacher (1983, 1984) argues that the similarities between the Ediacaran creatures and modern cnidarians are superficial and that the conventional claims for close biological relationships between the two groups have not been examined critically.

Consider the sea pen classification proposed for Ediacaran frond fossils. The individual branchlets in a sea pen frond are separated, allowing water to pass between the branchlets and permitting each individual polypoid of the sea pen colony to capture food from the water streaming by. This mode of food capture is called filter feeding. In most Ediacaran fronds, the branchlets extending off of the main stalk of the frond are fused together. This does not mean that the organism could not filter feed, but filter feeding, if it occurred at all in an Ediacaran frond, must have been much different from the filter feeding in a modern sea pen. Without gaps between the segments of the frond, filter feeding of an Ediacaran frond would be a very inefficient process, at best.

Further, Seilacher (1984) maintains that there are fundamental differences between Ediacaran discoids and modern medusae. As outlined above, modern jellyfish medusae have radial gut structures in the middle of the "umbrella," and concentric structures representing annular muscle bands at the periphery of the medusae. The opposite is true for many of the discoid Precambrian fossils such as Cyclomedusa and Mawsonites, in which the sculpture in the central part of the disc is predominantly concentric, and the marginal sculpture is chiefly radial (Glaessner 1979).

Seilacher (1984) also takes a dim view of the hypothesis that Dickinsonia and Spriggina were worms. No definitive evidence has been found for eyes, mouths, anuses, locomotory appendages, or guts in these or related fossil organisms. These physical and behavioral traits are usually seen in segmented, annelid worms, as well as in unsegmented flatworms, but are not seen in the Ediacaran wormlike creatures.

The most radical part of Seilacher's (1984) critique of the modern phyla model is his suggestion that all of the Ediacaran soft-bodied fossils — disc, frond, and "worm" shapes —are related to each other and have little or no relationship to Cambrian or later animals. Seilacher (1985) calls these Precambrian soft-bodied fossils the "Ven-dozoa" (after the Soviet name for the last and only period of the Precambrian, the Vendian), an extinct taxon (or unit of classification) of ancient life perhaps comparable in rank to one of the five kingdoms such as fungi, animals, and plants discussed earlier. According to Seilacher, vendozoans went extinct at or near the Precambrian-Cambrian boundary, leaving few or no living descendants in a world overrun by "normal" animals (Seilacher 1984).

Seilacher (1984, 1985) sees vendozoans as variants of a single constructional principle; that of inflatable or pneumatic structures whose shapes are maintained by internal "quilting." In other words, the Ediacaran fauna consists of organisms that have a lot more in common (in constructional terms) with quilted air mattresses and inflatable rafts than they have with sea anemones and annelid worms. The pneumatic structure is a good way to make a body that holds its shape and yet is essentially two dimensional.

There is good evidence for this pneumatic architecture. Many Ediacaran fossils are apparently "deflated"; the upper membrane has collapsed downward between the more rigid baffles or walls between adjacent sections of the quilted pneumatic structure (figure 2.7). Like a flexible air mattress, Ediacaran soft-bodied creatures were capable of substantial expansion and contraction (Runnegar 1982) and perhaps did resemble inflatable rafts (except that they would have been inflated with water instead of air).

Seilacher's (1983) Vendozoa hypothesis caused quite a commotion when it was first presented at the annual Geological Society of America meeting in Indianapolis, and continues to be a topic of contention. Prominent articles discussing, and in general supporting, his views appeared in Science magazine (Lewin 1983) and Natural History (Gould 1986). There was no immediate response from sup-

FIGURE 2.7. Seilacher's (1984) interpretation of the structure of Ediacaran soft-bodied organisms. LEFT: inflated as in life. RIGHT: deflated as in many fossil specimens. Note the relatively rigid vertical walls.

