NICHE, MORPHOLOGY AND LOCOMOTION IN LACERTID LIZARDS 
Ss 
Fig. 6 Lateral view of the bones of the distal part of the digit of a 
climbing lacertid, showing dorsal and ventral tendons (black) attaching 
to deep, claw-bearing distal phalanx. The sesamoid bone (s), which can 
slide on the surface of the penultimate phalanx, displaces the dorsal 
tendon away from the hinge-line of the articulation between the two 
phalanges, increasing its moment arm around the centre of rotation and 
its efficacy in raising the distal phalanx and its claw. 

Fig. 7 Lateral and dorsal views of fourth hind digits. a., c. Ground- 
dwelling lacertid, Lacerta agilis. b., d. Specialised climber, Lacerta 
oxycephala. 
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The dorsal tendon of each digit which inserts on the final claw- 
bearing phalanx encloses a small sesamoid bone that lies close to the 
articulation of this phalanx with the penultimate one (Fig. 6). This 
digital sesamoid acts like a more familiar one, the patella (knee cap) 
of many mammals, in enhancing the efficacy of the tendon by 
moving it away from the hinge-line of the articulation and increasing 
its moment arm around the centre of rotation of the distal phalanx 
(Curry, 1984). 
There are considerable differences in proportions of the pes and in 
shape and relative orientation of the phalanges and claws. The 
extremes are found in strictly ground dwelling forms, especially the 
more advanced members of the Armatured clade, and in those that 
climb extensively on steep open continuous surfaces. Basic differ- 
ences are summarised in Table 2. 
The pes in ground dwelling lacertids from open situations (Figs 
Ta,c, 8a, 9a, 10a ). 
In advanced members of the Armatured clade, like Acanthodactylus, 
the whole foot is large and metatarsal bones | to 4, and the digits 
arising from these, are especially long and increase successively in 
length. In some instances, such as Heliobolus lugubris the metatarsal 
bones are more or less parallel and bound closely together. Digits 1— 
4 are also elongated but digit 5, which arises from the highly modified 
fifth metatarsal bone, is frequently short and may be miniaturised, its 
phalanges and claw being much smaller than those of other toes. In 
extreme cases like Heliobolus lugubris, the whole fifth toe only 
extends as far as the distal end of metatarsal 4. Similar substantial 
reduction also occurs in /chnotropis capensis. Toes are straight or 
gently curved ventrally when at rest (Fig. 9a) and are rounded in cross 
section (Fig. 10a). The phalanges themselves are robust (Fig. 9a) and 
tend to become steadily shorter distally in each digit.Although the pre- 
penultimate phalanx of toes 3 and 4, may sometimes be a little shorter 
than contiguous ones this is not very marked. The terminal phalanx of 
each digit and the claw that covers it is relatively long, shallow and 
curves gently downwards. The prominence on the terminal phalanx, 
to which the ventral tendon of the digit is attached, is relatively close 
to the centre of rotation of the claw (Fig. 17c). 
Articulations within the digits are double consisting of two hori- 
zontally arranged protruberences on the distal end of each phalanx 
that fit into two hollows on the proximal end of the adjoining one. 
Although the articulations all appear at first sight to be ginglymi, 
only the most distal one totally restricts movement to the vertical 
plane. The others in digits 24 allow these toes to be flexed laterally 
so they can curve quite easily in this direction, even though they are 
rather stiff basally. However, mesial flexion of these digits is more 
restricted and they can only form a gentle curve in this direction. The 
different extents of lateral and of mesial movement within these 
digits presumably depends on the degree of restriction produced by 
the ligamentous connections on each side of the articulations and by 
accessory tendons. Digit 5 swings easily around its base but joints 
within it, while allowing some movement, are generally stiffer in the 
horizontal plane than those in digits 24. All digits can be flexed 
extensively downwards and upwards when the muscles controlling 
them are relaxed. 
Similar structure of the pes is found throughout the open-ground 
forms that constitute the clade made up of Latastia and its advanced 
sister group; it is also approached in many aspects in such ground- 
dwelling primitive Palaearctic species as Lacerta agilis (Fig. 9a, 10a). 
The pes in lacertids regularly climbing on steep open surfaces 
(Figs 7b, d, 8b, 9b—-e, 10b). 
In Lacerta oxycephala, a species that habitually climbs on precipi- 
tous rock outcrops (Arnold, 1987), the foot is small and metatarsal 
