In late July of 1978 Jack Horner visited the Rock Shop and Museum in Bynum, MT. Local resident Marion Brandvold had discovered baby dinosaur bones that she showed Jack. Laurie Trexler had also found an adult duckbilled dinosaur skull nearby. The baby bones were the first to be found in North America, and the first in a nest anywhere in the world. Both the babies and the skull were discovered while Dave and Laurie Trexler and Marion Brandvold were working on an adult dinosaur they planned to display in their museum. At the time, this display would have been only the second dinosaur on display in Montana.
Bob Makela and Jack Horner collected the skull and site materials from the baby locality. They also borrowed the baby bones collected by Marion. These were studied that winter and a preliminary report naming “Maiasaura peeblesorum” was published in Nature in 1979, before any eggs were discovered. A volunteer of Jack’s, Fran Tannenbaum, found the first eggs at Egg Mountain, approximately 3/4 of a mile from the original Maiasaura locality, in July of 1979.
Both Egg Mountain and the Maiasaura locality are rich in egg, baby, and adult fossils. The animals themselves roamed a broad flat coastal plain along the edge of the Cretaceous Interior Seaway approximately 77 million years ago. The Egg Mountain locality has produced remains of adult and embryonic Troodon (a small meat-eater) and adult Orodromeus (a small plant-eater), as well as remains of cretaceous mammals, lizards, and pterosaurs. The Maiasaura locality has yielded literally thousands of individual fossils, all believed to be from the single species of Maiasaura. The original Maiasaura nest contained remains of baby dinosaurs that had been hatched for some time before their demise. Because these individuals had remained in the nest after hatching, some individual (most likely the mother!) had to have been caring for the babies. This was the first indisputable evidence that dinosaurs were capable of any sort of complex behavior.
The Maiasaura’s round nests were six or seven feet wide and could hold up to 25 eggs. The hatched babies were about one foot long. Adult Maiasaurs weighed almost 6,000 pounds and were almost 30 feet long. The nests of Troodon at Egg Mountain itself are about half the diameter of the Maiasaura nests, but they contained roughly the same numbers of eggs.
Egg Mountain is located near Choteau, Montana, and dinosaur research continues in the area to this day. Both of Montana’s dinosaur research facilities are actively working on projects in the local Two Medicine Formation. While the Egg Mountain area is closed to the public, access to area fossils is available through public programs at the Two Medicine Dinosaur Center in Bynum, and undergraduate and graduate studies are available through Montana State University/Museum of the Rockies.