In 1985, three years before Littlefoot and gang would set off for The Great Valley, paleontologist Jack Horner, writer James Gorman, and artist Doug Henderson told the story of a different baby dinosaur on a perilous journey. Her name was Maia – short for Maiasaura – and while her adventure was fiction, it was informed by the science of what Horner and his colleagues were digging up at “Egg Mountain” in Montana.
Maia was not a miniature version of her parents. She hatched as a big-eyed, wobbly-legged infant, and spent her early days in the warmth of the nest as she grew and her limbs became stronger. But her salad days quickly ended when hungry Troodon raided the nesting ground, and as Maia matured she encountered even bigger, toothier dangers in the surrounding forests. Ultimately, though, Maia survived long enough to become a mother herself, and this, according to a new study by paleohistologist Holly Woodward and colleagues, would have made Maia a very exceptional member of her species.
The site that inspired the story of Maia isn’t just a horizon of tiny bones and eggshells. Over the past three decades paleontologists from Montana’s Museum of the Rockies have dug up hundreds of Maiasaura bones from a second horizon just above the nesting grounds. So many bones have been found here, in fact, that Woodward and her collaborators have been able to assemble the first good picture of what Maiasaura population biology was like.
Of all the dinosaurs that ever lived, only a fraction of a fraction of a fraction are preserved in the fossil record. And of the dinosaurs that have come down to us as petrified skeletons, many are only known from fragments or partial skeletons. This makes studying dinosaur biology incredibly difficult. While a biologist working on living animals might have hundreds of specimens to study for demographic information – and can often go observe their chosen species in life – paleontologists are lucky if they have a small smattering of beat-up bones to work with. And even well-known dinosaurs can be hard to study. While 50 or so partial Tyrannosaurus rex skeletons are known, for example, they don’t represent a single population but are from different places and slices of time from a window of about two million years. So if you’re a paleobiologist wanting to study variation between individuals, population structure, and other details, more often than not there just aren’t enough dinosaurs to get at those puzzles.
But the Egg Mountain bonebed is another story. Even though the bones were strewn into a jumble when buried, paleontologists have been able to recover elements from animals of different sizes and ages that died very close to each other in time. Within that sample the researchers have extracted at least 50 tibiae – the larger of the two lower hindlimb bones – and, as Woodward and colleagues demonstrate, this is a very useful bone to have.
Histology, or the study of bone structure, is one of the most important subfields within paleontology right now. By cutting into bones and looking at them under the microscope paleontologists can see how fast a dinosaur was growing, estimate its age, and get other biological information otherwise invisible to us. The most useful for studying dinosaurs have been leg bones – the femur and the tibia – as they often record a great deal of detail about an individual animal’s life in a way that’s relatively easy to see. To that end, Woodward and coauthors cut into and scrutinized the 50 leg bones – representing at least 32 distinct individual Maiasaura ranging from about 10 to over 23 feet in length – to investiage what the osteological details would reveal about this dinosaur population.
As it turns out, the fictional Maia was even luckier than even Horner initially understood. Within the sample, 31 of the bones lacked any lines of arrested growth (or LAGs) that are usually taken as markers of each passing year. This means that the majority of the dead dinosaurs were yearlings that were still growing extremely quickly. From this Woodward and her colleagues calculated that baby Maiasaura faced an 89.9% mortality rate in their very first year. If baby dinosaurs even survived to the moment they pushed out of their eggs, the odds were set against most of them surviving to adulthood.
Old dinosaurs didn’t fare so well, either. Woodward and colleagues counted 11 tibiae from Maiasaura that were seven years old or more, and five of these had a special growth marker – the external fundamental system (EFS) – which signaled that these were old dinosaurs whose skeletons had stopped growing. These fully mature dinosaurs had a mortality rate of 44.4%, Woodward and coauthors concluded, and probably represent the attrition that comes with old age.
There were a few dinosaurs in the middle of these two extremes, but, for the most part, young, vigorous Maiasaura were absent from the sample. And this seems to fit with the general picture of how the bonebed formed. The profile seems to suggest some kind of local catastrophe, like a drought or disease. Yearlings and old individuals were hit hardest, as is the case with many large animals alive today, while the young dinosaurs in their years of “peak performance” were better able to withstand the stress. Not that being a young, vigorous dinosaur was without its challenges, though. The few prime-aged Maiasaura in the sample might have been undergoing some unique growing pains that made them more susceptible, Woodward and colleagues write, which might be related to the onset of sexual maturity at around three years of age.
This sort of life history wasn’t unique to dinosaurs. Maiasaura grew at rates comparable to their living avian cousins, and the survivorship estimates Woodward and colleagues were able to calculate compare well with those biologists have recorded for red deer and Dall sheep. And the fact that such conclusions are possible at all is not only owed to the fortuitous discovery of so many dinosaurs in one place, but to the decades of work Museum of the Rockies researchers and volunteers have put into the site.
Best of all, all that hard word raises the possibility that paleontologists can apply the same lessons to other places where dinosaurs died en masse. The disarticulated remains of at least 48 individual Allosaurus have been found at the Cleveland Lloyd Dinosaur Quarry in Utah, for example, and New Mexico’s Ghost Ranch quarry is famous for producing hundreds of Coelophysis skeletons. These collections of bones, as well as others scattered around the world, offer additional opportunities to gain new insights into dinosaur life if we can grit our teeth and comes to terms with this simple fact – we won’t really be able to understand dinosaurs without slicing up a few bones.
Woodward, H., Freedman Fowler, E., Farlow, J., Horner, J. 2015. Maiasaura, a model organism for extinct vertebrate population biology: a large sample statistical assessment of growth dynamics and survivorship. Paleobiology. doi: 10.1017/pab.2015.19