Braincase anatomy of Pterodaustro guinazui

CODORNIÚ, L.; PAULINA CARABAJAL, A.

Rio de Janeiro

Simposio; III International Symposium on Pterosaurs; 2013

Universidade Federal du Rio de Janeiro y Museo Nacional

The braincase anatomy is poorly known in pterosaurs. Compared to the large number of species and specimens that have been recorded so far for Pterosauria, the braincase is known for a few genera within the clade. Detailed descriptions are known especially for 3D preserved material of pterodactyloids such as Anhanguera (Kellner, 1996; Kellner and Tomida, 2000; Witmer et al., 2003), Tapejara (Eck et al., 2011; Kellner, 1996; Wellnhofer and Kellner, 1991), Pteranodon (Bennett, 2001), Dsungaripterus (Lü et al., 1997), two naturally preserved endocranial casts from Israel (Lewy et al., 1992), and Pterodactylus (Edinger, 1941; Wellnhofer, 1978). The braincase anatomy in non-pterodactyloid pterosaurs is known in Parapsicephalus (Newton, 1888), Rhamphorhynchus (Wellnhofer, 1978; Witmer et al., 2003), Cacibupteryx (Gasparini et al, 2004), Dorygnathus (Padian, 2008) and Carniadactylus (Dalla Vecchia, 2009). New material of the pterodactyloid Pterodaustro guinazui (Lower Cretaceous of Lagarcito Formation, San Luis Province, Argentina) allows a detailed description of the braincase anatomy based mainly on an undescribed specimen with an exposed braincase in latero-ventral view (Fig. 1). Although several skulls were previously described for this taxon (Bonaparte, 1971; Sánchez, 1973; Chiappe et al., 2000), the braincase remains are poorly known, as in most pterosaurs. The new specimen (MIC V 250) corresponds to a partial skull in right lateral view, which is missing the squamosal and postorbital. The material is not highly depressed and also exhibits some aspects of the skull roof and the occipital plate. Other 13 specimens allow the description of the dorsal, posterior, and partially ventral braincase views. The supraoccipital is a moderate-sized bone, slightly concave and oval shaped. It is a posteroventrally inclined element in the occipital table that encloses (probably together with the parietal on each side), a large oval foramen for the dorsal head vein. At the dorsal end of the supraoccipital, there is a medial small and low ridge running vertically (supraoccipital knob). The exoccipital forms most of the surface of the occipital condyle and the lateral margin of the foramen magnum. The exoccipital-opisthotic complex expands laterally forming the paroccipital process which is posteroventrally projected, short with an expanded and rounded distal end. There is a distinct notch on the dorsomedial margin of the paroccipital process that corresponds to the preserved margins of the postemporal fenestra. This opening has a similar or slightly smaller diameter than the foramen for the dorsal head vein. On the lateral surface of the braincase, the posterior branch of the opisthotic is separating two recesses. The dorsal recess is subdivided in two depressions, one corresponding to a pneumatic recess and another bearing two small foramina, probably for the middle cerebral vein. The ventral and small oval recess corresponds to the columellar recess. The foramen magnum is approximately the same height as the occipital condyle. The occipital condyle is circular in posterior view, suggesting a hemispheric shape for this structure. Laterally to the occipital condyle there are two foramina. The posterior foramen is located near the base of the neck of the condyle and corresponds to a single opening for the passage of the cranial nerve XII. The anterior foramen is larger and corresponds to the metotic foramen, which is internally subdivided in two smaller foramina for cranial nerves IX - XI. The basioccipital forms the small ventral surface of the occipital condyle. The neck of the occipital condyle is short and posteroventrally projected. Below the condyle, the basioccipital projects two short and slightly divergent basal tubera. These processes are separated by a median triangular projection of the basisphenoid, resulting in a clear V-shaped basioccipital-basisphenoid suture. It seems that the basal tubera are also formed by a lateral projection of the basisphenoid. The basisphenoid is a long bone extended anteroventrally from the basioccipital, forming a triangular medial projection between the basal tubera posteriorly, and contacting the pterygoids ventrally. Posteroventrally, the fused basipterygoid processes form a subrectangular and slightly expanded plate, ending in a rounded bulge. The distal end of this plate shows two free and short projections, slightly divergent. In non-pterodactyloids pterosaurs the processes are separated over their entire length and not connected by a bony web or plate. Pterodaustro exhibits the pterodactyloid condition as observed in Gnathosaurus (Wellnhofer, 1978), Pteranodon (Bennett, 2001) and Anhanguera (Kellner and Tomida, 2000). On the lateral aspect of the basisphenoid there is a shallow and elongated lateral tympanic recess. The recess is subdivided in at least three minor recesses. Related to the recess there is a well-developed preotic pendant, which is forming a crest-like structure. The sutures between the laterosphenoid and prootic are obscured by fusion. The postorbital process of the laterosphenoid is a low crest (anterodorsally projected on the lateral aspect of the braincase). A possible depression or oval recess for cranial nerve V is observed in the specimen MMP 1086, and inside there is evidence of tree very small foramina.The ethmoidal elements (sphenethmoids and mesethmoids) in Pterodaustro are ossified, as in Anhanguera and Tapejara (Kellner, 1996). They are firmly fused to the orbitosphenoid-laterosphenoid posteriorly and the frontal dorsally, enclosing the relatively long olfactory tract and the olfactory bulbs (specimens: MMP 1097, MMP 1086, MIC V 777, MMP 3562). There are no visible sutures between the elements of this complex. There is evidence of the presence of an ossified interorbital septum in the specimen MIC V 250. The surface of this bone is flat and smooth, without striations (they are thin bones, almost transparent). The shape of the interorbital septum is unknown, because it is not complete in the available specimen. The new specimens of Pterodaustroguinazui reveal neuroanatomical features and are informative concerning the pterodactyloid condition.