190 INTRODUCTION TO EVOLUTION 



Accordingly, a general practice has been to draw the line between 

 therapsid reptiles and mammals at that point at which the lower jaw came 

 to consist of but one pair of bones, the right and left dentaries, articulat- 

 ing directly to the skull. We have seen that reptiles have several bones 

 in each half of the lower jaw (p. 172). The dentary is the principal tooth- 

 bearing bone but the connection of the lower jaw to the skull is made 

 by one of the other bones, the articular, which is hinged to the quad- 

 rate bone of the skull. In therapsid reptiles the bones other than the den- 

 tary became progressively reduced in size, while the dentary itself became 

 progressively larger, and extended back toward the squamosal bone of the 

 skull (Fig. 8.25, p. 171; in this lateral view the quadrate is hidden by the 

 squamosal, "sq," and the articular bone of the lower jaw is posterior to 

 the surangular, "sa"). Eventually the dentary became hinged to the 

 squamosal, and the articular and quadrate bones, greatly reduced in size, 

 lost their function of hinging the jaw and became the malleus and incus 

 ("hammer" and "anvil") of the chain of three little bones in the middle 

 ear. Animals having the dentary articulating directly with the skull in this 

 way are mammals. Interestingly enough, Mesozoic vertebrates have been 

 found having two jaw articulations side by side: articular with quadrate, 

 dentary with squamosal. Were such creatures reptiles or were they mam- 

 mals? If as we have stated animals having the dentary hinged to the 

 squamosal are mammals, they were mammals (Simpson, 1959). But in 

 calling them so we are drawing an arbitrary line. 



This difficulty of distinguishing certain therapsid reptiles from mam- 

 mals is highly significant. It arises from the existence of a series of transi- 

 tional stages linking typical reptiles to typical mammals. Such a series of 

 transitional stages occasions no surprise if mammals evolved from rep- 

 tiles by gradual process of change but would be entirely inexplicable if 

 mammals had been separately created. 



Evidence accumulates that several groups of therapsid reptiles gave rise 

 to descendants that would be regarded as mammals by the criteria men- 

 tioned (Olson, 1959; Simpson, 1959). Some of the lines became extinct; one 

 apparently led to modern monotremes, and another to the Pantotheria 

 (see below) and thence to marsupials and placentals. 



Undoubted mammals lived in the Jurassic, but they were not a prepos- 

 sessing tribe compared to the ruling reptiles of the time. All of them were 

 small, most of them of the sizes of mice and rats. One species approached 

 the cat in size and apparently in carnivorous food habits, while one herbiv- 

 orous species resembled a woodchuck in many ways. One hopeful portent 

 for the future was presented by the brains of these early mammals. Al- 

 though small and primitive, judged by modern standards, nevertheless the 



