The mysterious sex life of dinosaurs

(CNN) – Dinosaurs must have had sex to reproduce, but how they did it – with ruffles on their necks, armored plates and spiked tails – is unclear exactly.

No fossil has shown two dinosaurs caught in the act. In fact, the only known vertebrates that have been unequivocally conserved during mating are a pair of 47 million year old turtles that were joined by their genitals when they were buried alive.

It is also not possible to easily determine whether a dinosaur is male or female from fossilized bones.

Fossils that preserve elements of dinosaur behavior are very rare. However, with detailed analysis and elements of what we know about living animals, particularly birds, paleontologists are beginning to piece together the sex lives of dinosaurs.

Sex differences

Many species of animals show a difference in appearance between different sexes, a phenomenon called sexual dimorphism. Think of the mane of a lion, the feathers of a peacock, or the antlers of a deer. Such characteristics, however, are surprisingly difficult to determine in extinct species.

Despite many previous claims, including the fact that female dogs T. rex were larger than males, these findings are now considered inconclusive. Differences in anatomy could point to a young and an older individual or individuals of different species, or simply differences that have nothing to do with sex.

“We really don’t know 100%. I couldn’t confidently raise my hand and say ‘you know what? T. rex is male, this T. rex it is female’. It’s unfortunate because as a paleontologist it’s a fascinating and fun area to explore, “said paleontologist Dean Lomax, a visiting scientist in the Department of Earth and Environmental Sciences at the University of Manchester.

An exception to this is Confuciusornis, a 125-million-year-old dinosaur that has many characteristics in common with modern bird species and shows a remarkable difference in plumage between male and female specimens.

Some fossils show tail feathers with the length of the body and shaped like ribbons. These feathers had been interpreted to be used during sexual display and scientists were able to find indisputable evidence that females did not have this ornamental plumage.

The researchers identified evidence of medullary bone – a calcium-rich tissue present for a short period in a reproductively active female bird that was used to make eggshells – in birds that did not sport the long plumage.

Work done in the last decade on cells containing color pigments in exquisitely preserved fossils of feathered dinosaurs has revealed that some were brightly colored. This is perhaps surprising, given that popular culture historically portrayed dinosaurs as grayish-green in color. Lomax believes that it is possible that in the future we will find a fossil that shows clear evidence of sexual dimorphism.

“In the future, probably from China, I imagine you will find two different colored dinosaurs, their anatomies will match, but they will be very different in coloration,” said Lomax, who is also the author of “Locked in Time: Animal Behavior Unearthed in 50 Extraordinary Fossils“.

‘Prehistoric foreplay’

Thanks in large part to the discovery of feathered fossils in China (which were once the subject of controversy) in the 1990s, we now know that birds are the only living relative of dinosaurs. Specifically the therapods, part of the same family as the T. rex and Velociraptor.

“You go back 20 or 30 years, and there were still scientists who said that birds are not dinosaurs, but now we have much more evidence that they are. So you can observe the behavior of the birds and find out how some of the birds behaved. dinosaurs, “Lomax said.

An example is a type of scratch made by male ground-nesting birds to indicate that they are strong and good nest builders. It is part of a behavior called lekking, when males, usually in groups, dance competitively and perform other courtship rituals to attract the attention of females.

Dinosaurs had similar mating behavior, according to fossilized “scratches” left in 100-million-year-old rocks in the Dakota Formation of western Colorado. One site revealed more than 60 distinct scratches in a single area up to 50 meters long and 15 meters wide.

“The scratch evidence has significant implications,” said Martin Lockley, emeritus professor of geology at the University of Colorado in Denver, when the study was published in 2016.

“This is physical evidence of prehistoric foreplay that is very similar to (those of) today’s birds. Modern birds that scratch as ceremonial courtship usually do so near their final nesting sites. So the scratch evidence fossil offers a tantalizing clue that dinosaurs in ‘heat’ may have gathered here millions of years ago to reproduce and then nest nearby, “he explained.

Flirting flyers

It is believed that the large bony frill that borders the skull of the dinosaurs Protoceratops, from the same family as him Triceratops, was also used as a signal for potential mates, a recent study of 30 complete skulls suggests.

This is not a feature found in living animals today, and paleontologists have long debated what the function of the diverse array of frills and horns was in ceratopsians. Perhaps, the scientists thought, it was to regulate body heat or for defense.

Three-dimensional analysis showed that the frill formed an independent region of the skull that grew much more rapidly than any other region of the head, a pattern often seen with sexual selection, that is, the idea that certain traits are favored by the opposite sex and because of that, over time, they become more elaborate.

In the case of ProtoceratopsHowever, the researchers concluded that both males and females would have sported the distinctive frill and that it would not have varied dramatically between the sexes.

Dinosaur sex

So what would dinosaur mating actually look like?

While most mammals have separate holes for different bodily functions, many other animals, including birds and reptiles, only have one which is known as the cloaca.

A big clue to understanding dinosaur sex was revealed this year when paleontologists from the University of Bristol and the University of Massachusetts Amherst announced in the journal Current Biology that they had found a dinosaur cloaca belonging to a Psittacosaurus, a dinosaur the size of a farmer.

Most birds mate by “cloacal kisses”, pressing their openings. Some paleontologists think that dinosaurs may have mated like this.

However, Jakob Vinther, a paleontologist and senior lecturer in the Faculty of Earth Sciences at the University of Bristol, believes that the Psittacosaurus male would have had a penis. The fossilized opening is more similar to that of a crocodile, which they have, and some birds, such as ostriches and ducks, which also have a penis.

“From what we can see, this cloaca would not have been suitable for cloacal kissing,” Vinther said. “It looks like it was penetrative sex,” he explained.

But it was the first time the cloaca of a dinosaur had been studied, and much of the mechanics of dinosaur sex defies imagination, especially in the case of creatures like the stegosaurus, with its armored plates and pointed tail.

“If the female doesn’t like the male, and she’s wagging her pointed tail, that’s a problem. Look at the potential angles. It could be that they were moving tail to tail together for a cloacal kiss … a quick encounter and that’s it. everything, “Lomax said.

“It could potentially have been rear-mounted, but I think it’s more unlikely due to friction from the spikes. Another possibility is that the female Stegosaurus could have been lying down and the male mounted from the side. But it’s hard to tell. We don’t really know. the sexual life of these animals, “he explained.