Supersaurus, Patagotitan, Sauroposeidon. The list could go on for pages. These were the largest animals to ever walk the Earth, immense sauropod dinosaurs that were unlike anything that had come before or has evolved since. But, despite our admiration, there’s still a great deal we don’t know about these giants - for example, the energy requirements of animals that pushed the boundaries of what’s biologically possible.
To a species, sauropods were plant eaters. So far, so good. Paleobotany offers a list of potential menu items in various times and places these dinosaurs roamed. And we know something about the basics of herbivorous energetics. Small sauropods - think of something like a juvenile Diplodocus - would require a steady diet of high quality, nutritious food while adult sauropods and the most immense species could subsist on greater absolute quantities of lower-quality food. To pick an illustration from our own time, this is why small primate species feast on fruit and insects, for example, while gorillas can munch on a wider array of comparatively energy-poor foods.
But it’s difficult to get beyond basic patterns and principles when you’re dealing with animals that have been dead for over 66 million years. Determining how much a given sauropod would eat in a day, what nutrition that diet offered, and how many sauropods would be able to coexist in the same place are all pressing questions that are difficult to approach. This is a matter of dinosaur ecology, in other words, where we go beyond anatomy to how these animals interacted with their world. And in an attempt to better understand this facet of the fossil record, geologist Fiona Gill and colleagues have investigated the nutritional value of what sauropods were probably eating during their Mesozoic span.
Sauropod dinosaurs didn’t attain such huge sizes out of some sort of destiny or evolutionary inertia. Their growth was fueled by the plants they ate, and they must have consumed incredible amounts. And, given what we know of the Triassic, Jurassic, and Cretaceous, the Mesozoic seems like it was favorable to increased plant productivity. Global temperatures were generally warmer during the Mesozoic, and, while levels fluctuated with time, the amount of atmospheric CO2 - essential plant food - was sometimes far higher than even our own elevated modern levels. These factors could have raised what ecologists call net primary productivity - in short, the rate at which an ecosystem can trap carbon in the form of plant biomass.
If the Mesozoic really was an exceptional time for plant growth, then, sauropod dinosaurs would've had plenty of biological fuel to expand to hitherto unknown sizes within what their anatomy and physiology allowed. The problem is that there’s far more to plant productivity than temperature and CO2 availability. Some experts have disputed the scenario I’ve just laid out, adding that even if overall plant growth were higher in the Mesozoic it doesn’t necessarily follow that the available dinosaur chow would have been high quality. Non-angiosperm plants like conifers and ferns might not have offered enough nutritional value, or plants grown under high-temperature, high-CO2 conditions might have lacked sufficient Nitrogen.
So, to test this idea, Gill and colleagues set about creating a little slice of the Mesozoic in the lab, growing plants like ferns, horsetails, and ginkgo under concentrations of CO2 meant to mimic different parts of the Mesozoic. These plants were then prepared and analyzed for their nutritional value. It turns out that the typically high atmospheric CO2 levels in the Mesozoic didn’t reduce the quality of the forage dinosaurs had available to them. Nitrogen levels in the plant tissues didn’t drop across the board - it appears to vary from species to species - and the available metabolizable energy of the Mesozoic-mimicked plants was not inherently low.
To figure out what this meant for dinosaurs, then, Gill and colleagues estimated the daily needs of a big sauropod. Envisioning a hypothetical 30 ton sauropod - think something like Apatosaurus - the researchers were able to approximate the daily intake of such an animal based on the nutritional quality of their experimental plants. While this dinosaur would need to eat about 242 pounds of monkey puzzle tree foliage each day if that tree grew under atmospheric CO2 concentrations of 2000 parts per million, for instance, the same sauropod would need only about 112 pounds of horsetails a day grown under atmospheric CO2 concentrations of 1200 parts per million to get the same amount of nutrition.
Naturally there’s variability involved - for example, whether a given sauropod primarily browsed high or vacuumed up low-lying plants - but the general upshot is that elevated atmospheric CO2 levels didn’t result in worse food choices for these dinosaurs. Based on the estimates of Gill and colleagues, sauropod-rich ecosystems like that of the Late Jurassic Morrison Formation may have been made possible, at least in part, by relatively high concentrations of atmospheric CO2. And even as such estimates are sure to be revised, I like this approach. Dinosaurs were not insulated from the world around them. They were part of it. Studies like this add a little more context to the epic story of the terrible lizards.