Fossils are rarely found in the exact position that they are deposited, with all the bones in the correct positions. Processes that affect the fossils, such as water currents, scavenging etc are known as taphonomic processes. For weird remains, the taphonomic processes have to be understood in order to explain how a fossil came to be in the position that it is seen in the rock. In this post, I am looking at a new theory that may explain some unusual ichthyosaur remains.
The remains in question are unusual in the fact that they consist of a well articulated, well preserved adult specimen that was pregnant when it died. The odd part is that the remains of the embryos (ichthyosaurs gave birth to live young) are scattered and some are preserved outside of the parent (Fig. 1). What could cause this type of positioning of the bones??
Previous theories have suggested that ocean bottom currents could have relocated the lighter bones. However, this would result in the smaller lighter bones of the parent also being moved (such as smaller vertebrae towards to tip of the tail, or small phalanges (finger bones) located at the tips of the paddles). This is not always the case.
Another suggested theory was carcass explosion caused by sudden release of gases built up in the body as a result of decomposition. A recent paper by Reisdorf et. al. (2012) showed that (a) ichthyosaurs would likely sink on death, and (b) it would be impossible for a carcass explosion to happen in deeper water (which is a shame, because explosions are cool). This is because of high water pressures surrounding the remains. However, Reisdorf did not suggest a mechanism that could explain the features of the fossil in Fig. 1.
A paper this year by van Loon (2013) offers an alternative explanation, implosion! van Loon agrees with Reisdorf and co. that the corpse would sink and that decomposition would begin. However, van Loon goes on the say that at some point, when gases have built up, the outer wall of the ichthyosaur would break. (This could either be through scavenging or natural decomposition). When this happens it would result in an in-rushing of water due to high pressures outside the body compared to inside. This in-rushing of water would be turbulent enough to move some of the small embryonic bones and deposit them near the parent. It is suggested that this small movement of water would not affect the remains of the parent, resulting in the fossil seen above.
There are many other remains of ichthyosaurs where this process has not happened and embryos are still contained within the abdomen of the parent or appear as though they are giving birth (Fig. 2).
Fig. 2: Ichthyosaur ‘giving birth’. Photo by Sam Bennett from the Staatliches Museum fur Naturkunde Stuttgart, Germany.
It is likely that the ichthyosaur in Fig. 2 did not die whilst giving birth but that the embryo was pushed along the natural path by decomposition gases. However, from the two specimens shown in this post, it is clear that not all specimens underwent the same taphonomic processes after death. There are many different scenarios that can make life of a palaeontologist harder, but also more interesting!
If you’ve reached this part, thank you for reading it all, or maybe just quickly scrolling to the bottom. Either way, you deserve a cup of tea/coffee and biscuit! Enjoy.
van Loon, A. J. 2013. Ichthyosaur embryos outside the mother body: not due to carcass explosion but to carcass implosion. Palaeobiodiversity and Palaeoenvironments. 93:103-109
Reisdorf, A. G., Bux, R., Wyler, D., Benecke, M., Klug, C., Maisch, M. W., Fornaro, P. & Wetzel, A. 2012. Float, explode or sink: postmortem fate of lung-breathing marine vertebrates. Palaeobiodiversity and Palaeoenvironments. 92:67-81