Qiu et al.’s [8] recent study on two new compsognathid-like theropods from the Yixian Formation provides support for a radiation of early diverging coelurosaurs that may have been restricted to Northeastern Asia. The authors describe two nearly complete and relatively well-preserved specimens that represent two new coelurosaur species classified either among Compsognathidae or Sinosauropterygidae (the latter being a clade of compsognathid-like coelurosaurs predominantly from the Yixian Formation), depending on the datamatrix that is used in the phylogenetic analysis. Gut contents combined with the morphology of the skull, teeth, neck and manus led Qiu et al. [8] to propose that these two new coelurosaurs and a third closely related species had distinct hunting strategies. The first, Huadanosaurus (∼1 m), was suggested to be a nocturnal predator that swallowed as a whole small prey items such as mammals. The second, Sinosauropteryx (∼1.2 m), likely hunted during the day and dismembered gracile prey such as lizards and insects. The third, Sinocalliopteryx (2.4 m), fed on larger animals such as dromaeosaurids and birds. The authors explained these distinct feeding ecologies among closely related and coeval carnivores, and the general diversification of theropods from the Jehol Biota, by the presence of separate ecosystems that were created by the formation of small and isolated rift basins during the destruction of the North China craton. Interestingly, the two new species described by Qiu et al. [8] were identified as juveniles and it is regrettable that the authors did not provide stronger arguments against the possibility that they instead represent immature individuals of already-known species. Indeed, besides the three compsognathids listed above, the Jehol Biota also includes the basally branching tyrannosauroids Dilong, Sinotyrannus and Yutyrannus—three distant relatives of the eponymous Tyrannosaurus—that are phylogenetically close to compsognathids. An alternative hypothesis is that the two newly described species possibly represent juvenile tyrannosauroids such as those of the large-bodied (9–10 m) and most certainly mature Sinotyrannus and Yutyrannus, whose feeding strategy may have changed during growth. Such an ontogenetic dietary shift was already proposed in two North American tyrannosaurids based on mandibular biomechanical properties and tooth morphology [9]. Ontogenetical changes in non-avian theropods remain vastly poorly understood, leading to heated debates about the validity of some taxa such as Nanotyrannus, which is probably a juvenile Tyrannosaurus [10]. Only the discovery of additional early diverging coelurosaurs of well-constrained ontogenetical stages from the Jehol Biota, combined with a much better understanding of theropod ontogeny, will enable validation of the separation of several coeval compsognathid-like species from this exceptionally preserved ecosystem and maintain the validity of Compsognathidae and Sinosauropterygidae as non-artificial monophyletic groupings.
 | Christophe Hendrickx Becario postdoctoral CONICET Paleontología |