Research reveals differences between male and female stegosaurs

The differences between male and female stegosaurs were revealed in research by an American undergraduate student. The study was done by 23-year-old Evan Saitta to complete the Paleontology course at Princeton University, USA, and was published Wednesday in PLoS One.

Stegosaurs, herbivorous dinosaurs that lived 150 million years ago in North America, have two rows of bone plates on their backs and two pairs of thorns at the tip of their tail. According to Saitta's study, the dorsal plates of males are broad and rounded, while those of females, with 45% smaller area, are high and pointed. Until now, scientists believed that the differences between the plates indicated the existence of two different species of stegosaurs.

While sexual dimorphism - the anatomical differences between males and females of the same species - is extremely common among animals, determining it in extinct species, such as dinosaurs, is considered a surprisingly difficult task, according to Michael Benton, professor of paleoliology at the University. from Bristol (UK), where Saitta is currently pursuing a master's degree.

"Evan made this discovery when he was completing his undergraduate studies at Princeton. It's very impressive when a student makes a scientific discovery of such weight, " said Benton.

The major difficulty in determining sexual dimorphism in extinct species, he said, is to eliminate other possible explanations for the anatomical differences between fossils. Two individuals with distinct anatomies may, for example, belong to two closely related species, or may be a young and a mature individual.

Saitta's work, however, was solidly grounded: He spent six seasons in the central Montana state of the United States as part of an excavation team that discovered the first "graveyard" of stegosaurus fossils. He was thus able to test - and refute - all alternative explanations for the anatomical differences between animals of the Stegosaurus mjosi species.

"When studying sexual dimorphism, you have to eliminate all other possible explanations. At this Stegosaurus site, I had the best data available to test all possibilities for explanation. A lot of people have tried to do this before with dinosaurs, but the problem is that there aren't many species that provide the necessary information. I was very lucky with the fossils I had access to, "Saitta told the report.

In the group of stegosaurs discovered in Montana, there was the coexistence of individuals with variations in their dorsal plates. According to Saitta, if these were two distinct species, there would be certain differences in skeletons that would indicate a separation of ecological niches.

The researcher also found that the differences were not a result of growth. Analyzing thin slices of fossil dorsal plaque samples under the microscope, Saitta showed that bone tissue growth had already ceased in both varieties - that is, even with the difference in plaques, they were all adults. The best explanation, therefore, would be sexual dimorphism.

"As males typically invest more in their ornamentation, the broader dorsal plates probably belonged to them. These larger plates should provide a large display surface to attract females. The taller, pointed plates probably functioned as a spiny protection against predators., for females, "said Saitta.

By Fábio de Castro - Sao Paulo

Via InAbstract