It is the belief of Steve Gould that these strange and bizarre animals in the Burgess Shale could well undermine our conventional view of evolution. But his first argument, that the problematic fossils have such distinctive body plans that they must represent extinct phyla is misleading. If a fly and a mouse have body plans that are laid down by the same fundamental instructions, then it is perhaps less likely that the self-evident differences in anatomy stem from really radical differences in genetic architecture.
The point I wish to stress is that again and again we have evidence of biological form stumbling on the same solution to a problem. Consider animals that swim in water… there are only a few fundamental methods of propulsion. It hardly matters if beetles, pelagic snails, squid, fish, newts, ichthyosaurs, snakes, lizards, turtles, dugongs or whales, the style in which the animal moves through the water will fall into one of only a few basic categories. The Burgess Shale was formed about 530 million years ago during the Cambrian period.
The Burgess Shale preserves the fossilized remains of a community of marine organisms that lived on mud banks built up against the outside of an algal reef. This algal reef had originally formed as vertical escarpment that eventually reached hundreds of meters in height. Three formations comprise Mount Field, where the Burgess Shale is found. These are the Cathedral, Stephen, and the Eldon formations. As illustrated on the diagram to right the Burgess Shale is found within the Stephen Formation. Examination of the reef escarpment reveals much about how the organisms inhabiting this area became so well Preserved.
Many of the fossilized organisms were living close to the reef and reef escarpement. The muds in this area were unstable and frequent turbidity currents carried the sediments downslope into deeper waters. Many of the associated fauna were caught up in these slumps and were transported with the mud to become redeposited as graded units. Through geologic time, mountain building processes along with erosion have resulted the shale that we see today in Yoho National Park. The Preservation of the Burgess Shale
The exceptional preservation of fossil material found within the Burgess Shale is the most significant feature of this deposit. Before discussing the processes which resulted in this unique deposition, it is necessary to understand the nature of the environment where these organisms lived. The presence of an algal reef indicates that depths were relatively shallow, well oxygenated, and well lit. In such an environment, preservation potential would normally be less than perfect. The fauna in this environment would not only be exposed to scavenging by other organisms, but would also be susceptible to rapid decay.
However, the place where the fossil organisms found in the Burgess Shale lived was much different from where, following transport, preservation occurred. Following periodic mud slumps Burgess Shale organisms were transported downslope and deposited into anoxic waters. In this anoxic environment, the amount of decay was limited and very slow. Predator and scavenger damage was also not a factor here. Under these conditions in conjunction with the compaction of overlying mud, the soft parts of the organisms were preserved as thin carbon films interspersed with fine mud layers.
The excellent preservation of these softer tissues as well as hard skeletal parts is what makes the deposit so unique. Fortunate protection from tectonism during the Mesozoic era has also influenced the preservation potential of the fossils. During this time, the Mount Wapta and Mount Field area, along with the Stephen Formation and the Burgess Shale, were moved along a thrust fault and brought to their present site. The fossils remained protected here from pressures and heats of surrounding tectonic events.
With about twenty-five species inhabiting shallow tropical and temperate oceans, the Cephalochordata are a very small branch of the animal kingdom. Known as lancelets or as amphioxus (from the Greek for “both ends pointed,” in reference to their shape), cephalochordates are small, eel-like, unprepossessing animals that spend much of their time buried in sand. However, because of their remarkable morphology, they have proved crucial in understanding the morphology and evolution of chordates in general including vertebrates. Note that cephalochordates have all the typical chrodate features.
The dorsal nerve cord is supported by a muscularized rod, or notochord. The pharynx is perforated by over 100 pharyngeal slits or “gill slits”, which are used to strain food particles out of the water. The musculature of the body is divided up into V-shaped blocks, or myomeres, and there is a post-anal tail. All of these features are shared with vertebrates. On the other hand, cephalochordates lack features found in most or all true vertebrates: the brain is very small and poorly developed, sense organs are also poorly developed, and there are no true vertebrae.
What role did chance play after the Burgess Shale era? Gould’s mused on the contingency of evolution–or what he called the chaotic nature of evolutionary history. Although undisturbed when told that weather patterns are chaotic, many become uneasy when told that our origin is also fraught with the uncertainties of a chaotic pathway. Gould asserts that with the acceptance of scientific orthodoxy, the idea of a creation fashioned for the purpose of man has been completely obliterated.
Radiometric dating reserved for man only the last one forty-thousandth of the earth’s history. And, some view evolution as a steady, inevitable pathway to human intelligence, Gould argues differently. If, as Gould argues, the evolutionary tape were played again, human life would not be expected. In fact, even if it were replayed a million times or more, man would not be expected again. In other words, Gould suggested that the Cambrian explosion is the biggest lottery of all time. There were high temperatures, environmental extremes and changes, and lucky genetics, which gave birth to the first cordate, Pichaia, the ancestor of vertibrates.
