Angiosperms – plants with flowers – make up more than 90% of the modern flora. From oak trees to tulips and from grass to potatoes, angiosperms are an immensely important part of the world around us. This was not always the case, however. Back in the Jurassic Period (150 million years ago), when Stegosaurus and Brachiosaurus roamed North America, angiosperms were hard to find. Instead, forests were made up of needled conifers, with ferns and cycads filling in the lower vegetation. Fast forward to the middle to late Cretaceous (100 to 66 million years ago), and suddenly flowering plants and trees are everywhere – and they have been ever since.
What changed? Some paleontologists think plant-eating dinosaurs caused the angiosperm explosion. During the Jurassic, long-necked sauropods were the most diverse plant-eating dinosaurs. More than twenty sauropod species are known from western North America alone. Sauropods are specially adapted for eating high-growing conifers, and the rise of coniferous forests may have been what prompted the evolution of sauropods in the first place. While sauropods didn’t disappear during the Cretaceous, they became less dominant, and had to share their salad with new plant-eaters like hadrosaurs, ankylosaurs, and ceratopsians. These newcomers were by no means small, but their mouths were situated closer to the ground, and they focused their attention on lower-growing vegetation.
When cattle ranchers let their herds graze in one area for too long, the bushes and shrubs get wiped out. Fast-growing angiosperm grasses and weeds quickly take the place of the woody plants, and it takes years – even decades – for bigger plants to recover. Perhaps the new plant-eating dinosaurs did the same thing on a global scale. Low conifers, cycads, and ferns couldn’t grow back fast enough, so fast-growing angiosperms were able to take over the understory.
The problem with a scenario like this is that it is hard to test. The patchiness of the fossil record means they we can’t say for sure whether the new plant-eating dinosaurs arrived on the scene before angiosperms, or if it was the other way around. An expansion of fast-growing angiosperms could have just as easily caused new plant-eaters to evolve to exploit the new resource. Or maybe there’s no connection between dinosaurs and flowers at all, and the timing is just a coincidence.
That’s what Butler and colleagues found in 2009, when they used statistical tests to compare the diversity of flowering plants to the diversity of plant-eating dinosaurs at various points in time. Under closer inspection, the timing just doesn’t work out. Based on currently available data, there is no clear correlation between increases in abundance or diversity between the two groups.
Still, the spread of angiosperms didn’t come from nowhere. Perhaps insects or small forest-dwelling mammals had a role in ensuring the success of flowering plants. As always, the answer depends on finding more fossils!
Barrett, P.M. and Willis, K.J. 2001. Did dinosaurs invent flowers? Dinosaur-angiosperm coevolution revisited. Biological Reviews 76:3:411-447.
Butler, R.J., Barrett, P.M, Kenrick, P., and Penn, M.G. 2009. Diversity patterns amongst herbivorous dinosaurs and plants during the Cretaceous: implications for hypotheses of dinosaur/angiosperm co-evolution. Journal of Evolutionary Biology 22:3:446-459.