E ARLY A RCHOSAURIFORM B RAINCASE E VOLUTION

When considering the evolutionary history of the diapsids leading up to archosaurs, many trends in braincase anatomy are recognisable. In order to try to evaluate braincase evolution in the group, comparisons will here be made between Captorhinus (Heaton 1979), Youngina (Evans 1987, Garnder et al. 2010), Prolacerta (Evans 1986), Euparkeria (Gower & Weber 1998) and Sphenodon (Säve-Södeberg 1947).

The braincase of Captorhinus is antero-posteriorly long and the braincase floor is flat, a pattern that is followed by Youngina (Figure 8C), Prolacerta (Figure 27) and Mesosuchus (Figure 11–13) braincases quite closely. In the latter two it is however possible to observe that the posterior region of the braincase floor becomes elevated, albeit subtly, as the basioccipital is short and tall. As a result, the occipital condyle is positioned more dorsally than the basal tubera. Euparkeria (Figure 15–18), in contrast, has a very short and tall braincase. It is also possible to observe that in basal taxa the para- and basisphenoid have a greater contribution to the braincase floor than the baisoccipital and that they gradually become restricted to the anterior region of the braincase, in particular the parasphenoid, while the basisphenoid tends to occupy the mid-portion of the ventral surface of the braincase floor. The occipital condyle becomes progressively more developed, along with the basal tubera and the basipterygoid processes. The processes of Mesosuchus and Euparkeria are strongly ventrally directed and much more developed than those of the other taxa. It is interesting to notice that the medial pharyngeal recess is already present and quite well developed in Youngina, although in Mesosuchus or Euparkeria they are still rather shallow. Overall, the basisphenoid becomes taller, but, at the same time, it also shows a decrease in its dorsal extension as the prootic expands anteriorly and forms most of the area ventral to the trigeminal foramen. The decrease in participation of the parabasisphenoid on the lateral braincase wall has been briefly discussed by Evans (1986).

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Figure 27: Prolacerta. A) skull in left lateral view; B–C) Briancase in posterior and left lateral view. Bocc:

basioccipital Fen vest: fenestra vestibuli; Met for: metotic foramen; Opisth: opisthotic; POP: paroccpital process; P/Bsphen: parabasisphenoid; Q: quadrate. From Clack & Allin 2004.

The prootic is very short in Captorhinus, and much of the antero-dorsal portion of the braincase remained unossified. Youngina shows a significant increase in its development but, in comparison with other archosauromorph taxa, this extension is still limited. Also, with the development of the paroccipital process, the prootic becomes restricted to the anterior portion of its base. The paroccipital process itself becomes distally elongate and more dorso- posteriorly directed and located progressively more dorsally. In Mesosuchus, the well- developed cristae prootica and parotica, as well as the anterior border of the fenestra ovalis, form a recess in which the foramen of the CN VII is found, a pattern followed by Euparkeria. There is a change in the topographical relationships of the trigeminal and facial foramina and the fenestra ovalis: in Captorhinus, the nerves are closer together and the facial foramen lies dorso-anteriorly to the fenestra ovalis, close to the paroccipital process. In Mesosuchus and Euparkeria, however, the facial foramen is positioned more ventrally and more anteriorly relative to the paraoccipital process, while the facial foramen is also located further anteriorly. The stapes does not shorten, but becomes progressively more slender. In Youngina it still shows a perforating foramen, which has been lost in Prolacerta. The footplate of the stapes is also reduced in Prolacerta, and a delicate neck separates it from the remaining of the shaft. The opisthotic becomes taller, forming a proper lateral braincase wall. However, with the dominance of the exoccipital in the posterior region of the braincase, it also becomes restricted to the paroccipital process and to its ventral ramus. Like the prootic, the supraoccipital also shows an anterior extension and its dorsal surface becomes flat, although still dorsally directed. In Captorhinus and Youngina, the facial foramen is very small and, though absent in the Prolacerta specimen studied by Evans (1986), it is relatively more prominent in Mesosuchus and Euparkeria and the routes for its palatine and maxilla- mandibular branches are also more well-marked.

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As the prootic, opisthotic and exoccipital become taller and develop the lateral braincase wall, certain posterior braincase elements such as the foramina of the CN XII and the metotic foramen rotate laterally and some inner ear structures such as the fenestra ovalis expands dorsally. In this regard, the morphology of Meosuchus is intermediate: its metotic foramen is laterally directed, but the foramina of the CN XII remain slightly posteriorly directed, as in Captorhinus or Prolacerta, but unlike Euparkeria. The elements forming the rims of the fenestra ovalis decrease in number. In Captorhinus, the rims of the fenestra ovalis are poorly ossified, but the parasphenoid, basisphenoid and basioccpital form much of its ventral, anterior and posterior borders. In Youngina, the fenestra is likewise poorly delimited, but it is formed only by the prootic and opisthotic. Both Prolacerta and Mesosuchus show a well-marked fenestra ovalis with no contribution from the baisoccipital, but in Eupakeria the basioccipital and parabasisphenoid contribute again to the postero- and antero-ventral borders of the fenestra. The metotic foramen becomes progressively more developed, and in Captorhinus it is formed by the opisthotic and basioccipital, but in Youngina the latter is substituted by the exoccipital. In Prolacerta it is unknown, but the pattern is followed by Mesosuchus. Euparkeria shows a further enlargement of the foramen, and the basioccipital takes part in its formation again, albeit restricted to its floor. Since the metotic foramen can to a certain extent act as a pressure-relief mechanism, its enlargement may indicate an increase in selective forces for the refinement of, and thus an improvement in, its hearing system.

The semicircular canals are elongate in Euparkeria in comparison to Youngina, but those of Mesosuchus are the longest and most slender of them. In Youngina, the lateral semicircular canal is the longest, conforming to basal amniotes and tetrapods in general, but in Mesosuchus and Euparkeria there is an evident development of the anterior semicircular canal. The common crus becomes also more dorsally positioned and as a consequence the utricculus becomes elongate. The vestibule of Mesosuchus seems to be more developed than that of Euparkeria, but the cochlea is much more elongate than in the latter.