Thursday, February 28, 2008

Why Palaeontologists Love Mudslides

The Burgess shale was deposited as a series of mudslides. This was the conclusion reached after a detailed sedimentological analysis by researchers from Univ. of Leicester and the Royal Ontario Museum. The Burgess shale is one of the most famous and important fossil localities in the world. Science Daily and Live Science carried reports of this paper. Most estimates based on molecular phylogeny and fossils place the origin of multicellular animals (metazoans) in the late Proterozoic Vendian period ( at or just before 600 million years ago). The subsequent Vendian-Early Cambrian fossil record is sparse (more on that later). The Burgess shale which is Middle Cambrian, dated to about 515 million years is however unusually rich in fossils giving us lots of information about various aspects of Middle Cambrian metazoans.

What makes the Burgess shale so special? Normally during fossilization organisms with hard durable skeletons have a higher chance of being preserved. The soft tissues of these organisms decay upon death or are consumed by scavengers. And organisms which are entirely soft bodied may not be represented at all. So we are left with a biased record in favor of the morphological characters of ancient organisms that are more easily preserved. I mentioned that fossilization normally preserves only the most robust materials of the organism. But occasionally it can preserve very delicate structures as well. One way this has happened in the early Cambrian is through phosphatization of body tissue. The process is delicate enough to preserve animal embryos. However large body fossils have not been found preserved in this manner. The Burgess shale fauna represent another type of preservation mode. The mud slides that formed the deposit buried the Middle Cambrian community rapidly and deeply. The deep burial protected soft tissue from oxidative decay and also from scavengers. This has enabled soft tissue forms to be preserved as impressions upon the mud along with skeletal fragments. Fossil assemblages preserved under such unusual circumstances where even the soft parts of organisms are preserved are called lagerstatte.

Since the Vendian-Early Cambrian body fossil record is sparse, scientists have relied heavily on lagersttate like the Burgess shale and the slightly older Chengjiang fauna from China to understand Cambrian animal evolution. Such lagerstatte have so far not been discovered in Vendian and Early Cambrian strata. This has led to many misunderstandings regarding early animal evolution. Creationists who insist that biological complexity cannot evolve through intermediate stages have used the Burgess shale as evidence that animals arose "suddenly", ignoring the older fossil record. The media often uses terms as "sudden appearance", "in the blink of an eye" without properly explaining what these terms mean in a geological context. The term Cambrian explosion hardly helps. Explosion does not mean that at one time nothing existed and then Poof suddenly complex animals appeared on the Cambrian sea-floor. Explosion refers to the evolutionary radiation of metazoans that took place in the Early-Middle Cambrian over a time span of about 15-20 million years. The image below summarizes the Vendian-Middle Cambrian fossil record.

The Burgess Shale and the Chengjiang fauna have assumed such iconic status that reading media reports I sometimes get the impression that without these fossil locales we would know nothing about early animal evolution. But that is just not true. The early fossil record although sparse does show a gradual increase in grades of complexity in animals. This is seen through an increase in size and complexity of trace fossils as well as body fossils. Representatives of most skeletonized phyla can be recognized in the fossil record by Mid-Late Cambrian times. Taxa representing lineages ancestral to phyla (stem-groups) however have not been recognized in many cases. This means that the fossil record of the early Cambrian does not by and large preserve the detailed stages of body plan evolution. This may partly be because the initial evolutionary radiation took place in soft bodied forms before skeletonization evolved. Early skeletons may also have been quite fragile, disintegrating in small pieces upon death of the organism, making reconstruction of form difficult. And finally, evolution of novel morphologies may have taken place geologically rapidly in ecologically restricted less diverse lineages reducing their chances of being preserved in the fossil record. This situation will change as more fossils are discovered and improved systematics place problematic Early Cambrian taxa to their correct position within various animal clades. The Burgess Shale and the Chengjiang faunas represent a time when complex body plans of animals had already been assembled. So although they tell us a lot about the disparity (range of morphological forms) and diversity during the Middle Cambrian, they are less useful in terms of understanding body plan evolution and the early diversification of animals which most probably took place in the latest Vendian and Early Cambrian. A notable exception to this so far has been the work of Graham Budd on Arthropods. Using an analysis of ancestral and derived character states of Arthropod stem taxa from early Cambrian and Burgess type deposits, Budd has reconstructed the various stages through which Arthropods acquired morphological features characteristic of their clade. Arthropods have an exceptionally rich fossil record in the Middle-Late Cambrian. Such types of analysis are constrained by the paucity of stem group fossils for many other phyla. A few more preserved fossil rich mudslides from the early Cambrian will go a long way in filling that gap.

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