Spotting the difference between and male and female dinosaur specimens has long been a source of confusion for palaeontologists, and with good reason. New research published in the journal PeerJ claims that previous guestimates about the sex of recovered specimens are likely inaccurate, owing to how poorly the fossil record preserves the key indicators of sex in extant species. Researchers, led by the Queen Mary University of London, looked at the skeletons of modern-day gharials to see what indicators can be gleaned from the fossil record when trying to sex a specimen, and found the results were slim pickings.
Gharials, sometimes known as gavials, are an endangered group of Asian giant crocodilians that demonstrate sexual dimorphism as the males are larger than the females and have a fleshy growth at the end of their snouts known as a ghara. The ghara is made up of soft tissue, meaning it’s not preserved in the fossil record, but it is supported by a bony hollow called the narial fossa that can be seen in their skulls post-mortem.
The research team studied 106 gharial specimens gathered from museum collections across the world to see if they could reliably identify the males from the females in this sexually dimorphic species. They found that, without the narial fossa, it was extremely difficult to identify which skeletons were male and which were female despite their known difference in size.
Dr David Hone, Senior Lecturer in Zoology at Queen Mary University of London and lead author of the study, said in a statement, "Like dinosaurs, gharials are large, slow-growing reptiles that lay eggs, which makes them a good model for studying extinct dinosaur species. 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. 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."
While many extant animals exhibit sexual dimorphism, which is obvious in a living specimen, most of these features wouldn’t be preserved in the fossil record, such as vibrant plumage. The researchers hypothesize that indicators of sex within dinosaurs may have been similarly dependent on the preservation of soft tissues, feathers, or keratinized features like horns meaning identifying male and female specimens from the fossil record alone is likely highly inaccurate.
“Our study suggests that unless the differences between the dinosaurs are really striking, or there is a clear feature like the fossa, we will struggle to tell a male and female dinosaur apart using our existing dinosaur skeletons,” Dr Hone said. "Many years ago, a scientific paper suggested that female T. rexes 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.”