Fossil find in California shakes up the natural history of cycad plants

For a long time, it was believed that cycads, a type 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. Now, a perfectly preserved 80-million-year-old pollen cone found in California is changing how scientists think about plants.

Two paleobotanists from the University of Kansas describe their findings in a piece that was just published in the journal New Phytologist.

Andres Elgorriaga, lead author, is a postdoctoral researcher at the KU Department of Ecology & Evolutionary Biology, KU Biodiversity Institute, and Natural History Museum. "Cycads aren't well-known but make up a significant part of plant diversity, accounting for around 25% of all gymnosperms," he said.

"Cycads are plants with small statures, thick, palm-like leaves on top, and thick stalks. They are linked to other seed-bearing trees that don't also produce blooms, such as Ginkgo and the monkey puzzle tree, and they also generate cones similar to pine cones. However, they are also the most endangered plant group, with the highest level of endangerment. Cycad trafficking is a serious problem as well.

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 categorize some ancient species. Cycads now are almost identical to their prehistoric predecessors, according to one popular theory.

"The prevailing school of thought is that cycads did not change much in deep time," said Brian Atkinson, co-author and assistant professor of ecology and evolutionary biology at the University of Kansas Biodiversity Institute and Natural History Museum.

"However, the fossil record of cycads is poorly known, and many items that have been mistakenly identified as cycads have since been found to be something else entirely. Here, we have a cone that has been preserved in three dimensions and is unmistakably a cycad because of its interior structure and pollen grains. The exterior morphology of this pollen cone, however, is distinct from that of currently existing cycads. This discovery shows that cycads aren't actually 'living fossils' and that their evolutionary history has probably been more dynamic than previously believed.

The KU scientists contend 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 realistic account of the natural history of cycads, one in which the species underwent diversification during the Cretaceous.

With this kind of discovery, Elgorriaga said, "we realize that there were cycads at this time that were really different from the ones today in their size, in their number of pollen sacs, in a lot of different ways." Perhaps cycad fossils haven't been discovered as frequently, or perhaps we're finding them but aren't recognizing them because they were so unlike modern cycads. These people are not "living fossils." Back then, they were different.

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.

They identified the ancient plant as belonging to Skyttegaardia, a recently defined genus based on solitary cone scales discovered in Denmark and dated to the Early Cretaceous (approximately 125 million years ago), in part based on the forms of the cone's scales, pollen, and pollen sacs. They also dispel some early confusion over the new genus' classification within the cycad family.

Because there were only two pollen sacs on each cone scale in the 3D reconstruction, Atkinson noted, "the cone scale reminded us of a fossil described from Scandinavia called Skyttegaardia." The original in Scandinavia was only reported in 2021 using solitary cone scales, despite the fact that there were numerous similarities. The reason they were hesitant to draw a clear conclusion about the fossil's cycad affinities was because it only possessed two pollen sacs per cone scale, whereas modern cycads have 20 to 700. This fossil was about half a centimeter long, in contrast to the majority of cycad pollen cones that are rather enormous.

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 demonstrates how our comprehension of evolutionary patterns is significantly impacted by the data we get from the fossil record, according to Atkinson. "Time can provide insights that aren't obvious from examining simply live plants or species, just like fossils. This case study is a great illustration of how fossils may help us better comprehend evolution over long stretches of time.