Reconstructing how dinosaurs lived, grew and reproduced is a challenging task. Often species are represented in the fossil record by just a few individuals, so drawing any conclusion for a species as a whole is wrought with difficulties and inaccuracies. But decades of research and fossil discoveries from a huge bonebed in Montana has allowed scientists to produce the most detailed life history of any dinosaur yet.
Known as Maiasaura peeblesorum, or “good mother reptile,” the animal lived in Montana around 76 million years ago in large herds that nested in colonies and cared extensively for their young, giving them their scientific name. Despite the dinosaurs’ large size, the nests in these colonies were packed tightly together, resembling those of modern sea birds. It’s even thought that adults covered their nests with rotting vegetation to incubate the eggs. It is in part due to these large nesting sites that researchers have been able to get their hands on a large selection of Maiasaura bones that span many age groups, from infants to adults.
“This is a dramatic step forward from studying fossilized creatures as single individuals to understanding their life cycle,” said Jack Horner in a statement. Horner is the curator of the Museum of the Rockies at Montana State University, where many of the Maiasaura fossils are held. “We are moving away from the novelty of a single instance to looking at a population of dinosaurs in the same way we look at populations of animals today.”
The researchers looked at the fossil bone microstructure, or histology, from 50 Maiasaura shin bones. These demonstrated aspects of the animals' growth that can’t be deduced by looking just a the shapes of the bones alone. Ultimately, they were able to answer questions about the animals' growth rate, metabolism, sexual maturity, and how long they took to reach adult size.
They believe that the dinosaurs reached sexual maturity at about the age of three, and were fully sized – thought to be around 2,300 kilograms (5,000 pounds) – by the age of eight. While mortality for the very young was very high, a depressing 90% for yearlings, if they could survive through their second year the dinosaurs could expect a six year period of peak performance where mortality was just 13% on average. By the time the dinosaurs reached old age, eight years and older, their mortality rate increased again to around 44%.
“By studying the clues in the bone histology, and looking at patterns in the death assemblage, we found multiple pieces of evidence all supporting the same timing of sexual and skeletal maturity,” explained Elizabeth Freedman Fowler, who coauthored the paper, which is published in the journal Paleobiology.
This study kicks off the Maiasaura Life History Project, which aims to add as much information as possible to how the dinosaurs lived, and create a histological “map” to see if different bones grew at different rates. They hope that the data presented in this study could be used as a model against which other dinosaurs can be compared.
Image in text: Reconstruction of an adult Maiasaura caring for its young. Credit: Dudo~commonswiki/Wikimedia Commons
