Trace fossils may help detect life elsewhere

Arecent, fascinating article linked two areas of science that are not often connected — paleontology and the search for life elsewhere in the universe.

When we think of fossils, we usually think of things such as bones, teeth and shells, which are known as body fossils. But there is another large group called trace fossils, including footprints, burrows (made in soft sediment) and borings (made in hard substances such as wood and rock). They also are known as ichnofossils, and their study is called ichnology.

Andrea Baucon of the University of Modena, Italy, and 10 other scientists, writing in the journal Earth Science Reviews (171: p. 141-181), made a good case that, when searching for signs of life on other planets, the most likely evidence may well be trace fossils.

Most discussions of the search for signs of extraterrestrial life favor looking for things such as an abundance of oxygen in the atmosphere or for organic compounds, but that assumes that life on other planets is chemically like that on Earth, which may be an unwarranted assumption.

But all life must eat, and in so doing may well leave traces that resemble the feeding traces of Earth’s organisms. The same kind of meandering traces are made by organisms as different as worms, snails, insects and echinoderms. That pattern is not tied to a specific biochemistry, but is the most-efficient way to cover ground in the search for food.

Most organisms on Earth are soft-bodied — 40 to 70 percent of most faunas consist of soft-bodied organisms — and have little chance of leaving body fossils. Also, during the first 80 percent of the history of life on Earth, life was soft-bodied. Hard parts originated fairly recently, only about 500 million years ago. Such may also be true of extrasolar planets, so body fossils may be rare to nonexistent, but soft-bodied organisms can leave huge numbers of trace fossils.

One example is a succession of weakly metamorphosed rocks in Portugal more than 1,300 feet thick. In all of those rocks, only two fragmentary body fossils have been found, but those same rocks have produced more than 5,000 trace fossils.

Similarly, an area of Cretaceous rock in Brazil covering more than 6,000 square miles has produced hundreds of fossil footprints of dinosaurs, mammals and arthropods, but not a single fossil bone or tooth.

One reason ichnofossils can be more abundant than body fossils is because an animal can contribute just one (or perhaps half a dozen, if it molts) skeleton to the fossil record, but during its lifetime it might make millions of footprints.

Ichnofossils also can be much larger than the organisms that make them. For example, a common burrowing shrimp is only about 2 inches long, but it makes burrows that can be more than 16 times larger.

Furthermore, diagenesis, the processes that occur after the original soft sediment is deposited and is slowly turned into solid rock, often alters or obliterates body fossils but enhances trace fossils, making them stand out from the surrounding material.

All in all, trace fossils might be the best thing to look for when hunting for signs of life on other planets.