EVOLUTION OF HOLASPIS 



161 



effect is increased when the tension is both along and across the 

 lamina. It can be demonstrated by placing a strip of paper on plane 

 surface and putting it under longitudinal tension, after which dis- 

 placing the intermediate area laterally, even to a small extent, 

 becomes very difficult. 



Such rigidity appears to be developed in the vertebral area of 

 Holaspis. which extends over the convex dorsum of the body. 

 Tension is generated by the lateral skin being pulled outwards during 

 rib spreading. This movement distorts the large plates and their 

 hinge regions slightly, so that there is a small widening and longitu- 

 dinal contraction of the vertebral band. As this is firmly attached at 

 the occiput and the tail, tension within it is thereby increased. The 

 slight movements of the plates exhaust the very limited internal 

 mobility of the band, increasing its lateral rigidity further. These 

 processes can be discerned in the detached dorsal skin of an alcohol- 

 preserved Holaspis. Lateral tension alone, produces a longitudinal 

 contraction of the vertebral band whereas, if it is applied when the 

 ends of the band are fixed to the substrate, the band becomes 

 laterally rigid. 



The rigid vertebral band ensures that the extended lateral skin is 

 spread evenly on both sides of the body, again helping to avoid the 

 tendency of the tense skin to wrinkle. It may also act to keep the 

 body straight during gliding by restricting lateral bending. This 

 effect can be simulated by attaching a strip of adhesive paper tape 

 along the side of an elongate rubber balloon. When this is inflated, 

 the stretched wall of the balloon exerts tension on the paper strip, 

 which represents the vertebral band of Holaspis and the air pressure 

 provides support for this in an analagous manner to the body of the 

 lizard. A balloon modified in this way is substantially harder to bend 

 sideways than an unmodified one. 



When the lateral skin of Holaspis is unstretched, the vertebral 

 band is slack and capable of rucking upwards at its hinges. This 

 permits the lizard a normal amount of lateral movement, when for 

 instance walking rather than gliding, since the band now lacks 

 lateral rigidity. 



The surface: weight ratio ( ' wing'-loading) of Holaspis was roughly 

 assessed on the assumption the whole animal acts as an aerofoil. 

 Area was found by placing straightened preserved lizards belly 

 downwards on squared paper and tracing their outline; maximum 

 lateral extent of the body was then estimated by comparison with 

 photographs of animals basking with their bodies fully expanded, 

 and by stretching the lateral skin. Weight was calculated on the 

 assumption that live lizards weigh 10% more than alcohol preserved 

 ones (Colbert, 1 967) The loadings for four individual adult Holaspis 

 varied between 0.26 and 0.37 gm/cm 2 . These are relatively small 

 figures when compared with those for Draco (Colbert, 1967). Such 

 low loading is probably necessary to compensate for the relatively 

 poor general aerodynamic shape of Holaspis. 



Functional significance of other characters. A minority 

 of derived features of Holaspis are not functionally associated 

 with its main distinctive behaviours The presence of a window in 

 the lower eyelid has developed in a wide range of small lizards 

 that bask directly in the sun in relatively dry microclimates 

 (Arnold, 1973; Greer, 1983). This means that such lizards can 

 reduce the extensive water loss associated with these situations by 

 closing their eyes but still retain vision to detect predators, pass- 

 ing food items etc. In agreement with this explanation, the window 

 is better developed in H. laevis which extends into relatively dry 

 savannah, than in H. guentheri which appears to be confined to 

 forest. Loss of pterygoid teeth in lacertids tends to correlate with 

 the general reduction in ossification found in Holaspis and may 

 be a concomitant of this. 



EVOLUTION OF HOLASPIS 



The order in which new features develop and the situations in which 

 they do so can often be reconstructed by examining states on side 

 branches on the lineage of the taxon concerned. This cannot be done 

 with many features of Holaspis as they have evolved within its 

 exclusive lineage, which by definition lacks side branches, so other 

 cues have to be used for these autapomorphies. However exam- 

 ination of the relatives of Holaspis does give some information. 

 Thus, a degree of climbing is widespread in lacertids as is a modest 

 amount of crevice use. This makes it most parsimonious to assume 

 these behaviours precede gliding, which is unique to Holaspis. 

 These activities and the morphological adaptations associated with 

 them are better developed in Holaspis itself. Improvement in climb- 

 ing modifications may possibly have begun first, as climbing steep 

 tree boles and branches must precede exploiting crevices in them. 



Animal gliders and fliers can be stable or unstable. In stable ones, 

 there is a long lift surface behind the centre of gravity. This means 

 that, as an animal glides, any tendency to pitch in the sagittal plane 

 around the centre of gravity is self-correcting. In pitching, the long 

 posterior lift surface will rise or fall, but the air pressure produced by 

 forward locomotion will return it and the animal as a whole to its 

 original orientation. Unstable fliers with short lift surfaces behind 

 the centre of gravity gain in manoeuvrability but do not self-correct 

 and so require sophisticated neurological mechanisms to maintain 

 appropriate posture in the air, something that is unnecessary in stable 

 forms (Smith, 1952). Unsurprisingly, stable forms evolved before 

 unstable ones in most of the main groups of flying animals, namely 

 insects, pterosaurs and birds, and possibly bats too (Smith, 1952). 



As might be expected from this, Holaspis is a stable glider. The 

 centre of gravity of preserved Holaspis appears to be just behind the 

 midpoint between the two pairs of legs. There is therefore a consid- 

 erable area of lift surface posterior to the centre of gravity, made up 

 of the hind body, hind limbs and tail. Experiments were conducted 

 with models made out of laminated cardboard and weighted to give 

 a wing loading and weight distribution similar to that of Holaspis. 

 When gently launched in the appropriate position, these glided well, 

 confirming that a glider of the dimensions and shape of Holaspis is 

 stable. 



Given inherent stability, gliding ability seems to require only an 

 aerofoil and ability to reach and maintain an appropriate belly-down 

 posture with limbs slightly raised, as well as some ability to trim, at 

 least initially. In tree frogs Cott, (1926) found adoption of initial 

 posture very critical: Phrynohyas venulosa spread its limbs and 

 glided when dropped whereas Hyla arborea, which is morphologi- 

 cally similar, dropped vertically with legs flailing. It might be 

 expected that the ability to adopt the appropriate posture would be 

 confined to Holaspis among lacertids as it is the only known glider, 

 but when tests were carried out on a number of lacertids this 

 propensity was found to be widespread, being present in completely 

 terrestrial lizards such as Lacerta agilis and Acanthodactylus 

 erythrurus, as well as climbing ones (Arnold, 1989a). The wide 

 distribution of this ability suggests it confers advantage in another 

 more general context and may have arisen there. This context may 

 be terrestrial locomotion. Certainly running lacertids seem to have 

 to continuously adjust their body positions and, at some points in the 

 stride cycle, they may be balanced on only a single toe (Arnold 

 1998b), so good neurological control of posture seems to be essen- 

 tial in this activity. 



The production of an aerofoil is likely to largely result from direct 

 adaptation, as many of the features of Holaspis appear to confer 

 advantage only in gliding. However rib spreading, like balance, is 



