Talking about dinosaurs sometimes reminds me of sports fans going over a player’s vital statistics. It’s important to know a player’s RBIs or interceptions (I think; I know practically nothing of sports), just like any hardcore dinosaur fan is going to know the diet, length, and other details of a particular species.
Some of these particulars are relatively easy to come by. Curved serrated teeth? Then the dinosaur’s likely a carnivore. But others are trickier. How do we know how much non-avian dinosaurs weighed when we can’t ask a Triceratops or Mononykus to hop on a scale?
The first issue is one of completeness. Finding complete or near-complete dinosaurs – especially the biggest ones! – is a very rare occurrence, and so palaeontologists often have to reconstruct skeletons on the basis of close relatives and what they expect the rest of the animal’s body to look like. The trouble is that some dinosaurs keep changing. Mass estimates for the latest version of Spinosaurus are bound to be different from those of previous reconstructions, and future ones will probably continue to change as more of the animal changes.
And that’s just to get to the basics of size. Palaeontologists also have to consider how much cartilage was between a dinosaur’s bones, how porous those bones were, and how much muscle mass is appropriate for the given animal. It’s not an endeavour for scientific lightweights.
Palaeontologists have used a variety of different methods to estimate how much dinosaurs weighed. Some studies have even turned to scaled-down dinosaur models to investigate how hefty Allosaurus and kin were. Others have looked to the relationship between body mass and limb size. Few have used multiple methods, however, and that’s why, palaeontologists Charlotte Brassey, Susannah Maidment, and Paul Barrett have used two different methods to give Stegosaurus an appropriate weight.
The palaeontologists picked Sophie to be their Stegosaurus. This is the exceptional skeleton that went on display at London’s Natural History Museum late last year, and having a near-complete Stegosaurus to start from helps limits the uncertainty involved in replacing missing parts from other specimens.
Brassey, Maidment, and Barrett used two different mass-estimation methods on Sophie. One relied on estimates based on the volume of a digital dinosaur model. The other was based on the relationship limb dimensions and body mass in living animals – a popular method in palaeontology for specimens that are incomplete, but include both fore- and hind limbs.
The problem with limb-based method, the palaeontologists write, is that dinosaur anatomy was often very different from those of the modern animals the technique was derived from. There’s no plate-backed animal like Stegosaurus alive today. Are methods useful for modern creatures really applicable to non-avian dinosaurs? That’s why the researchers tested the two against each other, and each attempt came up with different results.
The team’s preferred model for volume-based methods estimated that Sophie weighed 3,439 pounds – about a ton and a half – including 34 pounds of armour plates and spikes. (The skinnier and fatter models in the study gave lower and higher weights, ranging from 2, 890 pounds in the emaciated version and 4,175 pounds in the bulked-out one.) But the estimates from the limb proportions were much higher – from 5,191 to an extreme 8,269 pounds. That’s more than four tons!
The four ton estimate seemed far too heavy for Sophie. The largest model the team reconstructed for their volume model, excepted to represent the maximum extent of the dinosaur’s soft tissues, came out to two tons, and when they adjusted to digital model to be fit the four ton estimate, the researchers wound up with an unreasonably circular Stegosaurus. So what happened?
One of the recurring themes in palaeontology over the past few years has been an emphasis on ontogeny, or how an organism develops as it ages. Among other things, changes in an organism’s proportions as it ages are important to consider in terms of its mass and how it carries that weight. The extremely heavy, limb-based Stegosaurus estimates were caused by purposefully not taking into account that Sophie was relatively young and still growing. Once the palaeontologists corrected for this, the limb-based weight estimates came out between 4,019 and 4,757 pounds – much closer to the volume-based estimates. There’s always going to be variability in these matters, but Sophie probably weighed between one and a half to two tons.
The upshot of all this number-crunching is that limb-based mass estimates from incomplete or even fragmentary specimens should be taken with a grain of salt, especially if the life stage of the dinosaur in question isn’t known. Ideally, a good candidate for mass estimations should be a single individual represented by the majority of a skeleton and of a known age, preferably an adult. And this isn’t just fossil nitpicking. Body mass estimates play into everything from how dinosaurs moved to what kind of physiology they may have had. Figuring out what Stegosaurus weighed is a major step in bringing the dinosaur back to life in our scientific imagination.
Brassey, C. Maidment, S., Barrett, P. 2015. Body mass estimates of an exceptionally complete Stegosaurus (Ornithischia: Thyreophora): comparing volumetric and linear bivariate mass estimation models. Biology Letters. doi: 10.1098/rsbl.2014.0984