PALEOBIOLOGICAL IMPLICATIONS OF DIMINUTIVE INVERTEBRATE BURROWS WITHIN PROBABLE GUT CONTENTS OF A HADROSAURID DINOSAUR FROM THE UPPER CRETACEOUS (MIDDLE CAMPANIAN) JUDITH RIVER FORMATION OF MONTANA

Tiny distinctive burrows have been found within organic-rich material in the body cavity of an exceptionally-preserved specimen of a hadrosaurid, Brachylophosaurus canadensis, from the Judith River Formation of Montana. This association is particularly interesting because the organic-rich material appears to represent, at least in part, the gut contents of the dinosaur.

Over 200 burrows were observed in 17 samples of gut region material. These traces have several notable characteristics: almost all are 250 to 350 µm in diameter; many have thin calcareous linings; fine parallel striae are present in the linings of some burrows; and at least a dozen burrows share walls with adjacent burrows, often for several mm. Only one type of burrow was found. The absence of other types of traces is consistent with other taphonomic evidence that the dinosaur carcass was buried rapidly with little time for scavenging. The burrow characteristics indicate that they were made by small, terrestrial, soft-bodied vermiforms; modern analogues include annelids and nematodes. It seems unlikely that such vermiforms would have been the only scavengers, so it seems most parsimonious to interpret them as parasites that were stranded in the body cavity of the dinosaur. The fossil paired burrows have no precedent. Several lines of evidence, such as constant burrow diameters, and matching changes in direction, suggest that they were made by two individuals at the same time. These, coupled with the length of some examples, would suggest sustained intraspecific contact, which may be related to a social interaction such as mating.

These burrows provide a record of the activity of otherwise unknown terrestrial meiofaunal-sized soft-bodied invertebrates. The evidence that the vermiforms may have parasitized a hadrosaur and left traces of reproductive behavior adds unique information to our understanding of Mesozoic trophic interactions and vermiform paleobiology.