Dinosaur Park

Why Was Astrodon So Big?

2 Digs

Artwork by Clarence Schumaker.

Astrodon johnstoni, Maryland’s best-known dinosaur, was a giant. The largest bones attributed to this animal put in the same size class as Brachiosaurus altithorax – around 80 feet long and weighing as much as 30 tons. There is no land animal today that even comes close to the size of Astrodon and its sauropod brethren, and Dinosaur Park visitors frequently ask why. Although past scientists have floated vague ideas like higher oxygen levels or lower gravity contributing to the great size of some dinosaurs, there is no evidence to support this (and plenty of evidence to the contrary). Instead, the size of Astrodon and other sauropod dinosaurs comes down to two things: body architecture and reproduction.

To start, sauropods were more lightly built than today’s big mammals. Their bones contained large air pockets, reducing the amount of weight these animals had to carry around. In fact, a now-discarded name for the Arundel Clay sauropod, Pluerocoelus, is a reference to these open spaces. Modern birds inherited this ancient air-filled skeleton, which helps lighten their load for flying. Sauropods also had a very useful tool for gathering the food needed to power their giant bodies: a long neck. Astrodon could save energy by standing in one place and collecting plants from the wide area it could reach with its neck.

Perhaps most importantly, sauropods didn’t reproduce the way large mammals do. Elephants, rhinos, and horses are placental mammals, which means they give birth to live young. The bigger the animal, the bigger the baby, and bigger babies require longer pregnancies. Longer pregnancies require more energy from the mother, and increase the risk of fatal complications. At 640 days, an elephant’s gestation period may well approach the maximum possible pregnancy length before the energy expenditure and inherent risk become too great.

Sauropods, by comparison, had none of those problems because they laid eggs. A female Astrodon would lay dozens of comparatively tiny grapefruit-sized eggs, then abandon them to their fate. Very few of the resulting hatchlings would survive, but enough would beat the odds to keep the species going. This reproductive strategy has a much lower cost on individual mothers, and allowed sauropods to grow to be many times larger than a mammal ever could.

References

Sander, P., Christian, A., Clauss, M., Fechner, R., Gee, C., Griebeler, E. Gunga, H., Hummel, J., Mallison, H., Perry, S., Preuschoft, H., Rauhut, O., Remes, K., Tütken, T., Wings, O., Witzel, U. (2011). Biology of the sauropod dinosaurs: the evolution of gigantism. Biological Reviews. 86: 117-155.

Werner, J. and Griebeler, E. (2011). Reproductive biology and its impact on body size: Comparative analysis of mammalian, avian and dinosaurian reproduction. PLoS One. 6(12): e28442