Boy or Girl? Can We Tell Male And Female Dinosaurs Apart From Fossils Alone?

Unveiling the Secrets of Sexual Dimorphism in Dinosaurs

I know most people don’t think about these topics, but if you are anything like me, you can relate to my story.

I don’t remember how old I was, but I was probably very young, as my passion for dinosaurs started early in life. Thanks, Steven Spielberg! I was reading about the most important fossil discoveries in the US, and of course, I learned about Sue.

According to the Field Museum, “at more than 40 feet long and 13 feet tall at the hip, SUE is physically the largest Tyrannosaurus rex specimen discovered, out of more than 30 T. rex skeletons that have been found. SUE is also the most complete — around 90 percent.”

Being born in a patriarchy, I immediately caught on something that got my curiosity: “Sue,” a female name. How did they know it was a female? To my surprise, I quickly learned that “SUE’s sex is unknown; this T. rex is named for Sue Hendrickson, who discovered the dinosaur in 1990 during a commercial excavation trip north of Faith, South Dakota.”

Oh, so they don’t know? Why, though? Is it that hard to tell from fossils alone?

A recent study led by Dr. David Hone and his team sheds light on this intriguing topic by examining modern-day gharials, a type of large crocodilian that live in South Asia. The findings offer fresh insights into the challenges of identifying sexual dimorphism in extinct species.

Sexual dimorphism: distinct difference in size or appearance between the sexes of an animal in addition to difference between the sexual organs themselves

To tackle this puzzle, the researchers turned to the gharial (Gavialis gangeticus), an endangered crocodilian known for its distinct physical traits that dramatically vary between sexes.

Male gharials are not only larger than females but also sport a unique structure called a ghara on their snouts. This ghara is a fleshy growth supported by a bony hollow known as the narial fossa (see figure above). By analyzing 106 gharial skulls from museum collections worldwide, the team aimed to determine whether these characteristics could reliably indicate sex. They took both manual and digital measurements and employed various statistical methods to assess the data.

The study revealed that while the presence of the narial fossa is a reliable indicator of male gharials, distinguishing between sexes based solely on other skeletal features is far more challenging.

Dr. David Hone explains, “Our research shows that even with prior knowledge of the sex of the specimen, it can still be difficult to tell male and female gharials apart.” The findings highlighted that aside from the narial fossa, males, and females showed few significant differences in skull morphology that could be used to identify sex confidently. In other words, even experienced biologists couldn’t tell these animals apart from skeleton remains alone.

More importantly, the research underscored the complexity of detecting sexual dimorphism in extinct species like dinosaurs, for which we only have usually incomplete bone remains.

Gharials, which share several reproductive and growth characteristics with dinosaurs, provided a valuable model. However, even with this modern analog, the task proved difficult. “With most dinosaurs, we don’t have anywhere near that size of the dataset used for this study, and we don’t know the sex of the animals, so we’d expect this task to be much harder,” Hone adds.

So, what do these findings mean for our understanding of dinosaurs? The study suggests that identifying dinosaurs’ sex from fossils alone is incredibly difficult unless they exhibited extreme sexual dimorphism or had distinct sex-specific features like the ghagra in gharials.

This poses a significant challenge for paleontologists who seek to understand these ancient creatures’ social and reproductive behaviors. Having a wife who dedicated several years of her life to paleontological research, I can attest that this couldn’t be more true.

The study also calls into question previous claims about sexual dimorphism in popular dinosaur species. For instance, some studies have suggested that female T. rex were larger than males, based on limited fossil evidence.

However, the findings from the gharial study suggest that such conclusions may be premature. “Many years ago, a scientific paper suggested that female T. rex are bigger than males. However, this was based on records from 25 broken specimens and our results show this level of data just isn’t good enough to be able to make this conclusion,” says Dr. Hone.

The research highlights the importance of robust datasets and clear morphological indicators when studying sexual dimorphism in both living and extinct species (yes, there are complications in living animals, too).

For paleontologists, this means that caution is needed when drawing conclusions about the sex of dinosaur specimens based on skeletal features alone. Future research will benefit from more comprehensive datasets and may require new methodologies beyond morphological characteristics to unlock the secrets of dinosaur dimorphism.

Moreover, this study highlights the value of using modern animals as models to understand extinct species. While the gharial study reveals the complexities and limitations of this approach, it also opens the door to further research that could refine our methods and enhance our understanding of creatures that no longer roam the Earth.

By learning from the present, we inch closer to unveiling the mysteries of the past, one fossil at a time. And if anything, nature is teaching us, once again, that sex is more complicated than our culturally driven binary view.