506 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



SYSTEMATIC DESCRIPTIONS OF GENERA AND SPECIES 

 IN THE ALLOPS PHYLUM 



AUops Marsh 



Plates XX, XXI, XXXVIII, XC, CXI-CXXII, CXXXII; 

 text figures 184, 189, 197, 207, 375, 378, 381, 387, 389, 391, 

 393, 394, 397, 399, 409, 413, 429-434, 608, 612, 615, 712 



[For oi-iginil description and type references see p. 224. For ske:etal characters sec 

 p. 678] 



Generic characters. — Mesaticephalic, progressively 

 brachycephalic. Lateral pair of superior incisors 

 persistent; canines compressed anteroposteriorlj^, flat- 

 tened on posterior face; grinding teeth with moder- 

 ately distinct cingula; premolars simpler than in 

 Menodus but with progressive tetartocones; molars 

 with elevated and pointed cusps, the transverse 

 slightly exceeding the anteroposterior diameters. 

 Nasals broad and intermediate in length, abbreviat- 

 ing in progressively brachycephalic types; horns short, 

 broadly trihedral in section, directed obliquely out- 

 ward. 



This genus was estabhshed in 1887 by Marsh, who 

 selected the species A. serotinus as the genotype. In 

 1891 this author strengthened the genus by adding the 

 species AUops crassicornis, a more advanced form. 

 In 1902 Osborn described the species Megacerops 

 marshi, named in honor of Professor Marsh, which 

 was later referred to the AUops phylum. Finally, in 

 1917, Osborn added the most primitive species AUops 

 walcotti, named in honor of the former Director of 

 the United States Geological Survey. Although these 

 species are grouped within a single genus, they ap- 

 parently do not constitute a direct phyletic succession. 

 There are also puzzling affinities to Menops varians, 

 the type of the genus Menops. 



Progressive brachycephaly is characteristic of these 

 scattered and loosely related species, as shown in the 

 following ascending series in the Titanotherium zone: 



Zygomatic 

 index 



Upper beds: AUops crassicornis (type), brachycephalic. _ 76 



Upper beds: UTenops wonons, subbrachycephalic 73 



Svimmit of middle beds: AUops serotinus (type), sub- 



brachycephahc 72 



Middle beds, lower levels: AUops marshi, mesa,ticepha.\\c- 64-69 

 Base of lower beds: AUops walcoUi (type), mesatice- 

 phalic (?) 



Geologic distribution of AUops. — AUops walcotti 

 Osborn: A. walcotti, from the lower levels of the 

 lower Titanotherium zone, is a small, very primitive 

 form, distinguished by a narrow head, long, broad 



nasals, and relatively long face, horns with elongate 

 oval section, and two superior incisors. 



AUops marshi (Osborn): A. marshi, from the upper 

 levels of the lower beds, is distinctly mesaticephalic. 

 Nasals long and broad, horns subtrihedral in section, 

 premolars more progressive than in A. walcotti. This 

 animal is finely represented by eleven skulls in several 

 museums, which present a series of ascending muta- 

 tions. 



AUops serotinus Marsh: In A. serotinus the nasals 

 are still elongate and the horns are slender, elongate, 

 subtrihedral, preserving the section characteristic of 

 A. marshi. The premolars are still in a retarded stage 

 of evolution. This species is represented by five speci- 

 mens, two of which appear to present transitional 

 stages toward A. crassicornis, as follows: Skull Am. 

 Mus. 520 appears to be in a transitional stage between 

 A. serotinus and the more brachycephalic species A. 

 crassicornis, for the horns are in an intermediate stage 

 of development and the dental measurements have 

 the degree of development of the^premolar tetartocones 

 coincident with those in A. serotinus. Transitional 

 skull Nat. Mus. 4938 nearly equals in some of its meas- 

 urements the A. crassicornis type, but the premolar 

 tetartocones are still decidedly retarded. 



AUops crassicornis: The type specimen of the species 

 A. crassicornis is distinguished both by the more 

 massive proportions of the skull, the obtuse, short and 

 massive horns, the abbreviation of the nasals, and 

 especially by the more advanced development of the 

 tetartocones upon the premolars. It is by no means 

 certain that this massive, broad-headed animal is a 

 descendant of the types- named above. 



Stratigraphic position of species of AUops 



