The Garden of Ediacara

Anatomy is destiny. sicmund freud

Every body needs food. Less simply put, all living things require energy-rich molecules that can be broken down by organisms to provide energy. As mentioned in the introductory chapter, some autotrophic organisms are able to create these molecules by using energy sources and chemical building blocks taken from the nonliving environment that surrounds them. The bodies of heterotrophs are dependent on autotrophs to fabricate these energy-rich molecules for them. Because photosynthesis is by far the most common process of autotrophy, nearly all of the food on earth is ultimately fashioned from the energy of sunlight.

Photosynthesis is generally considered to be a characteristic of plants in the traditional usage of the term "plant." Nonbiologists are sometimes surprised to learn that animals such as the blue dragon sea slug also are photosynthetic, as was discussed at the end of Chapter 1. One might argue that marine animals with zooxanthellae (symbiotic protists) are not truly photosynthetic because it is the protists that do the photosynthesis, not the animal. The protists just happen to be inside the animal. We would argue that this is not an important consideration, since photosynthesis in all eukaryotic (nucleated) cells is accomplished by chloroplasts, tiny organelles that are the cell's photosynthesis factories. Chloroplasts are now thought by many biologists to have arisen by a symbiosis event in which a small, photosynthetic moneran took up symbiotic residence within a larger microbe (Margulis 1981). The symbiotic relationship eventually became so well established that it became an obligatory relationship for both the host microbe and the smaller symbiont mo-neran. Reproductive provisions were made to pass the genetic material of the symbiont, as well as the host, on to succeeding generations. It would sound strange to describe an oak as a "multicellular alga invaded by photosynthetic moneran symbionts," but that is —in essence — what a tree is. Animals with photosynthetic protists in their bodies are able to create food internally, in the same way that an oak tree can, so we feel that these animals can be correctly called photosynthetic.

Trophic strategy can influence anatomy in important ways. The bodies of many multicellular animals have been modified to enhance their sunlight-capturing abilities. Guy Narbonne has suggested to us (1988, personal communication) that exclusively heterotrophic feeding may not be as basic an attribute of animals as one might expect. Many of the most primitive types of living metazoa contain photo-symbiotic microbes or chloroplasts derived from microbes. Examples include the tiny green turbellarian worm Convoluta, the green hydra Chlorohydra, the sedentary jellyfish Cassiopeia, and the beautiful, green, sea anenome Anthropleura (Pearse et al. 1987). The blue dragon sea slug, with its symbiont-packed cerata, is yet another example of photosynthesis in an animal (Rudman 1987). The clam CoTculum fabricates "windows" in its shell to admit more light for its internal protists (Seilacher 1972).

Flattened bodies with large, exposed surface area are also found in many animals with photosymbiotic protists. As noted earlier, Fischer (1965) first suggested that the flattened shapes of Ediacaran creatures would have helped photosymbiont tenants gain sufficient light. Seilacher (1984) championed this idea by claiming photosynthesis as a possible trophic strategy for these apparently gutless and mouthless Ediacaran creatures.

The most obvious reason for any organism, regardless of what kingdom it belongs to, to evolve a leaf-shaped body is to maximize its surface area. Leaf shape evolves in response to factors in addition to surface area requirement, but the surface area requirement, in all cases we are aware of, is the most important factor. Most of the Ediacaran frond fossils are leaf- or fern-frond-shaped — recall the derivation of the genus name Pteridinium (figure 2.2) from the Greek word for fern. Even Dickinsonia (figure 2.5) roughly resembles an elm leaf. Leaves of modern plants and Ediacaran animals probably evolved similar shapes for the same reason, namely, maximization of surface area.

It is important to emphasize that we are making an inference when we argue that the soft-bodied creatures of the Ediacaran fauna were maximizing their surface area for the purposes of autotrophic feeding. An inference in the sciences is a statement or assumption that cannot yet be unequivocally confirmed, but which seems to fit the facts available. As such, most scientific inferences have the same status as an educated guess in other forms of human discourse. An educated guess may seem a shaky foudation on which to base a major theory, but many breakthroughs in science have occurred when scientists were willing to gamble by looking at a scientific problem from a conjectural and unconventional perspective. With' regard to the Ediacaran biota, it is certain that "normal" heterotrophic animals were present because of deposit-feeding burrows — clearly not all Vendian animals needed to maximize their surface areas. The unusual shapes of the soft-bodied fossils, however, require explanation, and the inference of photosymbiosis best accounts for the strange shapes of the Ediacaran body fossils.

