Shape-Shifting Dinosaurs: Unveiling the Secrets of Dinosaur Growth

For decades, museums have showcased impressive dinosaur skeletons, sparking the imaginations of children and adults alike. But a closer look reveals a fascinating puzzle: where are all the baby dinosaurs?

The Mystery of the Missing Juveniles

In the early 1900s, museums engaged in a competitive race to acquire the biggest and best dinosaur specimens. This focus on large, mature dinosaurs led to a significant oversight. By the 1970s, scientists began to question the conspicuous absence of smaller, juvenile dinosaurs in museum collections.

The prevailing assumption was that identifying juvenile dinosaurs would be straightforward – simply smaller versions of their adult counterparts. However, this assumption proved to be a major stumbling block.

The Role of Scientific Ego

Adding to the confusion, scientists often have a natural inclination to name new species. This tendency resulted in the classification of numerous dinosaurs based on what turned out to be variations within the same species at different growth stages. Every slight difference was seen as a completely new species.

A Breakthrough Discovery: Dinosaur Ontogeny

In 1975, Dr. Peter Dodson made a groundbreaking observation: dinosaurs grow more like birds than reptiles. He used the cassowary as an example, noting that the crest on its head doesn't begin to grow until the bird reaches about 80% of its adult size. This means that juvenile cassowaries lack the defining characteristic of their adult form.

Allometric Cranial Ontogeny

Dodson's insight highlighted the concept of allometric cranial ontogeny, or relative skull growth. If scientists were to find a cassowary at 80% grown, they might incorrectly assume it was a different species altogether.

Dodson demonstrated this principle using duck-billed dinosaurs (Hypacrosaurus), showing that the crest size varied significantly between juveniles and adults. This discovery should have prompted a reevaluation of dinosaur classification, but the desire to name new species persisted.

Cutting-Edge Research: Bone Histology

Fortunately, a new method emerged for determining the age of dinosaurs: examining their bone structure. Cutting into dinosaur bones reveals valuable information about their maturity. Juvenile bones are spongy, while mature bones are dense and solid.

Overcoming Museum Resistance

However, museums are understandably reluctant to allow researchers to dissect their precious dinosaur skulls. Undeterred, some scientists with their own collections began to explore this technique.

Case Studies: Shape-Shifting Dinosaurs

Pachycephalosaurus, Stigimoloch, and Dracorex

Three dinosaurs from the Hell Creek Formation – Pachycephalosaurus, Stigimoloch, and Dracorex – were long considered distinct species. Pachycephalosaurus had a large, thick dome on its head, Stigimoloch had spikes and a small dome, and Dracorex had spikes but no dome.

However, when the skulls are lined up, a clear pattern emerges: Dracorex is the smallest, Stigimoloch is medium-sized, and Pachycephalosaurus is the largest. Cutting open the skulls revealed that Dracorex and Stigimoloch had spongy, juvenile bone, while Pachycephalosaurus had solid, mature bone.

This evidence suggests that Dracorex is a juvenile, Stigimoloch is a subadult, and Pachycephalosaurus is the adult form of the same dinosaur. This discovery effectively renders Dracorex and Stigimoloch extinct, as they are simply growth stages of Pachycephalosaurus.

Triceratops and Torosaurus

Similarly, Triceratops and Torosaurus were once considered separate species. Triceratops had a solid frill, while Torosaurus had large holes in its frill. However, no juvenile Triceratops had ever been found before the year 2000.

After finding juvenile Triceratops, scientists observed that their horns curved backward, gradually growing forward as they matured. The triangular bones on the frill also flattened out with age. Furthermore, while adult Torosaurus skulls exhibited mature bone with holes, juvenile Triceratops skulls showed signs of hole formation.

This evidence indicates that Torosaurus is simply a fully grown Triceratops. According to naming conventions, the original name (Triceratops) takes precedence, rendering Torosaurus extinct.

Edmontosaurus and Anatotitan

The same principle applies to Edmontosaurus and Anatotitan. Bone histology reveals that Edmontosaurus is a juvenile or subadult, while Anatotitan is an adult. Thus, Anatotitan is now considered a mature Edmontosaurus.

Tyrannosaurus Rex and Nanotyrannus

Even the mighty Tyrannosaurus Rex has been subject to this reevaluation. Nanotyrannus, a smaller tyrannosaur with more teeth, was once thought to be a separate species. However, cutting into the bones revealed that Nanotyrannus had juvenile bone, while T-Rex had more mature bone.

Furthermore, the number of teeth in T-Rex specimens decreases with size, with the largest specimens having the fewest teeth. This suggests that Nanotyrannus is simply a juvenile T-Rex.

The Revised Dinosaur Count

By applying these principles, the number of recognized dinosaur species from the Late Cretaceous period has been reduced from twelve to seven. While this may disappoint some dinosaur enthusiasts, it provides a more accurate understanding of dinosaur growth and evolution.

This research highlights the importance of considering ontogeny when classifying extinct animals. By studying bone structure and growth patterns, scientists can gain a more complete picture of the lives of these magnificent creatures.