Early Devonian fossil provides first evidence of advanced reproductive biology in land plants

A plant species that grew about 400 million years ago (Early Devonian period) produced a spectrum of spore sizes, which is an essential innovation necessary for all advanced plant reproductive strategies, including seeds and flowers.

In this image of one of the reproductive structures of one of the New Old Species, elliptical impressions of sporangia can be seen in one row, while to the right another row displays preserved charred spore masses. Photo credit: Andrew Leslie.

The Devonian period is one of the most important periods for the evolution of land plants. He witnessed the diversification from small mosses to towering complex forests.

The development of different spore sizes, or heterosporia, represents a major alteration in reproductive control – a feature that later evolved into small and large versions of these reproductive units.

“Think of all the different types of sexual systems that are found in flowers – it all relies on the presence of small separate spores, or pollen, and large spores, which are found inside the seeds,” he said. lead author, Dr. Andrew Leslie, researcher. in the Department of Geological Sciences at Stanford University.

“With two discrete size classes, it’s a more efficient way to pack resources because large spores can’t move around as easily as small ones, but can feed offspring better.”

The newly discovered plant species belongs to the herbaceous barinophytes, an unusual group of extinct plants that may be related to lycopods, and is one of the most complete examples of an apparently intermediate stage in plant reproductive biology.

“Usually when we see heterosporous plants appear in the fossil record, they kind of show up,” Dr Leslie said.

“We think this may be a kind of snapshot of this very rarely seen transition period in evolutionary history where you see strong variation among spores in reproductive structure.”

Dr. Leslie and his colleagues analyzed fossilized plants from the collections of the Smithsonian National Museum of Natural History.

From around 30 small rock chips originally collected from the Campbellton Formation in New Brunswick in Canada by paleobotanist and co-author Dr. Francis Hueber, they identified over 80 sporangia (reproductive structures) .

The spores themselves range from about 70 to 200 microns in diameter.

While some of the structures contained large or small spores exclusively, others contained only intermediate sized spores, and others contained the full range of spore sizes – perhaps some producing sperm and some producing eggs. .

“It’s rare to have as many sporangia with well-preserved spores as you can measure,” Dr. Leslie said.

“We were just lucky in how they were preserved.”

With the group of plants described above heat, the new species represents the first evidence of more advanced reproductive biology in land plants. The next example does not appear in the fossil record until 20 million years later.

“These types of fossils help us pinpoint when and how exactly plants achieved this type of partitioning of their reproductive resources,” Dr Leslie said.

“The very end of this specialization evolutionary story looks like a flower.”

The team’s paper was published in the May 4, 2020 issue of the journal Current biology.

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Nikole K. Bonacorsi et al. 2020. A new reproductive strategy in an early Devonian plant. Current biology 30(9):388-389; doi:10.1016/j.cub.2020.03.040