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So now I’m not content with the Paleocene, and I want to deal with the lower Cretaceous?
Actually, this comes from a blog discussion I got sucked into on at SVPOW, on a really interesting Sauropod reconstruction by Brian Engh. As I noted over there, I’m posting some first thoughts on dinosaur-plant interactions over here.
For fun, I’ve been writing a time travel story set in the Paleocene, and so I’ve gotten interested in the weirdness one runs into going back in time. It’s another facet of worldbuilding, except that, instead of setting it on an alien planet far, far away, I’m trying to figure out the deep past.
The central problem is, I think, one of modern perceptions. To demonstrate, I’m going to choose three very different ecosystems: redwood forest, Midwestern prairie, and California needlegrass grassland. The redwoods have been around for a *very long* time, and even into the Paleocene, they were dominant in a lot of places, with ferns growing in the open fields around them (grasses didn’t really show up until the Oligocene, if I remember correctly). Most people’s view of the redwoods is this quiet place where the biomass is 99.9% plant and the herbivores are mostly absent. Fire is mostly absent too: they call the redwoods “the asbestos forest” for good reason.
Contrast the redwoods with the prairie, where the interaction between grasses and grazers pretty much dominates the system. Prairie grasses tolerate grazing and fire, usually much better than other plants tolerate grazing and fire. But it’s really about grazing, and when you remove the grazers, it’s hard to keep the woody plants from taking over. It’s a neat trick: everything gets eaten, but the grasses simply regrow better.
Contrast both with the California grasslands, where there were few (if any) grazers for the last 10,000 years or so. California native grasses are horrible at tolerating grazing. They can be mowed once per year, and that only about 1ft. off the ground. More mowing than that, and they die. But they tolerate fire just fine.
Getting back to the sauropods, I criticized Brian’s reconstruction of the great beasts knocking down the forest, and he (quite properly) took exception. But to me it is a paradox: why would there be a coniferous forest there at all? All those enormous dinosaurs, which ranged from the size of hippos to medium-sized whales, were living on a diet of conifers, possibly ferns, and possibly cycads and cycadeoids. If you think about this for a while, you realize how very bizarre it sounds. From the fossil evidence, it appears that conifers and ferns are good food, and that doesn’t make a lot of sense. Sure, deer go after arborvitae and yews, but they kill the conifers. And ferns are full of fascinating carcinogens, while cycads are possibly even more nasty and tough. And very slow growing. If animals don’t routinely eat conifers and ferns today, why were the biggest land animals of all time chowing down on them 24/7 a hundred million years ago?
So, how do you grow a sauropod on such a crappy diet? The best answer I can come up with is that, back in the Mesozoic, there must have been another guild of plants around. These were conifers and ferns (if not cycads and gnetales) that used the prairie grass trick. They tolerated browsing, because it killed off their competitors and they could regrow. With the dinosaurs extinct, the plants collectively switched to a strategy of repelling smaller herbivores (mammals and insects), just as the native grassland species did in California. This is the Paleocene environment I’ve been researching, and we still see it in places like Papua New Guinea.
What did the mesozoic browsed conifers look like? That’s another problem I’m having. I don’t know of a modern conifer that tolerates browsing well. However, there were lots of extinct conifers back then. Ferns too. Still, I’m guessing that the Mesozoic forests had more in common with prairies than they do with the redwood cathedrals of today. They were heavily browsed, and seedlings probably recruited in meso-sites where the dinosaurs couldn’t eat them. They probably also root-sprouted readily, just as modern redwoods do.
As for size, I suspect that the tallest mesozoic conifers simply overtopped even the giant sauropods. But I don’t think they got as big as do modern trees, because of the respiration problem. Plants respire just as animals do, and plant respiration rates depend on temperature. There’s a reason that the really big trees grow where it’s cool and wet. It’s easy for them to generate a huge carbon surplus for wood in an environment like that, and the wet weather and fog helps them maintain the positive water balance to keep the tree tops hydrated. A tree in a hot, dry environment simply can’t grow that big, because it has less surplus carbon to put into wood (more carbon went into feeding hot cells), and there is less water to feed the high branches. As I understand it, the Jurassic was (on average) hotter and drier than today, so I’d guess that Jurassic conifers weren’t the giants we see today. And while some of them could overtop even Sauroposeidon, I suspect that many had to live out their lives in the browsing zone, forming some sort of weird multilayered conifer fern multiprairie/savanna.
Long, rambling post, and I’d welcome your thoughts. As I noted above, I’m ultimately interested in trying to get my head around what the Cretaceous looked like. The Paleocene I understand to some degree, but the Mesozoic is a new world to me, and a very strange one. It’s fun to figure out how to do justice to it properly in art or literature.
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Putting the life back in science fiction 
Again, just to be a stickler, Sauroposeidon was early cretaceous. That said, interesting post. I’m all for people doing more in-depth studies of mesozoic plant life. I’m regularly frustrated by the lack of detailed information on paleobotanical systems.
Comment by brian engh December 15, 2010 @ 9:38 pmAbsolutely Brian, and thanks for the clarification.
The only reason I framed this entry as “the Mesozoic” is that the dinosaur food issue crops up all the way to the end of the Cretaceous. So far as I can tell, the angiosperms don’t really dominate until some time in the Paleocene.
And I’m not even talking about the dearth of mesozoic nitrogen fixing plants. Compared to legumes et al. cycads aren’t exactly N-fixing champions. Was most of the N-fixing done by Azolla?
Heck, maybe the reason dinosaurs were so big is that the food quality was lousy, and only bulk processors could get enough nutrition out of leaves to live.
I still have this itchy feeling that we’re missing something.
Comment by Heteromeles December 15, 2010 @ 9:52 pmOne word: fungi.
Comment by Duane March 31, 2012 @ 4:37 amActually, Duane, not fungi.
I’m not sure whether you’re commenting with respect to nitrogen fixing or nutrient cycling. It’s a common misconception that fungi fix nitrogen.
If you’re talking about nitrogen fixation, you’re off base. Only bacteria fix nitrogen. The confusion came from some bacteriologist naming a group of bacteria the “actinomycetes.” They’re simply nitrogen-fixing bacteria that include some species that grow in branching filaments that look somewhat like fungi. They’re now called “actinobacteria,” but the damage has been done. Just remember: only bacteria fix nitrogen, and whenever you see nitrogen fixation, a bacteria is involved. The plant is the host. Most of today’s nitrogen fixing plants (the angiosperms) evolved at the very end of the Cretaceous or in the Paleogene, and it’s not at all clear what was fixing nitrogen before that. Certainly cycads and some ferns supported nitrogen fixing bacteria, but for the most part, they don’t produce as much as legumes do.
As for nutrient cycling (what fungi do a very good job at), you’re probably not quite on it. After I posted this, I read Sampson’s Dinosaur Odyssey. It’s an interesting pop science book, and he mentions a study of Cretaceous hadrosaur poop that contained highly decayed wood. The implication is that these dinosaurs (and many others) ate very dead trees, because the fungal tissue in the trunks was sufficiently nutritious. These days, fungi (along with termites and ants) do the major work of breaking wood down into soil, but it appears that dinosaurs did much of that work back in the Mesozoic. This would speed up nutrient cycling considerably. As I said, the Mesozoic was an alien place.
Comment by heteromeles March 31, 2012 @ 2:37 pm