FIGURE 3.1. Number of spurious Precambrian animal fossil descriptions plotted against year. The three highest peaks in the data primarily result from the work of Dawson (1860-1870), Walcott (1890-1900), and various overenthusiastic recent workers (1960-1970).
stone, contained four elongate lobe-shaped objects, each with concentric U-shaped ridges at the end of the lobe. The U-shaped ridges looked a lot like the concentric layers of sediment that form in many types of animal-built burrows. I immediately showed the specimens to the leader of the field expedition I was on. He agreed that they looked very much like trace fossils. When I returned from the desert, I showed the objects to Preston Cloud. Cloud, who by 1982 had acquired a ferocious reputation as the premier debunker of Precam-brian "fossils," agreed that they could be biologic. With youthful enthusiasm, I published a photograph of these objects (along with other discoveries from Mexico), describing them as "probable meta-zoan traces" (M. McMenamin et al. 1983). After publication, I returned to Sonora and attempted to find more specimens of convincing trace fossils in the Clemente Formation, the Precambrian rock unit that had produced these specimens, but without success. I showed the specimens again to Cloud. He still felt that they could be biologic, although he was bothered by the fact that all four of the lobes
were oriented in approximately the same direction. This made him suspicious, but he wasn't sure why. This later proved to be an astute observation. In 1984, while I was completing my doctoral thesis, the Australian paleontologist Malcolm R. Walter visited Santa Barbara. I showed him the enigmatic structures, and he immediately came up with a way to form these lobes without invoking animal activity. Walter has seen flow structures, resembling tiny lava flows off of the flanks of volcanoes, forming in association with sediment fluid-escape cones, also called sand volcanos. Sand volcanos can form when water-saturated sediment is exposed to air, and then disturbed by compactional forces or jostled by earthquakes. When this occurs, the sediment settles and forces water to move upwards. The water will sometimes follow a cylindrical conduit, roughly resembling the vent of an igneous volcano. Sediment entrained in the water stream will be deposited where the dewatering flow meets the air, and can be deposited in a broad-sediment cone, or sand volcano. These sand volcanos are often only a few centimeters in diameter, much smaller than their igneous counterparts. At the center of the sand mound is a small collapse pit, which looks like the vent, or caldera, in an igneous volcano.
When small sand volcanos are preserved in ancient sediments, they are called pit-and-mound structures. Sometimes a particularly fluid slurry is ejected from the sand volcano vent. This slurry can flow down and beyond the flanks of the sand volcano, forming a lobe of fluidized sediment that can settle to create a sedimentary structure that looks very much like a trace fossil with backfilled layers (figure 3.3). Ancient sand volcanos can be recognized in Precambrian sediments of Australia (Walter 1972). Walter (1976) has described similar structures forming today at Shoshone Geyser Basin, Yellowstone National Park. Pit-and-mound structures (figure 3.4) are known from the same locality in the Clemente Formation that yielded the trace-fossillike structures. I am now convinced that these objects are pseudofossils (M. McMenamin 1984), and I plan to be more skeptical in the future when I see Precambrian fossillike objects that are from rocks older than the Ediacaran soft-bodied fossils. Careful attention to the mode of formation of inorganic sedimentary structures can help one avoid the misidentification of pseudofossils.
Even with intense study by a number of different investigators, there are some Precambrian structures that remain possibly biologic and genuinely enigmatic. These unidentified fossillike objects (UFLOs) are continuing sources of dispute. Body fossils can be mimicked by
sedimentary structures of mechanical origin. Circular "medusoids" have been recently reinterpreted as gas pits (Sun 1986) and very ancient "frond-fossils" may be inorganic ice tracks, formed by the scraping of a block of ice across sediments (Jenkins 1986). The UFLO called "Brooksella" canyonensis is perhaps the most famous of these. Described in 1941 by R. S. Bassler, this object (see Hantzschel 1975; his figure 89) consists of a set of radial, overlapping petallike lobes that look something like the remains of a daisy that has had its yellow center removed. "Brooksella" canyonensis has been var iously interpreted as a jellyfish body fossil (Bassler 1941), a pseudofossil formed by gas bubble escape through soft sediment (Cloud 1968, 1973), and a starlike trace fossil, possibly a feeding burrow (Seilacher 1956). Kaufmann and Fursich (1983) support the trace fossil interpretation because of the presence of curved layers in one of the lobes which they interpret as evidence for metazoan burrowing activity. As shown in the discussion above, however, such layering does not rule out formation by an inorganic process.
