Vertebral pneumaticity of the paravian theropod Unenlagia comahuensis, from the Upper Cretaceous of Patagonia, Argentina
Introduction
Dromaeosaurids are highly derived theropods related to birds. The first discoveries of dromaeosaurids began in the ‘20s of the last century, with Dromaeosaurus albertensis (Matthew and Brown, 1922) and Velociraptor mongoliensis (Osborn, 1924) in Canada and Mongolia, respectively. Since that time, all dromaeosaurid remains were found in the Northern Hemisphere, mostly in Asia and North America (Gianechini and Apesteguía, 2011).
The separation of the supercontinent Pangea into a northern Laurasia and a southern Gondwana, during the Jurassic, possibly generated a vicarious evolution event between the unenlagiines in the south and the remaining dromaeosaurids in the north (Makovicky et al., 2005; Novas and Pol, 2005).
In the ‘90s, a new theropod (Unenlagia comahuensis) with characters shared with dromaeosaurids and birds was found in Cretaceous outcrops of Argentina (Novas and Puerta, 1997). In later years, other theropod specimens were found in southern landmasses, which had common features with U. comahuensis. Bonaparte (1999) observed that U. comahuensis shares characters with the Malagascan taxon Rahonavis ostromi (Forster et al., 1998), and considered that they are members of the same family, which he established as Unenlagiidae. With the discovery and description of Buitreraptor gonzalezorum, new phylogenetic analyses clustered these taxa in a monophyletic group, within Dromaeosauridae, which they established as Unenlagiinae (Makovicky et al., 2005).
Among extant tetrapods, only birds have postcranial skeletal pneumaticity (Duncker, 1971; O'Connor, 2006), However, this feature is present in the evolutionary line of the ornithodirans, specifically in pterosaurs and saurischian dinosaurs, from the Late Triassic (Britt, 1997; Wedel, 2009; Benson et al., 2012). The pneumatic system consists of bone correlates, such as fossae, foramina, camellate tissue or camerae, and soft tissue (respiratory system), including the lungs and their nine air sacs. Epithelial extensions, called ‘diverticula’, are released from these air sacs, which penetrate the bones through foramina (King, 1966; Duncker, 1971).
In the case of non-avian theropods, postcranial pneumaticity is restricted to the axial skeleton and some scarce appendicular bones, and the location of foramina is similar to that of foramina in birds (O'Connor and Claessens, 2005; O'Connor, 2006; Benson et al., 2012).
The goal of this work is to present a study of the vertebral pneumaticity of Unenlagia comahuensis. This includes a detailed description of the fossae and foramina and a visualization of the internal pneumaticity using CT scans. These results will be compared with that seen in other dromaeosaurids, and especially in other unenlagiines.
Section snippets
Anatomical description and computed tomography scans
The holotype specimen of Unenlagia comahuensis (MCF-PVPH 78) was found in outcrops of the Portezuelo Formation (Turonian–Coniacian), in the Sierra de Portezuelo, center-east of Neuquén Province, Argentina (Novas and Puerta, 1997; Novas et al., 2021; Fig. 1). The materials here analyzed are part of the holotype and consist of three dorsal vertebrae (Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6), which were interpreted as 8th, 10th and 13th in position (Novas et al., 2021). As the 13th vertebra is the
Description of the external vertebral pneumaticity
The anterior surface of the neural arches shows right and left centroprezygapophyseal fossae (cprf). These are bounded laterally by the centroprezygapophyseal lamina (cprl) and the prezygoparapophyseal lamina (prpl), dorsomedially by the intraprezygapophyseal lamina (tprl), medially by the neural canal, and dorsally by the prezygapophysis (Fig. 2). The cprf of the 13th vertebra are less developed. Inside each cprf of the 8th and 10th vertebrae, two foramina are observed, one of them is larger
Discussion
The presence of internal camellate tissue in the vertebrae of the unenlagiines is not unexpected, since it is a common trait in derived coelurosaurs (e.g. Britt, 1993; Sues and Averianov, 2016a; Forster et al., 2020; Smith et al., 2021; see below). U. comahuensis confirms that the lpf communicates with the internal camellae. Previous authors have explained that the only non-ambiguous evidence of pneumaticity in bones is the presence of internal chambers that are continuous with the exterior
Conclusions
The external and internal pneumaticity of the dorsal vertebrae of Unenlagia comahuensis were described in this work. Notable external pneumatic traits are the presence of well-developed lpf throughout the dorsal series along with the presence of a fossa (posdf) at both sides of the neural spine of the last (13th) dorsal, all of which have internal foramina that seem to communicate with the interior. The lpf are extended through the entire dorsal series in several groups of non-avian theropods,
Acknowledgments
The authors of this work thank Dr Rodolfo Coria and Ludmila Coria (MCF) for allowing us access to the material of Unenlagia comahuensis, and Daniel Cabaza (MML), Carlos Muñóz (MPCA), Jorge Calvo (MUCPv), Carl Mehling (AMNH), and Daniel Brinkman (YPM) for allowing access to the fossil collections under their care. We also want to especially thank Sanatorio Plaza Huincul for allowing us to perform the CT scans. Thanks to Mattia Baiano and Guillermo Windholz for assistance in the review of the
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