Photosymbiosis is not the only possible departure from hetero-trophic feeding, the usual method of food acquisition for modern animals. Seilacher (1984) notes that flat bodies are good for absorption of simple compounds such as hydrogen sulfide, needed for one type of chemosymbiosis. In chemosymbiosis as in photosymbiosis, microbes (in this case bacteria) are held within an animal's tissues as paying guests. The bacteria are able to use the energy stored in hydrogen sulphide molecules that diffuse into the host animal's tissues. The bacteria use the hydrogen sulfide to create food, using biochemical reactions that would be impossible for animals to do by themselves. The bacteria use some of the food for themselves, but great excesses are produced and passed on to the host animal's tissues.

The greatest zoological discovery of this century is that of the deep-sea vent faunas (Weisburd 1986 reviews a recent discovery).

These faunas include giant, gutless pogonophoran worms and clams. Such large animals are able to live in these lightless waters thanks to internal bacteria that metabolize the hydrogen sulfide percolating up from the volcanic activity of a mid-ocean rift as it creates new sea floor. The vent faunas appear to survive largely independent of sunlight-derived food, and the clams and pogonophoran worms have been termed "autotrophic animals" by Felbreck (1981).

There may be important similarities between the ecologies of these flattened Ediacaran creatures and the modern deep sea vent faunas. Most Vendian soft-bodied fossils are from sediments that were deposited in shallow well-lit marine environments, and photo-symbiosis would have been easy for flat, soft bodied creatures living under these conditions. A few Vendian fauna localities, such as the Mistaken Point fauna of Newfoundland, were deposited in deeper water (Anderson and Conway Morris 1982). Assuming that they lived near where they were deposited, photosymbiosis would have been impossible for these creatures because sunlight does not penetrate to these great ocean depths. Must one then conclude that these organisms were all heterotrophic?

Almost none of the Mistaken Point soft-bodied fossils are also known from shallower water deposits. Of the soft-bodied, high surface area forms, only one has been found amidst shallower water Ediacaran faunas. One interpretation of these differences is that the Newfoundland soft-bodied creatures used their flat bodies, as Seilacher (1984) suggests, for the absorption of hydrogen sulfide or other nutritious gases. The presence of Ediacaran soft-bodied fossils in deep water sediments is therefore no proof that they were exclusively heterotrophic feeders.

The Mistaken Point fauna is deposited in a turbidite, a type of deep-sea sediment that would not necessarily form near sea-floor volcanic activity. If the organisms were indeed absorbing gases from the sea floor, this presupposes a constant source of gaseous nutrients. There is no evidence for hydrothermal activity in the Mistaken Point strata; where could such gases have come from? There are a number of possible sources for such gas. Sulfides can be produced in sediments by the microbial degradation of buried organic material, and can then slowly percolate to the sediment-water interface. Chemosymbiotic tube worms have been reported living over "breaches" or open rifts, in sediments off the Oregon coast (Anderson 1985). No hydrothermal activity or hot water springs are associated with the Oregon tube worms, and they seem to be feeding off of methane-enriched waters that are percolating up through the sediment rifts. There are tremendous reservoirs of methane and other natural gases held in sea floor sediment as gas hydrates (Kvenvolden and M. McMenamin 1980); these gases may leak to the sea floor over time, particularly after times of glaciation when the temperature of sea floor sediments is changing. Perhaps some or all of the Newfoundland organisms were trapping hydrogen sulfide, methane, or other nutrients that were escaping from sediments below, just as their shallow water counterparts were capturing sunlight radiating from above.

