Trace fossils are left behind by ancient organisms and are found in sedimentary rock. They provide important information about behavior and anatomy, and can be divided into categories such as dwelling structures and feeding traces. They also offer unique insights, such as the possibility of life on land during the Cambrian era. Some of the most spectacular trace fossils are left by eurypterids and even human footprints.
As the words suggest, trace fossils are trace fossils left behind by organisms, many ancient and extinct. The oldest are from the Twitya Formation in northwestern Canada, dated to 610 million years ago. Within paleontology, the formal name for trace fossils is ichnofossils, from the Greek ichno meaning “trace” or “trace.” Trace fossils are found in the sedimentary rock and dated by examining the isotope ratios of the zircons embedded in the same layer.
While trace fossils may not be as fantastical as more familiar body fossils, they are much easier to find and provide a crucial reference point for unraveling the behavior and anatomy of organisms. On the other hand, many trace fossils may be of ambiguous origin, with thousands of unidentified organisms. Under these circumstances, scientists have to settle for educated guesses and debates.
Trace fossils can be divided into various categories. There are Domichnia, fossil traces of dwelling structures, such as underground chambers; Fodinichnia, three-dimensional structures of animals that burrow through sediment for food, such as worms; Pascichnia, traces of feeding left by surface organisms; Cubichnia, left by organisms resting on soft sediments; and Repichnia, surface tracks left by organisms that crawl across the surface.
Trace fossils can give us important information that other fossils cannot. For example, there are soil tracks, called Climactichnites, left by a large snail-like animal dating back to 510 million years ago, in the Cambrian era. This is quite unique, as the earliest fossils of air-breathing animals – indicated by breathing holes – date back to just 428 million years ago. Was there really life on land as far back as the Cambrian, or perhaps life adapted to short periods on land, such as crawling between tidal pools? Perhaps these animals were so soft and fragile that they decomposed before leaving any fossil bodies. With no trace fossils, we would have to rely solely on body fossils to determine when the first organisms arrived on land.
Some of the most spectacular fossil traces are left by eurypterids, gigantic sea scorpions the size of small cars. There are also more recent fossil traces, such as human footprints a few million years old, left by the first homonids traveling on volcanic ash in Tanzania.
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