Medusoids

The most common Ediacaran body fossils were the first type discovered at the classic site in the Ediacara Hills, South Australia. Circular or discoid Ediacarans are conventionally called medusoids, in an intentional comparison to the free-swimming medusa phase of the jellyfish life cycle. This is perhaps unfortunate because it is by no means certain that Ediacaran medusoids were related to the jellyfish medusa. However, the name has stuck.8 The term medusa refers to the tentacles of living jellyfish, which resemble the snake-hair of Medusa, one of the three Gorgons slain by Perseus in Greek mythology.

In a dry and barren series of hillocks hundreds of kilometers north of Adelaide, Australia, R. C. Sprigg, who in 1946 was assistant government geologist of South Australia, was reassessing a series of abandoned lead-silver mines. The South Australian government was in the process of reviewing its mineral resources, and Sprigg had been sent to determine whether these mines were worth reopening.

While traversing the quartzite outcrops and flaggy slabs in the vicinity of the mines, Sprigg was pleasantly surprised to find unusual fossils in the quartzite exposures to the southwest of the mine area. Sprigg published several reports on the fossils and concluded that they all lacked hard parts and appeared to represent varied types of simple, ancient ani-mals.9 Sprigg went further to suggest that these fossils were some of the oldest evidence known of animal life.

Oddly enough, considering the importance of the find, more than a decade was to pass before a serious paleontological research expedition was mounted to the region. Led in October 1958 by Brian Daily, curator of paleontology at the South Australian Museum, the expedition returned with two trucks and a trailer full of fossils, a haul of over 1500 specimens.10

Sporadic collecting expeditions to this site continue to this day. Some specimens have made it onto the collectors' market; an acquaintance of mine recently purchased a small Dickinsonia specimen for $500 at a rock and mineral show in Springfield, Massachusetts. The specimen originally cost $600, but the vendor agreed to take $100 off the price if my colleague could identify the genus, which he proceeded to do.

Ediacarans from the type locality have also left Australia illegally.11 In the early 1990s German fossil smugglers removed Ediacaran specimens from outcrops with motorized rock saws and shipped them to Asia. The fossils fell into the hands of Japanese collectors who paid for the specimens a sum reported to be in the high six figures in American dollars.

The thieves had committed an error as well as a crime, selecting a specimen that, from photographs taken on the outcrop, was well known to the paleontological community. Australian authorities notified Interpol, the specimens were seized in Japan, and the German smugglers were apprehended. The Japanese collectors found themselves shy a substantial sum of money.

The same smugglers were apparently active elsewhere in Australia and, in a raid of an important Early Cambrian trilobite locality, inadvertently exposed the first known occurrences of the Cambrian predator fossil Anomalocaris from the southern hemisphere. Fortunately for paleontologists, the smugglers did not recognize what they had found as they furtively dug for trilobites and tossed aside the anomalocarid specimens as if they were worthless matrix.

Medusoids are both the youngest and the oldest of the Ediacarans. They are also the most common, and were the first to be noted by scientists, although from the start there were questions about the biological nature of these structures. In 1877 E. Hill and T. G. Bonney reported "curious arrangements of concentric rings which have been supposed to be organisms,"12 but then dismissed them as being accidental and inorganic.13 These structures had been known to local quarrymen as ring stones.14 The structures, from the faces of the North Quarry, Woodhouse Eaves in Charnwood Forest, Leicestershire, England, are now known to be genuine Ediacaran fossils. Ironically, it was in Africa, where the usually common medusoids are rare, that Ediacaran fossils were first described and interpreted as fossils.

The oldest medusoid, approximately 600 million years old,15 was found by my field party in Sonora, Mexico (figure 2.1). The expedition leading to its discovery is the subject of chapter 9. Like the discovery of fossils in the Ediacara Hills of Australia, the Mexican find was an outgrowth of a government-sponsored effort (by the U.S. Geological Survey and the Mexican Recursos Minerales; color plate 1) to characterize the mineral resources of a remote area.

Figure 2.1: The oldest Ediacaran, a specimen of Cyclomedusa from the Clemente Formation of Sonora, Mexico. Fossil is viewed from the bottom. Greatest dimension of rock specimen is 6 cm.

The youngest Ediacaran fossils known are also medusoids (figure 2.2). Their discovery in Booley Bay, County Wexford, Ireland, led T. Peter Crimes to conclude that "there was no mass extinction at the end of the Precambrian."16 The specimens were transported and deposited in a deepwater setting, although it is possible that they originated in shallow water and were carried downslope by submarine currents.

The oldest and youngest medusoid fossils are actually quite similar. Both have concentric and radial elements. Of most interest here is the tubular or flamelike nature of the radial elements, which are concentrated on the periphery of the organism.

A. Seilacher was first to point out that, unlike a modern jellyfish, the radial elements in an Ediacaran medusoid are on the outer edge of the specimen. In true jellyfish, the structures on the outer periphery of the body are concentric muscle bands that contract the jellyfish's bell and, when contractions are rhythmically synchronized, allow it to swim.

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