Exploited Twice: Bored Bone in a Theropod Coprolite from the Jurassic Morrison Formation of Utah, U.S.A

Unusual compound trace fossils of bored bone within coprolites provide direct fossil evidence for successive exploitation of dinosaur tissues by vertebrates and invertebrates. These specimens are best described as matrix-supported conglomerates of dinosaur bone, and were found as float in the Brushy Basin Member of the Jurassic Morrison Formation in the San Rafael Swell of eastern Utah. The high concentration of bone, absence of sorting, paucity of inorganic clasts, and presence of microcrystalline phosphate point to a fecal origin, and the contents and large size of the coprolitic masses indicate that they were produced by a large theropod.

Several fragments of bone in the coprolites have been conspicuously bored. Casts formed by separation of lithified fill from some of the cylindrical boreholes reveal bullet-shaped termini; this distinctive morphology suggests that the cavities were utilized for protection. Inasmuch as pupating larvae of dermestid beetles make the closest modern analogs of comparable boreholes in bone, we deduce that ancestors of modern dermestids may have bored the Jurassic bone. Although these are the first reported fossils of boreholes in coprolites, a number of examples of bored bone have been described from the Mesozoic and the late Cenozoic. Such trace fossils may reflect changing patterns in the availability of large-bodied vertebrate carcasses over time.

It is not clear whether the bone fragments were bored before or after theropod consumption, but it is likely that the bone borers opportunistically exploited bone found in theropod feces. These compound traces reveal complex trophic interactions linking dinosaurs and insects, whereby refractory dinosaur tissues were exploited and recycled in a Jurassic ecosystem.