A form of chemotrophy (feeding on chemicals) that does not involve symbiosis is simple absorption of nutrients dissolved in sea water. Although this might not seem a particularly efficient way of obtaining food, there are tremendous amounts of "unclaimed" organic material dissolved in sea water. Monerans allow these nutrients to diffuse into their cells, a fact well known to microbiolo-gists. Less well known is the fact that larger organisms can feed in this way also. Benthic foraminifera up to 38 millimeters long from McMurdo Sound, Antarctica, take up dissolved organic matter largely as a function of the surface area of their branched bodies (Delaca et al. 1981). These protists live under semipermanent sea ice. Even though this environment is usually poor in dissolved organic matter compared to other environments, these forams are able to satisfy their food requirements by the direct uptake of dissolved nutrients. Members of the Ediacaran fauna, particularly those without access to sunlight, may have also fed by absorption of dissolved nutrients.

Although there is as of yet no unequivocal proof, it seems reasonable to infer from their shapes that members of the Ediacaran fauna used photosymbiosis, chemosymbiosis, and direct nutrient absorption to satisfy their food needs. Since these methods do not involve killing, eating, and digesting other living things, we will refer to them as "soft path" feeding strategies. Heterotrophic organisms use "hard path" feeding strategies because they need to use up the bodies of other organisms for energy. The higher in the food pyramid, the "harder" the feeding strategy, on up to the keystone predator (top carnivore) at the top of any particular ecosystem's trophic pyramid. It is important to note that the term "hard," as used here, does not necessarily imply that autotrophic organisms have any easier a time obtaining their food than do heterotrophic organisms. Green plants are not very efficient at converting sunlight to food; sunlight can be thought of as an elusive prey because it is not a concentrated energy source (Ricklefs 1976). Low food concentrations are a major difficulty encountered by organisms employing soft path feeding strategies.

Deposit feeding is intermediate between hard and soft paths. Much of the edible organic material in sediments accessible to deposit feeding organisms is dead, in the same sense that dissolved nutrients in seawater are nonliving, although both were once derived from living sources. Ingestion of nonliving organics in sediment is the "soft" aspect of deposit feeding. This organic material, however, is usually coated with bacteria and other heterotrophic monerans that invariably get ingested and digested by deposit feeders, constituting the "hard" part of deposit feeding. As noted earlier, trace fossils indicate that deposit feeding undoubtedly occurred during the Vendian. Filter feeding, or capturing food suspended in the water, also has components of both hard and soft paths because suspension feeders can take both living and nonliving food from the water.

Many marine animals and protists today utilize photosymbiotic, chemosymbiotic, or direct nutrient absorption feeding strategies. Why, then, are flattened, soft-bodied forms with these food-producing habits rare in modern seas? Flattened forms are rare today, the main modern exception being the fleshy, multicellular marine algae that are both soft-bodied and reliant solely on soft-path feeding.

Glaessner (1984) puzzles over the reasons why soft-bodied organisms up to one meter in greatest dimension were able to colonize the Vendian sea floor unmolested, and he notes that there is no evidence for large predators in the Vendian. There is not a single exclusively hard path organism known from the Vendian, particularly if one questions the jellyfish affinities suggested for some Edi-acaran fossils. What are the implications of a predator-free Vendian?

One way to approach this question is by asking another question. Have increases in the numbers, types, and abilities of predators caused long-term changes in marine ecology? This question has led to a proposal by G. J. Vermeij (1987) that the intensity of predation has continually escalated throughout the last 600 million years, resulting in improvements in the ability of prey to avoid being eaten. Vermeij's hypothesis of evolutionary escalation can be viewed as a positive feedback cycle, with ecological conditions within a given habitat becoming more rigorous with the passage of hundreds of millions of years. This view of predator-prey escalation as a dominant theme or trend in the history of life has been criticized as being limited and suggestive rather than decisively convincing (Kohn 1987), in part because of the difficulty of obtaining accurate paleoecological information from the imperfect fossil record. Nevertheless, an analysis of the paleoecology of Ordovician to Pleistocene brittlestar (a type of echinoderm similar to starfish) communities shows that long-term changes in predation pressure are likely to have been responsible for the demise or restriction of certain marine communities, particularly those dominated by filter feeders living near the sea floor (Aronson and Sues 1987). Brittlestars are sea-floor filter feeders that are very susceptible to predation by swimming predators such as fish. Since dense fossilized stands of brittlestars are less common after the Paleozoic than before, Aronson and Sues (1987) conclude that the frequency of brittlestar-dominated communites has decreased as a result of increases in the numbers and efficiency of swimming predators.

