terrestrial

Co-evolution with dinosaurs?

In 1986 Bob Bakker suggested that angiosperms and dinosaurs may have co-evolved, with dinosaur feeding habits influencing the evolution of angiosperms. A change in herbivore communities occurred just before the evolution of angiosperms, and he argued that the new dinosaur groups were instrumental in stimulating the explosive radiation of the angiosperms.

The fossil remains from around 160 million years ago (Late Jurassic) tell us that around 95% of the herbivore biomass was made up of high-browsing stegosaurs and sauropods with cranial and dental adaptations for feeding on conifers. This browsing pattern put pressure on the mature trees but still permitted the gymnosperm seedlings to flourish.

Text Box: Ultrasaurus, an example of a high-browsing dinosaur
http://home.no.net/paleo/sauropod/sauropod.htm

Around 144 million years ago (Jurassic-Cretaceous boundary), the herbivore community changed from high browsers to low-browsing ornithischians. Low browsing increased mortality among the gymnosperm seedlings, preventing them from growing to their full height and maturity, thus creating more substantial gaps in the canopy and thinning out the forest structure. The rapid life cycle of angiosperms may have enabled them to reach maturity before being browsed, permitting at least some of the plants to produce seeds, unlike the gymnosperms. The low-browsing dinosaurs may also have aided in the dispersal of angiosperms by eating some of the plants and excreting the seeds, which, with their improved seed coat, came out relatively unscathed.

This model has been rejected by many scientists because there is no proven direct relationship between the change in dinosaur communities and the evolution of angiosperms.