The skeleton was one of the most beautiful I’ve ever seen. Propped up on a table in the Yale Peabody Museum of Natural History’s old, dusty collections area and lit by senior collections manager Christopher Norris, the ancient alligator looked as if it had become petrified in the middle of swimming. One white skeletal hand even reached out in front, as if it were about to reanimate at any moment. This was not a new skeleton. The alligator had lived in Wyoming during Eocene time, Norris explained, back when the local climate was more amenable to such reptiles. Another reminder of how much the world has changed with time, yes, but I was stunned by the aesthetic beauty of the articulated skeleton. How could such bones remain so perfect through 50 million years?
One of the key components of fossilization is rapid burial. Every paleontologist knows this. To become a fossil, you have to be covered up before scavengers and other destructive forces can do their dirty work. But how rapid is rapid, and what conditions have to prevail to keep the bones from falling away from each other?
These questions fall within the realm of taphonomy – the fate of an organism between death and discovery – but, frustratingly, the afterlives of crocodiles have been relatively little-studied. Most previous studies on crocodile taphonomy have focused on details such as the pattern of insect damage to the body and what happens to their bones when buried, but not and what keeps crocodiles together or tears them apart. That’s why University of Queensland paleontologists Caitlin Syme and Steven Salisbury dunked eight dead saltwater crocodiles in sand-filled aquariums.
The juvenile crocs, as Syme and Salisbury report in a Palaeogeography, Palaeoclimatology, Palaeoecology paper, had been euthanized as part of a different experiment on bone tissues. Not wanting to let good reptiles go to waste, the paleontologists prepared three different treatments to see how the conditions of burial altered the ways in which crocodile skeletons held together. While they buried crocodiles in fine sand for “treatment one”, Syme and Salisbury let “treatment two” crocodiles bloat, float, and sink before burial. “Treatment three” crocodiles were left uncovered for the duration of the experiment.
All of the carcasses broke down along the typical pathway – fresh, bloated, active decay (in which internal organs start to become visible), advanced decay (when bones can be seen), to remains. There were no big scavengers to tear at the bodies or even currents to jostle the bones. The aquaria were the equivalents of calm, cool, shallow pools where the crocodiles could rest in relative peace. Even so, some still wound up as osteological jumbles while others maintained their anatomical composure.
Floating made all the difference. After about five days, Syme and Salisbury write, all of the unhindered crocodile carcasses had filled with enough gas that they started to float belly- or side-up. Even one of the buried, treatment one crocodiles eventually broke free of its encasing sediment and floated for a short time. This gave insects a shot at the crocodiles. Flies landed on the exposed reptiles, laying eggs on what would become both food and shelter for the resulting maggots.
The larvae were surprisingly rough on the flesh, tugging and pulling as they fed. And given that they started around the crocodile’s cloaca, it’s no surprise that they consumed enough flesh to allow a some of the hip bones and nearby ribs to fall off the bodies. For a short time, the crocodiles rained bones.
But a crocodile carcass can’t float forever. By about the 24th day the crocodiles started to sink back to the bottom, and now they were rotten and soft. The crocodiles didn’t settle gently. Their bodies buckled onto the sand, bones spilling out into new configurations. Each skeleton varied in terms of how well they stayed together, but, in general, the crocodiles from treatments two and three had skulls and limbs disarticulated from their spines.
The results go beyond the afterlives of crocodiles. Syme and Salisbury argue that the crocodiles can tell us something about the formation of fossil lagerstätten – sites where multiple organisms have been found in excellent, articulated preservation.
One explanation for exquisite lagerstätten fossils is that the organisms preserved therein settled to the bottom and decayed undisturbed in a quiet, oxygen-poor environment before being buried. But as Syme and Salisbury point out, their crocodiles were in calm, cool environments and they still wound up as broken-up puzzles. If a carcass gets a chance to float, it’s going to start coming apart at the seams.
It would be too simplistic to say that rapid burial is the deciding factor in keeping skeletons together, though. The critical factor, Syme and Salisbury write, is something to prevent a carcass from floating. There are multiple ways this might be achieved. Underwater, the body might get stuck beneath a logjam or mat of weeds. Provided that scavengers don’t get to the body, it may then sink back down relatively intact. Or if conditions are cold enough, they may suppress the gas buildup that makes bodies float. Then again, the carcass might bloat on dry land and not have the chance to float at all. (There is no evidence for decaying, airborne vertebrate carcasses in the fossil record.) And, of course, the body might be buried with enough sediment within the first few days of death to prevent it from bobbing up to the surface.
The latter option is probably what was suffered by the alligator that so transfixed me in the Yale basement. The crocodylian did not float on. To come down to us so complete, it must have been buried at death or within a few days thereafter. Eventually locked into stone, it has survived the ages as a dual symbol – a totem of both life and death from prehistory.
For more, read Syme’s summary of her paper here.
Syme, C., Salisbury, S. 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