Convincing trace fossils are known from the late Precambrian, sometimes in association with the soft bodied Ediacaran fossils (Glaessner 1969). These trace fossils are generally simpler, less common, and less diverse than Cambrian trace fossils. There is a significant difference in the complexity and depth of burrowing between Cambrian and Precambrian trace fossils, and it has been argued that the changeover from simple trace fossils to more complex types of traces occurred at more or less the same time as the Cambrian explosion, the first appearance of abundant Cambrian shelly fossils. In a paper that now seems surprisingly ahead of its time, Seilacher (1956) was first to point out the transition in the trace fossil record that occurred across the Precambrian-Cambrian boundary.
Marine trace fossils are generally of three varieties: locomotion traces, deposit feeding traces, and dwelling traces. The three types can and do grade into each other, but in most cases any particular trace fossil is more of one type than of another. Deposit feeding, for instance, nearly always has some component of locomotion because the animal needs to move around to find fresh, unmined organic deposits in the sediment for food. An example of a pure locomotion trace would be scurry marks made by a many-legged arthropod as it moves across a stiff mud, leaving claw marks behind.
Dwelling burrows are actually a form of skeleton made of sediment. The animal has excavated a cavity in the substrate, and this hole or tube in the mud serves as "home base" from which the resident can mine sediment or capture food particles suspended in the water with a filtering device. This type of burrow is also useful for avoiding predators that cannot burrow into the sediment.
Nearly all of the convincing trace fossils from the Precambrian are of the deposit feeding and locomotion varieties. Most of these are simple, tubular burrows of an animal that once moved through the soft (but now cemented into rock) sediment in a more-or-less unidirectional or slightly meandering fashion (Crimes 1987). These types of traces are commonly formed by many different types of unrelated, wormlike organisms (including some annelids or true worms), so it is impossible to learn much about the identity of the tracemaker from these simple traces. Sometimes these traces show evidence of peristalsis, or motion by rhythmic contraction of circular muscle bands along the length of the animal's tubular body. Earthworms move by peristalsis, and food is moved down your esophagus by a similar sequence of muscle contractions.
Few Precambrian animals had learned the "trick" of excavating a home from the sediment. Deep vertical, cylindrical burrows are very rare in Precambrian sediments. Organisms that make these kinds of burrows are often sessile, which means that they spend large amounts of their lives staying in one place. The simplest dwelling trace fossil of this type is called Skolithos (figure 3.5). Skolithos is an un-branched vertical, tilted, or curving cylindrical burrow. The oldest deep (greater than 1 cm in depth), undoubted burrows are Precam-
brian in age. Steeply inclined burrows several centimeters deep called Skolithos declinatus are known from the late Precambrian of the Soviet Union (Sokolov and Ivanovskii 1985). But deep trace fossils such as these are exceptional in Precambrian sediments. Other late Precambrian tracemakers were able to shallowly penetrate sediments. Neonereites threaded a sine-wave-shaped path through the sediment. This tracemaker pratically swam through the sediment, something like a porpoise swimming through surface waters. A Neo-nereites fossil usually looks like a train of holes running across the bedding plane surface of a rock sample, where fracturing of the rock along a horizontal bedding plane surface has cut through the vertically sloping portions of the trace fossil.
Precambrian deposit feeding burrows are quite shallow, most being restricted to the upper centimeter of the sediment surface. The shallow depth of Precambrian deposit feeding burrows, like the near absence of Precambrian vertical burrows, is puzzling. With the beginning of the Cambrian, the depth of burrowing for both deposit feeding burrowers and dwelling burrow formers took a leap downward. Skolithos specimens 10 cm in length and longer have been reported from Early Cambrian rocks in Scandinavia and elsewhere. The increased depth of Cambrian deposit feeding is exemplified by deep, ploughing trace fossils from northern Mexico (figure 3.6). Traces such as these, in which an animal greater than one centimeter in diameter is able to muscle its way through the sediment, are unknown before the very end of the Precambrian.
