Description and Classification

Hypacrosaurus was a large, plant-eating dinosaur that could walk on two legs (bipedal) or four legs (quadrupedal). Adults could reach lengths of about 9 meters (30 feet) and weigh up to 4 metric tons (about 4.4 short tons). One of its most notable features was a tall, hollow, bony crest on top of its skull. This crest was different in shape and size between the two species and likely varied with age and sex. The crest was formed by an expansion of the nasal bones and may have been used for display to attract mates, to recognize members of its own species, or possibly to make sounds, similar to other crested hadrosaurs like Lambeosaurus or Corythosaurus.

Hypacrosaurus belongs to the order Ornithischia, the group of “bird-hipped” dinosaurs. Within this group, it is classified as a member of the Hadrosauridae family, commonly known as duck-billed dinosaurs due to their wide, flattened snouts. More specifically, Hypacrosaurus is placed in the subfamily Lambeosaurinae, which includes hadrosaurs with hollow cranial crests, distinguishing them from the solid-crested or crestless Hadrosaurinae, like Edmontosaurus.

Distinguishing Features

Hypacrosaurus had several features that help paleontologists identify it:

• A prominent, hollow, plate-like crest on its skull. In H. altispinus, the crest was tall and somewhat rounded, while in H. stebingeri, it was lower and longer.
• Very tall neural spines on the vertebrae of its back and tail, making its back appear quite high. This feature is reflected in the species name altispinus, meaning “high-spined.”
• A typical hadrosaur “duck-bill” snout, useful for cropping vegetation.
• Complex dental batteries with hundreds of tightly packed teeth designed for grinding tough plant material.

Paleoenvironment and Diet

Hypacrosaurus lived in what is now western North America during the Late Cretaceous. H. altispinus fossils are found in the Horseshoe Canyon Formation of Alberta, Canada, an area that was a coastal plain with rivers, swamps, and forests. H. stebingeri fossils come from the Two Medicine Formation of Montana, USA, which also represented a coastal plain environment, but perhaps slightly further from the sea. The climate was likely warm and seasonal. Hypacrosaurus shared its habitat with a variety of other dinosaurs, including predatory tyrannosaurids like Albertosaurus, other hadrosaurs, ceratopsians (horned dinosaurs), and ankylosaurs (armored dinosaurs).

As a hadrosaur, Hypacrosaurus was an herbivore. Its powerful jaws and multiple rows of grinding teeth were well-suited for processing tough plant matter. Its diet likely consisted of conifers, ferns, flowering plants, twigs, and seeds found in its environment. It could browse on vegetation at various heights, using its bipedal stance to reach higher branches or its quadrupedal stance for lower-growing plants.

Significance and Ongoing Research

Hypacrosaurus is significant for several reasons. The discovery of numerous nests, eggs, embryos, and juvenile specimens, particularly for H. stebingeri at sites like “Egg Mountain” in Montana, has provided invaluable information about hadrosaur nesting behavior, growth rates, and development. These findings suggest that Hypacrosaurus hatchlings grew very rapidly and that these dinosaurs may have exhibited some form of parental care. The growth rate studies indicate that Hypacrosaurus reached adult size relatively quickly, possibly within a decade.

Ongoing research continues to explore the function of the cranial crest, using CT scans to reconstruct the internal nasal passages and model potential vocalizations. Scientists also study the bone histology (microscopic structure of bones) to refine estimates of growth rates and age at maturity. The well-preserved fossil record of Hypacrosaurus helps paleontologists understand the diversity and ecology of hadrosaur dinosaurs and the ecosystems they inhabited during the final stages of the dinosaur era, providing important comparisons with other lambeosaurines like Parasaurolophus.