August 9, 2011
In almost every general dinosaur book I can remember from my childhood, there was at least one page dealing with the making of fossils. The story was always straightforward—almost like a recipe. Take one dead dinosaur, add a copious amount of sediment, throw in millions of years and, voilà, you’ve got a beautiful fossilized skeleton. Easier to follow than a Julia Child recipe, so long as you’ve got the time.
A few books added additional details. Scavengers might nibble at the carcass before burial, for example, though the scavengers almost always took the form of small, bird-like dinosaurs. I can’t remember any book mentioning the hordes of scavenging insects that helped break down dinosaur bodies. Maybe that’s because the role arthropods play in the breakdown of a dinosaur body has only been relatively recently appreciated. Over the past few years, paleontologists have reported a growing number of cases of interactions between dead dinosaurs and insects. Just last month one team of paleontologists reported fossil cocoons preserved inside a broken dinosaur egg—a possible indicator that parasitoid wasps used the rotting egg to give the next generation a head start—and an in-press Palaeogeography, Palaeoclimatology, Palaeoecology paper suggests that the dead dinosaurs of Cretaceous Mongolia often played host to insects.
The new paper, by Mototaka Saneyoshi, Mahito Watabe, Shigeru Suzuka and Khishigjav Tsogtbaatar, focuses on the damaged bones of Protoceratops, Velociraptor, Bagaceratops and an unidentified ankylosaur found in the Djadokhta and Barun Goyot Formations of Mongolia. All of these dinosaurs lived during the Late Cretaceous during a span of time between about 80 million and 70 million years ago. At this time the area was a sandy desert, and these conditions likely contributed to the rapid burial and preservation of the dinosaurs. When a dinosaur died, its body dessicated relatively quickly in the arid habitat and blowing sands soon covered up the carcass. Then the scavengers set in.
Saneyoshi and colleagues report on pits, notches, channels and borings from several dinosaur specimens. These are not the first such traces to be found on dinosaur skeletons from Mongolia; in the huge New Perspectives on Horned Dinosaurs volume published last year, paleontologists James Kirkland and Kenneth Bader described a well-preserved Protoceratops skeleton that had clearly been damaged by insects. In addition to chambers that may have harbored developing insect pupae, many of the surfaces along the dinosaur’s joints had been eaten away.
The damage to the skeleton described by Kirkland and Bader occurred after burial—if the insects had started chewing on the carcass before that, the skeleton would have fallen apart. This is how things may have gone down: The insects searched out the buried dinosaur carcass, dug down into the sand to reach it and then began their dirty work underground. Scarab or darkling beetles appeared to be the most likely candidates based on the behavior of modern scavengers. (Experiments were carried out by Bader with flesh-eating dermestid beetles to see if they would dig below the surface to feed on a carcass. “[M]ost of the beetles,” he reported, “either died in the cage or escaped in search of an alternative food source.” No word on whether the beetle exploratory mission was successful.) The additional specimens described by Saneyoshi and co-authors show similar types of damage, particularly around the joints, and these findings reinforce the notion that dead dinosaurs were an important staple of some flesh-eating insects of the day.
So why did the bone-chewing insects like dinosaur joints so much? That is difficult to say. As the authors of the in-press paper point out, very little is known about how desert insects utilize vertebrate carcasses. Studies of modern-day necrophagous insects will be needed to better understand what happened in Cretaceous Mongolia. In the past, though, some researchers have suggested that insects like termites have been attracted to fresh bones and cartilage as a handy source of nitrogen in dry environments in which the element may be relatively hard to come by. The positive evidence for this hypothesis is relatively thin at the moment, though, and further investigations will be needed to test the idea.
Mammals, according to Saneyoshi and co-authors, may have tucked into one dinosaur carcass, too. Even though the quick burial of the dinosaurs led the scientists to state “damage caused by vertebrate scavengers and transportation processes can be excluded in the case of present study” early in the paper, in a later section they mention a relatively large, 1.2-inch hole in one of the Protoceratops skeletons which they hypothesize was left by a mammal. The hole is near the shoulder blade, between the ribs and vertebrae, and is cited as being too large to have been made by an insect. Perhaps a multicuberculate—a variety of small mammal that became extinct many millions of years ago—dug out such a hole. Or perhaps not.
The large hole in the Protoceratops skeleton is difficult to interpret—it is not a clear bite mark, and the fact that the space is between bones makes it difficult to determine how the damage was created. Saneyoshi and co-authors mainly point to the size of the gap as being indicative of a mammal, but there don’t appear to be any tell-tale mammal toothmarks like those reported on Cretaceous dinosaur bones by Nicholas Longrich and Michael J. Ryan last year. (The paper by Longrich and Ryan is not cited in the in-press manuscript by Saneyoshi and collaborators.) There’s also the problem of timing. If Kirkland and Bader are correct about the timing of insect scavenging in these kinds of environments—namely that these types of insect traces were made after the dinosaurs were buried—then it is significantly less likely that a mammal created the damage in question. Tooth scores or gouges on a bone—a limb bone, a rib, a shoulder blade, or something similar—would help make the mammal connection, but as it presently stands, I think the large hole on the Protoceratops skeleton cannot be confidently attributed to a little, gnawing multituberculate.
We are only just beginning to understand how insects made the most of dinosaur bodies. There are plenty of damaged fossils out there which need description, and there is much we don’t yet understand about how modern insects utilize vertebrate carcasses. We need to know more about the prehistoric past as well as processes still in action today. There are still many mysteries for fossil forensic investigators to solve.
Kirkland, J.I. and Bader, K., 2010. Insect trace fossils associated with Protoceratops carcasses in the Djadokhta Formation (Upper Cretaceous), Mongolia. In: Ryan, M.J., Chinnery − Allgeier, B.J., Eberth, D.A. (Eds.), New Perspectives On Horned Dinosaurs. Indiana University Press, Bloomington, pp. 509-519.
Saneyoshi, M., Watabe, M., Suzuki, S., & Tsogtbaatar, K. (2011). Trace fossils on dinosaur bones from Upper Cretaceous eolian deposits in Mongolia: Taphonomic interpretation of paleoecosystems in ancient desert environments Palaeogeography, Palaeoclimatology, Palaeoecology DOI: 10.1016/j.palaeo.2011.07.024
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