The hypothesis that the Cambrian skeletonization of animals was related to the appearance of predators has a long history, dating back over eighty years (Evans 1910; Schuchert and Dunbar 1933; Hutchison 1961). Glaessner (1984) dismissed this idea as simplistic and "anthropomorphic." The rarity of fossilized Cambrian predators encouraged arguments against the importance of predation in the Cambrian (Valentine 1973), in spite of Hutchison's (1961) suggestion that early predators were likely to have been entirely soft-bodied. Hutchison's inference has been largely borne out, thanks to recent analysis of Middle Cambrian soft-bodied fossils in the Burgess Shale of British Columbia, Canada. In addition to typical Cambrian shelly fossils such as trilobites, the Burgess Shale has yielded soft-bodied or lightly sclerotized predators such as the priapulid worm Ottoia and Anom-alocaris (Whittington 1985).

Anomalocaris is a huge (by Cambrian standards) animal whose maximum length approaches half a meter. Its name means "anomalous shrimp," and was first applied to vaguely shrimplike segmented objects first recognized in the Burgess Shale and later found in the Lower Cambrian Kinzers Formation of Pennsylvania. These objects later proved to be Anomalocaris' paired frontal appendages. These appendages are usually found detached from the main Anomalocaris body. Also usually found separate is the circular mouth of Anomalocaris, which was originally described in error as the jellyfish Pey-toia. Several complete specimens from the Burgess Shale have allowed Whittington and Briggs (1985) to combine the various fossil fragments and reconstruct Anomalocaris to its previous glory. The reconstruction of Anomalocaris (figure 7.1) resembles no living ani-

mal. The predator had a body shaped like a flattened teardrop, both sides of which were flanked by swimming fins. At its broad head were a pair of jointed appendages for drawing prey into its circular mouth, which was shaped like a pineapple ring, lined with teeth. Anomalocaris has been blamed for injuries to the carapaces of Lower and Middle Cambrian trilobites. These trilobites have gouges taken out of them, and the margins of the wounds display a raised rim, indicating the the trilobite survived the attack and lived to heal the injury (Rudkin 1979]. An unusual trilobite specimen —described as the species Olenellus pecularis by Resser and Howell (1938) —from the Kinzers Shale, is actually a specimen of the familiar Lower Cambrian trilobite Olenellus thompsoni with a damaged left side of the cephalon. Since Anomalocaris appendages occur in the same beds, it seems reasonable to infer that this predator was responsible for damage to the peculiar Olenellus.

Although examples of damaged prey and possible antipredatory adaptations greatly outnumber actual fossils of predators in the Lower Cambrian, some of the oldest known phosphatic small shelly fossils probably belonged to predators. Spines belonging to the genus Pro-tohertzina (figure 4.6) are found in the earliest Cambrian strata, and, as noted earlier, have similarities in microstructure to the grasping spines of modern chetognaths or arrow worms. Arrow worms are a living phylum of voracious micropredators. They are free swimming and can attack prey under a few millimeters in length.

In addition to Anomalocaris and Protohertzina, there are a few other possible Lower Cambrian predator fossils. The Lower Cam-

FIGURE 7.1. Reconstruction of the 45 cm-long Cambrian predator Anomalocaris.


FIGURE 7.1. Reconstruction of the 45 cm-long Cambrian predator Anomalocaris.

brian trace fossil Teichichnus resembles the traces made today by mobile, carnivorous polychaete worms (Seilacher 1957). The trace-maker of Dolopichnus, a cylindrical burrow filled with fragments of trilobite carapaces, seems to have attacked Lower Cambrian trilob-ites (Alpert and Moore 1975). The Dolopichnus burrow may have housed a sea anemonelike predator capable of stunning trilobites and drawing them into its gut. An object from the Kinzers Formation described by Resser and Howell (1938) as a chela (claw) of possible crustacean origin has turned out not to be a chela, but may be a fragment of a predatory species such as Anomalocaris (D. E. G. Briggs, personal communication, 1986). Fossil chelae such as crab and lobster claws did not become abundant until well after the Cambrian, but D. H. Collins has recently discovered the fossil of a predatory arthropod, with five pairs of claws on its head, in the Burgess shale (Middle Cambrian). This new fossil is informally called "Santa Claws" (Collins 1985); this has been playfully echoed in the new formal taxonomic name for the species — Sanctacaris uncata (or "Santa Claws shrimp"; Briggs and Collins [1988]).

