Early Devonian Fossil Provides Earliest Evidence for Advanced Reproductive Biology in Land Plants
A species of plant 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.
The Devonian period is one of the most important time periods for the evolution of land plants. It witnessed diversification from small mosses to towering complex forests.
The development of different spore sizes, or heterospory, represents a major modification to control reproduction — 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 in flowers — all of that is predicated on having separate small spores, or pollen, and big spores, which are inside the seeds,” said senior author Dr. Andrew Leslie, a researcher in the Geological Sciences Department at Stanford University.
“With two discrete size classes, it’s a more efficient way of packaging resources because the big spores can’t move as easily as the little ones, but can better nourish offspring.”
The newly-discovered plant species belongs to the herbaceous barinophytes, an unusual extinct group of plants that may be related to clubmosses, and is one of the most comprehensive examples of a seemingly intermediate stage of plant reproductive biology.
“Usually when we see heterosporous plants appear in the fossil record, they just sort of pop into existence,” Dr. Leslie said.
“We think this may be kind of a snapshot of this very rarely witnessed transition period in evolutionary history where you see high variation amongst spores in the reproductive structure.”
Dr. Leslie and colleagues analyzed fossilized plants from the collections of the Smithsonian National Museum of Natural History.
From about 30 small chips of rock originally collected from the Campbellton Formation of New Brunswick in Canada by paleobotanist and co-author Dr. Francis Hueber, they identified more than 80 sporangia (reproductive structures).
The spores themselves range from about 70 to 200 microns in diameter.
While some of the structures contained exclusively large or small spores, others held only intermediate-sized spores and others held the entire range of spore sizes — possibly with some producing sperm and others eggs.
“It’s rare to get this many sporangia with well-preserved spores that you can measure,” Dr. Leslie said.
“We just kind of got lucky in how they were preserved.”
Together with the previously described plant group Chaleuria, the new species represents the first evidence of more advanced reproductive biology in land plants. The next example doesn’t appear in the fossil record until about 20 million years later.
“These kinds of fossils help us locate when and how exactly plants achieved that kind of partitioning of their reproductive resources,” Dr. Leslie said.
“The very end of that evolutionary history of specialization is something like a flower.”
The team’s paper was published in the May 4, 2020 issue of the journal Current Biology.
Nikole K. Bonacorsi et al. 2020. A novel reproductive strategy in an Early Devonian plant. Current Biology 30 (9): 388-389; doi: 10.1016/j.cub.2020.03.040