Putting the life back in science fiction

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.