Body mass (tonnes)

FIGURE 2.4. A graph showing (humerus circumference plus femur circumference) plotted against body mass, for various quadrupedal mammals. Data from Anderson et al. 1 985.

the circumferences of their bones. However, appearances can be deceptive. Imagine that we do not know the masses of the two largest mammals in the graph, and want to estimate them from their bone circumferences. The estimates that the equation would give us are 1.2 tonnes for the 2.0-tonne hippopotamus, and 9.0 tonnes for the 5.9-tonne elephant. We must expect errors as bad as this, or worse, if we use the equation for estimating dinosaur masses.

The circumferences of the humerus and femur of Brachiosaurus are 654 and 730 millimeters, giving a total 1384 millimeters. The mass of the same individual Brachiosaurus has already been estimated, from the volume of a model, to be 47 tonnes. These data are represented by the hollow circle in figure 2.5, which lies well below the mammal line. The line predicts a mass of only 32 tonnes, for the observed circumferences, but the discrepancy is no worse than the examples of the hippopotamus and elephant led us to expect.

The line is based on quadrupedal mammals, and it seems reasonable to apply it to quadrupedal dinosaurs. It would be a mistake to expect it to give accurate estimates of body mass, but even rough estimates are interesting. For bipeds, a different line is needed. The Anderson team produced a biped equation by modifying the quadruped one. They used only the femur circumference (because bipeds walk only on their hind legs) and adjusted the factor in the equation to make the predicted mass for one particular dinosaur match the mass that had been estimated from a model. Their biped equation is body mass in kg = 0.00016 (femur circumference in mm)2™.

This underestimates the masses of kangaroos and overestimates the masses of ostriches.

Table 2.2 shows the masses of dinosaurs estimated from the volumes of models by Colbert and myself, and calculated from their equations by the Anderson group. For each of the species in the table, at least two estimates of mass have been made. For some of them, the estimates agree well, but for others there are big discrepancies. In the worst cases {Diplodocus and Brachiosaurus) the largest estimate is about three times the smallest.

Some of the discrepancies can be explained. Photographs in Colbert's paper show that his model of Stegosaurus was skinnier than the one I used, and his model of Triceratops was more portly than mine. In each case, one model may be more realistic than the other, but it is hard to say which. Even if a model is based on accurate skeleton measurements, its volume depends a lot on the judgement of the modeller. Some other discrepancies may be due to estimates being based on different-sized specimens of the same species.

TABLE 2.2. Masses (in tonnes) of dinosaurs.

Colbert"

Alexander'

Anderson et al.h

theropods Allosaurus Tyrannosaurus sauropods Diplodocus Apatosaurus Brachiosaurus ornithopods Iguanodon 'Anatosaurus' stegosaurs

Stegosaurus ccratopians Styracosaurus Triceratops fragilis rex carnegiei louisae brancai hernissartensis copei ungulatus alhertensis 'prorsus'

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