CHAPTER IV 



SYSTEMATIC CLASSIFICATION OF THE TITANOTHERES 



SECTION 1. 



PHYLETIC VERSUS LINNAEAN SYSTEM 

 OF CLASSIFICATION 



NEO-LINNAEAN SYSTEMATIC DIVISIONS (ZOOLOGIC) AND 

 EVOIUTIONARY PHYIA (PALEONTOIOGIC) 



As explained in the introduction, the Linnaean 

 system was based on the theory of the special creation 

 of all systematic divisions coinciding in geographic 

 space, so that its application to our modern paleonto- 

 logic phyla, which succeed one another over long 

 periods of geologic time, is beset with great difficulties 

 and has led to different uses of systematic terms by 

 different authors. The present monograph employs 

 a phyletic system which has been used by the author 

 since 1892 in the classification of the Perissodactyla 

 (Osborn, 1892.67, pp. 90-94). 



The taxonomic principle is that ancestral affinity is 

 stronger than contemporary resemblance. Thus an 

 animal that is directly ancestral to the titanotheres is 

 placed in the family Brontotheriidae; an animal that 

 is directly ancestral to BrontotJierium is placed in the 

 subfamily Brontotheriinae; a series of ascending 

 species in the same line are placed in the genus 

 BrontotJierium; a series of "ascending mutations" may 

 be placed within the single species BrontotJierium 

 gigas. 



Such a vertical or phyletic application of the Lin- 

 naean system involves, it is true, a departure from the 

 traditional Linnaean methods, but in the author's 

 opinion it is far preferable to the introduction of a 

 new systematic terminology. If necessary the author's 

 system may be distinguished as neo-Linnaean. It is 

 an adaptation of the Linnaean system to phylogeny 

 as revealed by paleontology. 



The degrees or steps in the evolution of neomorphic 

 and heteromorphic characters, or rectigradations and 

 allometrons, afford the real basis of our division of the 

 great family tree of the titanotheres into branches 

 and subbranches as follows: 



Family, a branch of the Perissodactyla having a large num- 

 ber of similar characters and similar tendencies of evolution. 



Subfamily, a branch of the main family embracing one or 

 more genera retaining certain similar characters and developing 

 certain peculiar evolutionary tendencies. 



Genus, a branch of a subfamily or a stage of a subfamily 

 distinguished by the prominent position of certain distinctive 

 characters, which may be in widely different stages of develop- 

 ment — for example, Brontotherium leidyi, B. platyceras. 



Species and subspecies, divisions distinguished by certain 

 gradations in the development of characters common to the 

 genus, also by certain rectigradations and allometrons. 



Ascending mutations, divisions distinguished by various 

 intermediate stages of development of rectigradations and 

 allometrons. 



These principles of phyletic classification as devel- 

 oped and adopted in this monograph are also fully 

 explained in Chapter I. 



Classification is simply a convenient and condensed 

 expression of our knowledge of hereditary lines of 

 descent. It is constantly shifting and changing with 

 discovery. The final classification can be attained 

 only after we have worked out all the lines of descent 

 of this great family. In the meantime we may review 

 the history of the successive attempts at classification 

 made up to the present time. 



SUPEEFAMILY NAMES PROPOSED BY OSBORN (1898) AND 

 HAY (1902) 



Superfamily Titanotherioidea Osborn, 1898 



Original reference. — Am. Mus. Nat. Hist. Mem., vol. 

 1, pt. 3, p. 79, 1898 (Osborn, 1898.143). 



Osborn divided the Perissodactyla into five super- 

 families : 



I. Titanotherioidea ("including the single family Titano- 

 theriidae"), understood by Osborn to include both Eocene and 

 Oligocene titanotheres. 



II. Hippoidea, including Equidae and Palaeotheriidae. 



III. Tapiroidea, including Tapiridae and Lophiodontidae. 



IV. Rhinocerotoidea, Including Hyracodontidae, Amyno- 

 dontidae, Rhinocerotidae. 



V. Chalicotherioidea, Chalicotheriidae. 



Present determination. — Superfamily names are 

 formed by adding oidea to the stem of the family 

 name, and as Brontotheriidae is now regarded as valid, 

 it appeared necessary to Doctor Hay to substitute for 

 Osborn's term Titanotherioidea the term Bronto- 

 therioidea, first used by Hay in 1902. 



Superfamily Brontotherioidea Hay, 1902 



Original reference. — U. S. Geol. Survey Bull. 179, 

 p. 629, 1902 (Hay, 1902.1). 



The content of this term is as follows: 



Brontotherioidea : 

 Brontotheriidae: 



Lambdotheriinae (Eocene titanotheres) . 

 Brontotheriinae (Oligocene titanotheres). 



The content of the term Brontotherioidea Hay, 1902, 

 is thus the same as that of Titanotherioidea Osborn, 

 1898. 



FAMILY NAMES PROPOSED OR ADOPTED BY MARSH (1873), 

 FLOWER (1875), COPE (1879-1889), AND OSBORN (1889) 



Family Brontotheridae Marsh, 1873 



Original reference. — Am. Jour. Sci., 3d ser., vol. 5, 

 p. 486, 1873 (Marsh, 1873.1). 



Included genera. — Titanotherium Leidy and Bronto- 

 therium Marsh. 



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