A BURROW RUNS THROUGH IT
A system of burrows was recently excavated during the Holland Dinosaur Expeditions (2010-2012) of the Natural History Museum of Los Angeles County, led by Dr. Luis Chiappe of the Dinosaur Institute. These burrows are attributed to a fossorial mammal based on their large size, complex architecture, and internal morphology. Besides these features, what distinguishes these burrows from those previously described is their close association with the skeleton of a sauropod dinosaur. The Royce Quarry is located in southeastern Utah. The sediments are part of the Late Jurassic Morrison Formation, though its precise stratigraphic location is difficult to determine, but it is in the lower Morrison, most likely the Salt Wash Member.
The sediment which forms the Royce Quarry is composed of a pale grey-green mudstone overlain by a thin, finely-laminated redgreen mudstone, and finally a mottled red mudstone with prominent root casts (a paleosol). The bonebed itself contains the bones of a single camarasaurid sauropod. To date, no body fossils of the burrow makers have been found.
The largest burrows are 10-20 cm wide and occur within the quarry mudstone and overlying paleosol. Smaller burrows (<10 cm) are prevalent throughout the lower portions of the bonebed and have been found in close association with many sauropod bones. The uppermost burrows form a complex network at what would have been close to the paleosurface of the original soil. These are connected to the surface through multiple vertical or oblique passageways. All burrows are infilled with a grey-green sediment speckled with reddish-brown or black areas interpreted as trapped organic matter. All burrows are cemented with calcite. Grain size varies between different burrows suggesting multiple, separate infilling events.
Modern and fossil mammal burrows are characterized by a relative complexity in geometry and architecture. Burrows of modern fossorial mammals typically have: vertical shafts leading to horizontal tunnels; enlarged chambers used for nesting or latrines; and multiple-branching systems. Both size and facies occurrence of these burrows – in a paleosol – mean these more likely belong to mammals. These burrows confirm the notion that some Jurassic mammals utilized a fossorial lifestyle.
Whether the mammals gnawed on the sauropod bones or not is to be determined, as the bones have yet to be prepared. Furthermore, the large size of the burrows suggests that there remains undiscovered mammalian diversity, especially at larger body sizes. Burrow allometry is closely correlated with the body mass of the tracemaker, which provides a mass estimate of 400-500 g, about the size of a large pocket gopher.
Work on describing the burrow system is ongoing and we hope to get it published soon.” - Chris Noto, Assistant Professor teaching human anatomy at University of Wisconsin-Parkside
Martin, A.J., Noto, C.R., and Chiappe, L.M. 2011. A burrow runs through it: unusual co-occurrence of a large mammal burrow system and dinosaur skeleton in the Morrison Formation of Utah. Society of Vertebrate Paleontology Annual Meeting, Las Vegas NV.
Photos by Chris Noto. Permitted work conducted in the Monticello Field Office by the Dinosaur Institute under permit number UT10-009E-Sa, held by Luis Chiappe.