80 
the pes may be small in some of these forms and the digits are often 
not strongly kinked and have robust subequal phalanges and rounded 
cross sections. The syndrome is best developed as whole in 
Psammodromus hispanicus and L. agilis. 
In some cases, a mixture of features typical of ground-dwelling 
and other activities occur. This may be a result of functional compro- 
mises, for instance in forms that are substantially ground dwelling 
but also occur in other situations. In Lacerta vivipara, a ground form 
that spends substantial time in dense grassy vegetation, many features 
associated with ground dwelling are present but metatarsal 4 is short 
and toe 5 quite long. Possibly the way the feet of this species are used 
in traversing vegetation has similarities to climbing. In Poromera, 
the foot has some features associated with ground locomotion and 
some with climbing quite strongly developed. However, in spite of 
probably sometimes climbing in vegetation, the hands and feet of 
Philochortus are essentially of the ground type. 
Overall, direction of change in foot morphology appears to follow 
closely that of structural niche in lacertids (p. 00). 
The variations in the pes found in lacertids are paralleled quite 
closely in some other families. For instance, within the sister group 
of lacertids, the Teiioidea, the Tetidae which are mainly ground 
dwelling in open situations have the pedal characteristics of lacertids 
occupying similar structural habitats. As here, the fifth toe is usually 
miniaturised and in Jeius disappears entirely, something that also 
occurs in the ground running agamid Sitana (Russell and Rewcastle, 
1979). 
Special structures of the digits 
In primitive Palaearctic lacertids and more basal members of the 
Armatured clade including Nucras, the toes are covered above with 
a single row of unkeeled scales along their length and below by a row 
of scales or lamellae that correspond more or less to those above. The 
lower row is often tubercular and each scale may be divided cen- 
trally, although this feature varies considerably, sometimes even 
among subdigital scales on the same toe. A number of modifications 
of this primitive external toe structure occur. 
Expanded subdigital lamellae 
Takydromus kuehnei is unique among lacertids in having the more 
proximal subdigital lamellae of the digits clearly expanded laterally 
to form a narrow pad superficially similar to those of geckoes such 
as Cyrtodactylus. This feature, towards which there is a slight 
tendency in some other Takydromus, may possibly enhance adhe- 
sion on the surfaces of the vegetation, among which these lizards are 
often found, by increasing the lower surface of the toes. However, 
SEM studies reveal no microornamentation of adhesive setae on the 
subdigital lamellae of Takydromus kuehnei (pers. obs.), such as are 
found in other pad bearing climbing forms including many geckoes 
and anoles, and the skink, Prasinohaema virens (Williams & Peter- 
son, 1982). 
Keeling of subdigital scales 
Instead of being tubercular, the scales beneath the digits of lacertids 
may bear keels which, in ventral view, appear more or less parallel 
to the axis of the digit. In these cases the free edge of each scale and 
its keels are directed obliquely downwards, the latter ending in 
projections. When a toe is put down on a smooth flat surface, contact 
with this is largely limited to these points. Downwardly directed 
scales with keels ending in projections also occur on the palms and 
soles. 
A tendency to keeling, often with considerable individual varia- 
tion occurs in most Psammodromus species and in Philochortus. 
E.N. ARNOLD 
Fully developed and consistent keeling is found in the advanced 
clade of ground dwellers in the Armatured clade that constitutes the 
sister group of Philochortus. Full keeling has evolved independently 
in Psammodromus hispanicus (presumably from the intermediate 
condition in other members of the genus), in Omanosaura cyanura, 
and in Lacerta cappadocica; there are thus four origins of the 
condition within the Lacertidae. 
The number of keels on subdigital lamellae varies: two is most 
frequent but there are sometimes several, something which is com- 
moner on the manus than the pes. Single keels also occur, in Lacerta 
cappadocica and in dune dwelling species of Meroles, 
Acanthodactylus and Eremias in which they are associated with less 
downward projection of the edge of the scale and little development 
of projections at the tips of the keels. In at least the first two genera, 
the shift to single keels has happened more than once. 
Keeling on subdigital scales may vary within a species, for 
example there may be one to several in different populations of 
Acanthodactylus grandis (Arnold, 1983). This suggests keeling is 
quite labile in detailed form. Species that live exclusively on very 
fine aeolian sand may lose keeling secondarily, something that has 
developed independently in Meroles anchietae and Eremias 
(Scapteira). 
Evolutionary shift to keeling does not appear to be related to 
changed locomotory requirements and instead may be more import- 
ant in protecting the toes from high temperatures (Arnold, 1973). 
Some desert lacertids are at least briefly active on surfaces as hot as 
60°C (pers. obs.), even though their digits incorporate delicate blood 
vessels and nerves. In this situation, limiting contact with the ground 
largely to the projections at the end of keels is likely to reduce heat 
intake, especially as keratin, of which the subdigital lamellae are 
formed, is a good insulator. If this is so, keeling may not be important 
as such but only as a means of providing support for the projections 
that actually contact the ground. Similar support of projections by 
keels is found in the belly scales of many Takydromus species, 
although here the projections appear more important in increasing 
frictional contact rather than in insulation (Arnold, 1997). 
In the Armatured clade, the shift to keeling is associated with 
movement into hot open ground habitats and the same is true in 
Psammodromus. The Lacerta and Omanosaura with keeled digits 
are rock-dwellers but in particularly warm areas. 
It is not clear why aeolian sand species often exhibit reduction 
from double or multiple to single keels with less downward inflexion 
of the free edges of the subdigital scales, and sometimes totally lose 
these features. One possibility is that the keeling and the associated 
projections will not be able to keep the digits substantially out of 
contact with the ground, because the toes of running lizards usually 
sink into soft sand, at least to some extent, so projections supported 
by keels will not restrict contact. In fact, the sinking may also reduce 
the problem of heat load since the digits are only briefly in contact 
with the very hot uppermost layer of sand and pass rapidly through 
it into the rather cooler layers below. 
Outside the Lacertidae, digital keeling occurs in many other lizard 
families and is usually associated with hot substrata. It is found in 
many iguanians, scincids and cordylids that occur in sunny situa- 
tions, but is absent in largely nocturnal or mesic clades such as 
gekkotans, xantusiids and anguids. The development of full keeling 
is probably associated with modest body size, a situation in which 
the problems of overheating of the extremities are likely to be 
particularly acute. 
Digital fringes 
Lateral and often mesial fringes of pointed scales on the digits have 
developed in at least five separate clades of the Lacertidae: in 
