Forebrain and Midbrain of Lizards 
59 
teiids and varanids are active foragers, many 
of the iguanoids are clearly sit-and-wait 
predators. Additionally, it should be noted 
that the agamids and iguanids also include 
the most striking examples of herbivory 
among lizards and the most complex social 
systems. 
While the present iguanoid and varanoid 
adaptations are distinct, they do not preclude 
the possibility that similar selective pres- 
sures were operating at some point in their 
past history. This possibility can be explored 
only with further information regarding the 
fossil record of these families, information 
which, in the long run, is also likely to be 
most fruitful in determining if the draco- 
morphs constitute an assemblage of parallel 
forms. 
The derived brain characters that charac- 
terize the dracomorphs do not appear to re- 
flect adaptations to a specific ecological situa- 
tion. They clearly cannot be accounted for 
in terms of hypertrophy of any single sen- 
sory modality, nor are they associated with 
motor specializations of any single region of 
the body. The brains of dracomorphs can 
best be described as the most complex among 
living lizards, with increase in both size and 
differentiation of most sensory modalities. 
The one exception is the reduction or loss 
of the vomeronasal system in iguanoids. The 
hypertrophy of this system in varanoids is 
the single most useful neurological sorting 
character in separating varanoids from 
iguanoids. 
In summary, the dracomorphs are char- 
acterized by extensive and complex derived 
brain characters that do not relate to any 
single sensory or motor adaptation, nor do 
the living taxa share any obvious ecological 
adaptation. For these reasons, I believe the 
families assigned to this division share a 
more immediate ancestry with one another 
than with other lizards and are not an 
assemblage due to parallel evolution from a 
more remote common ancestor. 
It is possible to recognize three major 
groups or superfamilies within the lacer- 
tomorphs: gekkoids, lacertoids, and lanthan- 
otoids (Fig. 22B). The gekkoids include the 
pygopodids, gekkonids, and xantusiids. Rep- 
resentatives of these three families exhibit 
an enlarged nucleus dorsolateralis and medial 
cortex. Both pygopodids and xantusiids ex- 
hibit specialized medial ridge plates, and 
gekkonids and pygopodids possess almost 
identical caudal ridge plates. Finally gek- 
konids and pygopodids possess similar peri- 
ventricular tectal laminae. These neural 
characters are derived with respect to 
Sphenodon with the exception of the caudal 
ridge plate. The close affinity of gekkonids, 
pygopopids, and xantusiids is also supported 
by considerable nonneurological data as well 
(McDowell and Bogert, 1954; Shute and Bel- 
lairs, 1953; Miller, 1966; and Underwood, 
1957). 
The lacertoids constitute a large group, 
six families, within which three trends can 
be recognized with regard to neural develop- 
ment. The lacertids and scincids possess very 
similar brain features. Both families are 
characterized by asymmetrical ridge plates 
in which the lateral division is the thickest, 
pronounced cell clusters of the ridge plate, 
and tectal laminae similar to Sphenodon. 
Skinks differ from lacertids in possessing a 
poorer developed lateral ridge plate and do 
not exhibit a distinct tectal lamina 4 or 
distinct subdivisions of tectal lamina 5. 
Both the anguids and xenosaurids exhibit 
specialized medial ridge plates with little or 
no cell clustering and the core cells are larger 
than those of skinks and lacertids. Xeno- 
saurids can be separated from anguids on the 
basis of a thinner tectal lamina 5. 
The cordylids may be closely related to 
the xenosaurids. Both possess well-developed 
ridges and similar tectal lamination How- 
ever the cordylid ridge, unlike that of 
Xenosaurus, is characterized by pronounced 
cell clusters. The cordylids are particularly 
interesting as they are diurnal insectivores 
exploiting a desert niche much like that of 
many agamids and iguanids. Thus cordylids 
might be expected to have developed a num- 
ber of neural similarities to agamids and 
iguanids. At present, my analysis suggests 
that while the cordylids do possess well 
developed brains, they do not share any of 
