564 Comparative Morphology of Chordates 



differentiated for crushing or chewing food. The degree and character 

 of the differentiation of the several types of teeth varies greatly in the 

 numerous orders of mammals and is one of the most important charac- 

 teristics used in defining the groups. These peculiarities of skull and 

 teeth have been most important in making possible the recognition 

 of mammalian fossils. 



The history of mammals has been traced, so far as the main facts 

 are concerned, in a fairly satisfactory way as far back as the latter part 

 of the Mesozoic. The fossil material is, so far, much more abundant 

 than that of birds. Earlier Mesozoic strata have as yet yielded only 

 scanty and fragmentary mammalian material — mostly incomplete 

 skulls and teeth — but it is unquestionably mammalian, so far as can 

 be judged from skeletal parts and teeth. It is likely, therefore, that 

 mammals came into existence in the Triassic Period, probably in the 

 early part of it. It is just here that facts of special significance come 

 to light. 



The earliest known reptiles (cotylosaurs), some of them hardly 

 distinguishable from amphibians, are found in the Permian. Their 

 skulls are of the anapsid type (see p. 501), and the occipital condyle 

 is single. As the primitive group began to differentiate, there appeared 

 some animals which, while unquestionably reptilian, yet had some 

 peculiarities suggestive of mammals — hence called Theromorpha, 

 meaning "mammal-like." The skull was of the synapsid type (Fig. 

 394). In some later (Triassic) theromorphs, the Therapsida, there 

 were two occipital condyles. In some of these "mammal-like" 

 reptiles there was a rudimentary secondary palate, incomplete pos- 

 teriorly because the right and left palatine bones were not fully joined 

 in the median line. But Cynognathus (Fig. 437B), a Triassic therap- 

 sid, had a complete and distinctly mammalian secondary palate. The 

 theromorphs retained the reptilian joint between the articular of the 

 lower jaw and the quadrate of the upper, but in Cynognathus and others 

 the quadrate was very small and apparently not firmly joined to 

 neighboring bones, while the dentary, chief dermal bone of the lower 

 jaw, was greatly enlarged, at the expense of other bones of the lower 

 jaw, and came into such close relation to the squamosal as to indicate 

 that the dentary and squamosal must have served to augment the 

 joint between articular and quadrate. The tympanic mechanism, how- 

 ever, is reptilian, only a single auditory ossicle, the columella, being 

 present. 



The earlier theromorphs show a tendency toward a heterodont 

 dentition, and in some of the later (Triassic) therapsids, such as 

 Cynognathus, the dentition is definitely of the mammalian pattern and 



