INTRODUCTION TO MAMMALIAN PALEONTOLOGY 



23 



origin of similar characters in other families of Peris- 

 sodactyla, has accordingly been studied with great 

 care. 



Retrogressive characters. — The retrogression or disap- 

 pearance of characters is illustrated in the history 

 of the titanotheres by the features enumerated below. 



1. Reduction of the canine teeth in many later titanotheres. 



2. Reduction and occasional loss of incisors. 



3. Reduction and frequent loss of first lower premolar. 



appearance in North America and western Europe of 

 members of nine different families of Perissodactyla, 

 the odd-toed ungulates, which were probably all de- 

 scended from a common ancestral or stem form which 

 lived in Upper Cretaceous time. The probable charac- 

 ters of this stem form are fully described in Chapter X, 

 where it is shown that the ancestral perissodactyl was 

 a comparatively small and simple quadruped not ex- 



Perissodxictyls 



CRETACEOUS 



Peris sodaxtyL Stem, 



Figure 17.- 



-Successive invasion of nine perissodactyl families in Nortli America and western Europe between 

 latitudes 40° and 50° 



Th2 chalicotheres (aberrant clawed perissodactyls with affinities to the titanotheres) are regarded as members of a separate superfamily, the 

 Chalicotheroidea. Diagonal shading indicates the extent to which each phylum is represented by fossil remains. 



4. Reduction and loss of protoconule and metaconule in 

 upper molars. 



5. Reduction of nasals and their coalescence with frontals. 



6. Reduction of the trapezium in later titanotheres. 



PHYLOGENY OF THE NINE TYPICAL FAMILIES OF THE 

 PERISSODACTYLA 



The competition of the titanotheres through natural 

 selection was naturally closest with other members of 

 the order Perissodactyla. As shown in the ordinal 

 phylogenetic tree (fig. 17), we observe the successive 



ceeding half a meter in height, and that it was origi- 

 nally confined to a definite geographic area, feeding 

 ground, and range, very possibly in northern Asia. 

 The eight families that appear in North America and 

 the paleotheres, which appear only in western Europe, 

 were by no means equally distinct from one another. 

 They were originally separated from the stem form 

 not into nine branches but into five great main 

 branches, termed superfamilies, as shown in Figure 17 

 and in the accompanying table. 



