Hypacrosaurus

About Hypacrosaurus

Features: High Back Sail and Resonating Crest

The genus name Hypacrosaurus means "near the highest lizard." This comes from the fact that it had a large body height comparable to Tyrannosaurus, the strongest carnivorous dinosaur living in the same era and region.

Visually, the biggest feature of Hypacrosaurus is the extremely long upward projections of its backbone (neural spines). When viewed from the side, it had a unique silhouette as if a giant sail were towering on its back. This back sail is thought to have functioned as a visual symbol to distinguish companions in the herd and to appeal to the opposite sex.

The hollow, round crest on its head is also a major feature. It was once thought that the internal airway was straight, but recent studies using CT scans have revealed that a complex "S-curve" airway, different from other closely related species, runs like a maze.

Hypacrosaurus could resonate this crest like a giant wind instrument to produce powerful low-frequency sounds. In addition, analysis of brain fossils (endocasts) revealed that it had excellent hearing. Even in dense forests, they likely engaged in advanced communication, calling to distant companions with low voices and warning each other of danger.

Growth Speed and "Youth-Only Herds"

Hypacrosaurus is a rare dinosaur in the world, for which fossils of all growth stages have been found, from nests and eggs to babies and adults.

Fossils of babies whose teeth were already worn down have been found in nests where eggshells were crushed. This indicates that babies did not leave the nest immediately after hatching, but were cared for by their parents who fed them chewed plant food. Like their close relative Maiasaura, they were "good parents."

By examining the tree-ring-like patterns (Lines of Arrested Growth) carved into the cross section of the bones, their growth speed was revealed.
When hatched, they were about 1.7 meters long, but in just 2 to 3 years, they grew to about 40% of their adult size and could reproduce. They reached their maximum size of about 9 meters at 10 to 12 years of age. Their greatest defense to protect themselves from carnivorous dinosaurs was "to grow large as quickly as possible."

Also, in Alberta, Canada, fossil beds (bonebeds) have been discovered where "only growing juvenile individuals" gathered. It seems they had an interesting lifestyle of separating from adult herds and acting in groups of their own generation - youth only.

The Changing Crest and the Phantom Dinosaur "Cheneosaurus"

In the history of dinosaur research, hadrosaurids, including Hypacrosaurus, caused taxonomic confusion for a long time.
In the past, many fossils of small dinosaurs with low, flat crests on their heads were discovered and consecutively named as new species such as "Cheneosaurus."

However, detailed bone measurement studies in the 1970s revealed the truth. The dinosaurs called "Cheneosaurus" were not a separate species, but were actually the children (juveniles) of Hypacrosaurus.

The crest of Hypacrosaurus hardly inflates during childhood. When its body size approaches half that of an adult, the crest suddenly begins to grow rapidly. This dramatic change in appearance was the cause of tormenting scholars for years. The fact that the crest grows rapidly matching the maturity period suggests that it was an appeal organ (secondary sexual characteristic) for attracting the opposite sex.

Ancient Cells? The Astonishing Discovery of DNA Markers

In 2020, a research team observed a baby Hypacrosaurus skull fossil in detail under a microscope and discovered three-dimensional cell traces preserved in the "cartilage" before it became bone. Surprisingly, the appearance of the exact moment of cell division and even structures like chromosomes were preserved inside.

Further chemical tests confirmed a reaction (marker) indicating that not only cartilage-specific proteins but also fragments of dinosaur DNA were preserved inside the cell nucleus.

It does not mean we can extract a complete gene (genome) to revive dinosaurs like in the movies. However, the fact that the special environment of "cartilage," which is less exposed to oxygen, protected molecular information for 75 million years, shocked the paleontological community and became a major discovery opening a new door in fossil research.

Discovery and Paper Publication

In 1910, paleontologist Barnum Brown excavated skeletal fossils such as a giant backbone and parts of a pelvis along the Red Deer River in Alberta, Canada. At this time, no skull was found, but he judged this fossil to be a new species and described it in a paper as "Hypacrosaurus (near the highest lizard)" in 1913. Later, in 1924, a skull with a large crest was finally discovered.

Diverse herbivorous dinosaurs crowded the North American continent during the Late Cretaceous. To avoid competition for limited plants, it is thought they coexisted by neatly dividing their dietary styles (niche partitioning), where relatives like Saurolophus "selected" good quality plants to eat, while Hypacrosaurus ate "large amounts" of slightly lower quality plants.