The scientists interpret the coccoidal cells as being sheathed cyanobacteria or possibly green algae. "The filaments have reproductive characteristics that make us think they are fungi," Xiao said.
Taylor said, "Clearly, there are two kinds of organisms living together and, we believe, interacting in more than a chance association."
In modern lichens and in the 400-million-year-old Scotland fossils, the coccoidal cells provide the nutrients and the fungal filaments provide protection against dehydration. But in the marine environment, dehydration is not an issue and the 600-million-year-old rocks also contain many fossils of coccoidal cells that are not surrounded by filaments. "So it is a loose lichen-like association," said Xiao. "The organisms are not obliged to live together."
Now there is a new question. "We know that this symbiotic relationship was forged 600 million years ago or earlier. But, was it carried over to land, or did each organism invade land and forge a new relationship independent of the marine relationship? If the latter, then the 600-million-year-old relationship may not be the direct ancestor of the 400-million-year-old relationship," Xiao said.
Fungi and algae in modern lichens can easily marry and divorce, he said. "Given the ease with which the symbiotic relationship is formed, I wouldn't be surprised if the land-based relationships formed independently of the older marine relationships."
"In fact, studies of modern lichens demonstrate that the lichen symbiosis evolved many times," Taylor said.
"The ability to form a symbiotic