Fossils place plants on land over 500 million years ago. The oldest known traces of land plants are encased in midCambrian rocks about 550 million years old. They are microscopic fossils of what paleobotanists interpret to be plant spores. Fossilized plant spores are plentiful in mid-Ordovician (460-million-year-old) deposits from around the world, suggesting that by then plants were abundant and widespread. Some of these fossilized spores occur in aggregates of four, which is the way some modern bryophytes release their spores. The tough walls of sporopollenin explain why spores are the plant structures most likely to leave a fossil record. However, fossilized spores are sometimes ascociated with scraps of tissue that many represent the remains of the sporophytes that produced the spores.