A lot can happen in over 500 million years. Fossils rest in rocks moving around as parts of the continents or spreading out over the ocean floor. These rocks run the gauntlet of time, facing numerous obstacles to preservation. Buried deep, they can be subjected to heat and pressure, distorting their precious contents beyond recognition. Out on the surface, they encounter erosion caused by all manner of environmental effects. Many fossils, if not most, are destroyed by nature before any hopeful fossil hunter can get their hands on them. This means that studying the past can get trickier the further back we look and the Cambrian explosion is no exception.
Study of the diversification event at the start of the Cambrian Period has advanced significantly in recent decades. In 1989, Harvard palaeontologist Stephen Jay Gould brought the bizarre beasts of the Burgess Shale – an assortment of beautifully preserved fossils from the middle Cambrian – into the public eye, presenting an image of the Cambrian explosion which has persisted to the present in many popular depictions. The fossils from just before the Cambrian, the Ediacaran biota, were presented as a failed evolutionary experiment, not as precursors to life in the Cambrian. The fossils of the Burgess Shale were understood to represent many more body plans than we see in the world today, many of which became extinct. This is an increase in disparity, rather than diversity, and it happened rapidly in evolutionary terms. How rapidly it was thought to have occurred varied dramatically, as the data just was not available.
Gould and other scientists, whose work he helped publicise, had three key hurdles which palaeontologists have since leapt over, changing the way we see the Cambrian explosion significantly: dating the rocks in question was hugely problematic; how we classify fossil organisms has changed; and how these fossils were preserved was poorly understood. Since then, genetic data has also been used, along with other techniques, to try to understand the Cambrian.
Problems with Dating
To get a good idea of what was going on in the late Precambrian and early Cambrian rocks have to be correlated and dated accurately. During the 1980s, the base of the Cambrian was thought to be around 570 Ma and the timings of individual events were difficult to discern. Deciding which rock sections represented the base of the Cambrian was mired by nationalistic clashes, large sections of missing periods of time (unconformities) were common, fossils were difficult to use for correlating rocks as they were restricted geographically, and there was a considerable lack of volcanic rocks which could be used for radiometric dating.
In 1994, an international commission agreed that the base of the Cambrian would be marked by a trace fossil, a burrow called Treptichnus pedum, which is one of the earliest types of burrows displaying vertical penetration into the sediment – a strong indicator that behaviour had become more complex. Radiometric dating has been increasingly used, along with data from fossils and stable isotopes, to give a much-refined timescale for the events of the Cambrian and the Ediacaran periods; the boundary between the two, for example, is now dated at 541 Ma.
With an increase in understanding of the timings of events in the Cambrian, we now know that over twenty million years had elapsed between the start of the period and the first exceptionally preserved faunas (Chengjiang is dated at 515 Ma and the Burgess Shale at 508 Ma).
A Systematic Issue
How we classify organisms can determine how we interpret the events of the Cambrian. In school, we tend to learn the Linnaean approach to systematics, resulting in a hierarchy often taught through a mnemonic device such as “Kings Play Chess On Fancy Gold Squares” relating to the classifications “Kingdom Phylum Class Order Family Genus Species”. Fitting living organisms into this hierarchy can be tricky, so when you factor in extinct organisms it only exacerbates the problem. The animals we see in the Cambrian are a prime example, with many given the designation of ‘phylum’ based on a single fossil – they just could not be placed in any existing phyla, yet were clearly animals and could be placed within ‘super-phyla’ which encompassed a number of closely related phyla. This is the mess which palaeontologists were trying to clear up during the 1980s and which Gould was describing in his book. The number of animal phyla appeared to far exceed our modern assembly of approximately 35, perhaps even three times as many. This rapid increase in disparity made the Cambrian diversification appear even more explosive, as this huge number of high-level taxa implied an even greater lack of antecedents and perhaps even novel evolutionary mechanisms.
It is now expected that extinct organisms will not fit into a grouping based on extant organisms. Using cladistics, we can place living organisms together in what is known as a crown group, where they share traits which they all inherited from their common ancestor, whilst extinct close relatives can be placed as stem groups as they possess many of those traits of crown group organisms but lack some which would result in their inclusion in the crown group.
When this approach is applied to the Cambrian, we find that it is populated mostly by stem group taxa and that many of those supposed new phyla are actually stem groups closely related to modern crown groups, such as arthropods. This has reduced the number of phyla in the Cambrian down to around 14 crown groups and has helped to elucidate some of the evolutionary steps taken during this critical period in the history of animal life.
Going from dead organisms to fossil assemblages is a messy business. The environment of deposition dictates which organisms are preserved. Sometimes there is a size bias, where only larger organisms or only smaller organisms are preserved. Commonly, the bias is towards organisms with hard parts, resulting in a lack of understanding of soft-bodied organisms. The Burgess Shale of Canada is one of those exceptional sites of fossil preservation (Lagerstatten), displaying the anatomy of soft-bodied organisms in abundance. Since its discovery, it has been joined by other exceptional Cambrian deposits, most notably Chengjiang in China and Sirius Passet in Greenland. The Ediacaran biota are soft-bodied and have been preserved in very different ways to Cambrian Lagerstatten, and between those are the SSFs (Small Shelly Fossils) which are exclusively little bits of hard shell.
It is often difficult to compare fossils which are preserved through different processes, with different biases, and a lot of work has gone on in the last few decades to work out how they were preserved and how they can be compared. Take these assemblages out of their temporal context and it is difficult to make sense of what was happening in terms of their evolution. A better understanding of how they were preserved, more assemblages to compare, and a more refined timescale, have all contributed to painting a more up to date picture of the Cambrian explosion.
Through our increased understanding of the ways in which Cambrian and Ediacaran fossils are preserved, a more refined timescale, and a better understanding of how to classify these organisms, we have a different picture to that of Gould and the palaeontological community of the 1980s. It is no longer a diversification resulting in an unprecedented number of phyla, but one in which stem groups abound. We now have a better understanding of the timings of key events over tens of millions of years, and of those beautiful sites of exceptional preservation. I will continue to flesh out our modern understanding of the Cambrian explosion in future blog posts.