Dinosaur thigh bones could be the secret to revealing their sex, study finds

An interesting new study of a dinosaur mass grave may have found a way to sex dinosaurs in the future by comparing their femurs.
Christopher McFadden
Example of Gallimimus (a genus of Ornithomimidae) fossils.

Gary Todd/Wikimedia Commons 

A team of researchers from the Muséum National d’Histoire Naturelle in France has found what they believe to be definitive evidence of sexual dimorphism in a group of non-avian dinosaurs. By studying a group of at least 61 individual ornithomimosaurs (ostrich-like theropod dinosaurs), the skeletons could be put into two main groups based on the dimensions of their thigh bone fossils. These groups, they claim, represent individuals that are probably either example of males or females of the species.

However, the researchers cannot yet tell which is which. The dinosaur skeletons are also from the same time and place and were likely the victims of some horrific mass dying event like a natural disaster.

The researchers cannot tell which are male or female, however

A study by Romain Pintore, a Ph.D. student at the Muséum National d’Histoire Naturelle, and his co-authors analyzed the shape of preserved thigh bones. The study revealed differences in curvature that could be categorized into two distinct groups, with variations existing on a spectrum. The width of the epiphysis (the end part of a long bone) at the bottom of the bones showed the same pattern. The surviving relatives of dinosaurs, such as birds and crocodiles, exhibit enough variations in these features that they can be used to distinguish males from females with a reasonable degree of accuracy.

So, the researchers argue, the same could be true for dinosaurs too. Typically, birds with similar differences have wider epiphyses in males than in females. However, ostriches seem to have the opposite pattern. Surprisingly, the authors discovered an equal number of bones from each group in their sample, which was also unexpected.

Dinosaur thigh bones could be the secret to revealing their sex, study finds
Image showing the two major groupings based on femur dimensions.

Unlike modern flocking relatives of dinosaurs, which have higher survival rates for adult females, dinosaurs likely hatched similar numbers of each sex. While some paleontologists have labeled dinosaurs who died incubating eggs as female, this judgment is questionable considering the caring nature of fathers in other species, such as penguins and emus.

Like birds, female dinosaurs are also thought to have possessed unique medullary bones that aided in the creation of eggshells. These bones were only present during the egg-laying phase and were likely consumed while making eggs. As a result, a small number of female dinosaurs can only be identified by discovering these bones in specimens that died at the right time. It is possible that the discovery of an ornithomimosaur with medullary tissue-producing eggs could allow for the identification of female thigh-bone shapes. Still, as of yet, no such finding has been made.

The larger ones are necessarily males

In the past, size was commonly used to test, but nowadays, most paleontologists do not consider it a reliable method. The fact that males are usually larger in many mammal species does not apply to all animals, as evidenced by spiders and smaller hummingbirds.

“Skeletons are often found in different locations, and the preserved animals may have lived several thousand or even millions of years apart, making it impossible to tell whether any variations in the skeleton are due to regional, temporal, individual, age-related or sex-related differences,” an accompanying editorial notes.

You can view the study for yourself in the journal eLife.

Study abstract:

"Sexual dimorphism is challenging to detect among fossils due to a lack of statistical representativeness. The Angeac-Charente Lagerstätte (France) represents a remarkable ‘snapshot’ from a Berriasian (Early Cretaceous) ecosystem and offers a unique opportunity to study intraspecific variation among a herd of at least 61 coeval ornithomimosaurs. Herein, we investigated the hindlimb variation across the best-preserved specimens from the herd through 3D Geometric Morphometrics and Gaussian Mixture Modeling. Our results based on complete and fragmented femora evidenced a dimorphism characterized by variations in the shaft curvature and the distal epiphysis width. Since the same features vary between sexes among modern avian dinosaurs, crocodilians, and more distant amniotes, we attributed this bimodal variation to sexual dimorphism based on the extant phylogenetic bracketing approach. Documenting sexual dimorphism in fossil dinosaurs allows a better characterization and accounting of intraspecific variations, which is particularly relevant to address ongoing taxonomical and ecological questions relative to dinosaur evolution."

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