Study: Megarraptors dominated the dinosaur community of Chilean Patagonia 70 million years ago

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 Artistic interpretation of Patagonia during the Upper Cretaceous. Animals depicted include non-avian dinosaurs, birds and other vertebrates that have been discovered in the fossil record of the region. Credit: Mauricio Álvarez and Gabriel DíazArtistic interpretation of Patagonia during the Upper Cretaceous. Animals depicted include non-avian dinosaurs, birds and other vertebrates that have been discovered in the fossil record of the region. Credit: Mauricio Álvarez and Gabriel Díaz
By Camila Buvinic
The enigmatic ankylosaur Stegouros elengassen, which was on the cover of Nature magazine in December 2021; primitive mammals such as Magallanodon baikashkenke and Orretherium tzen; frogs belonging to the Calyptocephalellidae group and the genus Kuruleufenia; and turtles of the genus Yaminuechelis, are part of the described fauna that inhabited Chilean Patagonia toward the end of the age of dinosaurs, a record to which the discovery of remains of sauropods and other herbivorous dinosaurs is added. Despite the numerous paleontological discoveries made to date in this area near Torres del Paine, in the Magallanes region, a fundamental piece of this ecosystem had yet to be identified: the predators that dominated the food chain in this Cretaceous world.
This is the contribution of a study prepared by researchers from the University of Texas, the University of Chile, the University of Concepción, the Chilean Antarctic Institute (INACH) and the National Museum of Natural History, based on fossil pieces rescued in the Las Chinas River Valley sector in paleontological campaigns organized by INACH and carried out between 2016 and 2020. The work, recently published in the Journal of South American Earth Sciences, describes a series of theropods (meaning beast foot), a suborder of dinosaurs -mostly carnivores- whose most popularly known representatives are Tyrannosaurus rex and Velociraptor. In fact, all modern birds are also theropods that evolved into feathered, flying forms with larger brains. 
The remains found in Chilean Patagonia correspond mainly to teeth and postcranial bone remains that show a diverse community of carnivorous theropods that lived there between 66 and 75 million years ago, a time when the geography of this area had a large river delta and abundant vegetation. Marcelo Leppe, director of the Chilean Antarctic Institute and co-author of the article, emphasizes that the present work is an enormous advance in the understanding of the enigmatic paleontological complex of Cerro Guido-Las Chinas, a site in southern Patagonia that contains the last 20 million years of the Age of Dinosaurs, and adds that this impressive locality holds several other records, "for it is the southernmost Late Cretaceous locality with continental deposits (rivers, lakes, marshes and coastal deltas) in the world, as well as being a place that concentrates successive stages of the history of the physical connection between Patagonia and Antarctica that occurred intermittently between 83 and 66 million years ago."
Many questions still remain about this mysterious window into the past, the researcher assures, as these environments were under immense pressure from abrupt global cooling and temperature rises, fluctuating ocean levels, massive volcanic eruptions in India (which was east of Africa at the time) and culminating with the meteorite impact on the Yucatan Peninsula 66 million years ago, which produced the third largest extinction of life in all of natural history. "We still need to know how life made its way into that apocalyptic scenario, to give rise to our typical southern environments of South America, New Zealand and Australia, where theropods are still present, no longer with dinosaurs as impressive as the megarraptorids, but with a diversity of birds still living, after 70 million years, in forests, swamps and marshes of Patagonia, Antarctica and Australia," says Leppe.
The findings, which represent the southernmost fossil record of this type of dinosaur outside Antarctica, provide relevant information on the diversity of the fauna that inhabited the southernmost part of the world near the end of the age of dinosaurs, which occurred 66 million years ago.
Sarah Davis, a recent PhD graduate from the University of Texas at Austin, has collaborated with the University of Chile and INACH on this and other work since 2017. "The fossils we describe in this work are a further step in the knowledge of the diverse vertebrate fauna of the Las Chinas Valley and provide important data for studying the distribution of theropods in high southern latitudes. In addition to providing evidence for the coexistence of large and small predatory dinosaurs in the region during this time, we also describe remains of two distinct avian lineages during a key period for unraveling avian evolution. I would like to emphasize that this includes the southernmost South American records of the group of birds that includes the birds we have today, the only dinosaurs that survived the late Cretaceous mass extinction. We hope that work in this sector will help clarify the patterns of extinction and survival of dinosaurs, including the bird groups," says Davis.
Maip macrothorax, the world's largest raptor dinosaur.Maip macrothorax, the world's largest raptor dinosaur.
