Behavior and Neurology of Lizards 
N. Greenberg and P. D. MacLean, eds. 
NIMH, 1978. 
Forebrain and Midbrain Organization in Lizards and Its 
Phylogenetic Significance 
R. Glenn Northcutt 
Division of Biological Sciences 
University of Michigan 
SUMMARY. The telencephalon, diencephalon, pretectum, and optic tectum are examined in 
representatives of all living lizard families. Two patterns of organization in the central nervous 
system of lizards are suggested on the basis of the observed variation in these brain areas. A 
lacertid pattern is defined, which characterizes taxa of the following families: Anguidae, Cor- 
dylidae, Dibamidae, Gekkonidae, Helodermatidae, Lacertidae, Lanthanotidae, Pygopodidae, 
Scincidae, Xantusiidae, and Xenosauridae. These taxa are compared to Sphenodon and are be- 
lieved to retain more primitive characters than taxa possessing the second, or igtuinid, pattern. 
The iguanid pattern characterizes taxa of the following families : Agamidae, Chamaeleonidae, 
Iguanidae, Teiidae, and Varanidae. 
The major evolutionary changes in the central nervous system of lizards consist of a hyper- 
trophy and differentiation of a multiple sensory area in the telencephalon, the dorsal ventricular 
ridge and its related pathways, and pretectum and central and superficial zones of the optic 
tectum. The functional, behavioral, and phylogenetic implications of these trends are discussed. 
INTRODUCTION 
Most comparative neuroanatomical studies 
on lizards have been stimulated by the view 
that these animals represent an early and 
simple evolutionary grade of land verte- 
brates leading to mammals. The assumption 
has been that if the “simple” brains of rep- 
tiles could be understood, then study of more 
“complex” mammalian brains would be 
greatly facilitated. Such studies usually have 
utilized a single species chosen by availabil- 
ity, experimental convenience, and specula- 
tion regarding supposed primitiveness. 
While it is probably true that mammals, 
as an adaptive radiation, possess a more com- 
plex brain organization than do living rep- 
tiles, it is equally true that the living reptiles 
represent distinct and very different radia- 
tions than the reptiles that gave rise to 
mammals (Romer, 1966). Thus, an under- 
standing of the neural organization and its 
behavioral correlates in living reptiles may 
tell us little, if anything, directly regarding 
the brain and behavior of the reptilian 
therapsids that gave rise to mammals. 
If an examination of living reptiles will 
not allow us to reconstruct a model of the 
protomammalian brain, then why study rep- 
tiles at all? Besides the obvious answer that 
we may be interested in reptiles as a suc- 
cessful group of vertebrates occupying a 
variety of niches and possessing a wide 
variety of interesting behaviors worthy of 
study in their own right, the evolutionary 
biologist can remind us that reptiles were 
the first vertebrates to be truly terrestrial, 
and each reptilian radiation has solved many 
of the major obstacles to successful land 
invasion in a strikingly different manner 
(Gans, 1974). 
Recognition of these different patterns of 
adaptation and their advantages and limita- 
tions will allow us to understand how differ- 
ent solutions have occurred in response to 
similar biological problems and, when com- 
pared to avian and mammalian solutions, 
will clearly tell us not only a great deal about 
living reptiles, but will also add new mean- 
ing to the patterns we see in birds and 
mammals. Thus, I believe the major task 
of comparative neurobiology is not to recon- 
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