Coelurosaurs of Romualdo Formation, Early Cretaceous of Brazil (free pdf)

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

A new paper:

Free pdf:

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 (advance online publication)
doi: https://doi.org/10.1002/ar.70085
https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.70085

Free pdf:
https://anatomypubs.onlinelibrary.wiley.com/doi/epdf/10.1002/ar.70085

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.

=====

[Mickey Mortimer]

Very cool paper. Nice to finally get a detailed description of Santanaraptor after... twenty-six years. 

Also good to have official PhyloCode definitions for some major coelurosaurian clades. All look good except Eutyrannosauria, which is defined as "The largest clade containing Tyrannosaurus rex Osborn 1905, but not Megaraptor namunhuaiquii Novas 1998." It was previously defined (by this work's first two authors) as (Dryptosaurus aquilunguis + Tyrannosaurus rex). Both end up having somewhat similar taxonomic composition in this tree, but that's because Delcourt et al. have the currently unusual result of maniraptoromorph proceratosaurids (including Dilong). Note their trees also have Eotyrannus and Yutyrannus as eutyrannosaurs, which would not match the earlier definition. Maniraptoromorph proceratosaurids and Dilong have been recovered in some past analyses (mostly early 2000s), but I do wonder if adding taxa like Xiongguanlong and Suskityrannus might bring them back to the tyrannosauroid fold. Juratyrant is either a megaraptoran (Equal Weights) or a basal maniraptoromorph (Implied Weights), which has happened in the past with the usually related Eotyrannus. In any case, I think it would have been good to add Dilong as an external specifier of Eutyrannosauria, because right now Eutyrannosauria becomes a junior synonym of Tyrannosauroidea in many current phylogenies. 

The analysis has a lot of other interesting parts- Eoraptor as a theropod for the first time in a long time, Marshosaurus as the most basal tetanurine (either with zigongensis in EW or Condorraptor in IW), Yuanmouraptor and Yunyangosaurus as piatnitzkysaurids (EW), Leshansaurus as a spinosaurid, spinosaurids closer to birds than megalosauroids (IW; the 1990s called...), Metriacanthosaurus outside Avetheropoda (EW), Coelurus as an ornithomimosaur...
Like the Lori tree it has Tugulusaurus as a compsognathid, and Deltadromeus and Gualicho as ornithomimosaurs. I've figured the latter was just because I didn't incorporate ceratosaur characters, but maybe Sereno was right and Deltadromeus has been a coelurosaur this whole time. 

Mickey Mortimer

[Tim Williams]

Mickey Mortimer <therizino...@gmail.com> wrote:

> Like the Lori tree it has Tugulusaurus as a compsognathid, and Deltadromeus and Gualicho as ornithomimosaurs. I've figured the latter was just because I didn't incorporate ceratosaur characters, but maybe Sereno was right and Deltadromeus has been a coelurosaur this whole time. 

I assume this doesn't take into account the proposed synonymization of _Deltadromeus_ with _Bahariasaurus_.