The curious case of the shapeshifting dinosaurs

New research reveals that dinosaurs changed shape during their lives. This means that many dinosaurs, which have so far been considered to be different species, were actually just different growth stages of the same species. Rasmus Krag Jacobsen delves into the misclassification debate that might change the way we understand Triceratops.

Are palaeontologists on the verge of exterminating a dinosaur species? New bone microstructure research suggests that the giants of the Cretaceous period changed shape during their lives. In several cases palaeontologists have mistakenly believed that one single species was several separate species. Dinosaurs’ skulls were almost like modelling wax with variable bones, growing appendages and developing structures that morphed throughout their lives .

“Dinosaurs underwent radical changes in shape as they grew up from juvenile to adults. In some cases, these changes were so extreme that the juvenile form and adult form of a single species might be mistaken for two separate species,” explains PhD student John Scanella, from the Museum of the Rockies (MOR), in Montana, US.

Almost identical skeletons
Together with his colleague, Curator of Palaeontology Jack Horner, Scanella has inspired debate among palaeontologists with a study of the giant, three-horned Triceratops. His paper, published in Journal of Vertebrate Palaeontology, demonstrates that this dinosaur and its close relative, Torosaurus, were actually one single species. The researchers also placed a third species, Nedoceratops, under the same umbrella. For Scanella and Horner, Triceratops, Torosaurus and Nedoceratops are the same species.

This is not the first time that the Triceratops genealogy has been revised. When the first two-horned species were found in the late 1800s, palaeontologist Othniel Charles Marsh (1831-1899) mistook them for the remains of an ancient giant buffalo. It was not until giant skulls with duckbills, horns and frills were unearthed that Marsh realised he had discovered the first horned dinosaur. Later, several specimens appeared, and a total of 16 new species were described. However, the research was characterised by haste. In 1996, George Washington University palaeontologist Catherine Forster performed a major clean-up. It turned out that the species are merely variants of the two Triceratops species recognised today, Triceratops horridus and T. prorsus.

Scanella and Horner are studying the population of dinosaurs that inhabited the Hell Creek Formation. The North American site that spans across Montana, North Dakota, South Dakota and Wyoming, has so many fossils that, according to Scanella, it is impossible to walk around without stepping on the remains of a Triceratops. The many specimens found in the area are the reason why Scanella and Horner were able to map the development stages and morphological changes of these dinosaurs. They have studied more than 100 Triceratop skulls of different sizes — from soccer balls to small cars. Out of these, they have examined the bone structure of 29 Triceratops specimens and nine Torosaurus specimens. Their hypotesis suggests that Triceratops and Torosaurus have similar skull anatomies.

During the past 10 years, advances in bone microstructure have allowed researchers to determine the age of individual animals in a more precise way. Now researchers can see that the bones of young individuals do not have the same compact structure seen in older specimens of the same species. The bone analyses conducted by Horner and Scanella revealed that all Torosaurus specimens are aged individuals, whereas all Triceratops specimens are young individuals. The scientists also found signs of a transition to Torosaurus in several Triceratops specimens. In Torosaurus the frill is longer and has large holes in it; and in several Triceratops individuals the scientists discovered that the frill bone was clearly thinner in two corresponding places — and that might be the incipient formation of Torosaurus’ holes. According to the palaeontologists, this is a sign that the holes eroded as the frill bones were reshaped.

The skeletons and teeth of the two species are practically indistinguishable. According to Horner and Scanella, that is simply because the so-called Torosaurus is just an aged version of Triceratops. They suggest, therefore, that Torosaurus should not be defined as a separate species. Instead, both believe Triceratops developed from having rear-bent forehead horns and a small, spiked frill in its youth to having a larger frill without spikes and forehead horns pointing forward as an adult. It sounds dramatic, but great changes are often seen in the animal kingdom. For example, some horned mammals such as the deer and gazelle, and certain birds undergo similar changes; the Australian ostrich-like cassowary has a frill which is lying down in its juvenile years, but raises as it gets older.

Scientists disagree about kinship
Scanella and Horner’s analysis is well-founded; however, the intermediate stage between Triceratops and Torosaurus needs to be further investigated. The scientists point to a separate dinosaur species, Nedoceratops, which has been difficult to classify, but has two holes in its frill just like Torosaurus.

Palaeontologist Andrew Farke of the Raymond M. Alf Museum of Palaeontology in California, US, disagrees with Horner and Scanella. Consequently, he has made a thorough analysis of Nedoceratops which was published in January, 2011, in the scientific journal PLoS ONE. Farke argues that Nedoceratops is a separate dinosaur species because its nose horn is very small and its two forehead horns point vertically up in the air. The texture of the bones and the extent to which they are grown indicate that the skull belongs to an aged individual. “Triceratops, Torosaurus and likely Nedoceratops are all distinct taxa, indicating that species richness for chasmosaurine ceratopsids in the Lance Formation just prior to the Cretaceous-Paleocene extinction was roughly equivalent to that earlier in Cretaceous,” Farke said in his conclusions.

Scanella disagrees with Farke. He believes that differences are easy to spot, but they do not disclose information about the kinship of the species. Scanella says that the two small holes in the frill are natural for an individual developing the big holes of the Torosaurus. Among the many new Triceratops findings made in the Hell Creek Formation there are several overlaps to Nedoceratops, which have not been fully described.

Even if Farke is right about Nedoceratops being a separate species, he agrees with the hypothesis that Torosaurus could still have been an aged version of Triceratops. However, Nicholas Longrich from Yale University, US, and his colleagues recently analysed 35 Torosaurus and Triceratops specimens and found evidence, using the closing of sutures between skull bones, that some of the Torosaurus skulls belonged to immature individuals and some Triceratops skulls to adults.

The scientific debate concerning Triceratops is far from settled and will probably go on for many years. Much more is at stake than just the names of some obscure animals. Any reduction of the number of species means a lot for our understanding of the biology, evolution, habits, diversity and extinction of the dinosaurs. On his website (openpaleo.blogspot.com), Farke mentioned that he is revisiting his paper and has consulted Scanella. The new results will be published later this year. No matter the outcome of the debate, the idea of “˜dinosaur shapeshifting’ is popular. It dates back to Peter Dodson’s work in the 1970s and 80s, but our understanding of bone microstructures has improved.

In 2009, Horner and Mark Goodwin showed that two species of two-legged, thick-headed dinosaurs, Stygimoloch and Dracorex, were probably development stages of a third thick-headed dinosaur, Pachycephalosaurus. Similarly, extreme skull changes are known from duckbilled dinosaurs that vary between four and 15 metres. Several of the “˜small’ species have therefore turned out to be the first development stages of larger species. Farke and Scanella agree that there will be more studies in the coming years that will weed out more dinosaurs.

If you’d like to find out more about the shapeshifting dinosaurs, grab a copy of Issue 16

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