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.

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?