You have often seen me use the phrase “taphonomic distortion” to describe bones that have been crushed or altered to a degree during the fossilisation process. And yet taphonomy is often overlooked in favour of the more glamorous side of palaeontology as we tend to focus on the various skeletal remains displayed before us and wonder about every aspect of how these animals may have lived and died.
But that’s alright and we all do it and, indeed, there is nothing wrong with focussing on the animal since that is what palaeontology is – the study of ancient life. And yet the discovery of something like a dinosaur skeleton is so much more all-encompassing since there is a huge amount of detail in the quarry that needs to be retrieved and/or recorded. Taphonomic detail is a vital as the fossil remains themselves and can reveal a wealth of essential data. But what is taphonomy?
Simply put, taphonomy is the study of the processes that enabled living organisms to become fossilised. We need to understand these processes so that we can better understand a more complete picture about our long dead animals and, of course, the environment in which they lived.
There are many factors that are included in the taphonomic process. For example, were the remains transported after death? What was the process of decomposition? How did the remains get buried? What was the rate of compaction? What other processes were involved? What were the chemicals involved? How did permineralisation take place? The questions are endless…..
Taphonomy also throws up some other words and phrases that are difficult to follow – especially to the layman. For example what happens to an organism once it dies and ends up being buried is known as biostratinomy. Another oft quoted word is the process of diagenesis which is more of a geological term to describe changes to rocks during both their formation and afterward but has been used by palaeontologists to describe how organic remains are transformed after they have been buried.
My interest in taphonomy was sparked by the sometime amazing contrast in bone preservation in the same quarry. No matter how consistent fossil remains in a certain formation appear to be there are always anomalies and I am constantly aware not to get over confident when extracting fossils and also when preparing them. I am also always amazed how different the preservation can be in a single bone with one part being solid, well preserved and easy to prepare whilst the other end can be a crumbling mess that is a preparator’s nightmare.
But taphonomy not only reveals what may have happened to an individual animal but can also provide a wealth of detail regarding the environment it lived in. Numerous samples of the surrounding matrix within and around the quarry normally reveal microfossils – both plant and animal – which provide extensive data and enables a micropalaeontologist to build up a pretty accurate picture of the climate, the flora and fauna alive at the time.
As taphonomy goes, I have only scraped the surface of this fascinating and very important aspect of palaeontology. Anyway, as a palaeontologist, I should darn well have an interest in taphonomy if I want to understand more. For the record, some of the best papers I have read are Raymond Rogers’ work on the Two Medicine and Judith River Formations (1990, 1998) and Anna Behrensmayer’s 1988 paper on vertebrate preservation in fluvial channels – essential reading for any budding taphonomist out there.
The reason for this little venture into taphonomy was inspired in part by a new paper recently published by Frankie Johnson, of Montana State University, and her colleagues looking at the taphonomic influences on the incredible sauropod nesting site at Auca Mahuevo situated in Patagonia, Argentina. Professor Johnson is something of a specialist when it comes to fossilised eggs and the sedimentological processes that affect them and she has performed extensive field work at the locality.
Auca Mahuevo, if there is anybody out there who does not know of this amazing site, was discovered in 1997 by Luis Chiappe and his crew and has become one of the best known dinosaur nesting sites in the world. It is Upper Cretaceous in age and exposes terrestrial fluvial deposits of the Anacleto Formation.
The deposits at Auca Mahuevo are composed of fine grained mixed load siltstones, sandstones and mudstones which were deposited in floodplains and channels under semi-arid conditions during alternate wet and dry seasons. In these deposits are found tens of thousands of titanosaur sauropod eggs with various degrees of preservation but these do include those with embryos inside and there are even patches of wonderfully preserved embryonic skin that clearly displays the scale patterns and their various morphologies.
The study area features four distinct egg-bearing layers representing four different sequential nesting time periods. Sauropod skeletal remains are also found – in between egg levels 3 and 4 and also within level 4. The team wanted to know what affect vertisols, pedogenesis and the resulting taphonomic movement may have had on both egg morphology and the overall size and shape of an individual nest.
