February 25, 2011
Flowers, Pine Cones and Dinosaurs
A pair of Abydosaurus - Early Cretaceous sauropods - snag a conifer for lunch. Restoration by Michael Skrepnick.
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 When visitors to Yale’s Peabody Museum look at Rudolph Zallinger’s beautiful (if outdated) “Age of Reptiles” mural, their attention is drawn to the tubby Allosaurus and the “Brontosaurus” wallowing in the swamp.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
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A couple of quick notes, just to muddy the waters quite a bit.
First, according to Scott Wing’s work, we do have a bit of a paradox: Most angiosperm families were present by the end of the Cretaceous (possibly not including some of the biggest ones, such as the grasses and composites). However, where there is sampling data, they formed a minority of the plant biomass. Even at the end of the Cretaceous, angiosperms showed up in what we would today consider “weedy,” frequently disturbed habitats (such as the edges of intermittent streams), and the majority of the landscape was dominated by conifers and ferns. The paradox is more in our thinking than in reality. If you look at modern plant communities, a majority are dominated by a minority of families (grasses, oaks, etc), and most of the diversity is in uncommon to rare species. But so far as I know, angiosperms didn’t truly become dominant until well into the Paleogene.
Second, everyone misses the underground story, by which I mean ectomycorrhizae and nitrogen fixation. Yes, pines developed ectomycorrhizae, certainly by the Eocene. But ectomycorrhizae form the most widespread case of convergent evolution in history: the symbiosis evolved at least five separate times in multiple phyla of fungi, and at least a dozen independent times in seed plants from pines to manzanitas. Not that anyone’s noticed.
The important part here is that, while cycads and ferns do indeed support nitrogen fixing cyanobacteria, the real powerhouse nitrogen fixers–legumes, for example–are all angiosperms. Moreover, most are clustered in the Eurosids 2 branch of the angiosperm phylogenetic tree, along with most of the ectomycorrhizal anygiosperm species. And most of these groups evolved in the Paleocene or later.
What does this mean? Ectomycorrhizae speed up nutrient cycling, because the fungi involved can take nitrogen directly out of the leaf litter more efficiently than other mycorrhizal fungi, and they can strip nutrients out of rock better than other mycorrhizal fungi. Nitrogen fixers such as legumes also fix nitrogen, adding it to the soil and speeding up the nitrogen cycle that way.
So basically, we can assume that nitrogen cycling speeded up quite a bit, starting in the Tertiary.
My question is, what did the dinosaurs do? They lived in a world where nitrogen and possibly other nutrients cycled far more slowly than it does today. This may or may not have something to do with why there were so many huge herbivores around, but I do wonder.
But personally, I just wish that more botanists would get their heads away from the plant sex organs and pay attention to what went on with the roots. It’s not like those great herbivores were seed eaters, after all. Where did they get all their nitrogen?
Heterromeles,
Botanist have thought pleny about roots. Angiosperms are not the only plants that can do significant Nitrogen fixation. The Cycads are the most basal living seed plants and they have abundant N fixing bacteria in their roots. And nothing beats the aquatic fern Azolla for Nitrogen and Carbon fixation. Wing’s work is interesting but anyone working on Late Cretaceous floras can tell you that Angiosperms were major components of the biota by that time.
The Leslie paper provides some interesting insights but the pollen cone aspect of the story is too simple.infer Conifer pollen cones show several interesting trends in several groups including an unexplained shift toward gigantism in the Araucariaceae in the Cretaceous.