Fossilized spores from the Early Ordivician deposits of Australia. Image credit: Strother & Foster

 

Spores found in 480 million-year-old rock bring the fossil record in line with molecular estimates of when plants first adapted to life on land.

When plants first ventured onto the land, evolving from freshwater-dwelling algae, more than 500 million years ago, they transformed the planet. By drawing carbon dioxide from the air, they cooled Earth, and by eroding rock surfaces they helped build the soil that now covers so much land.

Until now, the first fossil evidence of land plants was from the Devonian period (420 million years ago). However, molecular evidence suggests an earlier origin in the Cambrian period. In a new paper in the journal Science, paleontologists described an assemblage of spore-like microfossils from Early Ordivician (480 million years ago) deposits in Australia; these spores are of intermediate morphology between confirmed land plant spores and earlier forms of uncertain relationship.

“These spore-like microfossils fill in a gap of approximately 25 million years in the fossil spore record, linking well-accepted younger plant spores to older more problematic forms,” said Dr. Paul Strother, a paleobotanist in the Department of Earth and Environmental Sciences at Boston College.

Dr. Strother and his colleague, Dr. Clinton Foster from the Research School of Earth Sciences at the Australian National University, examined populations of fossil spores extracted from a rock core drilled in 1958 in northern Western Australia.

“We found a mix of fossils linking older, more problematic spore-like microfossils with younger spores that are clearly derived from land plants,” Dr. Strother said.

“This helps to bring the fossil spore record into alignment with molecular clock dates if we consider the origin of land plants as a long-term process involving the evolution of embryonic development.”

“The fossil record preserves direct evidence of the evolutionary assembly of the plant regulatory and developmental genome.”

“This process starts with the evolution of the plant spore and leads to the origin of plant tissues, organs, and eventually macroscopic, complete plants — perhaps somewhat akin to mosses living today.”

“When we consider spores as an important component of the evolution of land plants, there is no longer a gap in the fossil record between molecular dating and fossil recovery.”

“Absent that gap, we have a much clearer picture of a whole new evolutionary step: from simple cellularity to complex multicellularity.”

“As a result, researchers and the public may need to re-think how they view the origin of terrestrial plants — that pivotal advance of life from water to land.”

“We need to move away from thinking of the origin of land plants as a singularity in time, and instead integrate the fossil record into an evo-devo model of genome assembly across millions of years during the Paleozoic Era, specifically between the Cambrian and Devonian divisions within that era,” he said.

“This requires serious re-interpretation of problematic fossils that have previously been interpreted as fungi, not plants.”

The study was published in the journal Science.

 
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