Landing the earliest plants and animals.

Landing the earliest plants and animals 

Traditionally, scientists have thought of the oceans as a sortof womb from which fairly advanced plants and animals emerged to colonize the land. But could land-based life have taken root early on from simple creatures that lived in well-drained soils? The fossil record of early land life is too sparse to answer this question, but a new discovery should keep the idea germinating in more than a few scientists' heads.

In the Jan. 2 SCIENCE, Gregory J. Retallack at the University ofOregon in Eugene and Carolyn R. Feakes at Harvard describe what they believe are the oldest known traces of land animals: a network of tubular burrows ranging in diameter from 2 to 21 millimeters, which are embedded in a 488-million-year-old formation in Potters Mill, Pa. While trace fossils from this formation were known before, Retallack and Feakes say they are the first to recognize that the burrows were made not by marine or lake-dwelling animals, but by organisms living on land.

Their most important evidence is that the chemical makeupand microstructure of the burrow strata are typical of fossil soils and not of lake or river sediments, according to Retallack. Moreover, the researchers believe the burrows were a permanent feature of these soils, rather than being created long after the soil formed, because some burrows are encrusted with dolomite nodules, which normally take thousands of years to grow. They conclude that the burrows were made in a semiarid to arid environment.

Rettalack and Feakes found that the burrow diameters comein specific sizes, suggesting that the animals that made them grew in spurts. From the patterns of dirt the animals threw back into the tunnels as they dug, the researchers also conclude that the animals were bilaterally symmetric. Their best candidate is an arthropod, possibly a millipede. But since the oldest millipede fossil is 436 million years old and may well be a marine organism, the researchers suggest that their burrower may be a completely unknown and extinct animal.

According to Retallack, the oldest uncontested evidence of aland-based ecosystem is 414 million years old. "Our burrows show that there were functioning ecosystems on land, with some sort of plant food and fairly large animals, 34 million years earlier," he says. As such, this find lends support to some controversial suggestions, based on fossils of spores, that land colonization began at least 458 million years ago. One implication of all these finds, says Retallack, is that "by looking in aquatic assemblages, we may be hunting in the wrong places for the ancestors of land organisms."