Some trilobites may have been predatory. Lower Cambrian specimens of the trilobite Redlichia from South Australia bear wounds that may have been inflicted by fellow trilobites (Conway Morris and Jenkins 1985). The Middle Cambrian trilobite Olenoides may have grasped small prey with the aid of spinose limbs (Whittington 1985).

Examples of possible antipredatory adaptations are common in Lower Cambrian faunas. If an animal can form a skeleton, forming spines as an extension of the shell is an effective way to deter predators. Some Lower Cambrian shelly fossils may have been devoted entirely to making a spiny protective coat, such as the Lapwor-thella (figures 4.8 and 4.9) scleritome discussed in chapter 4. The calcium carbonate sclerite Chancelloria belonged to an extremely spiny scleritome (Bengtson and Missarzhevskii 1981). The individual sclerites have six or more spines radiating from a central boss. Complete scleritomes (figure 4.10) of Chancelloria spicules are known from the Burgess Shale, and the Chancelloria animal must have been densely covered with spines.

Spines were in the defensive repertoire of other early shelly animals. The oldest articulate brachiopod, a specimen possibly belonging to the genus Nisusia (similar to the specimen shown in figure 4.20) from Lower Cambrian strata of the Siberian platform, has low tubercles on the shell exterior that have been interpreted as the bases of spines (Ushatinskaya 1986). Shell tubercles indicative of spines are well known from postCambrian brachiopods. The Lower Cambrian brachiopod Acrothele spinulosa from northwestern Africa has numerous slender, closely set spines radiating from the shell margin (Poulson 1960). Such spines could certainly serve a protective function.

One of the helicoplacoid echinoderms, Hehcoplacus curtisi, bears spines on the upper surface of its spindle-shaped, plated exoskeleton. Although these spines would not be as effective as the spines on a modern sea urchin, the spines on this helicoplacoid would still have afforded additional protection to the upper surface of the animal (M. McMenamin 1986). If it was able to expand and contract its spindle-shaped skeleton, this helicoplacoid may have expanded when threatened, enlarging itself into a swollen object covered with short spines in a defensive strategy similar to that of a modern pufferfish or blowfish. In addition to spines, Cambrian echinoderms protected themselves by increasing the rigidity of their shell. Ridges and folds on eocrinoids are certainly protective because they make the echi-noderm more rigid, and enable the inhabitant to live safely and "permanently inside the castle" formed by the calcite plates (Paul 1979:421). Later eocrinoids show greater degrees of rigidification than earlier ones. Helicoplacoids, the earliest known echinoderms (figure 4.18), may have gone extinct so soon because of an inability to rigidify their skeletal plates.

Many of the earliest known trilobites have elongate, pointed spines projecting from their carapaces. The "corners" or genal areas, of the head or cephalon of trilobites frequently developed into a pair of elongate genal spines. Long genal spines might have had uses as support on soft substrates (Clarkson 1979) or to aid molting (Robinson and Kaesler 1987), but they also would make the trilobite less susceptible to attack by predators (Clarkson 1979). Species of Fallo-taspis (figure 7.2) and fudomia (figure 4.13) are representative early trilobites with very long genal spines. The trilobite Callavia broeg-geri had a long projection—extending from the back of its cephalon —called the occipital spine (figure 7.3). The elongate genal spines and throracic spines of Olenellus yorkense (figure 7.4) made this trilobite less vulnerable to predators. Projecting genal spines and thoracic spines made it more difficult to flip a trilobite over and expose the softer underside. Although Lower Cambrian trilobites were unable to enroll (a common defense in Ordovician and later trilobites), even a slight flexure would have raised the spines into a

FIGURE 7.2. The cephalon of Fallotaspis [above], one of many Lower Cambrian trilobites with long genal spines. Width of specimen 5 cm. [After Harrington et al. 1959)
FIGURE 7.3. CaUavia [above], a Lower Cambrian trilobite with a prominent occipital spine projecting from the posterior of the cephalon. Width of specimen 3.5 cm. (After Harrington et al. 1959)
FIGURE 7.4. Olenellus [above], spiny Lower Cambrian trilobite. Length 5 cm. (After Harrington et al. 1959)

defensive posture. Some trilobites had to flex the cephalon downward in order to begin the molting process (McNamara and Rudkin 1984); this same reflex could have served a double duty by raising the genal spines into defensive position when the trilobite was threatened.

