A new study by researchers at Yale and the University of Oxford has added crucial information to the understanding of the origins of sauropods, which are dinosaurs known for their long necks, small heads, tree-trunk-like legs and large bodies.
Sauropods are one of the world’s best-known species of dinosaur — as one of two dinosaur species immortalized in emoji form — and were named by Yale paleontologist O.C. Marsh in 1878. The study on sauropods was published in February in Evolution: International Journal of Organic Evolution and found that the early forms of the sauropod species had characteristics of earlier dinosaurs at birth and only developed their unique skull features as adults. But as new species evolved, those unique cranial features were now found to emerge earlier in the developmental process, when the dinosaurs were in the embryonic or juvenile stage — a phenomenon known as predisplacement.
“This is the first time this process, which is called predisplacement, has been demonstrated convincingly among dinosaurs, and one of the few times it has been demonstrated in the fossil record,” first author of the study Matteo Fabbri GRD ’21, a doctoral student in Yale’s Department of Earth and Planetary Sciences, wrote in an email to the News. “Ultimately, we show how different groups of dinosaurs evolved different developmental strategies.”
According to Fabbri, the researchers decided to analyze the skulls of the Anchisaurus species — the ancient precursor species to the sauropod — because of the many questions surrounding a large part of the species’ skull, including the palate and the lower part of the braincase. The scientists wanted to study those skull elements in addition to features of sauropod skulls in order to fully understand the anatomy and the evolutionary relationships of species from the Anchisaurus to the sauropod.
Using a micro-CT scanner, the researchers digitally dissected and reassembled the delicate specimens without risking any mechanical damage.
“Yale happened to have these amazing specimens of Anchisaurus, which, upon examination, we quickly began to suspect capture a transition point between the small, fairly unremarkable early members of the sauropod line and their behemoth descendants, the sauropods proper,” senior author Bhart-Anjan Bhullar ’05, assistant professor in the Department of Earth and Planetary Sciences and assistant curator at the Yale Peabody Museum of Natural History, wrote in an email to the news. “Moreover, amazingly, we had baby and adult skulls of the [Anchisaurus] — an amazingly rare circumstance when dealing with fossils of this age. This digital dissection and reassembly was especially important for the baby skull, which turned out to include many small, thin bones that could easily have been damaged by mechanical preparation.”
The team’s modeling produced some unexpected results, according to researchers. Bhullar explained that they discovered that several distinctive features of the sauropod skull — such as the scoop-like face, buck teeth and the tilted jaw muscles — were already present in Anchisaurus, but only appeared after it had hatched. In contrast, these features are present in the earliest pre-hatching embryos of true sauropods.
Bhullar explained that they had no reason to expect that there would be a transformation of timing in the appearance of those features throughout the development of the two species.
“I think the biggest surprise was just how different the adult and juvenile skulls look,” co-author and graduate student Michael Hanson GRD ’21 wrote in an email to the News. “The adult’s skull has a boxy look like those of other early sauropodomorphs that lived around the same time, but is also tiny compared to those much larger species. Meanwhile the juvenile specimen has a very round head with large eyes and resembles the skulls of some of the earliest dinosaurs known, that difference was completely unexpected.”
Bhullar, Hanson and Fabbri all referred to the process of piecing the skull bones together and analyzing them as the most challenging part of the experiment.
“The Anchisaurus specimens that we were looking at had some bones that exhibited some degree of distortion so it was important to try to manipulate the 3D digital model to remove some distortion to ensure that they fit together, but also making sure these modifications could be justified,” Hanson wrote. “Additionally, 19th Century preparators often ended up abrading away a lot of bone from the specimen, which led to a lot of missing regions, that made them hard to piece together. It was also obvious that our reconstruction was not going to look much like earlier reference diagrams that previous researchers studying this specimen drew by hand, so we were on our own in that regard too.”
Bhullar emphasized that his personal philosophy favors projects that will yield “complete stories” instead of just turning up small pieces, which is reflected in the way every single bone piece his team studied was meticulously segmented and reassembled.
Going forward, the researchers are interested in delving deeper into figuring out when the shift in timing of the appearance of distinctive sauropod skull features in Anchisaurus occurred. They are also interested in studying if the shift was a sudden process and if it affected only the head of the animal. Further, they seek to learn more about the growth patterns in the Anchisaurus’ body to see if they were in fact a dwarf species related to the giant sauropods.
Dinosaurs are a diverse group of reptiles that are thought to have first appeared during the Triassic period, between 243 and 233.23 million years ago.
Mai Chen | firstname.lastname@example.org