Paleontologists have undertaken a thorough anatomical reevaluation of the fossilized remains of two coelurosaurian theropod dinosaurs that inhabited South America during the Early Cretaceous period: Santanaraptor Placidus and Milischia asymmetrica.
Hypothetical reconstruction of Milischia asymmetrica (left) and Santanaraptor Placidus (right), focusing on lizards that existed in what is now northeastern Brazil approximately 112 million years ago. Image credit: Guilherme Gehr.
Santanaraptor Placidus and Milischia asymmetrica thrived in Brazil during the Early Cretaceous period, roughly 112 million years ago.
The holotypes of these coelurosaurs were discovered in Romualdo Formation in 1996 and 2000, respectively.
Dr. Rafael Delcourt from the University of São Paulo and his team stated, “The Romualdo Formation is a renowned Early Cretaceous fossil hotspot situated along the edge of the Araripe Plateau in northwestern Brazil.”
“Its megafossil record encompasses plants, invertebrates, turtles, crocodiles, pterosaurs, theropod dinosaurs, as well as one of the most diverse and well-preserved ichthyofaunas from the Mesozoic era.”
“Spinosaurids are among the most notable Romualdian theropods, with numerous reference specimens including the well-preserved skull of Illitator Challenger and the snout of Illitator Challenger Angaturama Limai.”
“Both Santanaraptor Placidus and Milischia asymmetrica originate from the same deposits that produced Spinosaurus, specifically the upper carbonate condensation level of the renowned Romualdo Formation.”
In their new study, Dr. Delcourt and collaborators from Brazil, Argentina, and Germany revised and compared the anatomy of Santanaraptor Placidus and Milischia asymmetrica to examine their phylogenetic relationships and potential synonymy.
Their analysis positions both species within an early-divergent group of maniraptoromorph dinosaurs, alongside Juratirant Rangami and Taniko Lagreus Topwilsoni, dating back to the Late Jurassic of Laurasia, indicating an early diversification of coelurosaurs in this region.
While there is some skeletal overlap, notable anatomical differences affirm their classification as separate species.
“The clade containing Juratirant Rangami, Milischia asymmetrica, Taniko Lagreus Topwilsoni, and Santanaraptor Placidus appears to extend back to at least Laurasia in the Late Jurassic, prior to the formation of the Apulian sea route that facilitated faunal exchange in the Early Cretaceous,” noted the paleontologists.
“At that time, a variety of coelurosaurs existed in both Laurasia and Gondwana, including Megaraptorra and branches such as ornithomimosaur, alvarezaurid, and dromaeosaurid.”
“Our phylogenetic model aligns more closely with the existing biogeographical data than one that confines Romualdocoelurosaurids primarily within Laurasian compsognathids or tyrannosaurids.”
“However, further studies and extensive fieldwork are necessary to clarify the early radiation of Coelurosaurus and its affinities with Romualdo morphology.”
A paper detailing these findings was published on November 18, 2025, in Anatomical Records.
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Rafael Delcourt and colleagues. Coelurosaurian theropods of the Romualdo Formation of the Early Cretaceous (Aptian) of Brazil: Santanaraptor Placidus meets Milischia asymmetrica. Anatomical Records, published online on November 18, 2025. doi: 10.1002/ar.70085
Source: www.sci.news
