INSECT BURROWS AND BORINGS ON UPPER JURASSIC (MORRISON FORMATION) DINOSAUR BONES

Borings on Late Cretaceous dinosaur bones of Asia and burrows on Early Cretaceous bones from Utah, USA, are relatively common and generally attributed to beetles (Coleoptera) but have not been reported on pre-Cretaceous bones. Invertebrate trace fossils on an articulated front half of a sub-adult Camptosaurus (BYU 17945) show that insects, possibly beetles, also impacted bones in the Jurassic. The specimen was collected from a silty mudstone in a floodplain deposit within the Brushy Basin Member of the Morrison Formation, Albany Co., Wyoming. These preserved traces differ from other pits which occur abundantly on bones from the Morrison Formation in that: 1) pits are sparse and do not overlap, and range from shallow depressions -- to borings -- to tubes passing through a bone, 2) scratch marks are abundant and distinct, and 3) consumptive sinuous burrows are present on articular surfaces.

Fine scratches, 60 to 200µm wide and up to 2mm long, occur on laminar bone as star-shaped clusters radiating from a central point, crosshatched trails, and broad depressions scattered over the surface with concentrations along cracks and anatomical ridges. Pits and scratch marks along cracks indicate the cracks were present prior to infestation and facilitated bone harvesting. The star-shaped clusters are incipit points of attack and grade into pits and deep borings. Scratches occur in left and right pairs, with opposing scratches separated by a ridge, marking where mandibles are pulled toward the mouth of the scratch maker. The star pits/borings are roughly circular with diameters of 2-6mm and are up to 5mm deep with vertical to near vertical walls.

Similar, but smaller, insect trace fossils have been described on bones from the Pliocene of Tanzania and Oligocene of Bohemia, and were tentatively ascribed to termites. The differences between the Jurassic traces described here and those described from modern or fossil Jurassic termites is that these traces suggest larger individuals with stronger mandibulae. The trace-maker may be a larger termite species, or perhaps a beetle or other insect such as a hymenopteran. We rule out dermestid beetles because the borings are not flask-shaped and dermestids are not known to burrow and consume large quantities of bone.