The explosion in the number of flowering plant (angiosperm) species during the Cretaceous is credited with causing a dramatic decline in other vascular plant groups such as horsetails, cycads, and ferns. But new research published this week in Nature suggests that fern diversity underwent a renaissance following the rise of angiosperms and that the appearance of angiosperm forests triggered this revitalization.

“Paleobotanists had hinted at this possibility,” said Kathleen Pryer of the Department of Biology at Duke University, who led the study. “But it's very hard to prove it just with the fossil record.”

In an accompanying News and Views article Torsten Eriksson, from the Bergius Foundation, argued that the findings call into question the concept of evolutionary “cul-de-sacs,” a term applied to lineages that remain unchanged over long periods of evolutionary time.

“People have been saying 'Well, these guys have been around for so long and they [still] virtually look the same, so they probably are just slowly dying off,'” he told The Scientist. But on the evidence of this research, he said, that is “probably not true. If they get the opportunity, things will happen.”

Pryer's team focused on a group of ferns called polypods that comprise 80% of extant fern species, but that are poorly represented in Cretaceous deposits. To compare the timing of angiosperm and polypod diversification, the authors obtained DNA sequence data from two chloroplast genes (rbcL and rps4) for 45 polypods and other ferns to reconstruct the fern phylogeny using Bayesian sampling.

The branch lengths of such phylogenetic trees are traditionally determined using fixed molecular clocks. But because these cannot cope with the variable rates of evolution that are increasingly recognized to occur between lineages, the researchers instead calibrated their tree with fossil records, a method that functions as a relaxed molecular clock.

By estimating divergence times across 1000 randomly sampled Bayesian trees, the team was also able to demonstrate only a small degree of uncertainty within their chosen phylogeny. “In the past, people have just taken their best estimate of phylogeny, applied the fossil constraints, and gotten a point estimate for a particular divergence time,” said Eric Schuettpelz, coauthor of the paper.

The resulting chronogram could then be compared with a similar one for angiosperms. The comparison revealed that polypods also diversified in the Cretaceous — but after the radiation of the angiosperms.

Alexei Drummond, an evolutionary biologist at the University of Oxford, agreed, although he expressed concern that the degree of variation in evolutionary rates required to fit the tree to the fossil record is not presented in the paper. “Are they reasonable variations, or astronomically large and something we would be suspicious about?” he said.

For polypods, the opportunity to undergo renewed diversification could have arisen once the angiosperm flora became organized into forests. “Angiosperm forests are ecologically and structurally much more complex than the gymnosperm forests that were around prior to their arrival,” said Pryer. “They offer a much more diverse array of microclimates and habitats.” Many modern ferns are, for example, epiphytes that grow on the trunks and branches of trees.

The authors cite the recent discovery in a polypod fern of an unusual photoreceptor, homologues of which are found in other polypods but not in basal ferns or seed plants. The Duke team suggests that the photoreceptor could have assisted polypods to utilize the low light levels found under angiosperm forest canopies.

Harald Schneider, first author of the paper, stressed the need for a better understanding of the function and control of the photoreceptor, as well as of the types of habitats utilized by Cretaceous polypods. Until then, said Eriksson, “that's a story. But it's a plausible story.”