Louis Dollo

Louis Antoine Marie Joseph Dollo completed his degree in civil engineering in 1877. He devoted five years to industrial pursuits, then left his homeland of France for Belgium. Working at the Musée Royal d'Histoire Naturelle de Belgique, he concentrated primarily on Iguanodon, the dinosaur originally discovered and named by Gideon Mantell.

Photograph
From The Dinosaur Papers edited by Weishampel and White

Dollo's timing was fortunate. In 1878, an underground coal mine in southeast Belgium proved to be the final resting place of 31 well-preserved Iguanodon dinosaurs. Dollo oversaw the preparation and articulation of the skeletons, and in 1882, published the first of several papers about the fossils. He discussed the preservation of the skeletons, identified two separate species and addressed their posture. Dollo advocated a bipedal, kangaroo-like stance for Iguanodon. The posture has since been overturned, but paleontologists do acknowledge that Iguanodon probably stood primarily on its hind legs. Dollo speculated that Iguanodon was amphibious and used its powerful tail to swim like crocodiles. He also found that the "horn" that Mantell originally placed on the animal's snout was really a thumb spike.

It's easy to shrug off Dollo's task of mounting skeletons as no big deal, but articulating dinosaur bones is no trivial task. Fossil bones have two annoying characteristics that are individually irritating, and together produce a synergy of aggravation: They are very heavy and they are very fragile. Dollo put together a system of ropes, pulleys and beams to hoist the beefy bones into position before hammering them into place. The system was labor-intensive, but it worked. Today, paleontologists often use light, replaceable casts.

Dollo's engineering expertise was as evident in his writing as it was in his fossil mounts. His writing was concise, direct and usually in numbered paragraphs or lists. When addressing two different forms of Iguanodon, he reasoned as follows.

  1. Either the small form is a young animal and the large one represents the adult state;
  2. Or we are in the presence of variations of either sexual or individual nature;
  3. Or we are dealing either with two species, or with two distinct genera.

Besides his extensive studies of Iguanodon, Dollo helped establish what is now known as paleobiology. He found that detailed studies of fossil anatomy could be used to reconstruct the adaptations of extinct animals. In 1893, he established Dollo's Law, also known as the Law of Irreversible Evolution. He hypothesized that organisms could evolve particular specializations but could not later lose those specializations. Horses, for example, could not re-evolve the side toes they had already lost.

More than a century later, biologists debated whether Dollo's Law was true. A paper published in 2009 suggested that, on a molecular level, it was. Glucocorticoid receptors are proteins that help most vertebrates cope with stressful or dangerous situations by snaring cortisol. Researchers reconstructed the protein's gene history, including the mutations it had likely undergone then reversed some of the mutations. Without key mutations that enabled the protein to specialize in cortisol grabbing, the protein was useless for any purpose. Other mutations that may have refined the process added extra features to the protein that got in the way of anything else. Although these other mutations could theoretically be reversed, they weren't likely to be favored by natural selection. But the research continued, and another study highlighted in 2013 used the DNA of dust mites to assemble their family tree. That study suggested that free-living dust mites evolved from parasites, and developed traits that their parasitic ancestors had given up. So the debate continues.

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