Along with the increased depth of burrowing, there was an astonishing increase in the diversity of trace fossils at the beginning of the Cambrian. The number of different types of trace fossils soared at this time, and their abundance in any particular sedimentary environment also went up. Skolithos is so abundant in parts of the Early Cambrian Kalmarsund Sandstone of Sweden that it could be called "organ-pipe" rock. Arthropod trace fossils, some made by the earliest trilobites, became abundant for the first time. Cruziana, a trace fossil made by a deposit-feeding trilobite or trilobite-like organism as it "cruises" through the sediment, was commonly formed in the earliest Cambrian (figure 3.7). Deep furrowed traces appeared in abundance on the surface of sandstone deposits. Some of these fossils meander in complex, geometrically regular patterns virtually unknown in Precambrian traces. Skolithos is by no means the only new type of dwelling burrow. Diplocraterion appeared as a U-shaped burrow with two vertical arms, and other types of dwelling burrows
appeared, including one that flared upwards like an inverted cone. Some burrows such as Phycodes (figure 3.8) had fan-shaped arrays of deposit-feeding probe tracks radiating from a central dwelling tube. The tracemaker of Phycodes was based in the central tube and mined the sediment immediately nearby. This type of activity also forms star- or flower-shaped trace fossils (figure 3.9). Even the tubular deposit feeding burrows displayed some fancy new forms (Crimes 1987). Treptichnus, the "feather-stitch" trace fossil, consists of linked, alternating segment deposit feeding tubular burrows. The maker of Gyrolithes spiralled downward into the sediment like a corkscrew.
Even shallow, sediment surface burrows in the Cambrian show a marked change in character over their Precambrian predecessors. In the late 1840s and early 1850s, the oldest fossil known was Old-hamia antiqua, a delicate fan-shaped fossil discovered by Thomas Oldham in the Lower Cambrian sediments of Bray Head, Ireland (Secord 1986). These radiating or fan-shaped trace fossils imply a fairly complex "advance and retreat" type of deposit feeding behavior, and are known only from the Cambrian (Hofmann and Cecile 1981).
Net-shaped trace fossils or graphoglyptids (figure 3.10) are distinctive because of their unusually regular geometric pattern. The first graphoglyptid trace fossils appear in the earliest Cambrian. Grapho-glyptid traces get their name from their resemblance to writing carved in stone, and indeed they can look like written messages from
the history of past life. A typical graphoglyptid trace consists of a hexagonal network of cylindrical burrows a few millimeters below the sediment surface. This network connects to the open water by a series of tubes that rise upwards from the points where three of the horizontal canals meet to form one of the corners of a hexagon. Crimes and Crossley (1980) have shown that the graphoglyptid burrow geometry is engineered to make currents flow through the buried tunnel network. Flow through the burrow network is passive; in other words, it results from open water currents passing over the vertical risers, or pipelike tubes, in the graphoglyptid system. This aquatic ventilation of the buried tunnel system, a simple consequence of the burrow geometry, is of great use to the maker of the burrow. Unfortunately, the actual feeding strategy of the graphoglyp-tid tracemaker is unknown, even though modem graphoglyptid traces (identified as hexagonal patterns of holes, the terminations of the vertical tubes) have been spotted by cameras surveying the deep-sea floor (Weisburd 1986). A recent oceanographic expedition successfully collected an entire graphoglyptid burrow system in a sediment
grab sample, but the sample was inadvertently washed down a drain by a technician before the tracemaker could be identified!
One hypothesis for the food-collecting strategy of the graphoglyp-tid tracemakers is filter feeding. The flow through the tunnel network functions anytime that there is a bottom current on the sea-floor above, and a filter feeder would be supplied with suspended goodies without having to expend energy pumping water through filtering devices. Once the tunnels were built, passive flow from sea floor currents would provide all the energy necessary for filter feeding.
An alternative feeding strategy hypothesis has been proposed by Seilacher (1977), who suggests that the tracemaker's burrow system has been designed to cultivate a moneran "crop" on the interior burrow walls. Monerans are capable of breaking down resistant organic substances that metazoans cannot digest. Graphoglyptid animals may be using their burrow walls in much the same way that cows use microbes in their complex digestive system to break down indigestible plant substances such as cellulose (cows can digest grass
FIGURE 3.10. The graphoglyptid trace fossil Protopaleodictyon, from the Lower Cambrian of western Canada. Burrow geometry in many graphoglyp-tids is such that water flows continuously through the interconnected passages. (Photo courtesy J. Magwood)
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