Bull. not. Hist. Mus. Land. (Zool.) 65(2): 165-171 



Issued 25 November 1999 



Modes of ear reduction in iguanian lizards 

 (Reptilia, Iguania); different paths to similar 

 ends 



E.N. ARNOLD 



Department of Zoology, Natural History Museum, Cromwell Road, London SW7 5BD 



Synopsis. New observations are presented on interspecific variation in ear structure in Phrynocephalus (Agamidae) and the 

 Chamaeleonidae. The tympanum has probably been obscured at least fourteen times in the Iguania and more extensive ear 

 reduction has occurred independently at least once in each of five separate clades. Combining information on ear reduction with 

 estimated phylogenies of the groups concerned demonstrates that the process has been initiated in at least two quite different ways 

 in the Iguania. Modifications to the ear are congruent with hypotheses of phy logeny based on other characters in Tympanocryptis, 

 the Cophotis-Lyriocephalus-Ceratophora clade, phrynosomatid sand lizards, and to a large extent in the Chamaeleonidae. In 

 Phrynocephalus there is evidence that some modifications show partial reversal in one or more lineages. 



INTRODUCTION 



Different lineages of organisms often evolve a number of similar 

 traits independently, but the order in which these are assembled may 

 often be different, even in ecological analogues, especially if the 

 taxa concerned are not closely related (Arnold, 1 994). This phenom- 

 enon of equipotentiality, where more or less the same overall 

 condition is reached by different routes, will be demonstrated in the 

 external and middle ear of iguanian lizards, using some new ana- 

 tomical observations and recent information on the phylogeny of 

 this assemblage. Reduction of the external and middle ears can be 

 shown to have occurred a number of times but by at least two 

 primary initial routes, the various changes making up the process 

 taking place in different sequences. 



Versluys (1898) and Mertens (1971) listed cases where the tym- 

 panum is obscured in the Iguania and more extensive ear reduction 

 in the group has been surveyed by Smith (1938). More recently, 

 Wever (1978) discussed selected instances of ear modification in 

 greater detail and gave information on the ability of such altered 

 organs to transmit sound. The present account corrects some errors 

 in Smith's otherwise useful paper and describes the wide variety of 

 previously unreported ear conditions found in the genus 

 Phrynocephalus and some more modest differences among chame- 

 leons. 



put forward by Frost and Etheridge ( 1 989), is accepted. On the other 

 hand, these authors' use of Chamaeleonidae is not followed. They 

 employ the name for the whole of the Acrodonta, which comprises 

 Chamaeleonidae in its usual sense plus what has generally been 

 called Agamidae, including the Uromastycidae (Borsuk-Bialynicka 

 & Moody, 1984). 



Moody ( 1 980) allocated the wide range of lizards which had long 

 usually been referred to the genus Agama to a number of separate 

 genera: Stellio, Agama, Xenagama, Pseudotrapelus and Trapelus. 

 This course was followed by several authors, but the name Stellio is 

 unavailable (Stejneger, 1 933) and the group it was used to denote by 

 Moody is paraphyletic, comprising distinct Palaearctic and mainly 

 African assemblages (Joger, 1991) of which the former is probably 

 a clade and the members of the latter more closely related to such 

 taxa as Agama, Pseudotrapelus and Trapelus (personal observa- 

 tions). Leviton, Anderson, Adler& Minton (1992) argue for the use 

 of Laudakia Gray, 1845 for the Palaearctic forms, a course followed 

 here. The more recent suggestion (Henle, 1995), that Laudakia 

 should be confined to some members of this assemblage and the rest 

 placed in Placoderma Blyth, 1854, requires more thorough assess- 

 ment of the relationships of these lizards. The name Acanthocercus 

 Fitzinger, 1843 is available for the remainder of the forms that 

 Moody allocated to Stellio (see Schatti & Gasperetti, 1994; Henle, 

 1995;Baig&B6hme, 1997). 



MATERIAL EXAMINED 



Material examined forms part of the collection of the Natural 

 History Museum, London and a list of specimens checked is depos- 

 ited in the Reptile Section there. 



NOMENCLATURE 



Various changes in iguanian nomenclature have been suggested 

 recently and, to avoid confusion, usage in this paper will be specified 

 here. The use of Phrynosomatidae for what were previously infor- 

 mally called sceloporine iguanids (Savage, 1958), which has been 



DISTRIBUTION OF EAR REDUCTION IN THE 

 IGUANIA 



Basic structure of the unreduced Iguanian middle ear is shown in 

 Fig. 1. In extreme cases of reduction, the tympanic area is covered 

 by the anterior slip of the depressor mandibulae, the tympanum 

 itself is absent and the columella becomes more robust. The distal 

 part of the extracolumella may virtually disappear while at the same 

 time its attachments to surrounding structures are thickened and 

 sometimes ossified. These attachments are the internal or quadrate 

 process, connecting to the lower quadrate bone, and the dorsal 

 process which may connect to the paroccipital process, the interca- 

 lary ligament, or both. 



At least some reduction of the ear occurs within the following 

 apparently holophyletic groups of the Iguania. 



© The Natural History Museum. 1999 



