Mapping the Lives of a Population of Dinosaurs

Maiasaura peeblesorum – The Maiasaura Life History Project

Years of excavation at the quarries identified as nesting sites for the Hadrosaur known as Maiasaura (M. peeblesorum) has yielded an immense amount of data.  However, a team of scientists from Montana State University, Oklahoma State, and the Indiana Purdue University have taken the field research in a different direction and used an immense fossil deposit covering over two thousand square metres to report on the largest dinosaur population growth study ever undertaken.

The team’s findings make quite sombre reading for any would-be duck-billed dinosaur (not that they could read, we know).  Mortality rates for animals under twelve months of age were nearly 90%, whilst if you got passed your eighth birthday, the odds were beginning to stack up against you for living much longer.  These dinosaurs lived in tough times.  The numbers might sound frightening but mortality rates in extant antelope and other herbivores on the African savannah, are in some cases very similar.  Predatory dinosaurs probably did not fare any better.

Maiasaura peeblesorum – Model by Safari Ltd

Model of "Good Mother Lizard"

Model of “Good Mother Lizard”

Picture Credit: Everything Dinosaur

Maiasaura peeblesorum

Maiasaura was a large, flat-headed, duck-billed dinosaur that inhabited North America during the Late Cretaceous.  The first fossils of Maiasaura were found in the Badlands of western Montana in 1978, by a team of American scientists led by the famous palaeontologist Jack Horner.  The site the team discovered consisted of a number of nests, eggs, baby Maiasaura, juveniles as well as adult specimens.  The area was renamed “Egg Mountain”.  Approximately, 200 individual specimens have been excavated, providing evidence of the nesting behaviour of dinosaurs.  Published papers on these fossils were amongst the first to put forward the hypothesis that some types of baby dinosaurs were altricial, that is, heavily dependent on their parents for food and protection.  Here was evidence of dinosaurs being raised in nests.

This new research published this week in the journal “Paleobiology” provides the most detailed life history of any dinosaur and has created a framework to which all other dinosaurs can be compared.

The Badlands of Montana – Once Home to Thousands of Dinosaurs

The Badlands of Montana.

The Badlands of Montana.

Picture Credit: Holly Woodward Ballard/Karen Chin

Commenting on the significance of the research, Jack Horner, curator of the Museum of the Rockies and a man synonymous with all things Maiasaura stated:

“This is one of the most important pieces of palaeontology involving MSU [Montana State University] in the past twenty years.  This is a dramatic step forward from studying fossilised creatures as single individuals to understanding their life cycle.  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 research was led by Holly Woodward Ballard, Assistant Professor of Anatomy at Oklahoma State University, who prior to her appointment to this post, undertook her PhD at Montana State University.  Holly specialises in studying osteohistology (growth patterns of animals preserved in bone tissue) to map population growth dynamics in extinct vertebrates.  This data can then be used to create a model for palaeohistologic inferences, examining how individuals vary within a population, growth rates and survival rates.

To complete the research, the team analysed the fossil bone micro-structure (histology) of fifty Maiasaura tibiae (lower leg bones).  The bone histology reveals aspects of growth that cannot be ascertained by observation of the external structure and shape of the bone.  The histology reveals information such as growth rate, metabolism, age of maturity, and the age at death.

Assistant Professor Woodward Ballard explained:

“Histology is the key to understanding the growth dynamics of extinct animals.  You can only learn so much from a bone by looking at its shape, but the entire growth history of the animal is recorded within the bone.”

To a statistician a sample of just fifty may not sound like much, but to a vertebrate palaeontologist where a species can be known from a single bone or even a single tooth the Maiasaura fossil assemblage from the Badlands of Montana represents an absolute treasure trove of dinosaur fossil material.

The published paper provides an insight into how quickly Maiasaura babies grew up.  It had bird-level growth rates throughout most of its life, its bone tissue most closely resembles that of a modern warm-blooded (endothermic) mammal such as an elk.

The speed of growth might have something to do with the fact that the bigger you got the less chance of you ending up as a meat-eater’s lunch.  Everything Dinosaur has reported previously on a study into the histology of another Hadrosaur, called Hypacrosaurus that showed that these herbivores grew faster than the carnivorous dinosaurs that co-existed with them.

To read more about this study: Duck-Billed Dinosaurs Grew Fast to Avoid Tyrannosaurs

The bone histology also recorded major events in the life of individuals such as the different ages when animals died.

Elizabeth Freedman Fowler, curator of palaeontology at the Great Plains Dinosaur Museum in Malta (Montana), conducted the statistical analysis of the research data, she commented:

“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.”

With these dinosaurs, they probably were mature enough to breed within the third year of life and had an average adult body weight of 2,300 kilogrammes in eight years.  Life was tough for these herbivorous dinosaurs, especially the very young or the very old.  The average mortality rate for those less than twelve months of age was 89.9%, for individuals eight years and older it was 44.4%.  These figures sound alarming but most of the garden birds hatched this spring will not survive their first winter.

If a Maiasaura made it through two years, they enjoyed a six-year window of peak physical and reproductive fitness, when the average mortality rate was just 12.7%.

Assistant Professor Woodward Ballard added:

“By looking within the bones and by synthesizing what previous studies revealed, we now know more about the lift history of Maiasaura than any other dinosaur and have the sample size to back up or conclusions.  Our study makes Maiasaura a model organism to which other dinosaur population biology studies will be compared.”

Study Shows Considerable Variation with an Extinct Animal Population

The research also highlighted the extent of individual size variation within an extinct population of animals.  Earlier studies had linked age to the size of dinosaur limb bones, this method may not be that accurate based on this new data.  Histology studies examining a subset of dinosaur bones (such as femora or tibiae) had been carried out before with an assumed age for an animal calculated on the length of these key bones.  The length of the bone may be misleading, it is only by exploring the micro-structure of the bone that age details can be revealed.

Holly outlined how their research challenged the findings of earlier studies:

“Our results suggest you can’t just measure the length of a dinosaur bone and assume it represents an animal of a certain age. Within our sample, there is a lot of variability in the length of the tibia in each age group.  It would be like trying to assign an age to a person based on their height because you know the height and age of someone else.  Histology is the only way to quantify age in dinosaurs.”

Assistant Professor Holly Woodward Ballard at the Maiasaura Dig Site

At the Maiasaura bonebed.

At the Maiasaura bonebed.

Picture Credit: Holly Woodward Ballard/Karen Chin

The Maiasaura Life History Project

The Maiasaura research does not end with the publication of this paper.  This is only one of a series of proposed study areas.  Assistant Professor Woodward Ballard intends to lead a number of annual summer excavations up into the Badlands of Montana to collect more specimens.  The scientists want to keep working on the extensive bonebeds and build up a much more complete picture of the daily lives and struggles of these dinosaurs.

Clearly excited about the opportunity the huge bonebed presents, Holly stated:

“Our study kicks off The Maiasaura Life History Project, which seeks to learn as much as possible about Maiasaura and its environment seventy-six million years ago by continuing to collect and histologically examine fossils from the bonebed, adding statistical strength to the sample.  We plan to examine other skeletal elements and make a histological “map” of Maiasaura, seeing if the different bones in its body grew at different rates, which would allow us to study more aspects of its biology and behaviour.  We also want to better understand the environment in which the Maiasaura lived, including the life histories of other animals in the ecosystem.”

We at Everything Dinosaur wish all those involved in The Maiasaura Life History Project every success and we look forward to reporting on further research in the near future.

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