A British graduate student says he’s found a way to tell the difference between male and female stegosaurs, and perhaps dinosaurs in general. But critics are already attacking the study’s methodology and ethics. “I would have rejected this paper on a number of grounds,” says Kevin Padian, a paleontologist at the University of California, Berkeley.

The study began in 2009 when Evan Saitta, then a high school student, volunteered to help dig up dinosaur bones at a Montana quarry. Over the next several years, Saitta—now in graduate school at the University of Bristol in the United Kingdom—scraped five stegosaurs’ skeletons out of the rock and noticed something odd about their broad, jutting back plates. Most specimens of this particular species, Stegosaurus mjosi, have wide, oval plates, Saitta says. “We had plates like that, but we also had the exact opposite: tall, narrow plates. Something you don’t expect.”

When Saitta studied stegosaur fossils around the world as an undergraduate at Princeton University, he saw the same pattern: Some skeletons had the wide plates; others had the narrow ones. Noticing that his fossil plates from Montana had been evenly divided between the two types, he began to wonder if the plates marked a telltale difference between male and female stegosaurs, perhaps akin to moose antlers.

To support his hypothesis, Saitta went about excluding other possibilities. One alternative was that the plates changed shape over a stegosaurus’s lifetime, and thus had more to do with maturity than with sex. By looking at bone tissue under a microscope, Saitta says that he could tell that wasn’t the case because some of each type of plate had finished growing—hallmark of a mature animal. Based on this evidence, Saitta reports in PLOS ONE today that the plates could be used to sex stegosaurs. Ornamental features on other dinosaurs might also be sexual features, he says.

But Padian says Saitta has misidentified features in his bone tissue sections. “There’s no evidence the animal has stopped growing,” he says. And because the bones Saitta found were in a chaotic heap, Padian says, it’s impossible to use any other bone—such a femur—to figure out whether the dinosaur was an adult or a juvenile at the time of its death. “It’s very good that Saitta considers other hypotheses,” Padian says. “But his data, as presented, are not good enough to test any of them.”

Other critics also challenge the study’s methodology. For example, they note, Saitta used stegosaur skeletons from the Aathal Dinosaur Museum in Switzerland as a model to help sort out the jumble of bones dug out of the quarry. But Ken Carpenter, a paleontologist at Utah State University, Logan, says many of the Swiss specimens aren’t complete. Most of the back plates for those dinosaurs were never found, he says, so their bones are crowned with replicas from other dinosaurs. So, it could be that S. mjosi sported both round, large plates and long, thin ones on different parts of its back. Fossils like that have been found, Carpenter says—a fact he says Saitta ignores. “I applaud what he’s doing,” Carpenter says. “But he’s building on evidence that’s rather shaky.”

Saitta says he’s aware of the reconstruction done at the Aathal Dinosaur Museum, but he says that enough back plates were found on each of the stegosaurs there to support his hypothesis. As for the bone tissue slides, Saitta stands by his interpretation. “All one has to do is match images with the two in my study and see the similarity,” he says.

Padian and other critics also cite problems with the bones that Saitta excavated in Montana. They’re the property of a commercial fossil collector named Nate Murphy, the director of a company called the Judith River Dinosaur Institute. “These are private specimens and the ethics of our profession are you do not publish on them,” Padian says.

Some paleontologists worry that fossils in private hands could be auctioned off to the highest bidder and disappear from the scientific literature forever. Other privately owned fossils have faced the same controversy.

“It doesn’t matter that they call themselves the Judith River Dinosaur Institute or whatever the hell that is,” Padian says. “Those specimens are not in the public trust.”

But Saitta says being able to study dinosaur bones matters more than who owns them. “These stegosaurs are accessible and always will be accessible,” he says. “I want people to focus on the science.”