6 
MacLean 
Figure 6. Family tree of therapsids (from Romer, Fig. 275, 1966). 
In addition to the changes in the teeth, 
jaw, and posture that have been mentioned, 
other significant differences in the advanced 
reptiles include (1) a further widening of 
the temporal fossa, (2) the development of 
a secondary palate, (3) a disappearance of 
the pineal opening (see below), and (4) a 
phalangeal formula similar to mammals. 
What else can be said of these animals? 
Did they lay eggs? Did they care for their 
young? Or like the contemporary Komodo 
lizard, did the young have to escape to the 
trees to avoid being cannibalized (Auffen- 
berg, this volume) ? Since the most primitive 
existing mammals lay eggs, it is usually 
assumed that the mammal-like reptiles did 
also. At least one case has been reported of 
an adult skeleton — the cynodont called 
Thrinaxodon — close to the skull of an im- 
mature animal of the same kind. This finding 
has suggested that some of the mammal-like 
reptiles may have developed parental care 
(Colbert, 1969, p. 140). 
In the evolution of mammals, the develop- 
ment of vocalization and hearing became of 
the utmost importance for maintaining 
parent-offspring relationships under condi- 
tions of obfuscation. Could the mammal-like 
reptiles hear and vocalize, or were they essen- 
tially dumb like many of today’s lizards 
(Marcellini, this volume)? In the advanced 
forms, the quadrate and articular bones were 
becoming smaller, but were far from being 
transformed, respectively, into the incus and 
malleus of the mammalian internal ear. 
Hotten has devised experiments that suggest 
that mammal-like reptiles had auditory per- 
ception in the low range of frequencies 
(Hotten, 1959). Nothing can be surmised 
regarding the question of vocalization. 
Late in Triassic times, the mammal-like 
reptiles mysteriously became extinct. Fero- 
cious, and possibly cannibalistic as they 
were, it is probable that they did not bring 
about their own destruction. Rather it is 
presumed that a swifter, more ferocious kind 
of reptiles, the thecodonts (the forerunners 
of dinosaurs) began to outnumber and 
