I've just published my first academic paper in the journal 'Palaeogeography, Palaeoclimatology, Palaeoecology', with my co-author Steve Salisbury. Our paper is called "Patterns of aquatic decay and disarticulation in juvenile Indo-Pacific crocodiles (Crocodylus porosus), and implications for the taphonomic interpretation of fossil crocodyliform material."
What is the paper about?
Essentially, the paper looks at what happens to crocodile carcasses when they rot, undisturbed in fresh water – whether the carcasses float (they do), how long they float for (about a month), whether parts of the body fall off while they float (a few ribs and hip bones do, but not much), and what happens after the carcasses sink (they crumple into a pile of bones, but parts of the skeleton still stay attached to one another).
We also examined the difference between (1) crocodile carcasses buried in sand, versus (2) carcasses left in water to rot and buried once they sank, versus (3) carcasses left in water to rot and left unburied.
We were interested in how this applies to the fossil record: when we examine fossil crocodiles, can we tell where they died, and how long they were left to rot, and whether they were buried quickly or not?
We also examined the difference between (1) crocodile carcasses buried in sand, versus (2) carcasses left in water to rot and buried once they sank, versus (3) carcasses left in water to rot and left unburied.
We were interested in how this applies to the fossil record: when we examine fossil crocodiles, can we tell where they died, and how long they were left to rot, and whether they were buried quickly or not?
More details: 'Bloat and float'
Here are some diagrams that show what happened. The carcasses bloated due to bacteria inside the carcass eating soft tissue and creating gases (I'm sure you've all seen swollen or bloated roadkill before, it's the same process). These bloated carcasses float in water, a phenomena (not surprisingly) called 'bloat and float'.
Side view illustration of carcasses in the experiment bloating, floating, and sinking. Image from Syme et al. (2014).
The carcasses stayed intact while floating (except for a few ribs and hip bones that drifted away thanks to fly maggots eating flesh), and sunk after about 32 days. Large portions of the skeleton stayed intact after sinking, but some portions separated from one another - for example, the whole left leg may have stayed intact, but disconnected from the rest of the body.
Top-down photographs of one carcass as it decayed. You can see that while the carcass floated, the limbs, tail, and head stayed connected while slowing sinking below the water's surface. Once they sunk and we very gently drained water from the tank, the carcass looked a lot more disconnected! Image from Syme et al. (2014).
Burial: what difference does it make?
Part of the experiment was to determine the effect of burial on the carcasses. Two crocodile carcasses were buried in sand at the beginning of the experiment, and by the end of the experiment their skeletons looked like this:
Three crocodile carcasses rotted in fresh water, as I described earlier, but as soon as they sank they were buried in sand. At the end of the experiment, they looked like this:
And finally, three carcasses were left to rot in fresh water, and after they sunk, they were NOT buried. Here's what their carcasses looked like:
You can see that the carcasses buried at the beginning of the experiment look pretty much perfect – all the bones are connected and where they're supposed to be. That's not surprising, because the sand surrounding the body stopped anything from moving (there was an added complication as one of those carcasses actually floated up through the sand! But read the paper for more details).
The carcasses buried part-way through the experiment, as well as those not buried at all, look more similar to one another. Chunks of the skeleton stayed together, but other bits separated (disarticulated).
The carcasses buried part-way through the experiment, as well as those not buried at all, look more similar to one another. Chunks of the skeleton stayed together, but other bits separated (disarticulated).
What are the most important conclusions?
The buried carcasses 'survived' the best, and much better than the other carcasses decaying in water. This is a little surprising, because the water was left undisturbed during the entire experiment: there was no water current, no large animals were allowed to scavenge the carcasses, etc. It was purely the action of floating and sinking that caused the bones to separate and move around.
What does this mean for fossil crocodiles? We might think that a really well preserved fossil might have resulted from a carcass being quickly buried, which is a fair assumption. But we might also think that a carcass left to decay undisturbed in fresh water could also be really well preserved and look the same as a buried carcass. The results from this experiment show that this is not the case for modern crocs, and therefore may not be the case for ancient crocs!
What does this mean for fossil crocodiles? We might think that a really well preserved fossil might have resulted from a carcass being quickly buried, which is a fair assumption. But we might also think that a carcass left to decay undisturbed in fresh water could also be really well preserved and look the same as a buried carcass. The results from this experiment show that this is not the case for modern crocs, and therefore may not be the case for ancient crocs!
References
Syme, C. E., and Salisbury, S. W. 2014. Patterns of aquatic decay and disarticulation in juvenile Indo-Pacific crocodiles (Crocodylus porosus), and implications for the taphonomic interpretation of fossil crocodyliform material. Palaeogeography, Palaeoclimatology, Palaeoecology, 412:108-123. doi: 10.1016/j.palaeo.2014.07.031
