58 
Northcutt 
two divisions are considered sister radiations 
with the dracomorphs possessing the more 
derived characters. These characters are : 
(1) elaboration of tectal lamination with 
specific hypertrophy of lamina 14, subdivi- 
sion of lamina 7, and marked reduction of 
periventricular laminae; (2) hypertrophy of 
the pretectal nuclei with particular elabora- 
tion of the dorsal and ventral pretectal nuclei 
as well as nucleus geniculatus pretectalis; 
(3) expansion of the dorsal thalamus; and 
(4) complex migration and hypertrophy of 
the dorsal ventricular ridge of the telenceph- 
alon. 
The close affinity of teiids and varanids is 
suggested on the basis of the remarkable 
similarity in their brain structure. As noted 
earlier (with regard to the studied brain 
characters) it is possible to separate teiids 
and varanids only by differential develop- 
ment of nucleus sphericus, which is lamin- 
ated in varanids but not in teiids, by medio- 
lateral differences in the thickness of lamina 
14, and by a thicker tectal lamina 5 in 
teiids. While most taxonomic schemes have 
suggested close affinity between teiids and 
lacertids, Etheridge (1967) has suggested a 
close affinity between teiids and iguanids. 
Clearly lacertids and teiids exhibit the ex- 
treme range of brain variation seen in liz- 
ards, with the lacertids possessing charac- 
ters that appear very similar to gekkonids 
and scincids, and the teiids and varanids 
possessing characters identical or most sim- 
ilar to the iguanoids. Thus, the morphology 
of the central nervous system would indicate 
close pylogenetic affinity among teiids, 
varanids, and iguanoids. 
An alternate interpretation, however, 
must be considered. It is possible that the 
similarities in the assembly of taxa termed 
dracomorphs are due to parallelism and not 
to close phyletic affinity. Simpson (1961) 
defined parallelism as “the development of 
similar characters separately in two or more 
lineages of common ancestry and on the basis 
of, or channeled by, characteristics of that 
ancestry.” It is possible, therefore, that the 
derived brain characters of the dracomorphs 
have evolved three times independently due 
to similar selective pressures acting on sim- 
ilar genomes which can reasonably be as- 
sumed to exist within a monophyletic group 
such as lizards. 
Parallelism most frequently occurs when 
closely related taxa have invaded similar 
ecological zones so that similar selective 
pressures are operating. Thus, arguments 
accepting or rejecting suspected parallelism 
must include consideration of similarities in 
the ecological adaptations of the suspected 
taxa. Arguments based on the degree of 
similarity in characters, which are of major 
importance in establishing homology, are of 
little importance in determining parallelism 
since closely related taxa are expected to 
possess a high degree of similarity due to 
recent common ancestry. One additional test 
regarding parallelism may exist. This re- 
lates to the complexity of the character set 
suspected of parallelism, and the probability 
of its repeated independent occurrence. Mayr 
(1969) discusses a number of cases of par- 
allel evolution such as structural similarities 
related to brachiating in primates, indepen- 
dent adaptation for burrowing in insecti- 
vores, soldier mandibles in termite genera, 
and secondary jaw articulations in birds. In 
each case, the adaptations involve a limited 
number of morphological elements and are 
adaptations to a specific ecological regime. 
These examples suggest that parallel evolu- 
tion should occur most frequently in func- 
tional systems with fewer morphological 
features than in complex systems, and that 
parallel evolution most frequently occurs in 
response to very specific ecological adapta- 
tions. 
At present the behavior and ecology of the 
Varanoidea and Iguanoidea are distinctly 
different. There is no obvious common eco- 
logical adaptation (s) that links these two 
groups. While both groups are primarily 
diurnal, so are the lacertids as well as many 
other taxa grouped in the Lacertomorpha. 
Attention has focused at this conference on 
two alternate types of predation — active 
foraging versus sit-and-wait predators. The 
dracomorphs clearly do not fall into one of 
these two categories. While many of the 
