DISCUSSION

Comparisons between the Braincase of Youngina and Other Reptiles

In many Permo-Triassic higher stem-diapsids, the neurocranial anatomy is not well- known or well-described; however, Youngina shows many plesiomorphic conditions. For example, it compares well with captorhinids in the presence of a large, open and poorly ossified fenestra ovalis. Further, the basioccipital tubers are weakly developed unlike the elongated tubera found in Sphenodon and the archosauromorphs, the occipital condyle is kidney-shaped unlike the hemispherical condyle in archosauriforms, the paroccipital processes are not dorsoventrally expanded in posterior view unlike in squamates and archosauriforms. The open vidian canal on the basisphenoid and lack of fusion between the opisthotic and exoccipitals also distinguish it from squamates. The stapes is intermediate between captorhinids and more derived reptiles in being relatively slender and lacking a dorsal process, yet still possessing a large stapedial foramen. On the other hand, Youngina shows a number of derived braincase characters. The paroccipital processes contact the quadrate, unlike in more primitive reptiles. The abducens nerve occupies a groove on the dorsum sellae rather than piercing through as in captorhinids (Price 1935); in this, Youngina resembles basal archosauromorphs like Prolacerta and Tanystropheus (Evans 1986). The basipterygoid processes of the basisphenoid are fairly large as in crown-group diapsids, unlike the relatively smaller processes found in captorhinids (Price 1935), and they are not sutured to the palatal elements unlike in some derived groups (such as sauropterygians and turtles). New anatomical reinvestigations of previously described higher-stem diapsids for which the braincase is preserved would complement the anatomy we have described here for Youngina and permit more detailed comparisons as suggested by both Modesto and Sues (2002) and Bickelmann et al. (2009).

Hearing in Stem-diapsids

While Evans (1987) described many of the bony structures of the braincase, we present the first reconstruction of the vestibular system in Youngina, and in fact in any Permian diapsid, and the first complete stapes known for Youngina. Our HRXCT data show that earlier reconstructions of the stapes as a slender, gracile element (Evans 1987; Gow 1975) are not correct, and that the stapes is a supporting element within the skull, as illustrated by Carroll (1981). This has important implications for hearing in Youngina.

The stapes in Youngina was not specialized as a middle ear ossicle and consequently did not function in hearing--instead it served as a mechanical part of the skull architecture, a brace between the braincase and the quadrate. This is the plesiomorphic function of the stapes in tetrapods (Carroll 1980). Turtles and crown-group diapsids have freed the stapes from the quadrate so that it can swing freely and conduct airborne sounds between the tympanic membrane and the fenestrae ovalis (Rieppel 1993). Given the lack of the deep caudal emargination found in crown-group diapsids, or the caudolateral emargination found in turtles, the quadrate probably did not support a tympanic membrane (Reisz 1981). Impedance-matching hearing is not known among any other Paleozoic diapsid, let alone any amniote, except some parareptiles (Müller and Tsuji 2007). Recognizing the absence of these structures in Youngina agrees with its position as an early stem-diapsid, rather than an early lepidosauromorph as once suggested (Benton 1985).

Despite the fact that the stapes is not transformed into a middle ear ossicle, and the lack of a tympanic membrane, Youngina was not necessarily insensitive to sound. The gross structure of its auditory apparatus is similar to Sphenodon in that the stapes is not columelliform and articulates with the quadrate posteromedially and there is no tympanic membrane. Sphenodon has a range of auditory sensitivity in the lower frequencies of 100-900 Hz (Gans and Wever 1976), and it has been suggested that m. depressor mandibulae may perform a secondary function in sound absorption (Lombard and Hetherington 1993). Additional research could be performed to test the approximate hearing capabilities in Youngina based on the dimensions of the cochlear duct and comparison with the large data set of reptiles and birds published by Walsh et al. (2009). Comparing this data from Youngina to that of modern sauropsids could provide insight into the hearing capabilities and vocal complexity of derived stem-diapsids and the evolution of hearing in modern reptiles, and possible new interpretations of a previously reported aggregation of juvenile stem-diapsid specimens as evidence of group sociality (Smith and Evans 1996).