Pedogenesis is the term for soil creation and a vertisol, whilst not a palaeontological term per se, is a soil which has high clay content. Development of vertisols takes place today and the alternating temperatures in both wet and dry conditions cause this clay to shrink and swell which in turn leads to soil movement. This movement can be both vertical and lateral and creates visible features known as slickensides, microhighs and microlows. Slickensides are smooth, sometimes polished surfaces, which are caused by soil movement between two surfaces and are often accompanied by a striated surface which is indicative of the direction of travel. Microhighs and lows are the resulting deposits of sediments, both upward and downward (high and low), caused by soil movement and cracking.
|Partial egg on polished slickenside indicated|
by arrow (From Jackson et al 2013)
Sauropods used the floodplains for their nesting grounds and these were subject to flooding events which buried the nests and eggs. While this was catastrophic for the eggs and the embryos they contained it was spectacularly good fortune for us as these wonderful eggs survived as fossils. It is very easy to look at these clutches, see how they are situated and count the amount of eggs they contain and get a reasonable idea of clutch size and behavioural implications. So it would appear……
But this study reveals just how dangerous it is to take any fossil remains at face value without taking everything else into account. Vertisol development and the effects of pedogenesis had profound effects on the eggs and nests of Auca Mahuevo. Movement of eggs resulted in various changes in shape, size, shell thickness and ornamentation (amongst others) and this has parataxonomic implications especially when identifying oo-species. At the same time these changes also affect things such as egg volume and pore morphology which affects and influences incubation hypotheses.
Perhaps the most significant consequence of this sedimentary travel is the displacement of eggs from their original position in a clutch so that we have clutches that may be missing eggs or, indeed, have clutches that have combined to falsely increase the total number of a clutch. Not only that but there are clutches mixing from different stratigraphic horizons which further distorts the data.
Despite the enormous amount of fossil eggs involved at Auca Mahuevo there is hardly any evidence of a true nest – except for a few traces. Because of the enormous pedogenic vertisol disturbance over the many hundreds and thousands of years after the eggs were laid and then buried, evidence of nests has been mostly obliterated and this, in combination with the random displacement and distortion of the eggs makes behavioural hypotheses difficult.
With this enormous amount of post burial disturbance it is difficult to postulate the exact shape and size of the eggs, how they were laid and how big a clutch may have been. Since many dinosaur nesting sites are located in a floodplain environment, the authors stress that it is absolutely essential that an accurate record is made of those nests that are in situ and relatively undisturbed, as well as those that have been subsequently changed by pedoturbation and vertisol development (pedoturbation is the changes that take place to alluvial sediments that are not caused by the redistribution of water).
The authors back this up with a list of nine separate processes that should be followed when investigating any dinosaur egg-bearing strata and suggest that these guidelines will enable future workers to produce a much more comprehensive and accurate assessment about sauropod reproductive behaviour.
I liked this paper very much and learnt quite a bit which is always good. And, more than that, it is a timely reminder that when any fossil remains are found that it is essential that ALL details are recorded, that everything is taken into consideration and that not everything is as all that it may seem to appear.
Behrensmeyer, A. K., 1988. Vertebrate preservation in fluvial channels. Palaeogeography., Palaeoclimatology., Palaeoecology., 63:183 - 199.
Jackson, Frankie D., Schmitt, James G., Oser, Sara E., Influence of Vertisol development on sauropod egg taphonomy and distribution at the Auca Mahuevo locality, Patagonia Argentina, Palaeogeography, Palaeoclimatology, Palaeoecology (2013), doi: 10.1016/j.palaeo.2013.05.031
Rogers, R.R. (1990). Taphonomy of three dinosaur bone beds in the Upper Cretaceous Two Medicine Formation of northwestern Montana: evidence for drought-related mortality. Palaios, 1990, V.5, p. 394 – 413.
Rogers, R. R. (1998). Sequence analysis of the Upper Cretaceous Two Medicine and Judith River formations, Montana; nonmarine response to the Claggett and Bearpaw marine cycles. Journal of Sedimentary Research. doi:10.2110/jsr.68.604