Monday, November 3, 2008

Traces First #0: Preface

In this little series, I would like to review selected (may be popular) peculiarities of the fossil record that prove the relevance of trace fossils within earth system science as well as evolution. I hope I will find some time to review some instructive examples from the literature; it appears to be a rule: Evolutionary advances, pioneer colonisation patterns and other "events" or trends within earth history that are known from the body fossil record are frequently predated by findings of trace fossils.

After you have digested this arrogant statement, I will explain some concepts that are essential in understanding the distinctiveness of both archives of fossil record.

(1) The preservation potentials of body and trace fossils are different
In general, the preservation of a body fossil requires by far more lucky coincidences than preserving a track way. First of all, a trace or a track way records a short time span of an individual organism. Walking Reptiles or arthropods may produce thousands of trackways during their life time. Hence, the abundance of potential trace fossil specimens exceeds that of the corresponding trace maker by several orders of magnitude. Furthermore, it is not only a question of sheer probability. To fossilise a whole animal requires more peculiar conditions such as rapid burial, anoxic environment, and no benthic activity, for instance. Trace fossils (just) need to be created. They usually stick to the place where they have been maintained, called "in situ". However, they may be readily altered by subsquent benthic activity. Shallow marine sandstones are frequently thouroughly bioturbated, which actually means that they are full of trace fossils. As you see, to form trace fossils at all, well-oxygenated conditions are required to host a rich fauna. And this, by implication, means that animals itself are not very likely to become fossils. Dead organism would readily become consumed by benthic activity and oxygenation processes. To conclude, preservation potential for both types of fossils is not only different, virtually it is mutually exclusive but for some rare exceptions.

(2) Trace fossils are in situ structures
As outlined above, trace fossils are rarely transported. They are observed at the place where they have been created. The implications for ecology are, thus, more convincing than body fossils that are frequently transported and become buried at a different site.

(3) Similar trace fossils can be created by numerous organisms
In particular, invertebrate traces of the same ichnogenus or even ichnospecies can be produced by a variety of animals, which at a first glance diminishes their utility in palaeontological studies. But as they record principal strategies employed by a number of phyla, they are suited in the tracking of general trends in evolution such as infaunalisation, terrestrialisation etc. For me it does'nt really matter which critter got his feet at first on dry ground. It is more important that someone did it, is'nt it. However, I must admit that the "who done it?" question teases probably everyone.

(4) Trace fossils were created by organisms that otherwise won't have any fossils record
For an animal with no hard parts, it is fairly difficult to become a fossil. Preservation of soft tissues requires special circumstances. E.g. Worms are the main bioturbators in the marine realm. But in comparision to their impact on the ichnologic archive they left virtually no body fossil record.

These are just four concepts that explain the importance of ichnology in earth system science. May be I missed some points. I will keep this post updated if something pops into my mind.

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