What are "Lichens"? Lichens are structures composed of a unique symbiotic relationship between an alga and a fungus. The alga provides food, and the fungus provides other nutrients and protection. Although the partners in this symbiotic relationship have a long fossil record, lichens as we know them today do not. Their fossil record only extends into the Tertiary, partly because the relationship may be difficult to identify in fossils. Living on exposed rocks in dry environments, lichens help to weather rocks.
First known fossil occurrence: Tertiary.
Last known fossil occurrence: Quaternary. This group has living relatives.
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