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| Hypothetical reconstruction of Mirischia asymmetrica (left) and Santanaraptor placidus (right) disputing a lizard in what is now northeastern Brazil ~112 million years ago (Early Cretaceous, Aptian). Santanaraptor placidus Kellner, 1999 Mirischia asymmetrica Naish, Martill & Frey, 2004 in Delcourt, Grillo, Hendrickx, Kellermann et Langer, 2025. Santanaraptor Mirischia Illustration by Guilherme Gehr. |
Abstract
The upper carbonate concretion levels of the Romualdo Formation (Aptian, Brazil) have yielded several theropod dinosaur remains, including spinosaurids and the coelurosaurs Santanaraptor placidus and Mirischia asymmetrica, the phylogenetic affinities of which are controversial. Here, we present a comprehensive anatomical reassessment of the holotypes of both species (MN 4802-V and SMNK 2349 PAL, respectively), integrating newly observed osteological features and a detailed comparison of the pelvic and hind limb elements. Our preferred phylogenetic hypothesis places S. placidus and M. asymmetrica in the earliest-branching maniraptoromorph clade, along with Juratyrant langhami and Tanycolagreus topwilsoni from the Late Jurassic of Laurasia, suggesting an early diversification of coelurosaurs in that area, followed by Early Cretaceous dispersal events towards Gondwana. The comparative analysis of the two Romualdo taxa refutes their synonymy, given consistent differences in ischial (position and shape of the obturator plate and foramen) and tibial (condylar configuration) morphology. The observed morphological variation in the ischial obturator plate across early coelurosaurs further highlights a significant degree of homoplasy in this structure during the early radiation of the group. This revision underscores the need for additional research to further resolve the early evolutionary history of coelurosaur theropods.
Keywords: Araripe, Coelurosauria, Dinosauria, Gondwana, Theropoda
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| Hypothetical reconstruction of Mirischia asymmetrica (left) and Santanaraptor placidus (right) disputing a lizard in what is now northeastern Brazil ~112 million years ago (Early Cretaceous, Aptian). Illustration by Guilherme Gehr. |
Santanaraptor placidus Kellner, 1999
Mirischia asymmetrica Naish, Martill & Frey, 2004
CONCLUSIONS:
The comprehensive reassessment of Santanaraptor placidus (MN 4802-V) and Mirischia asymmetrica (SMNK 2349) presented here offers new insights into the anatomy, taxonomy, and phylogenetic affinities of these two coelurosaur theropods from the Early Cretaceous (Aptian) Romualdo Formation of Brazil. Despite the minor overlapping of skeletal elements, both specimens exhibit consistent anatomical differences that support their recognition as distinct species. These differences are particularly evident in the configuration of the ischial obturator plate and the structure of the proximal tibial condyles.
Some of our phylogenetic results place S. placidus and M. asymmetrica together with Juratyrant langhami and Tanycolagreus topwilsoni, from the Late Jurassic of Laurasia, in a clade of early branching maniraptoromorphs. This is the first time that these Brazilian taxa are found closely related within a lesser-inclusive clade, corroborating previous assumptions of close biogeographic relations between Laurasian and Gondwanan landmasses by the Early Cretaceous. Further biogeographic patterns are, however, harder to define, especially having in mind the still patchy record of Jurassic theropods in areas such as Africa, Australia, India, Antarctica, and South America.
Rafael Delcourt, Orlando Nelson Grillo, Christophe Hendrickx, Maximilian Kellermann, Max Cardoso Langer. 2025. The coelurosaur theropods of the Romualdo Formation, early Cretaceous (Aptian) of Brazil: Santanaraptor placidus meets Mirischia asymmetrica. The Anatomical Record. DOI: doi.org/10.1002/ar.70085 [18 November 2025]
