TRACE FOSSIL EVIDENCE FOR SOME OF THE EARLIEST KNOWN ANIMAL ACTIVITY BY SURVIVORS OF THE CRETACEOUS-PALEOGENE EXTINCTION EVENT, MUD BUTTES LOCALITY, SOUTHWESTERN NORTH DAKOTA

The extensive mass extinctions at the Cretaceous-Paleogene boundary reflect widespread environmental perturbations after the K-Pg bolide impact, and researchers continue to focus on reconstructing the pace and progression of terrestrial ecosystem recovery after this event. Paleobotanical evidence reveals that the earliest post K-Pg boundary terrestrial ecosystems in North America were dominated by low diversity, opportunistic, aquatic plant communities, but sediments from this early stage of recovery are nearly devoid of animal fossils. Now trace fossil evidence reveals some of the earliest animal activity after the impact event. Crisscrossing networks of horizontal Planolites burrows have been found about seven cm above the K-Pg boundary clay in the Fort Union Formation at the Mud Buttes locality in southwestern North Dakota. The burrows average around 4.8 cm in width and have rough, unlined burrow surfaces, no noticeable tapering, and very few branches. These three-dimensional burrow casts are evident at the interface of a facies change from a lignitic coal to siltstone, but a few burrows are also topped by a thin layer of coal; this suggests that the underlying coal may contain more burrows that are not readily apparent.

The sedimentary and palynological context of the burrows indicates that they were made in a peat-producing fen that was eventually buried by sediment. Abundant detritus would have provided a readily available food source, but the burrowers must have been capable of withstanding the challenging environmental stresses of flooded habitats, such as prolonged inundation, low oxygen, and acidic conditions. These features suggest clitellate annelids as possible burrowers; numerous extant oligochaetes are known to inhabit modern wetlands, and their size range and peristaltic mode of locomotion are consistent with the morphology of the Mud Buttes burrows. Regardless of the identities of the burrowing animals, these burrows demonstrate that opportunistic invertebrate survivors were like their botanical counterparts in their ability to exploit the widespread, ponded environments that characterized many early Paleogene environments in North America.