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 borrows. Below is a picture of the fossilized remains of a large network of borrows formed by a small animal. The rocks pictured here are the hardened remains of what filled the borrows 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).
Apparently hundreds of fossil burrows of various sizes and shapes have been found in a Triassic-aged layer of rock in Argentina. These are described in a paper in the open access journal PlosOne last year (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. No commentary on this today though if you follow any of my posts you can probably guess what I would probably say.