A Camptosaurus infested with dermisted beetles

When I think about taphonomy, the science that studies what happens to an organism after death (often summed up as "the laws of burial"), my thoughts most immediately turn to large scavengers, wind, and water. When an elephant dies on the African savanna, for instance, the carcass is sure to attract carnivores that will strip some of the flesh from the bones and depending on the location of the body parts of it may or may not end up being preserved. It's easier to ignore the chemical changes and smaller organisms that contribute to the breakdown of a carcass, but the action of these relatively more subtle agents is just as important.

Anyone who has had some experience with the art of taxidermy has probably at least heard of dermestid beetles. There are many genera and species within the Dermestidae, but some members of the genus Dermestes in particular frequently inhabit and consume carcasses. They are useful in everything from getting a fleshy skull clean to providing law enforcement officials with clues about the timing of someone's death. A new study published in the journal Ichnos (Britt et al. 2008) further increases the significance of these carrion-loving insects; during the Mesozoic they feasted on dinosaurs.

In 1995 the articulated remains of the dinosaur Camptosaurus were found on private land near Medicine Bow Wyoming, the remains ultimately coming into the possession of Brigham Young University. The bones were studied for a number of years and some of the marks on them were jokingly referred to as "bug bites," but it was only recently that this possibility was taken seriously. Of the 120 Camptosaurus bones that were recovered, only three show no sign of pits or marks made by insects, suggesting that this particular dinosaur was infested by bone-chewers. The fact that some of the matrix that encased the fossil filled in some of the marks meant that they were not made some time after death, and thus the team was dealing with bone-eating insects rather than fossil-eating ones.

Dermestid beetles aren't the only insects that like the nom on bone, though; termites, mayflies, and other insects also have been known to modify bone after death (some of the oldest bone modification by insects dating back to the Triassic). Insects can bite, bore, probe, and tunnel through bone, and in the process of investigating the probable insects that ate Camptosaurus from the inside out the investigators created a useful comparison chart of the different types of bone modifications insects cause (similar to the system of identifying hominid and carnivore bone modification proposed in Blumenschine et al. 1996).

After a useful review of insects that often infest carcasses and feed on bone, the authors firmly come down on the side of the dermestid beetles (although they admit that the traces could have been made by an as-yet-unknown beetle with convergent habits, but this would require positive evidence). The patterns of pits, bores, and grooves on the Camptosaurus bones most closely resemble damage done by Dermestes, and the realization that these beetles damage bone after the soft flesh supply has already been exhausted suggests that the Camptosaurus may have already been mostly skeletonized when the bone damage was inflicted. Likewise, if the insects were anything like living Dermestes they could provide paleontologists with some clues about the climate in the region at the time. According to the paper, the flesh-eating beetles prefer a climate of 60-80% relative humidity and about 25-30 degrees Celsius in temperature, although this creates more of a hypothesis about the ancient climate that would require more evidence to test.

Even if there are still some questions that require more evidence to answer, the new Ichnos paper is certainly an important one as taphonomic traces left by insects are often overlooked. Every fossil bone that is recovered can tell us much about the life of the organism that it once formed the frame of, but those bones also had a separate history that spans the gap between death and discovery; minute traces on ancient bones can tell us much more about ancient environments if we examine them carefully.

References;

Blumenschine, R>.J.; Marean, C.W.; Capaldo, S.D. "Blind Tests of Inter-analyst Correspondence and Accuracy in the Identification of Cut Marks, Percussion Marks, and Carnivore Tooth Marks on Bone Surfaces." Journal of Archaeological Science
Volume 23 (4), pp. 493-507

Britt, B., Scheetz, R., Dangerfield, A. (2008). A Suite of Dermestid Beetle Traces on Dinosaur Bone from the Upper Jurassic Morrison Formation, Wyoming, USA. Ichnos, 15(2), 59-71. DOI: 10.1080/10420940701193284