What’s Inside a Dinosaur Bone? (cont.)

What do these thin-sections show?
It depends on what questions you’re asking. For some years, I’ve been working on a team with a strong interest in how dinosaurs grew and how they lived. Another member of this team is Prof. Armand de Ricqlès of the University of Paris and the Collège de France. Since the 1970s he has been the world leader in fossil bone histology, and last year he spent two months with us as a Visiting Miller Professor. Also on the team is Prof. Jack Horner of the Museum of the Rockies in Bozeman, Montana. Our work began in 1991, when Jack was a Visiting Regents Professor with us; he had started to collaborate with Armand and realized that the three of us could combine resources and interests. And along with our students, we’ve found out some interesting things.
We realized that not a lot was known about how dinosaurs and their relatives grew, and how they might have been like and unlike various living animals in these respects. As it turns out, UCMP has very good collections from the Permian, Triassic, Early Jurassic and Late Cretaceous; the MOR has great Early and Late Cretaceous collections, as well as a growing library of
Maiasaura bone cross-section
A cross-section through a sub-adult thigh bone of the duckbill dinosaur Maiasaura shows white spaces where lots of blood vessels used to be, indicating that it was a fast-growing bone. The black wavy horizontal line in mid-picture is a growth line, reflecting a seasonal pause in the animal’s growth. (photo courtesy of de Ricqlès, Horner and Padian)
  living animals; and Armand’s lab in Paris has a tremendous collection of samples from living and extinct vertebrates from all around the world. So we went to work cutting up bones and looking at their microstructure.
Now, some readers are probably asking how we can destroy specimens like this. It’s a good question; a lot of museum curators at first wouldn’t think of letting us borrow their specimens to cut them up. But this is changing, because they’ve seen our results. And because our preparators make the specimens look as good as new, it’s really not a big problem. So, for example, the Smithsonian has let us cut up some of their stegosaur plates and spikes, the Natural History Museum in London has sacrificed some scutes from a rare dinosaur that they have, and Harvard’s Museum of Comparative Zoology has given us parts of a very early dinosaur relative to thin-section. We now have a library of thin-sections that sample all the major groups of archosaurs (dinosaurs, pterosaurs, crocodiles, and their relatives) plus many other living and extinct animals. And this is what they tell us.
Bones, a bit like tree trunks, record the history of growth. Like a tree, the limb bone tissues reflect the amount of growth in a given period, and how fast the bone was growing. The types of bone tissues vary, depending on how fast the animal grew. We know this from looking at the bone tissues of living animals and actually measuring how fast those tissues grow. So, when we see the same tissues in an extinct animal, we can estimate its growth rate. Most animals also lay down periodic growth lines, which can often be used as an independent marker of periodic growth (there are some cautions, because there are different kinds of growth lines and not all of them measure time regularly). Working with these lines of evidence, we’ve been able to determine how long it took dinosaurs to reach adult size. A complication is that as they grew, dinosaurs resorbed much of the early, inner bone tissues they laid down. Fortunately, we have a number of juvenile specimens, and even hatchlings and embryos of some dinosaurs, so we can reconstruct the whole growth history by comparing stages.

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