48 University of California Publications in Zoology 



Cyclura with wide parabasisphenoids, this bone becomes relatively wider during 

 postembryonic ontogeny. All basiliscines, crotaphytines, morunasaurs, and oplurines have 

 the parabasisphenoid strongly constricted behind the basipterygoid processes, indicating 

 that this condition is plesiomorphic for iguanines. 



A second part of the iguanine parabasisphenoid exhibits two distinct morphologies that 

 are constant within genera. The parabasisphenoids of all iguanines except Ctenosaura bear 

 large posterolateral processes that extend along the anterolateral edges of the lateral 

 processes of the basioccipital, reaching or closely approaching the spheno-occipital 

 tubercles (Fig. 21A,C,D). In Cyclura (Fig. 21D) and especially in Iguana (Fig. 21C), 

 widening of the parabasisphenoid obliterates the distinctness of its posterolateral processes; 

 their existence is inferred from the lateral extent of the parabasisphenoid along the lateral 

 processes of the basioccipital. Unlike other iguanines, the posterolateral processes of the 

 parabasisphenoid are very short or absent in Ctenosaura (Fig. 2 IB), a condition that may 

 be related to the elongation of the skull in this taxon. Only Crotaphytus (but not Gambelia) 

 among the outgroups examined exhibits a condition similar to that of Ctenosaura; therefore, 

 I considered the possession of long posterolateral processes of the parabasisphenoid to be 

 plesiomorphic. 



Basioccipital (Figs. 5B, 21). The basioccipital forms the posterior floor of the brain 

 cavity and makes up the large medial portion of the occipital condyle. It bears prominent 

 ventrolaterally directed lateral processes that are capped by the spheno-occipital tubercles. 

 These tubercles fuse to the lateral processes late in ontogeny. The basioccipital is sutured 

 to the exoccipitals dorsolaterally, to the prootics anterolaterally, and to the parabasisphenoid 

 anteriorly. Although iguanine basioccipital morphology is variable, I found no obvious 

 characters that bear on intergeneric relationships. 



Exoccipitals and Opisthotics (Figs. 5C, 21). The exoccipitals are indistinguishably 

 fused to the opisthotics in postembryonic developmental stages of all iguanines. These 

 compound bones form the posterior sides of the brain cavity and the lateral edges of the 

 foramen magnum. They meet the supraoccipital dorsomedially, the prootics anteriorly, and 

 the basioccipital ventromedially. The paroccipital processes of the opisthotics extend 

 laterally to contact the supratemporals, bracing the posterolateral comers of the skull. The 

 relative length of the paraoccipital processes varies among iguanine genera, but differences 

 are complicated by positive allometry of this feature both within and among taxa (though 

 the correlation is less precise in the latter case). Apparently the braincase widens more 

 slowly than the skull as a whole. As the paraoccipital processes elongate, they also become 

 more posteriorly oriented. 



Each exoccipital-opisthotic bears two prominent crests laterally: the crista 

 interfenestralis, which lies between the fenestra ovalis and the fenestra rotunda; and the 

 crista tuberalis, which bounds the antrum of the fenestra rotunda posteriorly. Variation 

 exists in the degree to which the crista tuberalis slants inward dorsally and to which it 

 obscures the crista interfenestralis in posterior view, but this variation is too great within 

 iguanine genera to be useful for inferring relationships among them. Dipsosaurus is unique 

 among iguanines in possessing a sharp, laterally directed point on each crista 



