I Like Big Plants and I Cannot Lie – Fruit Size Increases in Absence of MegaHerbivores

The megaherbivore gap after the non-avian dinosaur extinctions modified trait evolution and diversification of tropical palms

Renske E. Onstein, W. Daniel Kissling, and H. Peter Linder

Summarized by Makayla Palm

What data were used? Qualitative data from modern palm tree fruit, phylogenetic data, and palm tree fossils are used in order to observe changes over time in the taxon Arecaceae, or the palm tree, from the Paleogene Period. After the end-Cretaceous extinction that wiped out the non-avian dinosaurs, mega-herbivores, or any herbivore larger than 1,000 kg ( ~2200 lbs), were nowhere to be found. For the most part, small mammals were left foraging for food, and angiosperms (flower-bearing plants) were able to catch a break. The combination of mammalian seed-spreaders and lack of large herbivores preying on angiosperms (palms in this case) meant that the plants were able to increase in numbers without worrying about defenses. These furry seed-spreaders (small animals that pooped out their seeds) were still spreading, allowing plants to grow and didn’t evolve many defense mechanisms like rough leaves or spines. The researchers hypothesized that they would observe three things about palm diversity in the fossil material from this time: the origin of plant armature (or defense structures like spikes) in the Cretaceous Period because of many large herbivores, the decrease in armature during the Post-Cretaceous Paleogene Megaherbivore Gap (PMHG), and the change in fruit size over time as the plants were able to diversify. 

Methods: Measurements of the palm tree fruit fossil material were taken in order to compare how fruit size changed over time within the megaherbivore gap and observations were made on when these changes in size happened, which supplemented the phylogenetic analysis. Living palms were observed in modern habitats, as were  their interactions with larger herbivores of modern times to better understand how the fossil palms may have interacted with herbivores from the Paleogene.

Results: The hypothesis that the first armature appeared in the Cretaceous was confirmed by fossil material, which indicates an increase in defense likely due to megaherbivores. The armature of plants with larger fruit decreased over time, which also supports the hypothesis of losing these defense structures over time with less predation. Despite the disappearance of megaherbivores in the end-Cretaceous, fruit size stayed relatively large (above 4cm). Plants with larger fruit diversified on a constant scale over time, whereas plants with smaller fruit decreased in diversity, counter to the second hypothesis. Overall, some hypotheses were supported, and some were not. 

 The six graphs each have three columns representing before, during and after the Paleocene MegaHerbivore Gap. Graph (a) represents a consistent speciation rate among large fruit (defined to be >4cm in length). Graph (b) represents a speciation of armature in leaves and stems, showing a negative dip during the PMHG with an increase before and after. Graph (c ) represents speciation of stem armature, with a similar pattern to Graph (b), showing a dip during the PMHG. Graph (d) represents the rate of fruit size evolution (from small to large) increasing during the PMHG, and a constant state before and after the gap. Graph (e) represents a transition of evolving armature in leaf and stem, decreasing during the gap and increasing again afterward. Graph (F) represents the evolution of just stem armature, which stays constant before and after the PMHG, but dips significantly during the event itself.
This box and whisker plot tracks the changes of palm trees from before, during, and after the Paleocene MegaHerbivore Gap (PMHG) following the Cretaceous extinction. The median value (middle value of data)is represented by the bar across the yellow box. The graphs show that armature decreases immediately following the extinction ~66mya and the speciation of fruit staying constant. These also show the increased fruit size during the PMGH.

 Why is this study important? A lot of end-Cretaceous Period studies focus on the end of the dinosaurs, what caused the mass extinction, and how the age of mammals began. This study shows a different perspective on a well-studied time period by using a combination of paleobotany and vertebrate paleontology, and observing how the absence of large herbivores affected how ancient palm trees changed ecologically. This documented diversity opened new doors for angiosperm evolution and led to an increase in forests, setting the stage for the next era of geologic time in North America, the Cenozoic. 

The big picture: The Paleogene megaherbivore gap is a time in geologic history where the absence of large herbivores after the non-avian dinosaur extinction greatly affected ecosystems and the change in the landscape to more dense forests. The lack of large herbivores to eat plants allowed plants to evolve fewer defensive structures and larger fruit, which allowed them to spread farther distances and in greater numbers, because of the increase in seeds. 

Article Citation: Onstein, R. E., Kissling, W. D., & Linder, H. P. (2022). The megaherbivore gap after the non-avian dinosaur extinctions modified trait evolution and diversification of tropical palms. Proceedings of the Royal Society B, 289(1972), 20212633.

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