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E.N. ARNOLD 



more windows in the scleral ring (Fig. 2). These enable the eyeballs 

 to distort and flatten, so they can be housed in the narrow space 

 available within the head. Other shared features, which also contrib- 

 ute to low vertical dimensions, are depression of the body vertebrae 

 and reduction of the crests on their neural arches. 



Shortening and consequent decrease in mass of the adductor 

 muscles associated with reduction in head height probably plays a 

 critical role in facilitating the evolution of this cranial morphol- 

 ogy. Curtailed mass reduces the power of the muscles, so a 

 particularly strong, thick arched parietal area of the skull is no 

 longer necessary to resist their action, and this also permits the 

 posterior skull roof to become thin and flat. Similarly the mandi- 

 bles are subjected to reduced forces in biting and can consequently 

 be more slender with smaller vertical dimensions. However, such 

 shortening of the muscles carries penalties in terms of reduced 

 efficiency in biting and prey handling (Arnold, 1998a). Change in 

 geometry of the skull during the retraction phase of cranial kinesis 

 ameliorates this effect by improving their angle of action on the 

 jaw and the length of their excursion. This phenomenon is promi- 

 nent in Platysaurus, which has an expansion area in the skin on 

 the anterior cheek that accommodates the changes involved in the 

 substantial kinetic movement. The similarly orientated hinge 

 regions of Holaspis, between the small scales found in this region, 

 indicates that its skull is similarly highly mobile, as does the 

 simplified fronto-parietal suture. 



The downward flexion of the retroarticular process of the mandi- 

 ble may permit a longer and more efficient depressor mandibulae, in 

 spite of the flattened head, although this feature is not paralleled in 

 other very flattened crevice users. Other characteristics of Holaspis 

 could also plausibly be considered as adaptations to crevice use, but 

 are not repeated in functional analogues. Thus, the large plates along 

 the back which might possibly increase smoothness and so ease 

 mobility within crevices; although neither Platysurus or T. 

 semitaeniatus have this feature. The enlarged sternal fontanelle 

 could similarly be thought to increase flexibility in this region, but 

 Platysaitrus has no fontanelle at all. 



Another complex of Holaspis features may also be related to 

 use of crevices, specifically those beneath bark. This involves the 

 snout which is wedge shaped in lateral view (unlike that of rock 

 crevice dwellers), with the bizarre flattened boss formed from the 

 premaxilla, and nostrils set back from the snout tip and low on its 

 sides. Such features are not found even in extremely flattened 

 lizards using rock crevices, but they do occur in the flattened 

 lygosomine skink Aulacoplax, which habitually conceals itself in 

 the narrow interstices between the bases of the fronds of screw 

 pines (Pandanus spp.) (Brown and Fehlmann, 1958). This arrange- 

 ment may enable the skink to enlarge interstices so they are broad 

 enough to take the rest of the animal as it moves forwards. 

 Holaspis may possibly do the same when pushing beneath flexible 

 pieces of loose bark. The frequent presence of longitudinal 

 scratches on the dorsum of the head suggests this may be the case. 

 Fusion of the frontoparietal and interparietal scales may increase 

 strength and smoothness of the head surface but has no parallels 

 elsewhere. 



Gliding. Since Holaspis descends through the air in a controlled 

 way at relatively shallow angles it glides rather than parachutes. 

 Gliders depend on the possession of an aerofoil which extracts a lift 

 component as the animal moves through the air, the lift counteract- 

 ing the force of gravity. For gliding at shallow angles to be possible, 

 the ratio of surface: body weight needs to be high. Some other 

 gliding lizards have a specialised lift surface that provides this. In 

 the agamid Draco, this is formed from a membrane supported by the 



elongated abdominal ribs while in the gecko Ptychozoon there are 

 flaps attached to the sides of the belly that fold out, increasing 

 surface area. In Holaspis, it is the whole body that acts as an aerofoil 

 and some features that also confer performance advantage in using 

 crevices contribute to its formation. This is particularly true of 

 dorsoventral compression, but low ossification of the skull must 

 help increase the surface: weight ratio. Other features appear to be 

 specifically associated with gliding and are not found in crevice 

 dwellers, although they may occur in other gliders. Included here is 

 low ossification of the pectoral girdle and perhaps that of the 

 sternum and depression of the legs and tail. This last feature, 

 together with development of distinct trailing edges on the limbs, 

 also occurs in Draco and Ptychozoon, which do not enter very 

 narrow crevices. Surface area is further increased by the lateral flaps 

 on the neck and the webs of skin that form the trailing edge of the 

 proximal hind legs, both features again found in Draco and 

 Ptychozoon. The modified scales on the sides of the tail, on the 

 trailing edge of the crus and on the hind digits also increase surface 

 very efficiently, forming stiff lateral fringe-like extensions with 

 little increase in weight. They are exactly paralleled in structure and 

 function by scales on the hind side of the thigh and tail base in 

 Draco, while Ptychoz.oon has analogous cutaneous extensions along 

 the length of the tail. 



Holaspis is able to produce further temporary increase in sur- 

 face area by lateral expansion of the abdominal region so that this 

 becomes almost disc-like. The increase in surface area is brought 

 about by the long free dorsal ribs being rotated forward around 

 their articulations with the body vertebrae, so that instead of being 

 directed diagonally backwards they project more laterally. In 

 Holaspis, the gain in surface area this produces is large because 

 the ribs are long. The overlapping flexible rib tips form a continu- 

 ous lateral edge to the area supported by the ribs and this maintains 

 its continuity and longitudinal orientation in spite of the move- 

 ments of the ribs themselves. The rotation of the ribs is 

 presumably partly brought about by the intercostal muscles, as 

 seems to be the case in Draco (Colbert, 1967). But it is likely that 

 the well-developed slips of the m. intercostalis scalaris also play a 

 part. As they run somewhat diagonally outwards and backwards 

 from the in. rectus abdominis to the rib tips, their contraction 

 would also help swing the ribs forwards; at the same time the ribs 

 would tend to bow laterally and bend distally downwards. The 

 contraction would also raise the m. rectus abdominis and with it 

 the ventral integument which is closely attached. These move- 

 ments would produce a more aerodynamically efficient transverse 

 section in which the dorsal surface was more strongly convex and 

 the belly flat or slightly concave. 



The skin must stretch to allow for the increase in lateral area that 

 the rib movements produce. Its distinctive structure permits this, for 

 expansion occurs not only at the longitudinal lateral skin folds but 

 also at the extensible areas between the scales. The bridges that often 

 join the scales limit the direction and extent of expansion; they also 

 help distribute it evenly throughout the skin, discouraging wrinkling 

 and so contributing to a smooth surface. The looseness of the 

 connection between the skin and underlying structures usual in 

 lizards is also important in allowing skin tension to be evenly 

 distributed. 



It is probable that the band of large broad plates in the vertebral 

 region also has a function in producing as good an aerofoil as 

 possible. As the hinge regions between the plates are virtually 

 inelastic, the whole area can be regarded as a single lamina which is 

 firmly fixed at the occiput and at the tail base. When such an 

 elongate lamina is stretched over a flat or convex surface, and placed 

 under tension, it becomes very resistant to lateral deformation. This 



