Fossilized bones of dinosaurs, whales and mammoths get all of the attention but trace fossils provide important evidence for interpreting when and how organisms lived on Earth in the past. Trace fossils are not the fossilized remains of organisms themselves but rather are evidence of the past presence of organisms. The most recognizable trace fossils are footprints and leaf impressions. Many people are aware that 10s of thousands of preserved dinosaur footprints have been found but they may not be aware of how common other trace fossils are such as the preserved remains of mammal, insect and other animal burrows. Below is a picture of the fossilized remains of a large network of burrows formed by a small animal. The rocks pictured here are the hardened remains of what filled the burrows which was composed of different material than the surrounding rock. The latter rock eroded more readily leaving these hardened casts of the burrows behind. Paleontologists are interested in trace fossils such as these because they can often learn as much about the behavior of past life by studying such features as they can by studying the actual remains of the organisms themselves. See my post on elephant footprints for an example (Preservation of Behavior: Fossilized Elephant Tracks from the Arabian Peninsula).
The preserved burrows above are just a few of hundreds of fossil burrows of various sizes and shapes have been discovered thus far in a Triassic-aged layer of rock in Argentina. These are described in a paper in the open access journal PlosOne (see reference below). There are many additional images of these burrows in that paper. Below is another figure from that paper showing the geological context of these burrows. What is remarkable about the location of these burrows is that they are found in multiple layers of rocks and these layers of rock represent the bottom part of a much larger set of rock formations that extend more than 1000 feet above the location of the burrows.
The simplest explanation for the origin of these burrows is that they represent a time in the past before a 1000 feet of sediments was deposited on top of this region when animals burrowed into the ground on a relatively flat plain, much like Kansas today. Ripple laminations in rocks above the layers of rock where the burrows are found suggest that the region was covered with water at which time sediments would have been deposited to form more layers of rock. Today the regions is undergoing erosion, exposing these once hidden layers of rock and the fossilized burrows.
Large-Diameter Burrows of the Triassic Ischigualasto Basin, NW Argentina: Paleoecological and Paleoenvironmental Implications Colombi CE, Fernández E, Currie BS, Alcober OA, Martínez R, et al. (2012) Large-Diameter Burrows of the Triassic Ischigualasto Basin, NW Argentina: Paleoecological and Paleoenvironmental Implications. PLoS ONE 7(12): e50662. doi:10.1371/journal.pone.0050662
*Today’s Note is a revised version of a previous post from 2013.