No bigger than specks of dust, cryptospores are one of our largest windows into the deep history of plants. These ancient spores and pollen show up in the fossil record between 465 million and 407 million years ago, a key moment for Earth’s greenery. During the first half of that period, the nonvascular land plants—mosses, liverworts, and hornworts—held sway, dominating the landscape for 30 million years. But eventually, vascular plants—ferns and seed-bearing plants—evolved and gradually took over. By examining the shapes and structures of cryptospores collected during oil explorations, paleobotanists have been trying to pin down this transition. New finds now push back the date when vascular plants appeared by almost 30 million years, to about 450 million years ago, Philippe Steemans of the University of Liège, Belgium, and colleagues report in the 17 April issue of Science.
Cryptospores differ from modern spores and pollen in that they come in clumps of two or four, having failed to separate as modern pollen does into individual cells. Thus these dyads and tetrads (above; scale is 10 micrometers) are diagnostic for ancestral land plants. Spores that have disassociated from these clumps represent more modern plants, and folds and bumps on spore surfaces distinguish species and thus are indicative of diversity.
Since 1990, Steemans and Charles Wellman of the University of Sheffield in the United Kingdom have been retrieving fossils from boreholes dug for oil exploration in Saudi Arabia. They dissolve the rock in different solvents to remove carbonate and silicate materials and examine the remaining organic material under light and scanning electron microscopes. They typically find marine fossils, which help them determine the age of the rock, as well as spores and pollen.
In the most recent samples, the cryptospores at first seemed quite ordinary. But when they looked closely, Steemans and Wellman found single spores—called trilete spores (left; scale is 10 micrometers)—dispersed among the dyads and tetrads. “Our first reaction was to think that the samples had been contaminated by younger material,” Steemans recalls. But an independent analysis in a second laboratory turned up the same ancient trilete spores.
“These generally don’t appear until much later,” about 436 million years ago, says Wellman. Thus these newly described, 450-million-year-old cryptospores “probably represent the origin of vascular plants.”
Photo Credit: P. Steemans