Fossil captures plant in transition

Scientists have discovered an ancient plant species they say provides another piece in the evolutionary jigsaw.

Dating back some 400 million years to the Early Devonian period, it produced a spectrum of spore sizes, a precursor to the specialised strategies of land plants that span the world’s habitats.

As such, it is one of the most comprehensive examples of a seemingly intermediate stage of plant reproductive biology, a team led by Stanford University, US, reports in a paper in the journal Current Biology.

“Usually when we see heterosporous plants appear in the fossil record, they just sort of pop into existence,” says senior author Andrew Leslie. “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.”

The specimens likely belong to the herbaceous barinophytes, an unusual extinct group that may be related to clubmosses.

Leslie and colleagues made the discovery while analysing a collection of fossils that had been stored at the Smithsonian National Museum of Natural History for decades. 

From about 30 small chips of rock originally excavated from the Campbellton Formation of New Brunswick, Canada, they identified more than 80 reproductive structures or sporangia. 

The spores themselves range from about 70 to 200 microns in diameter – about a strand to two strands of hair. 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 other eggs.

“It’s rare to get this many sporangia with well-preserved spores that you can measure,” Leslie says. “We just kind of got lucky in how they were preserved.”

One of the most important time periods for the evolution of land plants, the Devonian witnessed diversification from small mosses to towering complex forests, the researchers say.

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,” Leslie says. 

“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 earliest plants, from between 475 million to 400 million years ago, lacked reproductive specialisation in the sense that they made the same types of spores, which would then grow into little plantlets that actually transferred reproductive cells. By partitioning reproductive resources, plants assumed more control over reproduction, according to the researchers.

The new species, together with the previously described plant group Chaleuria of the same age, 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.