Markings On The Bones Show Bugs Were Eating Dead Dinosaurs
If you look very closely at the bones on the wall of the Carnegie Quarry, you will notice many of them are covered with small markings. These markings have been proposed as the work of insects.
How are insects interpreted as the culprit? There is matrix from the surrounding rock located inside many of the markings, indicating they could not have been caused by excavation processes. This fact also makes it difficult to explain the markings as a result of abrasion from transport, or exposure to acidic conditions. The markings are just too consistently sized and shaped for either of those explanations. The markings do not match up with those of any known animal in the Morrison Formation ecosystem, so scavenging by vertebrates can be ruled out.
So if the possibilities can be ruled down to insects, who made them?
The scientists who have studied these markings and other traces made by insects on dinosaur bones in the Morrison Formation have used a variety of techniques. Photography with an intense light source positioned at an angle makes the markings more visible by highlighting their shadows. Molds are taken of the markings themselves and then photographed with powerful microscopes to reveal the finest of details.
The size, shape, and texture of the markings has been dutifully studied. How densely they cluster, is taken into account. The location of the markings is noted, whether they occur in the harder cortical bone or the softer cancellous bone at the ends where they articulate with other bones. Cancellous bone is surrounded by cartilage and other soft tissues and is much more vascular than cortical bones, so it is consumed by saprophagous (flesh or carrion-eating) insects more frequently.
The two groups of authors Britt et al., 2008, and Hasiotis et al., 1999, despite using similar techniques, arrived at separate conclusions, why?
Britt et al. conducted a thorough analysis of bones of a Camptosaurus specimen from the Morrison Formation. The authors noted the diversity of markings created by dermestid beetles, ranging from thin grooves made by the mandibles to elliptical pits created by pupal chambers. The authors found the pits on the Carnegie Quarry bones to be more consistent with marks made by termites during foraging. This interpretation was supported by the irregular and rough nature of the pits, consistent with the lopsided anatomy of the termite mouth. However, the authors did allude that the elliptical pits may have been consistent with dermestid pupal chambers.
Hasiotis et al. attributed the markings to be made by dermestid beetle pupal chambers. Dermestid beetles, when undergoing metamorphosis, will anchor themselves into bone. The authors compared the markings to analogs from the Tertiary period.
Whoever the culprit may be, an interpretation of the trace maker influences the interpretation of what the circumstances of the dinosaur bones entailed from death to fossilization. Termites will feed on the bone that is fresh and weathered. Dermestids, however require more specific conditions. For dermestids to use bone for pupation, the carcass must be partially covered with dried flesh. The carcasses must have been dry and above water, and must have been above the sedimentary surface.
An interpretation of the trace maker influences the reconstruction of the paleoenvironment. The traces are consistent with the record of the unionid clams, which occur in layers as a mass death assemblage. The river floodplain may have dried up seasonally, killing the unionids, and allowing insects to feed on dry dead flesh and leave their traces on the bone. When the flood returned they were washed down and deposited, beginning the process of fossilization.
Britt, Brooks B., Scheetz, Rodney D., Dangerfield, Anne. 2008. ‘A Suite of Dermestid Beetle Traces on Dinosaur Bone from the Upper Jurassic Morrison Formation, Wyoming, USA’, Ichnos, 15:2, 59-71.
Hasiotis, Stephen T., Fiorillo, Anthony R., Hanna, Rebecca R. 1999. ‘Preliminary report on borings in Jurassic dinosaur bones: evidence for invertebrate vertebrate interactions’, Vertebrate Paleontology in Utah. Utah Geological Survey. 99-1. Link to publication