The Lower Cambrian trilobite Laudonia (figure 4.15) has conspicuous "extra" spines (called metagenal spines) projecting from the front part of the cephalic margin. These spines increase the effective maximum width of the trilobite in much the same way that erectly-held fin spines increase the diameter of many modern bony fish. Spines such as these are present in the larval stages of several related trilobite genera, but only Laudonia retains elongate metagenal spines in adult stages. These spines may have dissuaded the contemporary predator Anomalocaris or other predators from frontal attack.

Spines, of course, are not the only strategy that prey can use to defend themselves against predators. Trilobites are the first organisms known to have had complex visual systems (Robinson and Kaestler 1987). A thorough search for eyes in the Ediacaran fossil Spriggina (a superficially trilobite-like form) has so far proved unsuccessful (Kirschvink et al. 1982), although if arthropods were present in the Ediacaran biota (Jenkins 1988), it won't be too surprising if Ediacaran body fossils eventually turn up sporting eye spots. Judging from the large, elongate eye regions (ocular lobes) of early trilobites such as fudomia, Nevadia, and Fallotaspis (figures 4.13, 4.14, and 7.2, respectively; M. McMenamin 1987b), the first trilobites had well-developed eyes that could have been used to avoid predators. Eyes can help animals that live on the sea floor to avoid being eaten, and well-developed eyes seem to have been rare or absent in the Ediacaran fauna.

Toxic secretions are also used by animals for protection. The inarticulate brachiopod genus Mickwitzia has dense punctae running through its shell valves (figures 4.19b, 7.5, and 7.6). These punctae have been interpreted (M. McMenamin 1986) as conduits for chemical deterrants used to discourage predators, because shells occurring in the same beds but belonging to different animals have been bored by parasites and predators (figures 7.7 and 7.8). Mickwit-ziid brachiopods, restricted to the Lower Cambrian, are some the the largest Lower Cambrian brachiopods known. A form from the Poleta

Cambrian Brachiopods

FIGURE 7.5. Shell structure of Mickwitzia, a Lower Cambrian inarticulate brachiopod (see also figure 4.19B). Note how punctae pass through all three shell layers. Width of valve 1 cm. 118

FIGURE 7.5. Shell structure of Mickwitzia, a Lower Cambrian inarticulate brachiopod (see also figure 4.19B). Note how punctae pass through all three shell layers. Width of valve 1 cm. 118

FIGURE 7.6. Enlarged view of a middle wall puncta in Mickwitzia, showing axial, hollow, phosphatic tube. The hollow tube may have carried chemicals to the exterior of the shell which were irritating or even toxic to boring organisms and other predators. From Lower Cambrian Puerto Blanco Formation, Mexico. Scale bar = 5 microns. (From M. McMenamin 1986; used with permission of the Society of Economic Paleontologists and Mineralogists)

FIGURE 7.6. Enlarged view of a middle wall puncta in Mickwitzia, showing axial, hollow, phosphatic tube. The hollow tube may have carried chemicals to the exterior of the shell which were irritating or even toxic to boring organisms and other predators. From Lower Cambrian Puerto Blanco Formation, Mexico. Scale bar = 5 microns. (From M. McMenamin 1986; used with permission of the Society of Economic Paleontologists and Mineralogists)

FIGURE 7.7. A specimen of the monoplacophoran Bemella with a possible bore hole near the apical end of the shell. Mickwitziid brachiopods (figure 7.6) found in the same beds have not been bored. From the Lower Cambrian Puerto Blanco Formation, Mexico. Scale bar = 100 microns.

