Senior thesis: Sexual differences in Stegosaurus
Under normal circumstances, Evan Saitta's senior thesis would seem ambitious.
The Princeton University ecology and evolutionary biology major has painstakingly analyzed 150 million-year-old fossils to determine whether a single anatomical difference found in a species of Stegosaurus indicates if an individual was male or female. For readers who lacked a childhood obsession with dinosaurs, Stegosaurus is a small-headed, large-bodied herbivore that stomped around what is now the North American West with a clutch of large spikes at the end of its massive tail and twin rows of substantial bony plates adorning its back.
Unusual for Stegosaurus, the back plates of the species Saitta studies, Stegosaurus mjosi, come in two varieties — short and wide, and tall and narrow. Saitta's hypothesis is that the distinctive plates were determined by the animal's sex. Females had one type of plate, and males the other. The work, which he plans to publish, could provide a new understanding of the physiology and lifestyle of Stegosaurus.
To support his idea, Saitta has dug up fossils in Montana every summer since his junior year of high school, examined Stegosaurus specimens in Switzerland, and pored over slides of late-Jurassic bone tissue in Princeton's Guyot Hall. A Montana medical clinic even let him use its X-ray computed tomography, or CT, scanner to identify the remnants of channels that once house blood vessels in the plates.
While Saitta acknowledges the scale of the challenge he undertook, if any feelings of frustration or fatigue exist they're undetectable beneath his pronounced enthusiasm for dinosaurs and natural history.
"This is what I want to do for the rest of my life," said Saitta, who will go on to study paleobiology at the University of Bristol in the United Kingdom. "It's definitely quite a challenge, but I'm confident it will all come together. Although a lot of this stuff is new, I think the general reception among people who study Stegosaurus has been pretty good."
James Gould, a professor of ecology and evolutionary biology and Saitta's adviser, said that Saitta's work could help establish understanding of a social system unique among dinosaurs — and of interest to modern ecologists — in which mate selection hinged on males attracting females rather than fighting other males for mates.
The sizable effort Saitta's thesis required was matched by his passion for the topic, Gould said.
"Evan is able to infer social structure from fossils of animals that have been dead for tens of millions of years. Moreover, it is the first instance of female choice in dinosaurs [wherein the female selects the mate], which is a mate-choice system of special interest to behavioral biologists like me," Gould said.
"It has required incredible hard work, the ability to get individuals and medical institutions to give him time and support, and remarkable vision. It's the sort of thesis we see in the department only once every five or 10 years," he said. "His enthusiasm is infectious, and it is a testament to my willpower that I was not out West with him this year digging up bones."
Saitta was a junior in high school in his native Jacksonville, Fla., when his thesis first took shape. He had volunteered to work at a site boasting an unusually rich collection of Stegosaurus remains in the foothills of the Little Snowy Mountains in central Montana. About two hours north of Billings, the site is in the northern extremes of the Morrison Formation, a massive wealth of Jurassic-period dinosaur fossils that extends southward from northern Montana to the middle of New Mexico, and stretches longitudinally from Utah to Kansas.
"I enjoyed it so much that I continued to go to Montana and developed a project. Even coming into Princeton I knew I wanted to develop a thesis based on this site," Saitta said. "In more than 100 years of studying Stegosaurus, this site is the first time that we have multiple [Stegosaurus] individuals in one location with no other animals around. These animals died together at that site, which suggests that they were part of a social group or 'herd' at the time of death. Even before people got into it, they knew there was something special about this site."
One special feature is the abundance of remains from an early species of Stegosaurus, the S. mjosi. First described in 2001 (and originally classified as Hesperosaurus), S. mjosi is distinguishable from later stegosaurs by its tail vertebrae, which have a teardrop shaped top as opposed to the two-pronged, or bifurcated, surface documented in other species. The animal's back plates also are not uniform, which is unusual. While other species have distinct plates, those of S. mjosi come in the two varieties that Saitta would study.
"It's the first time that multiple individuals were found together and we have two almost opposite kinds of plates, and you can see the difference from neck to tail," Saitta said.
"Every bone but the plates is similar, so it's one species," he said. "We have evidence that the one species of Stegosaurus comes in two varieties, so the question is what exactly causes this variation? My thesis was to eliminate alternative hypotheses because when you eliminate all other possibilities you're left with the one that is correct."
Drawing from a 2011 Japanese-led study outlining the analysis of Stegosaurus bones, Saitta set out to eliminate other possible causes of the plate variety. Using a CT scanner at the Billings Clinic, Saitta scanned 10 S. mjosi plates in search of "vascular piping" that had brought blood to the bone tissue beneath each plate's hard keratin exterior. Because Stegosaurus plates are thought to have partly served as a mating display, the Japanese study found that the presence of large, vascular piping within the plate did not appear until the individual reached sexual maturity. Once Saitta found the piping in all 10 plates, he knew that they all belonged to sexually mature individuals.
The next step was to further refine the age of the individuals. Dinosaurs age in a way that resembles both reptiles and mammals, Saitta said. They grow rapidly like mammals, but like reptiles, their bones exhibit internal layers that mark each year of growth, similar to tree rings. When an animal's full growth is reached, the rings become a tightly packed, less vascular outer layer called an "external fundamental system," or EFS. Saitta had three, 2-centimeter-square samples of each plate transferred to microscope slides. He saw reduced growth in both varieties of plates and an EFS in two of the tall and narrow plates. This established that the tall- and wide-plated morphs plates from the central Montana site came from both sexually mature, young adults and fully-grown, old adults.
"By eliminating the idea that these are two different species or that the morphs are young and old, that leaves just one other possibility — these are male and female," Saitta said.
The next step for Saitta is to determine which plate shape belongs to which sex — and he has a theory. He thinks the tall, narrow plates belonged to females, who would have needed the pointier plates to defend against predators. On the other hand, the plates of male S. mjosi were much larger in surface area and likely served as "billboard" displays intended to attract females, similar to the plumes of the male peacock, Saitta said.
Solving the mystery would reveal important information about Stegosaurus's social habits, such as whether males and females separated when not breeding. That would explain the puzzling absence of fossil sites with multiple individuals, Saitta said.
"Once you determine something as simple as sexual dimorphism [the distinction between two sexes], you can start to investigate all these other factors of [Stegosaurus] biology," Saitta said. "It's no longer saying they were just like other modern herbivores. That doesn't really tell you much because there are such a variety of those. We could start to narrow it down."
In the meantime, Saitta has analyzed 126 Stegosaurus plates to explore whether their shape is species specific or determined by an as-yet-undiscovered distinction between the sexes. He found no convincing evidence that any other species of Stegosaurus showed sexual dimorphism. This suggests that different species of Stegosaurus likely had different mating or social systems as well as different levels of predation risk. The analysis was a "reevaluation" that falls in line with his habit of questioning the details of what he sees and looking for new answers, an inclination that motivated his thesis.
"Princeton doesn't have an expert on dinosaur anatomy, but that's good because I wanted to come at it from a fresh perspective," Saitta said. "I was just always thinking about and reevaluating what I saw, so when I sat down to write I knew what I wanted to say."