30 University of California Publications in Zoology 



narrow frontals. As members of large species grow, relative orbit size decreases and 

 relative frontal width increases correspondingly. Nonetheless, Brachylophus has 

 seemingly wider frontals than would be predicted on the basis of its body size and 

 knowledge of frontal allometry in other iguanines. A detailed study of this allometry was 

 not undertaken, and I did not use variation in frontal width as a systematic character. 



Along with the prefrontals and postfrontals, the frontal forms the dorsal borders of the 

 orbits, which have a unique shape in Amblyrhynchus. In this genus, the dorsal orbital 

 borders are wedge-shaped, with the apices of the wedges pointing medially when viewed 

 from above (Fig. 11). In all other iguanines and all outgroups examined, the dorsal 

 borders of the orbits are more or less smoothly curved (Figs. 5A, 9), though the precise 

 shape varies among taxa. 



Dipsosaurus has a pair of relatively large openings at or near the suture between frontal 

 and nasals, one on each side of the midline. No other iguanine nor any outgroup that I 

 have examined has these, although some have much smaller foramina in roughly the same 

 location that may or may not be homologous. An area just anterior to the frontal on the 

 midline fails to ossify in certain other iguanids (e.g., Corytophanes and morunasaurs), but 

 it is not divided into paired openings as in Dipsosaurus. 



On the anteroventral surface of the iguanine frontal at the posterior end of the nasal 

 capsule, one or two pairs of crests may develop (Fig. 12). The lateral pair are the cristae 

 cranii, which form the dorsolateral walls of the olfactory tract and are invariably present. 

 In most iguanines these cristae extend continuously from the frontal onto the prefrontals 

 (Fig. 12 A); however, in Conolophus and in Ctenosaura defensor, the frontal portions of 

 the cristae project anteriorly, forming a step in each crista between its frontal and prefrontal 

 portions (Fig. 12B). Basiliscines, crotaphytines, morunasaurs, and oplurines all possess 

 cristae cranii resembling those in the majority of iguanines. 



A second pair of crests, located medial to the cristae cranii, is variably developed in 

 iguanines. These crests extend posterolaterally from the anterior end of the frontal at the 

 midline towards the cristae cranii. This medial pair of crests is absent in most iguanines 

 (Fig. 12A). Weakly developed medial crests are seen in Brachylophus and some 

 Ctenosaura, and somewhat larger ones occur in Conolophus (Fig. 12B). Amblyrhynchus 

 exhibits the strongest development of the medial crests and also undergoes considerable 

 ontogenetic change in this feature. At hatching, the medial crests of Amblyrhynchus are 

 similar to those of other iguanines that possess the crests, but are more strongly developed 

 and are united anteriorly to form a single median crest (Fig. 12C). During postembryonic 

 development the median portion elongates and grows ventrally, while the posterior ends of 

 the crests also grow ventrally and become continuous with the cristae cranii. The end result 

 is the formation of a pair of deep pockets separated by a median crest on the ventral surface 

 of the frontal bone (Fig. 12D). Large nasal salt glands (Dunson, 1969, 1976) probably fill 

 these pockets in entire specimens. 



Continuous morphological variation and a high degree of intergeneric overlap in the 

 range of this variation limit the usefulness of the medial pair of frontal cristae as characters 

 in phylogenetic analysis. The unique condition seen in Amblyrhynchus, however, is easily 