Formation of California has a shell valve measuring 3.7 cm in diameter, a size that dwarfs most other Cambrian brachiopods (figure 7.9). The Lower Cambrian evidence presented above for antipredatory

FIGURE 7.8. A specimen of Hyolithellus with a possible bore hole through the side of its shell. From the Lower Cambrian Puerto Blanco Formation, Mexico. Scale bar-50 microns.

defense and damaged prey stands in stark contrast to the lack of such evidence in fossil communities of the Vendian. The largest known organisms of the Vendian are the members of the Ediacaran fauna, and they are twice as large as the largest animals of the Cambrian.

FIGURE 7.9. The largest known Early Cambrian brachiopod; a large, partly crushed specimen of Mickwitzia. From the Poleta Formation of California. Greatest width 3.7 cm. (Photograph courtesy (. Wyatt Durham and Ellis L. Yochelsonl

Despite the large sizes of the Vendian fossils, there is no known evidence for predation on any of the numerous Ediacaran fauna specimens. Indeed, it is a wonder that these soft bodied organisms existed on the sea floor unmolested (Glaessner 1984). The Ediacaran fauna may have gone extinct because it lacked defenses against Cambrian predators (Brasier 1979), although a few members of the fauna seem to have lingered on into the Cambrian.

The unusual fossil Xenusion auerswaldae (figures 2.1 and 7.10) was originally described in 1927 as an arthropod with multiple paired appendages. The type specimen of Xenusion was found in a slab of thinly bedded sandstone from surficial deposits of northern Germany. Jaeger and Martinsson (1966) traced the distinctive pinstriped sandstone of the Xenusion cobble back to the Kalmarsund Sandstone of southern Sweden, and presented a convincing case that the fossil was originally part of this Swedish formation and was subsequently

FIGURE 7.10. Side view of Xenusion auerswaldae, showing spines on medial humps. These spines are the only possible defense against predators that have been seen in an "Ediacaran-type" soft-bodied fossil. Spine length is conjectural because the spines are broken in all known specimens of this species. Length of frond approximately 7.5 cm. (From M. McMenamin 1986; used with permission of the Society of Economic Paleontologists and Mineralogists)

carried south across the sea to Germany by the action of Pleistocene glaciers. Jaeger and Martinsson (1966) gave Xenusion an earliest Cambrian age because of the occurrence of abundant vertical Skoli-thos burrows in the Kalmarsund Sandstone, and also agreed that this fossil was an organism with walking or swimming appendages. They made a latex cast of the fossil, and "flexed" the cast to give it a more "natural" (read "arthropod-like") appearance (Jaeger and Martinsson 1966; their figure 1, p. 437), perhaps because they were bothered by the fossil's "missing head" and its unarthropodlike concavity.

Halstead Tarlo (1967) reinterpreted Xenusion as allied to Edi-acaran frond-shaped fossils such as Ranged and Charniodiscus (Glaessner 1979), in which case Xenusion would never have needed a head. Halstead Tarlo's (1967) interpretation seems most likely, particularly considering that other Ediacaran frond fossils have been confused with the thoracic regions of supposedly incomplete arthropod fossils. Pteridinium-Mke frond fossils from Stanly County, North Carolina (Gibson et al. 1984) were originally described as trilobite fossils lacking cephalons (St. Jean 1972). The individual "arms" in Xenusion are divided into eight or more segments, and all of these segments have faint parallel markings suggesting that they, too, may be divided into segments. Several orders of segment subdivision are seen in Ediacaran fossils such as Rangea (Jenkins 1985), and this feature accords with Seilacher's (1984) analogy between members of the Ediacaran fauna and air mattresses. Xenusion shares this "quilted" character, further supporting the hypothesis that Xenusion is a relict member of the Ediacaran fauna (M. McMenamin 1986). Note also the similarity of Xenusion to the Ediacaran 'pennatuloid organism' illustrated by Conway Morris (1989; his figure 2.4E).