The discovery of these pieces suggests that at the top of the food chain of this ecosystem would have been the megarraptors, predators similar to the famous tyrannosauroids, which could have reached 6 to 10 meters in length and weighed over a ton. "The postcranial evidence we have, mainly in the Chorrillos Formation in Argentina, indicates that these animals were large. We are talking about six or seven meters to nine or ten meters in length. Therefore, it is possible to expect that a large carnivorous dinosaur may have existed in Las Chinas. The main characteristic of its general morphology is that megarraptors have elongated skulls and large arms with powerful claws," said Jared Amudeo, researcher of the Paleontological Network of the University of Chile.
Dental remains from the middle and posterior part of the snout, belonging to different specimens, were part of the finding analyzed in this study. "One of the characteristics that allowed us to identify with great confidence that they belong to megarraptorids are, first of all, that the teeth are very curved towards the posterior part. Second, the distal part of the teeth, which face the back of the snout, have many serrations, which are like a knife used to cut meat, and the mesial part does not. This is one of the main characteristics of megarraptors," says Amudeo about these megarraptors, which are directly related to other South American species such as Orkoraptor burkei or Megaraptor namunhuaiquii.
The University of Chile researcher also explains that megarraptors are a very enigmatic clade of dinosaurs that, according to recent studies, would be related to tyrannosauroids and would have coexisted with the ankylosaur Stegouros elengassen, a species that was probably part of their diet. "Presumably the megarraptors hunted Stegouros, since it is a very abundant taxon in Las Chinas. We have found other dental crowns from the same levels of Stegouros, which are not published, but they are also from megarraptorids and coexisted in the same time span. It is also likely that it fed on ornithopods that have been found abundantly in Argentina, but we do not yet have direct evidence of this."
Digging up fossils at Cerro Guido, in Chilean Patagonia. Photo: INACHDigging up fossils at Cerro Guido, in Chilean Patagonia. Photo: INACH
They identified some teeth of the subfamily Unenlagiinae, which despite being non-avian dinosaurs probably had their entire body covered with feathers.They identified some teeth of the subfamily Unenlagiinae, which despite being non-avian dinosaurs probably had their entire body covered with feathers.
The avian factor
Jared Amudeo, who is currently studying for a master's degree in biology at the University of Chile, identified some teeth of the subfamily Unenlagiinae, which, despite being non-avian dinosaurs, probably had their entire body covered with feathers. The researcher, who in his short career has specialized in dental fossils, highlights the importance of the discovery of two specimens from the Unenlagiinae group, which represent the southernmost record of this peculiar group of theropods closely related to velociraptors and similar in appearance to them, as well as to other species that inhabited Gondwana such as Buitreraptor and Austroraptor, which reached more than six meters in length.
"One is possibly an unenlagen. Possibly, because there are some traits that are shared with described unenlagids, but it has others that are not common. It could be a novel evolutionary character that would tell us that this is a new species of unenglagine or perhaps a representative of a different clade. On the other hand, we have an unambiguous unenlagen, which also most likely represents another new species. There are many teeth that I reviewed, which are not published in this study, that present a uniqueness. This is probably another novelty of these specimens from Las Chinas, which allow us to suggest that what we can find in this area is different from what has already been published in Argentina and Brazil,”  emphasizes the member of the Paleontological Network of the University of Chile.
This work also made it possible to identify two groups of birds that lived during the Dinosaur Era: Ornithurinae and Enantiornithes. The former correspond to a group directly related to present-day birds and similar in appearance and size to a huala or a sage-grouse, also known as a loon or loon in North America. Enantiornithes, meanwhile, was the most diverse and abundant group of birds of the Mesozoic, "they are relatives of modern birds, but they are not the group that gave rise to them. They had a general shape similar to a sparrow, but with teeth," explains Jared Amudeo. "The important thing is that the molecular clocks of divergence of origin of modern birds is in this time span. The presence of birds, but not current birds, in this area and during this time gives us more clues that important things could be found here in this part of South America, for example, one of the direct common ancestors of today's birds," he adds.
For Sarah Davis, it is exciting that the youngest theropod fossil found at the site so far is from an ornithischian bird, the group of birds that includes the birds we know today. The presence of this group at such high latitudes, such as Las Chinas and parts of Antarctica, may point to these places as possible refuges or safe places for birds during the mass extinction, where they could escape the worst of the catastrophe and survive to become the groups we know today.  

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