Fossil find in California shakes natural history of cycads

Fossil find in California shakes natural history of cycads

Fossil find in California shakes natural history of cycads

Photo credit: SKsiddhartthan, CC BY-SA 4.0 via Wikimedia Commons

Cycads, a group of gymnosperms that can resemble miniature palms (like the popular sago palm houseplant), have long been considered “living fossils,” a group that had evolved minimally since the time of the dinosaurs. Now, a well-preserved 80-million-year-old pollen cone discovered in California has rewritten the scientific understanding of plants.

The findings are detailed in an article by two University of Kansas paleobotanists just published in the journal New phytologist.

“Cycads are not well known, but they represent a significant part of plant diversity, accounting for about 25% of all gymnosperms,” ​​said lead author Andres Elgorriaga, postdoctoral researcher at the KU Department of Ecology & Evolutionary Biology and KU Biodiversity Institute and Natural History Museum.

“Cycads are plants with thick stems and short stature, with thick, palm-like leaves at the top. They produce cones like pine cones and are related to other seed-bearing plants that also don’t produce flowers, like ginkgo and the monkey puzzle tree. But they are also critically endangered, with the highest level of endangerment of any plant group. The trade in cycads is also a significant problem.”

Despite their importance, a lack of fossil evidence and confusion over the classification of some fossil specimens have led to a murky scientific understanding of the evolutionary history of plants over the years. A prominent idea was that cycads today are almost identical to their prehistoric ancestors.

Three-dimensional reconstruction of the cone, first making the various tissues transparent and then digitally removing them to better understand its internal anatomy. Green: cone scales; Yellow: pollen sacs; Blue: vascular tissue. Photo credit: University of Kansas

“The prevailing school of thought is that cycads have not changed significantly in deep time,” said co-author Brian Atkinson, assistant professor of ecology and evolutionary biology and curator of paleobotany at the KU Biodiversity Institute and Natural History Museum.

“But the fossil record of cycads is poorly understood, and many things that have been called cycads have turned out not to be cycads at all. Here we have a three-dimensionally preserved cone that is unique to cycads because it has internal anatomy and pollen grains typical of that group. However, the external morphology of this pollen cone differs from living cycads. This finding suggests that cycads are not truly “living fossils” and they likely have a more dynamic evolutionary history than previously thought.”

According to the KU researchers, their analysis of an 80-million-year-old permineralized pollen cone found in the Campania woody shale formation in California’s Silverado Canyon tells a more accurate natural history of cycads — one in which plants diversified during the Cretaceous.

“With this kind of discovery, we’re finding that there were cycads at that time that were really different than today in size, in number of pollen sacs, and in a lot of things,” Elgorriaga said. “Maybe we didn’t find that many cycad fossils — or maybe we find them, but we just don’t recognize them because they were so different from today.” They are not ‘living fossils’. They used to be different.”

To conduct their analysis, Elgorriaga and Atkinson studied the architecture of the specimen’s cone, anatomical details, and organization of the vessels using serial sections, scanning electron microscopy, and 3D reconstruction. They also performed a series of evolutionary analyzes to place the fossil in the Cycad family tree.

Relying in part on the shapes of the cone scales, pollen, and pollen sacs, they assigned the ancient plant to Skyttegaardia, a recently described genus based on isolated cone scales found in Denmark and dated to the Early Cretaceous (about 125 million years ago). . In addition, they remove some initial doubts about the placement of the new genus in the Cycad clade.

“The 3D reconstruction was striking because it had only two pollen sacs per cone scale, and the shape of this cone scale reminded us of a fossil described from Scandinavia called Skyttegaardia,” said Atkinson. “There were many similarities, but the original in Scandinavia was only described in 2021 from isolated cone scales. They cautiously explored the idea that the fossil belonged to cycads, but felt uncomfortable concluding that firmly, especially because it only had two pollen sacs per cone on a scale — while cycads have 20 to 700 today. Most cycad pollen cones are quite large, while this fossil was only half a centimeter long.”

With the additional information from the new fossil plant, the KU researchers were “fairly confident” in their phylogenetic analysis, which showed Skyttegaardia’s positive relationship with cycads.

The researchers said their description of the ancestral plant shows how paleobotany can tell us more about how nature works in deep time.

“This shows us that the information we gather from the fossil record has a major impact on our understanding of evolutionary patterns,” Atkinson said. “Time, like fossils, can reveal insights not apparent from the study of living plants or organisms. This case study is an excellent example of how fossils can contribute to our understanding of evolution over time.”

More information:
Andres Elgorriaga et al., Cretaceous pollen cone with three‐dimensional relief sheds light on the morphological evolution of cycades in deep time, New phytologist (2023). DOI: 10.1111/nph.18852

Provided by the University of Kansas

Citation: Fossil find in California shakes up the natural history of cycad plants (2023, May 2), retrieved May 2, 2023 from cycad.html

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