Brain Research on Lizards 
5 
both correct and useful to refer to species- 
typical behavior. As ethologists have empha- 
sized, a species can be identified as readily 
by its behavioral patterns as by its morpho- 
logical characters. 
THE MAMMAL-LIKE REPTILES 
Curiously enough, in books and articles 
on evolution, one usually finds little emphasis 
placed on the mammal-like reptiles. Few 
people seem to be familiar with the mammal- 
like reptiles, and fewer still are aware that 
they once covered the face of the earth. 
Romer speaks to this point when he says, 
“The relatively late time at which the mam- 
mals took over the world’s supremacy from 
the reptilian dynasties would lead one to 
think that the stock from which they sprang 
must have been one developed at a compara- 
tively late date in reptilian history. This, 
however, is exactly the reverse of the true 
situation. The mammal-like reptiles, consti- 
tuting the subclass Synapsida, were among 
the earliest to appear of known reptilian 
groups and had passed the peak of their 
career before the first dinosaur appeared 
on the earth” (1966, p. 173). 
The synapsids (one temporal opening) de- 
veloped from the so-called stem reptiles 
(Cotylosaurs) in Pennsylvanian times — 
roughly 250 million years ago (see Fig. 5). 
During Permian and early Triassic times — 
for a period of about 100 million years — 
the mammal-like reptiles populated the world 
in profuse numbers. Today their remains are 
found on every continent including Antarc- 
tica, signifying that there once may have 
existed one massive continent called Gond- 
wanaland (see Colbert, 1972). Robert 
Broom calculated that there must be at least 
800 billion skeletons of mammal-like reptiles 
in the Karroo beds of South Africa (1932, 
p. 308). 
There were two main varieties of these 
animals — carnivores and herbivores. Remi- 
niscent of the Serengeti Plain, some of the 
carnivores are believed to have preyed on 
herds of herbivores. What were these ani- 
Figure 5. Family tree of reptiles 
(from Colbert, Figure 20, 1966). 
mals like? Some of the advanced carnivores 
are believed to have resembled dogs and 
wolves. Unlike their waddling predecessors, 
they had gotten up off their bellies and with 
legs supporting the body from underneath 
were able to run swiftly. The jaws and teeth 
were beginning to show mammalian charac- 
teristics. 
Figure 6 shows the family tree of the 
Therapsids. In regard to the evolution of 
mammals, the two lines of main interest are 
shown on the left, with the Gorgonopsia lead- 
ing up through the Cynodontia (dog-toothed) 
to the Tritylodontia, and the other line repre- 
sented by Therocephalia, leading up through 
Bauriamorpha to the Ictidosauria (weasel- 
like). The latter are said to be best repre- 
sented by Diarthrognathus discovered in 
South Africa. As the name implies, the jaw 
has dual characteristics, being a combination 
of the reptilian quadrate-articular joint and 
the mammalian squamosal-dentary joint 
(Colbert, 1969, p. 143). Both Romer (1966) 
and Colbert (1969) place this animal at the 
dividing line between reptiles and mammals. 
