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Restoration of Confuciusornis sanctus by Stephanie Abramowicz, NHM Dinosaur Institute.

Secret of the Dinosaur Sexes Locked in Bone

When Chicago’s Field Museum purchased the Tyrannosaurus rex “Sue” in 1997, the famous and legally-troubled dinosaur was thought to be a female. The key clue was a missing bone.

Like other dinosaurs, T. rex had a row of spike-like bones called chevrons that ran from the base of the tail to almost the tip. Living alligators and crocodiles share this feature, and, decades ago, paleontologist Alfred Sherwood Romer suggested that researchers could distinguish between the skeletons of male and female crocodylians by looking at the placement of the first chevron in the series. In females, the chevron was thought to be shorter and displaced further back from the hips, leaving more room for eggs to pass through her body. Peter Larson, part of the team that excavated Sue but lost her in the prolonged legal battle that followed, extended the same argument to Sue. Since the tyrannosaur’s first chevron was situated further back than in other specimens, Larson proposed, the big T. rex was a female.

When the museum unpacked and reconstructed Sue, though, Sue’s sex came under suspicion. The “missing” chevron was found among the dinosaur’s assorted bones, undermining Larson’s contention. And, as it turned out, the entire comparison was flawed. In a 2005 study of crocodylian chevrons, Gregory Erickson, Kristopher Lappin, and Larson concluded that there is no sexual difference between the first chevrons of male and female crocodylians, nor any detectable dimorphism in tyrannosaurs. At best, the researchers concluded, the “usefulness of chevron anatomy for sexing dinosaurs is tenuous.”

We really don’t know whether Sue was a male or female. The entire episode was just one debate within the long-running attempts of paleontologists to investigate the basic, yet extremely frustrating question of how to properly identify dinosaur sexes. Skeletal differences alone are of almost no help. Every proposed case of sexual dimorphism in non-avian dinosaurs has either been undermined by later studies or is extremely questionable. Either non-avian dinosaurs didn’t show sexual dimorphism in their skeletons, or we have yet to properly detect the differences.

But there actually is a way to identify some female dinosaurs. Medullary bone gives them away. This specific bone tissue was deposited in the long bones of pregnant female dinosaurs, just as it is in living birds. Not all female dinosaurs were developing medullary bone when they died, but the gravid ones were.

Medullary bone can do more than help paleontologists pick out a few females, though. Paleontologists Andrew Lee and Sarah Werning paired the tissue with other histological clues to figure out that most dinosaurs started reproducing while they were still growing, long before they reached skeletal maturity. Dinosaurs lived fast and died young.

Such studies made me wonder if medullary bone could help paleontologists finally detect sexual dimorphism among non-avian dinosaurs. By picking a highly-ornamented species and searching for traces of medullary bone within those dinosaurs, researchers would be able to see whether the females all shared any distinctive anatomical structures or looked just like the males. The process wouldn’t identify all the females, but, if the attempt identified any at all, the investigation might help researchers determine whether there was any conspicuous features to distinguish the sexes.

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A Confuciusornis with long tail feathers. Photo by Laikayiu, image from Wikipedia.

University of Cape Town histology expert Anusuya Chinsamy and colleagues have undertaken just such a study, published this week in Nature Communications. Instead of a non-avian dinosaur, though, the researchers picked Confuciusornis sanctus – an early bird preserved by the dozens in the 125 to 140 million year old rock of China.

The avian is a perfect candidate for such a study. Many of these birds are preserved with intact feathers, and their plumage comes in two forms. Some Confuciusornis are preserved with two elongated tail feathers, while others are not. Researchers have debated whether these ornaments are indicators of sexual dimorphism, species differences, individual variation, or simply different molt stages, but bone histology has finally solved the mystery.  The Confuciusornis with the long feathers were males, while females lacked the flashy adornments.

A particular Confuciusornis known as DNHM-D1874 yielded the crucial clue. Surrounded by feathers, this articulated skeleton lacked the long tail feathers seen on others. When Chinsamy and colleagues looked at the microstructure of specific bones, they found medullary tissue in the humerus (upper arm bone) and traces in the ulna (a lower arm bone), indicating that the bird had recently finished laying eggs when she perished.

The researchers also sampled the bones of three Confuciusornis preserved with long tail feathers. These specimens didn’t contain medullary bone. While it’s possible that these were females that were not laying eggs around the time of death, the presence of medullary bone in the non-ornamented form and its absence in the ornamented ones is consistent with the idea that the feathers underscore a real sex difference. And since relatively small Confuciusornis have been found with long tail feathers, Chinsamy and coauthors point out, these birds probably started reproducing relatively early in life.

Paleontologists can apply the same technique to non-avian dinosaurs. There are multiple specimens of dinosaurs such as Microraptor and Anchiornis that are preserved with feathers, and the fact that we can detect the color of dinosaur feathers has opened yet another avenue for comparison. Perhaps male and female dinosaurs sported different colors, much like many bird species today. Feathers aren’t a requirement for this kind of study, though – the same technique could conceivably be applied to other non-avian dinosaurs so long as they are known from large enough sample sizes. By breaking into bone, paleontologists may finally be able to discover the secrets of the dinosaur sexes.

References:

Chinsamy, A., Chiappe, L., Marugán-Lobón, J., Chunling, G., Fengjiao, Z. 2013. Gender identification of the Mesozoic bird Confuciusornis sanctusGender identification of the Mesozoic bird Confuciusornis sanctus. Nature Communications. 4, 1381. doi:10.1038/ncomms2377

Lee, A., Werning, S. 2008. Sexual maturity in growing dinosaurs does not fit reptilian growth models. PNAS. 105, 2: 582-587