December 7, 1857 (a Monday)

Louis Antoine Marie Joseph Dollo

On this date, the French vertebrate paleontologist Louis Antoine Marie Joseph Dollo was born. Dollo is noteworthy for establishing the intellectual foundations of paleobiology. He was a specialist in fossil fishes, reptiles, birds, and their paleoecology.

Dollo completed his degree in civil engineering in 1877. However, after 5 years in this profession, he left France to pursue his passion, paleontology, at the Musée Royal d’Histoire Naturelle de Belgique (Royal Museum of Natural History of Belgium) in Brussels, where he stayed most of his life. His timing was fortunate. In 1878, an underground coal mine at Bernissart, Belgium was discovered to be the final resting place of 31 well-preserved Iguanodon dinosaurs. Dollo oversaw the preparation and articulation of the famous skeletons, and in 1882, published the first of several papers about the fossils.

Dollo is also famous for the so-called Law of Irreversibility. This law was first stated by Dollo in this way:

An organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors.

In other words, Dollo proposed that evolution is irreversible – that is, once an organism has lost or discarded a structural feature during the course of evolution, then it cannot regain that feature. Although generally true, some exceptions are known so, in its modern form, this “law” should be understood as the principle that evolution tends to be irreversible – that is, it is highly improbable that an organism would regain a lost feature.


  • James Orville Farlow and M.K. Brett-Surman, The Complete Dinosaur (Bloomington, IN: Indiana University Press, 1999) 22.

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