The three-dimensional burrows were found at the interface of a layer of coal and a layer of siltstone in southwestern North Dakota by Pearson, who has spent many years studying K-T boundary sites in the state. The decomposing organic matter in the ancient environment would have provided a food source for the worms. A few of the burrows were topped by a thin layer of coal, suggesting that the underlying coal may contain additional, earlier worm burrows that are not readily apparent, Chin said.
The clay boundary layer laid down at the end of the Cretaceous Period is associated with high levels of iridium, an element rare in Earth's crust but abundant in asteroids. The Manhattan-sized asteroid plowed into Earth at 150 times the speed of a jet airliner and is thought to have released about a billion times more energy than the Hiroshima atomic bomb, triggering tremendous dust and ash storms, wildfires, tsunamis, mega-earthquakes and dark, cold "nuclear winter" conditions for a time.
The North Dakota fossil worm burrows indicate the creatures probably were about the diameter of an average earthworm. The burrows indicate horizontal rather than vertical movement through the substrate, likely reflecting feeding activity, Chin said.
The study indicates the burrows were made in a peat-producing, bog-like environment that eventually was buried by sediment. Chin said the worms must have been capable of withstanding the challenging environmental stresses of flooded habitats, including prolonged inundation, low oxygen and acidic conditions.
Since the ancient burrows were filled by sediment, they actually are "positive casts" of the trails made by the worms. The burrows are examples of "trace fossils," which also include tracks and fossilized feces, or coprolites. "When we reconstruct past environments, soft-bodied animals like worms get short-shrift since they don't stand out in the fos
|Contact: Karen Chin|
University of Colorado at Boulder