Taphonomy

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.

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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.

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Over another period of time these compressed layers containing fossils resurface and the fossils are then exposed.

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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.

Anomalocaris3-300x204 anomalocarisrom512113892059702499791776

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.

Anomalocaris

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.

References

Butterfield, N.J., (1995), Secular distribution of Burgess Shale-type preservation. Lethaia, 28, 1-13. http://onlinelibrary.wiley.com/doi/10.1111/j.1502-3931.1995.tb01587.x/abstract;jsessionid=1D8ECDC32369A9FD2951D3076EF7330C.f03t01

Butterfield, N. J., (2003), Exceptional Fossil Preservation and the Cambrian Explosion, The Journal of Integrative and Comparative Biology, 43, 166-177, http://www.ncbi.nlm.nih.gov/pubmed/21680421

Caron, J. B., (2008), Paleoecology of the Greater Phyllopod Bed community, Burgess Shale, Palaeogeography, Palaeoclimatology, Palaeoecology, 258, 222-256 http://0-ac.els-cdn.com.library.ucc.ie/S0031018207004579/1-s2.0-S0031018207004579-main.pdf?_tid=b1aaa3a8-a522-11e3-b785-00000aab0f6c&acdnat=1394105598_e31a2580b425604b3c1077b7f9208ccb 

Forchielli, A. et al., (2014), Taphonomic traits of clay-hosted early Cambrian Burgess Shale-type fossil Lagerstätten in South China, Palaeogeography, Palaeoclimatology, Palaeoecology, 398, 59-85, http://0-ac.els-cdn.com.library.ucc.ie/S0031018213003647/1-s2.0-S0031018213003647-main.pdf?_tid=3ad31afe-a521-11e3-b07a-00000aacb362&acdnat=1394104969_5aa357960f1bc95d39ee6c26707207aa 

Johnston, P. A., et al., (2009), Palaeontology and depositional environments of ancient brine seeps in the Middle Burgess Shale at The Monarch, British Columbia, Canada, Palaeogeography, Palaeoclimatology, Palaeoecology, 277, 86-105, http://0-ac.els-cdn.com.library.ucc.ie/S003101820900087X/1-s2.0-S003101820900087X-main.pdf?_tid=4c2c9fc0-a524-11e3-90e2-00000aab0f01&acdnat=1394106286_51b854c76a543b0d9523416da95c5118

Kloss, T. J., et al., (2009), Paleoecology and taphonomy of the Early Cambrian Maotianshan Shale biota chancelloriid Allonnia junyuani: Adaptation to nonactualistic Cambrian substrates, Palaeogeography, Palaeoclimatology, Palaeoecology, 277, 149-157, http://0-ac.els-cdn.com.library.ucc.ie/S0031018209000923/1-s2.0-S0031018209000923-main.pdf?_tid=89a7ed4e-a521-11e3-b785-00000aab0f6c&acdnat=1394105101_f23aba47b7fd38d5b92f92a8f885f29b

Moore, R. A., et al., (2009), Preservation of early and Middle Cambrian soft-bodied arthropods from the Pioche Shale, Nevada, USA, Palaeogeography, Palaeoclimatology, Palaeoecology, 277, 57-62, http://0-ac.els-cdn.com.library.ucc.ie/S0031018209000856/1-s2.0-S0031018209000856-main.pdf?_tid=09d53450-a521-11e3-b0ad-00000aacb361&acdnat=1394104887_6734c91f6ad73f91f5a2126b864f3821

Yoshiyuki, U., (2006), Theoretical study on the body form and swimming pattern of Anomalocaris based on hydrodynamic simulation, Journal of Theoretical Biology, 238, 11-17, http://0-ac.els-cdn.com.library.ucc.ie/S0022519305002109/1-s2.0-S0022519305002109-main.pdf?_tid=b8f8ae78-a524-11e3-9f50-00000aab0f01&acdnat=1394106469_11fd2ee43966efe932223a6f9e0a421a

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