For all that Charles Darwin figured out about life on earth, he was perpetually perplexed by flowering plants, calling their explosive evolution an “abominable mystery.” Now, a newly analyzed fossil species has shed light on where these plants, known as angiosperms, may have gotten their start. In water is the surprising suggestion.
For years botanists thought that angiosperms, which came to dominate the terrestrial landscape 160 million years ago, had arisen on dry land as they evolved from existing land plants. Bolstering the idea was the discovery in 1999 that a tiny land-dwelling shrub called Amborella sits at the base of the angiosperm family tree. “The consensus is that [flowering plants] originated on land and moved into water,” says Michael Donoghue, an evolutionary biologist at Yale University.
But David Dilcher, a paleobotanist at Indiana University, Bloomington, is now questioning that consensus. After an intense investigation of more than 1000 specimens of the fossil plant Montsechia vidalii, he and a team of researchers have concluded that the 125-million-year-old water dwellers are close relatives of the foxtail plant, a modern angiosperm. The connection between this fossil and the foxtail, as well as evidence from other ancient water plants, indicates that early on—perhaps at their very beginnings—angiosperms thrived in freshwater lakes and ponds, they suggest today in the Proceedings of the National Academy of Sciences.
Since Montsechia's discovery in rock deposits in Spain 150 years ago, botanists have classified it as a horsetail, a conifer, a tropical evergreen tree, and a liverwort, before finally deciding it was a flowering plant. To better understand these fossils, Dilcher and colleagues spent 6 years painstakingly dissolving the limestone surrounding them, revealing intricate structures beneath. The fossil species and current foxtail plants appear to share many of the same traits, Dilcher says. For example, analysis indicated that they are both pollinated underwater. Once released, their pollen sink and grow tubes, one of which eventually links to a hole in the female's seed-bearing structure, where fertilization occurs and the seed and fruit form.
This kinship with foxtail is intriguing, says Dilcher, because in the 1990s, foxtails were widely considered to be at the base of the flowering plants family tree. Then, molecular studies caused them and water lilies—another contender for the base—to give up their title to Amborella. Dilcher argues it’s time to reassess this assumption about Amborella using new morphological and molecular data.
Based on the Montsechia analysis, he wonders whether angiosperms made their first appearance in water. Two things in particular urge him on. If the close connection to Montsechia is true, then the foxtails would be 10 million years older than previously thought, he says, placing them closer to the time when angiosperms originated. In addition, other ancient aquatic angiosperms arose independently around the same time as Montsechia. Water lilies, for example, first made their appearance in Portugal about 125 million years ago. And another “first flower” contender—also an aquatic angiosperm—arose in China 125 million years ago, as Dilcher and his colleagues described in 1998.
Dilcher’s most recent work “demonstrates that aquatic angiosperms lived at the same time in very different regions of the Earth, [and that] adaptation to freshwater occurred early in angiosperm evolution," says Pamela Soltis, a plant biologist at the University of Florida in Gainesville, who was not involved with the work.
"The paper offers fresh evidence that early angiosperms invaded freshwater aquatic habitats," agrees Taylor Feild, a botanist at James Cook University in Townsville, Australia. But he's not convinced that flowering plants called water their first home. As he and others point out, even though land plants are just as old, aquatic plants are much more likely to be preserved in the fossil record, creating a potential bias.
Soltis, too, is skeptical that angiosperms started in water, but she says she is willing to keep an open mind. Finding more fossils of aquatic flowering plants and placing those fossils in the family tree “will tell us whether the first angiosperms were aquatic or terrestrial.”