The evolution of maximum body size on land
One of the questions I’ve been interested in researching for a while is why sauropods evolved their exceptional sizes. Why didn’t other groups on land evolve sizes that are even close? A first step in understanding the “why” is understanding the “where, how, and when.” A paper I’ve published in Current Biology is an attempt to answer those questions.
First off, it’s important to understand how big sauropods actually were. The icons to the right show a large sauropod next to (from left to right) the largest duck-billed dinosaur, T. rex, the two largest known terrestrial mammals (a relative of modern rhinos and a relative of modern elephants), and finally, a modern world-record African elephant for scale. Nothing even approaches sauropods in size. This is unusual for evolution—when something evolves, it often evolves more than once, even if it is a complex feature like warm-bloodedness, bipedalism, or complex eyes.
(Silhouettes by Scott Hartman, Nobu Tamura (vectorized by T. Michael Keesey), Steven Traver. https://creativecommons.org/licenses/by-nc-sa/3.0/)
Animals, including sauropods, come in all shapes and sizes. Rather than use a linear measurement (for example, length or height) to compare size among species, biologists often use body mass. Mass is easy to calculate for an extinct animal because how heavy an animal is is proportional to how thick its weight-bearing bones are, just as the strength of a column is related to its cross-sectional area.
I estimated body mass in about 200 sauropod species (out of the ~250 that are currently known) based on their limb dimensions. Then, I plotted these body masses onto an evolutionary (aka phylogenetic) tree, stretched across geologic time. The result is shown at right. Each line represents a species of the sauropod evolutionary tree. Branches highlighted in red are ones that crossed the body mass threshold of other terrestrial animals—in other words, these are lineages that started smaller than the biggest mammals, but evolved to be larger. There are about three dozen red lines, which means sauropods exceeded mammal body mass many, many times in their evolution!
What’s more, these largest-of-the-large sauropods differed in when and where they lived, their body proportions, what they ate, and how fast they grew. Over thirty times, when the “largest” niche became available, sauropods appear to have filled it, each time bringing their idiosyncratic evolutionary histories into the mix.
On a side note: to the right is a labeled version of Figure 1 from the Current Biology paper (here is a link to a higher-res version). The journal that I submitted to had an unusual rule about not including too many supplemental images, so I provide it here so one can easily compare the body masses of various sauropods. With nearly 200 species, the chart gets messy in some places. Where there was ambiguity about which line pointed to which taxon, I made the first letter of the taxon name touch the line. See the supplemental excel sheet from my paper for specimen numbers associated with each taxon name. Remember that each mass estimate comes with a 25% prediction error; for clarity, these error intervals are omitted here.
So why did sauropods evolve body sizes larger than any terrestrial mammals or other groups of dinosaurs? It wasn’t a one-off evolutionary event, since they exceeded the other groups’ sizes over 30 times. The answer lies in a combination of biological and historical factors (see this book for a lot more on this topic). Among other factors, sauropods had small teeth, meaning they didn’t chew; instead they fermented their food in their gut. Not chewing saves time and allows you to have a smaller head, which allows you to have a longer neck, which allows you to ingest food more efficiently, gathering food without moving much. What’s more, sauropod bodies were air-filled like those of birds, which meant they could get larger while saving mass, and that their breathing was more efficient. Finally, sauropods laid eggs—lots of eggs—which meant they could devote their energy to growing, not to parental care.
These factors all played a role in sauropods evolving to be generally large, but there is no set of factors that separates the sauropods that surpassed mammalian body mass from the ones that didn’t. Ecological conditions and histories likely instead played a role on a case-by-case basis in pushing the three-dozen sauropod lineages to evolve the largest body masses of all time, while the over two hundred other sauropods kept to more modest sizes. Future work will hopefully add to and revise my compilation of the largest (and smallest) sauropods, as more of these fascinating animals are discovered each year.
Note: the sauropod illustrations on this page are by Raúl Martin, originally completed for a Scientific American article I co-authored, published in 2012.