Morphology and allometry suggest multiple origins of rostral appendages in Sri Lankan agamid lizards



Greg Johnston, Vertebrates Section, South Australian Museum, North Terrace, Adelaide, SA 5000, Australia.



Rostral appendages occur in a very small number of species spread across the entire clade of iguanian lizards. The five species of Sri Lankan agamid lizards of the poorly known endemic genus Ceratophora show remarkable variation in the morphology and development of rostral appendages, which are absent in two species and present in the other three. Parsimony and Bayesian comparative methods do not robustly resolve whether the appendage evolved once (with two losses), twice (with one loss) or thrice independently. The appendage in C. tennentii is leaf-shaped, present in juveniles and monomorphic in adults. It is quite dissimilar to the appendages in C. aspera and C. stoddartii which are horn-shaped, absent in juveniles and dimorphic in adults. Ceratophora stoddartii is more closely related to C. erdeleni, which lacks the rostral appendage, than it is to C. aspera. The combined morphological, allometric and phylogenetic evidence suggests rostral appendages evolved three times within Ceratophora: perhaps once as a result of natural selection for crypsis (in C. tennentii) and twice as a result of sexual selection (in C. aspera and C. stoddartii). Our results suggest that these unusual ornaments can evolve by more than one mechanism and more readily than is suggested by their low frequency among iguanian lizards.