Paleontologists have carried out a comprehensive anatomical re-evaluation of the fossilized remains of two coelurosaurian theropod dinosaurs from the Lower Cretaceous of South America: Calm Santanaraptor And Asymmetrical Mirischia.
Hypothetical reconstruction of Asymmetrical Mirischia (left) and Calm Santanaraptor (right) discussing a lizard in what is now northeastern Brazil, about 112 million years ago. Image credit: Guilherme Gehr.
Calm Santanaraptor And Asymmetrical Mirischia lived in what is now Brazil during the Early Cretaceous, about 112 million years ago.
Holotypes of these coelurosaur species were discovered in the Romualdo Formation in 1996 and 2000, respectively.
“The Romualdo Formation is a world-renowned Lower Cretaceous Konservat Lagerstätte fossil that outcrops along the margins of the Araripe Plateau in northwestern Brazil,” said Dr. Rafael Delcourt of the University of São Paulo and colleagues.
“Its macrofossil record includes some of the most diverse and best preserved ichthyofauna of the Mesozoic, as well as plants, invertebrates, turtles, crocodilians, pterosaurs and theropod dinosaurs.”
“Spinosaurs are arguably the best-known Romualdo theropods, with many referenced specimens including the well-preserved skull of the challenger Irritator and the snout of Aintuji Liu.”
“Among the coelurosaurs, Calm Santanaraptor And Asymmetrical Mirischia come from the same deposits that gave rise to spinosaurs, that is to say the best known upper levels of carbonate concretion of the Romualdo Formation.
In the new research, Dr. Delcourt and his co-authors from Brazil, Argentina and Germany reviewed and compared the anatomy of Calm Santanaraptor And Asymmetrical Mirischiawith the aim of studying their phylogenetic affinities and their possible synonymy.
Their analysis places both species in a group of early branching maniraptoromorph dinosaurs, alongside Juratyran Langhami And Tanycolagreus topwilsoni from the Late Jurassic of Laurasia, suggesting early diversification of coelurosaurs in the region.
Despite the minor overlap in skeletal elements, they exhibit anatomical differences that support their recognition as distinct species.
“The clade containing Juratyran Langhami, Asymmetrical Mirischia, Tanycolagreus topwilsoniAnd Calm Santanaraptor appears to date back at least to the Late Jurassic of Laurasia, before the opening of the Apulian route, which allowed faunal exchanges in the Early Cretaceous,” the paleontologists said.
“In fact, several groups of coelurosaurs are present in both Laurasia and Gondwana at the time, including Megaraptora, as well as branches of Ornithomimosauria, Alvarezsauridae and Dromaeosauridae.”
“Our phylogenetic proposal corresponds better to the available biogeographic data than those relating the Romualdo coelurosaurs to the mainly Laurasian Compsognathidae and Tyrannosauroidea.”
“Yet additional research, including more extensive field work, is needed to unravel the early radiations of coelurosaurs, including the affinity of Romualdo forms.”
An article on the results was published on November 18, 2025 in The anatomical file.
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Rafael Delcourt and others. Coelurosaur theropods from the Romualdo Formation, Lower Cretaceous (Aptian) of Brazil: Calm Santanaraptor encounter Asymmetrical Mirischia. The anatomical filepublished online November 18, 2025; doi: 10.1002/ar.70085
