ORIGIN, RELATIONSHIPS, AND PHYLOGENY 



Figs. 2-7 



General: The age-old question of what constitutes a genus or a species in paleontology is one 

 of the many problems connected with a revision of the merycoidodonts. It is true that scarcely two 

 specimens of this group can be considered as exactly alike, and I am surprised, not that so many 

 genera and species have been described but that, under the circumstances, so few have been added 

 to the literature. 



Several thousand specimens of various genera of merycoidodonts have been studied, and I have 

 attempted to segregate the characters of the male and female, as well as to tabulate the changes in 

 old age, in order to reduce the number of species, but every scheme has failed of definite results 

 when applied to large numbers of specimens. As a consequence, while it seems certain that some of 

 the species are synonymous, I cannot furnish proof of this synonymy on the basis of sex or age. In 

 many instances I have indicated wherein I believe that synonymy lies, without always actually plac- 

 ing the species together. 



Matthew in 1930 said: "Once the species is placed on the books, it appears to be regarded as a 

 personal discourtesy for any subsequent reviser to question its validity." I sincerely trust that none 

 of my colleagues will feel any "personal discourtesy" in this revision. 



It was the great amount of fossil material and the large number of described species which led 

 Earl Douglass thirty-five years ago to write, in connection with his Limnenetes flatyce-ps, as follows: 



It is difficult to make comparisons or draw conclusions on account of the chaotic condition of this family. 

 It probably will remain so until some one .... can have the opportunity of studying the vast amount of material 

 that has been collected and who will undertake the enormous task of 'straightening things out.' 



Since that time the situation has become steadily worse. This status of the problem and the 

 difficulty of identification of the large amount of material in the Peabody Museum led the writer to 

 assemble illustrations of every described species, together with those of many undescribed speci- 

 mens, and to condense every reference to this family — a time-consuming occupation. The results of 

 this study I now pass along in the hope that they may be considered as a preliminary assemblage of 

 data on which may be erected a permanent and concrete understanding of the merycoidodonts. I 

 trust that there will be a supplement to this present work, after we have discovered more skeletal 

 material, which is after all the real basis for any phylogeny of this and all other families. 



I believe that the prevalent idea of considering that species and often genera among fossil 

 mammals seldom cross geologic horizons has led to the establishment of many species which other- 

 wise have very little basis for differentiation from older or younger species previously described. 

 The oreodonts oftentimes show a localization of forms within definite areas, but certainly this does 

 not always hold true. At most I believe that many of these species should be considered as muta- 

 tions or subspecies and not of higher rank. 



Loomis made a very real contribution to the classification of these genera in his study of the 

 premolar pattern, and he is convinced of variations between males and females and between youth 

 and old age. Others have come to the same conclusions, but the differences as outlined are inade- 

 quate when applied to a large series of skulls in different genera. The female supposedly has a 

 smaller, narrower skull and is more lightly built, but if this is followed to its logical conclusion, we 

 find in some groups a segregation of all males or all females in one locality or one horizon, which 

 leads to an absurdity. The wear of the teeth changes in different stages of life, but, except in a few 

 instances, I believe that errors along this line have been avoided. 



Origin: Of the ancestors of this family we know nothing of a certainty. In America no 

 remains of mammals which seem to be ancestral have been found in strata earlier than the Uinta, 

 and the evidence points to the merycoidodonts as being immigrants rather than members of the 



