Taphonomy is the study of decaying organisms over time and how they become fossilised. It must take into account how the organism died, decayed and the way it was buried might affect the organisms preservation in the rocks. The term taphonomy was introduced to paleontology in 1940 by Russian scientist Ivan Efremov. Taphonomy is crucial for understanding evolutionary trends.
The Burgess Shale has contributed significantly to the fossil record because of the large number of well preserved fossils found within its layers. Its fossils are preserved as dark organic films on thin layers of fine-grained shale; soft parts, such as muscle tissue or poorly calcified skeletons, are preserved in great detail. Most fossils found in the Burgess Shale are from the Cambrian period where there was an explosion of life. The method for preservation in Burgess Shale is unique. Organisms lived in phyllopod beds, which are underwater mud banks. The occasional water currents caused sediments to flow in mudslide-like form which rapidly buried the living organism.
This caused the fossils to be found in a random orientation. The method for preservation after this is uncertain, there are a few hypotheses. There is evidence to suggest that these organisms died instantly due to the absence of fossils found in the Burgess Shale exhibiting coiling. There is also no evidence of these organisms trying to burrow out of the mud. Therefore, preservation started immediately. The sediments surrounding the buried organisms were depleted of oxygen, this would have helped preservation as it kept scavengers and bacteria from digesting the remains. Additional layers of sediment accumulate over time and this causes compression of the deeper layers which causes fossilisation to begin.
Over another period of time these compressed layers containing fossils resurface and the fossils are then exposed.
Certain organs of some fossils are preserved in the Burgess Shale by a process called phosphatisation. This occurs because organs that were naturally high in phosphate were rapidly mineralised which retained the three-dimensional shape of that structure. The high level of preservation in the Burgess Shale is mainly due to the rapid burial and instant death of the organisms
Fossils found in the Burgess Shale are very well preserved, the majority of these fossils have preserved the exoskeleton, the limbs and the infillings of the gut and in rare fossils the gut contents and muscles are preserved. Most of these fossils consist of thin films of carbon, partially replaced by clay or other iron-rich mineral products, that preserve the shape of the organism. These carbon reflective films are opaque and silvery, composed of organic carbon. In order for soft tissue to be preserved, its volatile carbon framework must be replaced by something able to survive the stress of time and burial. Carbonaceous compressions are the main pathway for Burgess Shale type preservation.
These images show two Anomalocaris fossils found in the Burgess Shale.
From the well preserved fossils of Anomalocaris in the Burgess Shale, scientists are able to interpret the extinct species morphology and its mode of life.
An artists impression of an Anomalocaris living in the Burgess Shale. This was achieved through taphonomy as without this information we wouldn’t know the mode of life or morphology of this animal.
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