Egg Mountain is a famous fossil site in Montana that changed how scientists understand dinosaur reproduction and parenting. The site preserves nests, eggshell fragments, hatchling bones, and sediment layers from the Late Cretaceous Period. These remains suggest that some dinosaurs returned to nesting areas and may have cared for young after hatching.
Studying Egg Mountain helps paleontologists connect fossils to behavior, not just anatomy.
Key Facts
- Egg Mountain is located in Montana and is part of the Two Medicine Formation.
- The site is about 75 million years old, from the Late Cretaceous Period.
- Maiasaura means good mother lizard, a name based on evidence of nesting and young dinosaurs.
- Relative dating uses rock layer order: older layers are usually below younger layers when layers are undisturbed.
- Fossil density can be estimated as density = number of fossils / area.
- Sedimentation rate can be estimated as rate = layer thickness / time.
Vocabulary
- Paleontology
- Paleontology is the study of ancient life using fossils, rock layers, and other preserved evidence.
- Fossil
- A fossil is preserved evidence of a once-living organism, such as bone, eggshell, footprint, or plant material.
- Nest structure
- A nest structure is a preserved arrangement of sediment, eggs, eggshells, or bones that shows where animals laid eggs or raised young.
- Sediment layer
- A sediment layer is a sheet of deposited material such as mud, sand, or volcanic ash that can preserve fossils and record past environments.
- Two Medicine Formation
- The Two Medicine Formation is a rock unit in Montana that contains Late Cretaceous fossils, including the Egg Mountain nesting site.
Common Mistakes to Avoid
- Assuming every fossil egg at Egg Mountain belongs to the same species, which is wrong because identification requires eggshell texture, shape, location, and nearby bones.
- Treating a nest as proof of parental care by itself, which is wrong because scientists need multiple lines of evidence such as hatchling development, nest spacing, and repeated site use.
- Reading sediment layers out of order, which is wrong because undisturbed layers are usually oldest at the bottom and youngest at the top.
- Ignoring scale in excavation diagrams, which is wrong because nest spacing, egg size, and bone size are essential for interpreting animal behavior and growth.
Practice Questions
- 1 A paleontologist maps 18 nests in a 90 m² excavation area. What is the nest density in nests per square meter?
- 2 A sediment band above a nest layer is 2.4 m thick and represents 12,000 years of deposition. What was the average sedimentation rate in meters per year?
- 3 A site contains clustered eggshell fragments, hatchling bones that are too small for independent survival, and several nests spaced at regular distances. Explain why this combination of evidence is stronger than eggshells alone for interpreting dinosaur nesting behavior.