| Treptichnus pedum trace fossils from lower Cambrian portion of the upper Deep Spring Formation, Inyo County, California; feeding trails that resemble types made by modern priapulid worms. Such Treptichnus pedum trails begin to appear worldwide in strata deposited at the Precambrian-Cambrian boundary; stratigraphers use them to mark the precise moment in geologic time when the Paleozoic Era began--541 million years ago. Photograph from a specific scientific publication. |