Aided by strong jaws that could munch on nearly any foliage and supported by herd dynamics that included parental care, duck-billed hadrosaurid dinosaurs' global dominance contributed to a noticeable decline in herbivore diversity — or so it was thought until now.
The unearthing of a newly identified species challenges this long-held theory about hadrosaurids' cross-continental spread, according to research published in Science Advances.
"Our discovery clarifies there is no conclusive evidence that hadrosaurids ever reached as far as southern Patagonia and Antarctica. Instead, the only reliable evidence this far south is for the presence of a different, older lineage of duck-billed dinosaurs," said Alexander Vargas, a paleontologist at the Universidad de Chile and one of the study's corresponding authors.
Vargas and his colleagues now propose that this animal called Gonkoken nanoi appeared in what is now South America long before the emergence of hadrosaurids, which are sometimes called advanced duckbills.
"Transitional duckbills like Gonkoken survived as relicts in the far south until the last age of the dinosaur era," said Jhonatan Alarcón-Muñoz, a paleontologist at the Universidad de Chile and the first author as well as one of the corresponding authors of the study.
Planet of the Duckbills
As the Mesozoic Era ended but before an asteroid launched the Cretaceous-Paleogene extinction event roughly 66 million years ago, hadrosaurids surged in numbers, taking food and resources from other plant-eating dinosaurs. Scientists have documented the Hadrosauridae family's ascendancy especially well through examinations of fossils found in the North Hemisphere.
"In the Northern Hemisphere, their number of species rose steadily while that of other dinosaurs declined, especially other herbivores such as armored ankylosaurs, the horned ceratopsians, and earlier lineages of transitional duckbills," said Alarcón-Muñoz. "This has led to the suggestion that advanced duckbills were likely outcompeting other dinosaurs."
Yet, less is known about how much these animals affected dinosaur diversity in the Antarctic and subantarctic South. Some prior analyses found alleged remains of hadrosaurids in southern Patagonia and Antarctica, but those may need to be re-evaluated, the authors said.
"The pre-extinction ecosystems of southern continents are very poorly known," said Vargas. "We do not know if dinosaur diversity was changing shortly before the impact."
Based on the newfound evidence, the team said it seems less certain that advanced duckbills actually made it to the subantarctic, much less surpassed other dinosaurs including Gonkoken that had been living in the region for longer.
At the onset of their excavation in the Magallanes Region of Chile during 2013, Vargas and Alarcón-Muñoz were merely hoping to learn more about what hadrosaurid types existed in the subantarctic. They received quite a surprise in the form of 45 skeletal parts from three individual dinosaurs' duckbill-like remains that were skeletally and dentally distinct from hadrosaurids.
"Among other primitive features, our dinosaur had a smaller body size at about 3.5 to 4 meters long, whereas advanced duckbills were often double that size or positively gigantic at even 15 meters," said Alarcón-Muñoz.
In addition to its small stature, Gonkoken had fewer rows of teeth and less developed muscle than its advanced duckbill counterparts. Its unique morphology indicates it probably came to the ancient southern Gondwana landmass roughly 130 million years ago, before the Hadrosauridae family began to diverge. How it did so was at first a mystery.
"All possibilities to understand how its ancestors got there involve very long distances and marine barriers that blocked the way for most terrestrial species," said Vargas.
By constructing a duck-billed dinosaur family tree, synthesizing that data with maps of fossils and conducting statistical analyses, the group determined that Gonkoken's ancestors moved down from North America. Once in the remote, isolated area, Gonkoken would have lived with little struggle for resources up until the mass extinction began.
"Gonkoken's ancestors could have reached South America before the ancestors of advanced duckbills: a time advantage that allowed them to reach more to the south, where advanced duckbills did not have enough time to arrive before the mass extinction," said Vargas. "The Gonkoken lineage would have managed to survive until the very end of the dinosaur era."
Need for Research in Underexplored Regions
The fossilized remains of Gonkoken have a twofold impact. They indicate that hadrosaurids were not as geographically widespread as was once speculated. Perhaps this ecologically prominent taxonomic family did not make it to the subantarctic after all.
"It becomes clear that [a] tooth discovered in Antarctica and other fragmentary remains in southern Patagonia may not belong to advanced duckbill dinosaurs but may well correspond to transitional forms such as Gonkoken," said Vargas. "In short, there is no reliable evidence that advanced duckbills ever reached this far south."
Essentially, the results necessitate a new look at how the ecologically prominent taxonomic family diversified through space and time.
"This cautions against drawing conclusions at a global scale, from changes that are only documented in northern continents. Distant regions of the world were likely not undergoing comparable transformations," said Vargas.
Beyond questioning canonical ideas about the Hadrosauridae, Gonkoken exemplifies the need for more research in a frequently underexplored region of the world.
"Most countries in the global south have smaller research communities, with less access to funding," said Alarcón-Muñoz. "This could also be an artifact of this anthropogenic bias, for lack of more people and resources dedicated to exploration," said Alarcón-Muñoz. He described how roughly 10 years ago, when excavations started, no one expected to find much of anything besides marine deposits in subantarctic Chile.
Efforts to find and examine fossils in the subantarctic — and other places across the globe similarly overlooked — could continue to yield unexpected insights concerning what creatures roamed Earth millions of years ago.
"The bed bone of Gonkoken shows no signs of running out, and excavations continue every year since more specimens allow us to fill in missing body parts that remain unknown to us," said Vargas.
The authors also plan to 3D-print a reconstructed skeleton of Gonkoken. Modeling this diminutive yet transformative dinosaur will provide people with a valuable opportunity to experience their "paleontological heritage," Vargas added.