Paleontologists have conducted a comprehensive anatomical reassessment of the fossilized remains of two coelurosaur theropod dinosaurs from the Early Cretaceous of South America: Santanaraptor calm And Myriskhia asymmetrical.
Hypothetical reconstruction Myriskhia asymmetrical (left) and Santanaraptor calm (right) A dispute with a lizard in what is now northeastern Brazil approximately 112 million years ago. Image credit: Guilherme Ger.
Santanaraptor calm And Myriskhia asymmetrical lived in what is now Brazil during the Early Cretaceous, about 112 million years ago.
Holotypes of these coelurosaur species were discovered in Romualdo Formation in 1996 and 2000 respectively.
“The Romualdo Formation is a world-famous Early Cretaceous Konservat Lagerstätte fossil that occurs on the margins of the Araripe Plateau in northwestern Brazil,” said Dr. Rafael Delcourt from the University of São Paulo and colleagues.
“Its macrofossil record includes one of the most diverse and well-preserved ichthyofaunas of the Mesozoic, as well as plants, invertebrates, turtles, crocodiles, pterosaurs and theropod dinosaurs.”
“Spinosaurs are perhaps Romualdo's best-known theropods, with many referenced specimens including the well-preserved Irritator challenger skull and snout Aintuji Liu“
“Among the coelurosaurs Santanaraptor calm And Myriskhia asymmetrical come from the same sediments from which spinosaurs originated, that is, from the better known upper levels of the carbonate nodule of the Romualdo Formation.”
In a new study, Dr Delcourt and his co-authors from Brazil, Argentina and Germany reexamined and compared the anatomy Santanaraptor calm And Myriskhia asymmetricalin order to study their phylogenetic similarity and possible synonymy.
Their analysis places both species in the early branching group. Maniraptoromorph dinosaursalong with Yuratirant Langhami And Tanikolagreus topvilsoni from the Late Jurassic of Laurasia, suggesting early diversification of coelurosaurs in the area.
Despite the slight overlap of skeletal elements, they have anatomical differences that allow them to be recognized as separate species.
“The treasure containing Yuratirant Langhami, Myriskhia asymmetrical, Tanikolagreus topvilsoniAnd Santanaraptor calm appears to date back to at least the Late Jurassic of Laurasia, before the discovery of the Apulian Route, which allowed for the exchange of fauna in the Early Cretaceous,” the paleontologists said.
“In fact, there were several groups of coelurosaurs living in Laurasia and Gondwana at this time, including Megaraptora, as well as the clades Ornithomimosauria, Alvarezsauridae and Dromaeosauridae.”
“Our phylogenetic proposal fits the available biogeographic data better than those that associate Romualdo's coelurosaurs with the predominantly Laurasian Compsognathidae and Tyrannosauroidea.”
“However, further research, including more extensive field work, is needed to untangle the early coelurosaur radiation, including the affinities of Romualdo forms.”
A paper the findings were published on November 18, 2025 in Anatomical recording.
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Raphael Delcourt etc.. Coelurosaur theropods of the Romualdo Formation, Early Cretaceous (Aptian) of Brazil: Santanaraptor calm meets Myriskhia asymmetrical. Anatomical recordingpublished online November 18, 2025; doi: 10.1002/ar.70085
