Cycads are a group of plants that have been around for over 300 million years and are often referred to as living fossils. They belong to the family Cycadaceae and are characterized by their tough, leathery leaves that grow in a spiral pattern around a central stem.
For a long time, it was believed that cycads, a group of gymnosperms that can resemble small palm trees (like the common sago palm houseplant), were “living fossils,” or that they had undergone only minor evolution since the time of the dinosaurs. Cycads are native to tropical and subtropical regions around the world, including Africa, Asia, Australia, and the Americas.
Now, a well-preserved 80-million-year-old pollen cone discovered in California has rewritten scientific understanding of the plants. Cycads are also known for their symbiotic relationship with nitrogen-fixing bacteria, which allows them to grow in nutrient-poor soils.
The findings are detailed in a paper by two University of Kansas paleobotanists just published in the journal New Phytologist.
“Cycads aren’t well-known but make up a significant part of plant diversity, accounting for around 25% of all gymnosperms,” said lead author Andres Elgorriaga, postdoctoral researcher with the KU Department of Ecology & Evolutionary Biology and KU Biodiversity Institute and Natural History Museum.
“Cycads are plants that have thick stems and short stature, with thick, palm-like leaves on 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’re also highly endangered, with the highest level of endangerment among all plant groups. Trafficking of cycads also is a significant issue.”
Despite their significance, the scientific understanding of the evolution of plants is hazy due to a dearth of fossil evidence and disagreements over the years over how to classify some ancient specimens. Cycads now are almost identical to their prehistoric ancestors, according to one popular theory.
Cycads are plants that have thick stems and short stature, with thick, palm-like leaves on 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’re also highly endangered, with the highest level of endangerment among all plant groups. Trafficking of cycads also is a significant issue.
Andres Elgorriaga
“The prevailing school of thought is that cycads did not change much in deep time,” said co-author Brian Atkinson, assistant professor of ecology & 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 clearly assignable to cycads because it has internal anatomy and pollen grains typical of this group. However, the external morphology of this pollen cone is different from living cycads today. This finding suggests cycads aren’t really ‘living fossils’ and they probably have a more dynamic evolutionary history than previously thought.”
The KU scientists claim that their examination of an 80-million-year-old permineralized pollen cone discovered in the Campanian Holz Shale formation of Silverado Canyon, California, provides a more accurate account of the natural history of cycads, one in which the species underwent diversification during the Cretaceous.
“With this type of discovery, we realize during this time there were cycads that were really different than the ones today in their size, in their number of pollen sacs, in a lot of things,” Elgorriaga said.
“Maybe we haven’t found that many cycad fossils as well or maybe we’re finding them but we’re just not recognizing them because they were so different from how they are today. They aren’t ‘living fossils.’ They were different in the past.”
Elgorriaga and Atkinson used serial sectioning, scanning electron microscopy, and 3D reconstruction to analyze the specimen’s cone’s architecture, anatomical specifics, and vascular organization. A variety of evolutionary investigations were also carried out to locate the fossil within the cycad phylogeny.
Relying partly on the shapes of the cone’s 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). Further, they erase some initial doubt about the new genus’ placement in the cycad group.
“The 3D reconstruction was striking because it only had two pollen sacs per cone scale, and the form of this cone scale reminded us of a fossil described from Scandinavia called Skyttegaardia,” Atkinson said. “There were many similarities, but the original in Scandinavia was only described in 2021 based on isolated cone scales. They cautiously explored the idea that the fossil belonged to cycad but were uncomfortable with firmly concluding this primarily because it only had two pollen sacs per cone scale while cycads today have 20 to 700. Most cycad pollen cones are quite large, while this fossil was only half a centimeter in length.”
The KU researchers were “quite confident” in their phylogenetic analysis demonstrating Skyttegaardia’s favorable association with cycads after receiving the extra data from the new fossil plant.
The researchers said that their depiction of the earliest plant demonstrates how paleobotany might help us learn more about how nature has functioned across deep antiquity.
“This shows us that the information we collect from the fossil record greatly impacts our understanding of evolutionary patterns,” Atkinson said.
“Time, just like fossils, can reveal insights that aren’t apparent from studying only living plants or organisms. This case study is an excellent example of how fossils can contribute to our understanding of evolution over extended periods.”
