Telling a dinosaur’s sex is no easy task, research shows

The images are vivid: mother dinosaurs guarding their nests, males fighting each other for females or territory. But how do you tell the sex of a dinosaur?

Despite many claims to the contrary, recent research published in the journal Paleobiology by Jordan C. Mallon of the Canadian Museum of Nature shows that, for the most part, we can’t.

One exception is the presence of medullary bone.

When a female bird becomes pregnant, she produces a lacework of bone called medullary in the large hollow spaces of her thighbones. Toward the end of pregnancy, the medullary bone is absorbed and its calcium is used to make the shells for her babies.

Fossil medullary bone has been found only a few times, in a specimen, in an Allosaurus, in two iguanodontid dinosaurs and in a Mesozoic bird.

That’s it. Five specimens out of thousands that have been studied over the years.

Of course, fossil medullary bone would be extremely rare — it occurs only in females and for only a few weeks during pregnancy.

One of the most famous discoveries of supposed sexual dimorphism (differences in appearance between males and females) among dinosaurs were the hadrosaurs, the so-called “duck-billed” dinosaurs of the Late Cretaceous in western North America.

Differences in their head crests were thought to indicate whether a specimen was a male or female. That was until better data using rock layers showed that these “males” and “females” lived at different times and probably were different species, not different sexes.

The recent study looked at nine kinds of dinosaurs in which paleontologists suggested they could detect sexual dimorphism.

One was the famous deposit in New Mexico containing hundreds of skeletons of the small carnivorous dinosaur Coelophysis of the Triassic Period. Another was Protoceratops, the small herbivore from Mongolia. Scientists thought differences in the shape of its head shield marked sexual differences.

Others included Tyrannosaurus, in which limb or tail bones were thought to indicate differences; Stegosaurus, in which orientation and shape of plates were thought an indication; Allosaurus, with its jaw shape and tooth count; and Plateosaurus’ thigh bones.

However, none of the evidence of sexual dimorphism held up under close statistical scrutiny. Differences could be attributed to things such as age, injuries and disease, preservation and individual variation.

The most common problem was sample size, which for most dinosaur species just isn’t good enough. Mathematical modeling shows that even differences as large as 28 percent might not show up in sample sizes as large as 100 specimens, a number seldom reached among dinosaurs.

As the author points out, this is not to say that dinosaurs were not sexually dimorphic — both crocodiles and birds, their closest living relatives, certainly are — only that the available evidence is not sufficient to demonstrate it.

For all we know, even the most famous tyrannosaur skeleton might have been, as Johnny Cash sang, a boy named Sue.