Flowers, Pine Cones and Dinosaurs

When we think about the Mesozoic world, dinosaurs often dominate our attention. They are the stars of countless museum displays and restorations, and everything else about their world just seems like window dressing. When visitors to Yale's Peabody Museum look at Rudolph Zallinger's beautiful (if outdated) "Age of Reptiles" mural, their attention is drawn to the tubby Allosaurus and the "Brontosaurus" wallowing in the swamp. The plants and animals Zallinger painted around the dinosaurs simply provide the setting for the more charismatic monsters.

But as with modern ecosystems, we can't fully understand the lives of dinosaurs without knowing something about the supporting cast of organisms they lived alongside, especially plants. After all, plants were food to many, many dinosaur species, and plants undoubtedly influenced the evolution of dinosaurs just as dinosaurs influenced the evolution of plants. In fact, in a 1978 Nature paper, paleontologist Robert Bakker went so far as to suggest that dinosaurs had "invented" flowers.

Bakker's reasoning went like this. During the Jurassic and Early Cretaceous many of the large herbivorous dinosaurs—especially the stegosaurs and sauropods—fed on plants like cycads and conifers. Given the size of these dinosaurs, they would have consumed massive amounts of plant food, and their preferences at the prehistoric salad bar opened up opportunities for fast-growing plants that were able to quickly grow in disturbed environments—namely, the angiosperms, which include flowering plants. Dinosaurs effectively cleared away the competition and allowed flowering plants to proliferate, and in turn, the changes in the plant communities influenced the evolution of dinosaurs with heavy batteries of chewing power, such as the hadrosaurs and horned dinosaurs.

It is a lovely idea—we can thank dinosaurs for flowers—but studies conducted during the past 30 years have scrapped the hypothesis. Better sampling of the dinosaur and flowering plant fossil records caused the correlation between the two to fall apart. There is no strong evidence that dinosaurs had anything to do with the origin or initial spread of flowers. Many dinosaurs ate angiosperms at the end of the Cretaceous, but that is about all we know for sure about their relationship.

Nevertheless, dinosaurs probably did have some effect on plant evolution. Think of herbivorous dinosaurs as plant predators. Unlike animals, plants can't run away or otherwise evade their attackers, and so many plants have evolved defenses to discourage animals from eating them. Burning oils, toxic chemicals, thorns, microscopic spicules of silica and more—for plants, it's war, and a paper published this week suggests that sauropod dinosaurs may have influenced the evolution of one plant defense.

Published by Andrew Leslie in the Proceedings of the Royal Society B, the study looked at changes in the anatomy of conifer cones over the past 300 million years. Among the trends Leslie noticed was that seed-containing cones began increasing the amount of protective tissue around their seeds during the middle of the Jurassic. In particular, a group of trees technically known as the Araucariaceae and popularly called monkey puzzles was among the first conifers to develop large, well-protected cones, and these trees have been cited as an important food source for the large sauropod dinosaurs that proliferated during this time. Perhaps, Leslie suggests, the feeding habits of the large, long-necked dinosaurs of the Jurassic provided the evolutionary pressure for the development of well-protected seed cones.

But dinosaurs were not the only plant predators around. Early birds and small mammals may have fed on the seeds of conifers and been even more important to the evolution of well-armored cones, Leslie noted, and the diversification of insects with powerful piercing, sucking and chewing mouth parts during the Jurassic probably played a role in seed cone changes, as well.  Many animals, both large and small, fed on various parts of conifer trees, but figuring out the exact details of these interactions is extremely difficult from our current vantage point.

References:

Bakker, R. (1978). Dinosaur feeding behaviour and the origin of flowering plants Nature, 274 (5672), 661-663 DOI: 10.1038/274661a0

BARRETT, P., & WILLIS, K. (2001). Did dinosaurs invent flowers? Dinosaur–angiosperm coevolution revisited Biological Reviews of the Cambridge Philosophical Society, 76 (3), 411-447 DOI: 10.1017/S1464793101005735

Hummel, J., Gee, C., Sudekum, K., Sander, P., Nogge, G., & Clauss, M. (2008). In vitro digestibility of fern and gymnosperm foliage: implications for sauropod feeding ecology and diet selection Proceedings of the Royal Society B: Biological Sciences, 275 (1638), 1015-1021 DOI: 10.1098/rspb.2007.1728

Leslie, A. (2011). Predation and protection in the macroevolutionary history of conifer cones Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.2648

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.