Every bone has stories to tell. Some, such as size or the animal’s place in the evolutionary tree, you scrape off the surface. But others remain hidden until you cut inside.
Thanks to more than 30 years of fieldwork, Montana’s Museum of the Rockies has amassed a huge collection of Maiasaura bones. From the number of right-side tibiae – the larger of the two bones that make up the lower leg – they’ve been able to count at least 32 individual dinosaurs at the one site. (The sample probably contains a higher number, but 32 is the absolute minimum.) Most of these limb bones appear to be healthy, but, when MOR researchers cut into them, two of the tibiae showed signs of ancient injury.
The two bones – one from a one-year-old dinosaur, the other from a four-year-old – don’t show anything as simple as a fracture or bone infection. Instead, paleontologists Jorge Cubo, Holly Woodward, Ewan Wolff, and Jack Horner report, the pair of elements show rinds of extraneous bone that quickly grew over the outer surfaces. Not only might this be a sign of bone forming in response to biomechanical stresses and strains, but it could throw additional support to the idea that Maiasaura changed how they trotted around as they aged.
Thanks to studies of modern animals, such as sheep, pigs, and humans, osteologists know that a break in one bone can cause another bone to change. If you were to break your fibula but didn’t have the luxury of resting up until it was healed, the failure of the one bone would change the stresses on the neighboring tibia. This is what Cubo and colleagues propose for the Maiasaura. These young dinosaurs probably broke their respective right fibulae, and the extra bone formed in response to the altered strains on the tibia.
But the bone bulges aren’t in the same place in the two dinosaurs. The extra bone formed towards the outer side in the younger animal and on the back in the older one. Cubo and coauthors propose that this pattern is consistent with how such extraneous bone forms in bipeds versus quadrupeds. The bone growth in the one-year-old Maiasaura follows the pattern seen among bipeds, such as ourselves, while the growth in the four-year-old is more consistent with what’s seen in quadrupeds such as pigs and sheep.
Back in 2001 paleontologist David Dilkes suggested that Maiasaura changed postures as they aged. Muscle scars on the Cretaceous bones seemed to show that the young ran around on two feet and went down to walk on all fours as they got larger. With any luck, additional cases will help test this idea, but the pathologic bones seem to support this idea. Maiasaura was a dinosaur that walked on two legs in the morning and on four legs from the afternoon through the evening.
Cubo, J., Woodward, H., Wolff, E., Horner, J. 2015. First reported cases of biomechanically adaptive bone modeling in non-avian dinosaurs. PLOS ONE. doi: 10.1371/journal.pone.0131131