FIGURE 2.7. Seilacher's (1984) interpretation of the structure of Ediacaran soft-bodied organisms. LEFT: inflated as in life. RIGHT: deflated as in many fossil specimens. Note the relatively rigid vertical walls.

porters of the orthodox view of the Ediacaran fauna. Martin Glaess-ner and Mary Wade, long-standing proponents of the orthodox view of the Ediacaran fauna, have not yet published a response to the vendozoan hypothesis. Jenkins (1988) argues that arthropods, at least, must be present in the Ediacaran fauna, because of the presence of trace fossils that look as though they were formed by the legs of a Precambrian trilobite-like arthropod, but Jenkins has yet to publish photographs of these trace fossils.

The contrast between the conventional view and the Vendozoa hypothesis had not been fully resolved, but several paleontologists well-informed on the matter favor the orthodox viewpoint (Conway Morris 1987a, 1987b; Valentine and Erwin 1987). Seilacher himself takes the radical part of his Vendozoa hypothesis with a "grain of salt" and notes that the main function of this provocative idea is to stimulate research in the field of Precambrian paleontology and to free it from taxonomic preconceptions (Seilacher 1985). In a sense, Glaessner's and Seilacher's approaches are both extremes: the former sees the in the Ediacaran fauna mostly members of living phyla, and the latter sees mostly representatives of extinct phyla.

Which approach is closer to being correct? As Bengtson (1986) points out, the question cannot be answered with a simple verdict. Glaessner's philosophy of "shoehorning" Precambrian taxa into modern phlya is at least partly unjustified. Somewhat provocatively, Bengtson (1986) claims that instead of using "the present as the key to the past" (the fundamental geological mindset for interpreting past processes), Glaessner tends to use it as a "keyhole" to the past, letting our understanding of modern phyla narrow our field of vision to a small fraction of what it could have been, had we instead used it as a "key" to open the door to new interpretation. Indeed, some of the interpretations of the "Glaessner school" seem forced. Jenkins (1984; in his text, figure 2) reconstructs Kimberella with delicate cnidarian gonads, an interpretation that is in our opinion an overinterpretation. The vendozoan hypothesis merits serious attention, and we accept it as being closer to the truth than the conventional classification of Ediacaran fossils. At the very least, Seilacher has pointed out some important differences between the Ediacaran me-tazoans and their supposed cnidarian analogs.

We have much more information about the Ediacaran fauna than we did in the 1960s when much of the seminal taxonomic research was done by Martin Glaessner and Mary Wade. Nevertheless, it is not yet possible to unequivocally choose between such competing hypotheses as Seilacher's vendozoa hypothesis and the conventional view. Considering their impact on our understanding of early animal evolution, the unresolved questions of classification will continue to dominate discussion of the Ediacaran body fossils for some time to come.

Important as it is to know the biological affinities of the Ediacaran soft-bodied fossils, the Vendozoan controversy raises an even more important issue concerning these organisms. Many of the Ediacaran creatures were laterally compressed organisms, although a number of them were sac- or bag-shaped. Seilacher (1984, 1985) recognizes that flattened body shapes maximize surface area for the takeup of oxygen and food dissolved in seawater, and perhaps also for the absorption of light. "Normal" metazoan animals generally have plump, more or less cylindrical, bodies. For very small, thin skinned animals, cells near the body surface can get oxygen and expel waste by simple diffusion across the cell surface membranes. Waste products such as carbon dioxide will be supersaturated inside of the animal's body, and will tend to migrate out of its cells and into the open environment. The reverse is true for oxygen; it will tend to migrate into the cells because its concentration is greater on the outside than on the inside of an oxygen-respiring animal. Animals such as frogs and salamanders are able to respire (at least in part) in this way. But for most large, cylindrical animals, diffusion respiration will not work because diffusion is ineffective for cells buried deep within the animal's body. This is a consequence of the fact that as an animal increases its size, its total volume outstrips its surface area by a large margin.

When an animal grows larger without changing its shape, every linear dimension (length, for instance) will increase. But the surface area of the animal increases faster than the increase in linear dimension, and the volume of the animal increases faster still. More precisely, if the length of an animal is doubled, its surface area increases by a factor of four, and its volume goes up by a whopping factor of eight! The new animal twice as long has eight times as much body volume.

