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.