Paired columns of humps run down the midline of Xenusion (figure 2.1), each of which bears an outwardly directed, prominent spine (figure 7.10). Because the only known specimens of Xenusion are incomplete, the original length of these spines is not known. A defensive function for Xenusion's spines is plausible. They may, in fact, have been mineralized, although this cannot be proven with the fossil material currently available. Jenkins (1985) interprets the striate markings along the axis of one Ediacaran frond fossil as representing the impressions of stalk-supporting spicules. The spines of Xenusion may be stalk-supporting spines that grew at ninety degrees to the stalk axis, thus providing defense rather than support (figure 7.10). This type of defense would be compatible with an inferred soft path feeding strategy (such as photosymbiosis) in ways that an opaque, impermeable mineralized shell might not. On the other hand, such spines would not have been a particularly good defense. A new specimen of Xenusion has been found (again, from a glacially transported rock or glacial erratic; Krumbiegel et al. 1980; Schallreuter 1985) in which the secondary branches ("appendages") have been torn off and the central axis is contorted. This specimen was badly battered before being fossilized. Crashing waves might have damaged the new specimen in this way, but the injuries could also be the work of a marauding Cambrian predator.

Xenusion may be the last known member of an Ediacaran dynasty, a distinct period of earth history when flat or high surface area organisms (regardless of taxonomic affinites) had the sea floor to themselves, and survived using soft path feeding methods. Smaller predators were probably present at that time. Microfossils, known as heterocysts, reported from Vendian rocks, are thought to be the remains of heterotrophic protists (Bloeser 1985), and some Ediacaran animals were grazers and possibly filter feeders, but there is no evidence of predators capable of attacking the Ediacaran creatures. Why might this be the case? How could the trophic strategy of eating larger organisms remain undiscovered for millions of years? Unless there were large Vendian predators who have so far eluded paleontologists by avoiding fossilization or discovery, the time of large soft-bodied Vendian creatures can be called a largely predator-free "Garden of Ediacara." The uniqueness of this "garden" may go beyond a simple absence of large predators, however.

The Vendian-Cambrian transition records a profound turnover in the paleoecology of the marine biosphere, and this change is not solely expressed by the appearance of metazoa with biomineralized skeletons. Calcium carbonate-secreting algae appeared at the end of the Vendian (Riding and Voronova 1982), and later radiated to become important components of the earliest Paleozoic reefs.

A final example can illustrate the marine changes that took place during the Cambrian. Trace fossils belonging to soft-bodied organisms of enormous size (up to a half meter in length) are known from Upper Cambrian sediments deposited in very near shore, intertidal environments. One of these fossils, Climactichnites ("climax of trace fossils"), is nicknamed "Honda tracks" by paleontologists because it resembles the tracks made in sand by motorcycle tires (figure 7.11). Climactichnites is known from the Potsdam Sandstone of New York. The maker of Climactichnites was likely a mollusk with a very powerful crawling foot muscle. Also from the Late Cambrian are gigantic radular bites (tooth markings made by grazing mollusks; chitons [figure 4.27] can make these kinds of markings) from a tidal flat sandstone in Saudi Arabia. The creature that made these tracks was at least 10 centimeters in width (Seilacher 1977). Seilacher (1977:375) argues that these giant tracemakers were pioneers that left the water to graze monerans in an environment that was otherwise still uninhabited by higher organisms and "not yet endangered by terrestrial predators." In this last intertidal refuge of both large soft-bodied organisms and Garden of Ediacara monerans, metazoans seem to have been munching the monerans all the way back to the high tide mark.

If the decline of the Ediacaran fauna really was in part a result of the rise of Cambrian predators, the Vendian-Cambrian transition can be seen as a "fall" from the Garden of Ediacara. Quilted organisms such as the Vendian soft-bodied creatures are virtually unknown from later strata, suggesting that, in this case, anatomy was indeed destiny. This would be especially so if their soft path feeding requirements precluded the development of robust armor.

FIGURE 7.11. Climactichnites wihoni, the largest described Cambrian trace fossil, from the Upper Cambrian of New York. Width of each trace fossil 10 cm; the tracemaker was a very large animal (perhaps up to one half meter in length) by Cambrian standards. (From Walcott 1912)

FIGURE 7.11. Climactichnites wihoni, the largest described Cambrian trace fossil, from the Upper Cambrian of New York. Width of each trace fossil 10 cm; the tracemaker was a very large animal (perhaps up to one half meter in length) by Cambrian standards. (From Walcott 1912)

The fall from the Garden of Ediacara was a profound reorganization of our global ecosystem. What could have caused this transition from flattened, soft path feeders to a heterotroph-dominated, hard path biota? We will examine this question further in the next chapter.

0 0

Post a comment