Egg Mountain, the Two Medicine, and the Caring Mother Dinosaur

Article by John Dawson (2014 National Fossil Day Coordinator)
for 2014 Mesozoic Partner Highlights

An Illustration of life in a Maiasaura Colonial Nest.

Montana Geologic Road sign program

The area known as Egg Mountain is located in Montana just south of the town of Choteau. It is an outcropping of the Two Medicine Formation, which is present in much of northwestern Montana. The fossil site itself was discovered in 1977 by Marion Brandvold, owner of the Trex Agate Shop, aka "The Rock Shop," in Bynum, Montana. She found remains of juvenile dinosaurs at the site, and in 1978 showed her discoveries to paleontologist Jack Horner. Horner and his team went on to discover not just juvenile dinosaurs, but 14 dinosaur nests in a single area of the site. This is how the area got the name Egg Mountain. It supplied the first strong evidence that dinosaurs fed and cared for their young, and furthermore the first evidence that dinosaurs exhibited complex behaviors.

Model of Maiasaura nest at the Natural History Museum, London.

Photo from Wikimedia Commons (CC BY-SA 3.0).

These nests belonged to a dinosaur which has been named Maiasaura peeblesorum—the state fossil of Montana—which was discovered at Egg Mountain several months before the eggs themselves were found. Its genus name means "caring mother lizard." Since the time of this initial discovery, hundreds of specimens throughout all stages of life have been discovered, including eggs, juveniles, and adults. Maiasaura is in a family of dinosaurs known as hadrosaurs, a group of herbivorous dinosaurs often called "duck billed" dinosaurs. Hadrosaurs have been found throughout the world, and a great number have been found in North America. By studying the bones of Maiasaura it is has been determined that Maiasaura experienced a dramatic change in size as they aged. They would grow from their hatch size of less than 1 kg (2.2 pounds) to an adult size of 2,000 kg (~4,400 pounds) in a period of just 8 to 10 years. The discovery at Egg Mountain indicated that Maiasaura exhibited colonial nesting behavior, where large groups, likely herds, would all nest together in one area.

After hatching, the adults may have actively cared for their young for a significant amount of time. It has been suggested that Maiasaura seasonally migrated in order to meet the immense food needs of such a large group. Bone beds in the Two Medicine Formation have shown large groups of Maiasaura with individuals ranging from 3 meters (10 feet) to 7.5 meters (25 feet) in length. It is possible that Maiasaura would raise their offspring to the point that they could keep up with the herd, and then the herd would continue on their seasonal journey. It has also been suggested that Maiasaura behave in a way similar to modern sea turtles, returning to lay their eggs at the site that they themselves were born in.

An illustration of Cerasinops, an herbivorous ceratopsian dinosaur also from the Two Medicine Formation.

Image © N. Tamura. (CC BY-NC-ND 3.0)

Like other hadrosaurs, Maiasaura lived during the Cretaceous Period. Specifically, Maiasaura lived during a time called the Campanian Age, approximately 77 million years ago. By studying the surrounding sediment at sites like Egg Mountain, it has been determined that Maiasaura lived in environments described as upper coastal plains. While it is believed that Maiasaura only migrated within the same type of habitat, its journeys would have been both fascinating and dangerous. During this time, most plants were still either gymnosperms, such as conifers, cycads, and Ginkgo, or ferns. Flowering plants and broadleaf trees were just starting to appear in large numbers. A Maiasaura would have likely seen several other herbivorous dinosaurs during its travels. They would include ankylosaurs such as Scolosaurus, some ceratopsians (relatives of Triceratops and Protoceratops) including Cerasinops (pictured), and even another genus of hadrosaur known as Gryposaurus. However, they were likely to run into some dangerous carnivorous dinosaurs as well. These included Troodon, a 2 meter (6.5 feet) long relative to the famous Velociraptor. In addition, there is a chance that they might run into a larger Daspletosaurus, a 9 meter (30 feet) long "little" cousin of Tyrannosaurus. While their herds would have given them a great deal of safety, these animals could have still been a threat to Maiasaura. All of the dinosaurs mentioned earlier have been found in the Two Medicine Formation, in areas that indicate a similar environment to that of Egg Mountain.

Paleontologist Jack Horner excavating fossils at Egg Mountain.

Photo by L. Psihoyos

Thanks to Marion Brandvold's and Jack Horner's discoveries at Egg Mountain, we now have a much clearer view of the behavior of Cretaceous dinosaurs. Egg Mountain was truly an amazing discovery, and it showed that dinosaurs were far more complex than many had imagined. One can visit the site today and many parts are open to the public. For more information, you can contact either Two Medicine Dinosaur Center in Bynum, or the Museum of the Rockies, part of Montana State University in Bozeman. Both organizations are actively conducting projects in the Two Medicine Formation. If you ever find yourself in just south of Choteau, Montana, I highly recommend visiting this site and seeing for yourself the land of the Caring Mother Dinosaur.