With such dramatic increases in volume, how can a "thick" organism care for the nutrient needs of the cells buried deep inside its body? A large, cylindrical animal is clearly too thick for diffusion to work as a means of respiration. We metazoans have developed intricate systems of pipework and tubing to deliver nutrient and waste removal services to interior cells. Circulatory systems, digestive tracts, gills, and lungs are all solutions to the problems associated with volume increase. But this is not the only possible solution for large organisms. Dickinsonia, up to a meter in diameter, was flat and thin enough to use its expanded body surface for respiration and waste removal (Runnegar 1982), thereby avoiding reliance on complex internal plumbing.

A number of living organisms survive in this way. Some modern protists, up to 38 millimeters in length, are able to feed without "eating" anything and without photosynthesizing. These foramini-fera simply absorb nutrients dissolved in sea water (Delaca et al. 1981). Deep-sea worms, clams and other organisms living near mid-ocean hydrothermal vents absorb hydrogen sulfide (usually toxic to "normal" animals) from the vents, and with the help of internal, symbiotic bacteria, convert the sulfide into food. This arrangement between bacteria and metazoans is called chemosymbiosis. Photo-symbiosis, an arrangement in which interal "gardens" of photosyn-thetic monerans live within the tissues of a metazoan host and provide it with food, is utilized by a number of modern animals, including corals and some clams. These "solar-powered" animals are proving to be more abundant in marine communities than previously thought (Rudman 1987). A beautiful sea slug (or nudibranch) called the blue dragon (Pteraeolidia ianthina), is a one-time-only predator of coral polyps. Early in its life, the young blue dragon eats a coral polyp. But instead of digesting its victim's internal photosym-bionts (called zooxanthellae), the sea slug cultivates them and allows them to multiply within its own body. The blue dragon is apparently able to survive entirely on the excess foodstuffs made by the moner-ans, since adult blue dragons do not eat coral. The body of the blue dragon is covered with tubular outgrowths, called cerata. These cer-ata, filled with algae, form fanlike arrangements that optimize the light-gathering ability of the slug's body, and also give the slug its characteristic color (Rudman 1987). The adult blue dragon looks something like a sprig of blue spruce.

There is every reason to believe that some of the unorthodox feeding strategies noted above—passive nutrient uptake, chemo-symbiosis, photosymbiosis—were employed by members of the Edi-acaran fauna. A. G. Fischer (1965) was first to suggest that the flattened shapes of the Ediacaran fossils would have been conducive to the harboring of photosymbiont tenants. We recently asked Fischer to comment on his idea, now over two decades old:

Yes, so far as I know that thought of photosymbiont associations was original with me—not that that seems so important.

I find it hard to think of primitive things like that getting so big without some such mechanism —and I had no idea then of how big Dickinsonia actually gets or got (written communication, 1988).

Seilacher (1984) added that body shapes with high surface area are also good for absorption of simple compounds such as hydrogen sulfide, needed for one type of chemosymbiosis. A sea-floor "pancake" like Dickinsonia may have absorbed nutrients directly from sea water through its enhanced surface area, or alternatively, may have lived with an internal "garden" of symbiotic, photosynthetic, food-producing monerans or protists. Flatness of body is not, of course, a requirement for these feeding strategies. A newly named fig-shaped Ediacaran fossil Inaria karli (figure 2.8) was originally

Inaria Karli
FIGURE 2.8. Inaria karli has a sac-shaped, instead of flat, body plan. This figure shows a cutaway of a mature specimen in life position. Greatest width of specimen about 7.5 cm. (After Gehling 1988)

quite three-dimensional. Gehling (1988) suggests that Inaria possessed a bag-shaped body plan designed for photosymbionts, and may have acted like a "respiring culture-chamber." Strategies such as these may have rendered these organisms independent of external, digestible, living sources of food. The ecological implications of such strategies will be considered in the chapter 7 discussion of the "Garden of Ediacara."

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