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A.P. RUSSELL AND A.M. BAUER 



to Asaccus, but again concluded that the relationships of 

 Calodactylodes were obscure. 



Aristelliger was one of the taxa regarded as enigmatic by 

 Underwood (1954). He placed it in the Diplodactylinae and re- 

 garded it as an archaic form, possibly unable to compete with the 

 gekkonines, which he regarded as more derived. Indeed, he regarded 

 it as being a basal gekkonnid, retaining oil droplets in the eyes and 

 displaying vertebral amphicoely. In part. Underwood's (1954) as- 

 sessment of this genus may have been influenced by the fact that he 

 was, at the time, based in the West Indies and had more information 

 about it than most other geckos, and certainly more than any that he 

 also placed in the Dioplodactylinae. Aristelliger has been employed 

 in a variety of evolutionary (Hecht 1 952 ) and morphological (Ruibal 

 and Ernst 1965) studies, probably because of ease of availability. 

 These studies, however, have helped little to clarify the position of 

 the taxon. Although it has rather complex external digital structure, 

 anatomically it reveals a quite simple architecture. Thus more 

 detailed studies of the digits (Russell 1976, 1979; Russell and Bauer 

 1990. 1993) have not assisted in placing it with other genera that 

 typically show a more complex anatomy. 



BIOGEOGRAPHIC AND EVOLUTIONARY 

 IMPLICATIONS OF UNDERWOOD'S 

 CLASSIFICATION OF THE GECKOS 



Underwood (1954) pioneered a comprehensive approach to gecko 

 systematics. As a result of this, he was faced with issues of biogeog- 

 raphy and evolution that begged an explanation. For geckos, this 

 was essentially uncharted territory and the recognition of clusters, 

 especially within his Gekkoninae, generated new biogeographic and 

 evolutionary problems. Chief among these was the need to explain 

 the biogeography and evolution of his Diplodactylinae. This proved 

 especially challenging because, as noted above, this cluster of taxa 

 later proved to be the least stable of Underwood's ( 1954) proposed 

 units. 



Underwood (1954) interpreted eublepharids, with their scattered 

 distribution, as an ancient radiation with its own specialisations, 

 chiefly to arid conditions, rather than as a cluster of relicts. He 

 viewed the eublepharids as the primary, ancient Northern Hemi- 

 sphere radiation of the Gekkota. 



The sphaerodactylids were biogeographically non-problematic 

 as all occur in the New World. Underwood ( 1 954) viewed them as an 

 early New World offshoot of the Gekkota, based on his belief that 

 they were primitively diurnal, retaining certain plesiomorphic 

 lacertilian ophthalmological features. Kluge's (1967) demonstra- 

 tion that the sphaerodactyls are derived from within the gekkonines, 

 and subsequent recognition of secondary diurnality in the 

 sphaerodactylines (Roll, in press) has resulted in a reinterpretation 

 of sphaerodactyl biogeography and evolutionary history, with north 

 African affinities being supported by more recent systematic inves- 

 tigations (Arnold, 1993; Kluge, 1995). 



Underwood (1954) undertook to explain the distribution of the 

 Diplodactylinae which, in his view, included a large core of Australo- 

 Pacific taxa, but also genera from Africa and the Americas. He noted 

 that no genus occupied more than one continent and that most genera 

 had rather limited or patchy distributions. Only Aristelliger and the 

 New Zealand taxa did not co-occur with Gekkonines. He felt that 

 ovoviviparity might explain their ability to survive in New Zealand. 

 In the case of Aristelliger, he noted that its occurrence was basically 

 complementary to that of gekkonines, and suggested that it may 

 have formerly had a broader distribution but had subsequently 



withdrawn in the face of competition with gekkonine geckos. He 

 viewed the gekkonines as a more modern, expanding group that was 

 displacing diplodactylines from areas of previous occupancy. He 

 regarded New Caledonia as marking the periphery of the range of 

 the gekkonines, with Lepidodactylus and Eurydactylodes being 

 relatively recent invaders into diplodactyline (Rhacodactylus and 

 Bavayia) territory. He believed that Phelsuma, being chiefly insular, 

 diurnal, and arboreal, was ecologically segregated from the 

 gekkonines with which it co-occurs. He regarded its occurrence in 

 mainland East Africa as a recent event. Its arrival on islands of the 

 Indian Ocean was hypothesised to be as a nocturnal stock, an 

 offshoot of the southern African cluster of diplodactylines, with a 

 subsequent change in life style enabling it to coexist with gekkonines. 

 He regarded most continental diplodactylines as being terrestrial, 

 with arboreal forms being peripheral. 



The foregoing rather tortuous scenario developed by Underwood 

 (1954) to account for diplodactyline biogeography and evolution 

 was the direct result of the recognition of, as it was formulated at the 

 time, apolyphyletic assemblage. Removal of Aristelliger, Phelsuma 

 and a variety of other taxa (see above) from the Diplodactylinae 

 ( Kluge, 1 967) and inclusion of Eurydactylodes within it (Underwood, 

 1955) rendered biogeographic and evolutionary consideration of the 

 remaining diplodactylids more tractable (Bauer. 1990a), but left the 

 Gekkonidae (Underwood's Gekkoninae) yet more unwieldy. That 

 some gekkonine genera were present on multiple continents sug- 

 gested to Underwood (1954) that this was the dominant group. He 

 recognised four major digital morphologies among gekkonines, and 

 believed that each had reached most areas of the world and that most 

 had radiated in situ in each area, giving rise to numerous regionally 

 endemic genera. Thus, while expansion was an important theme in 

 the evolution of gekkonines, there was significant within-region 

 evolution as well. These ideas were obviously heavily influenced by 

 those of Darlington ( 1 948 ) and by the idea of competitive exclusion 

 (a more ecological than historical view). He noted the waif dispersal 

 capabilities of some geckos and opined that this complicated the 

 picture of dispersal via land bridges that served as his main para- 

 digm. The issue of waif dispersal, though recognised as being 

 restricted to certain taxa, remains to this day as a confounding factor 

 in the interpretation of the evolution of gekkonid spatial patterns. 



Further systematic consideration (see above) has resulted in an 

 increased complement of gekkonid genera, but has also resulted in 

 some level of internal resolution, which, in turn, has influenced 

 some aspects of biogeographic interpretation. For many regions, 

 local radiations of monophyletic clusters of genera have been recog- 

 nised, but resolution of pattern between these clusters remains 

 poorly understood. 



CONCLUSIONS 



Underwood's (1954) systematic, biogeographic and evolutionary 

 considerations of geckos marked the first attempt to comprehen- 

 sively assess this circumglobal and highly diverse cluster. His 

 analyses brought some degree of order to a previously very poorly 

 understood set of problems, and his choice of ophthalmological 

 characters as those of primary consideration resulted in the estab- 

 lishment of a basic pattern that has survived to the present in 

 modified form. Although Underwood (1968, 1970, 1971b, 1977a,b) 

 revisited the gekkotan eye repeatedly, the promise of phylogenetic 

 utility originally held out by ophthalmolgical data has not. until 

 recently, been pursued. Roll (1995, 1997, 1999) and Roll and 

 Schwemer (1999) have demonstrated that many diurnal geckos are 



