Chemical clues illuminate fossil plant relationships

by Lucas Joel
Friday, October 13, 2017

To reconstruct relationships among extinct plants and animals, paleontologists often compare genetic sequences from distinct organisms or analyze differences in fossil shapes. But both techniques have limitations: DNA does not last more than about a million years in the rock record, so genetic comparisons are typically limited to relatively recent species; and finding fossils intact enough to use for shape comparisons can be difficult. In a recent study, scientists describe a new technique that could help get around these issues — for some plants at least — using molecular remnants that are more robust than DNA and are preserved in fossil leaves.

“The major discovery is that the fossil leaves contain different organic molecules that are specific for different plant groups,” says Vivi Vajda, a paleobotanist at Lund University in Sweden and the Swedish Museum of Natural History who led the new study, published in Nature Ecology and Evolution. This means that “even in leaves that are more than 200 million years old, we can gain information on what plant group a specific leaf belonged to.”

Vajda and her team studied molecules that form part of the waxy sheath, or cuticle, that coats plant leaves and helps prevent water loss. Working first with modern plants, the researchers shined infrared light on cuticle samples, measuring the infrared spectrum that each absorbed. They found that mixtures of molecules in the cuticle produce unique spectral signatures, or “fingerprints,” depending on which plant made them. These cuticle fingerprints were successful at confirming relationships among living plants like pine trees and ferns, which are already known from DNA analyses, Vajda says.

Next, the team compared signatures from cuticle samples, carefully collected from fossils of extinct plants that have living descendants, to the signatures of those modern plants. And, again, the analyses confirmed the known relationships, Vajda says. This means that fossil cuticles have similar fingerprints to those of related plants alive today, even though these particular fossils have been subjected to burial temperatures up to several hundred degrees Celsius for up to 200 million years, says co-author Stephen McLoughlin, a paleobotanist also at the Swedish Museum of Natural History.

Lastly, Vajda and her colleagues analyzed fossils from species with no living relatives, including some belonging to groups like the Bennettitales and Nilssoniales, both of which lived during the Mesozoic and had leaf shapes, or morphologies, similar to cycads. “Our results show that the cuticle chemistry of Nilssoniales is much closer to Bennettitales than to cycads,” Vajda says. The team also clarified the relationship between fossil Ginkgo plants and Leptostrobales. Leptostrobales “have long been suspected to be related to Ginkgos [based] on morphological criteria, but have never been included in any formal phylogenetic analyses,” McLoughlin says. “Our study showed that Leptostrobales do indeed have a leaf chemistry that is much closer to Ginkgo than to any other group we sampled,” he says.

In addition to its success in clarifying plant phylogenies, the new method is also attractive because researchers only need a small sample of cuticle. “We can use about a millimeter-sized sample [of a fossil], and it’s relatively easy to run the analysis,” McLoughlin says. This offers a major advantage over using fossil morphologies to reconstruct relationships, because “for morphological analysis you need an entire leaf, but often you only find bits and pieces,” Vajda says.

The fact that the technique even works is a big accomplishment, says Evelyn Kustatscher, a paleobotanist at the Museum of Nature South Tyrol in Italy who was not involved in the research. The method will not entirely replace morphological analyses, she says, noting also that it “cannot be applied to all [plant] fossils since it necessitates the presence of a cuticle, which may not always be preserved.” Nonetheless, “it is a very important technique” with “amazing” potential.

The technique may help resolve a mystery involving the evolution of flowering plants, which, Vajda says, abruptly appear in the fossil record about 130 million years ago, seemingly without many ancestral flowering species preceding them. “But they must have started somewhere, and that’s a question that could be solved with chemistry, because it’s been shown to be really difficult to resolve it [using fossil] morphology,” she says.

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