The Hadrosaurus was a duck-billed, plant-eating dinosaur that weighed up to 4 tons and could reach between 23 and 32 feet in height. It was named in 1858 by palaeontologist Joseph Leidy from a skull-less skeleton and hundreds of teeth that were found in Haddonfield, New Jersey. The Hadrosaurus is New Jersey's state fossil. The current specimens are believed to belong to the Dendroolithidae paratoxonomic family.

Although rare, fossilised dinosaur eggs can be found in a small number of sites throughout the globe, most notably China's Gobi Desert, and the badlands of the western United States. For eggs to be preserved as fossils, the key requirement is that they are covered with soil relatively quickly after being laid; this will protect them from scavengers, the climate and oxygen, which will encourage decay. Not only do the eggs continue to be covered with successive layers of sand and dirt of millions of years, but they are also impregnated with ground water passing through the sediment. The result of this is that the short-lived biological structures within the egg are replaced by long-lived mineral structures. The relatively stable calcite shell, however, will remain almost unaltered.