154 AMERICAN MESOZOIC MAMMALIA 



The resemblance of the Stonesfield and Purbeck faunas in England is such as to 

 warrant the tentative conclusion that there were no changes of prime importance in the 

 distribution of the larger mammalian groups in this area during the middle and upper 

 Jurassic. They apparently belong to the same distributional wave or radiation. The 

 faunal relationships between the upper Jurassic and upper Cretaceous faunas and be- 

 tween the latter and the Paleocene faunas demand closer analysis. 



Marsh was the first to treat this question. He believed ( 1892A, pp. 250-1 ) that the 

 mammals of the Lance "are not transitional between Mesozoic and Tertiary forms, but 

 their affinities are with the former beyond a doubt. . . . The . . . Puerco is clearly 

 Tertiary, and the great break is between this horizon and the Ceratops beds of the 

 Laramie. ... I nstead of placing them close together, as some geologists seem inclined 

 to do, it will be more profitable in future to search for the great series of intervening 

 strata containing the forms that lead from one to the other. . . . Bearing in mind all 

 that is known to-day [1892] of the development and succession of Vertebrate life in 

 America, from the early Silurian on to the present time, it is safe to say that the faunal 

 break as now known between the Laramie and the lower Wahsatch [ Puerco] is far 

 more profound than would be the case if the entire Jurassic and the Cretaceous below 

 the Laramie were wanting. ..." The essential elements of this view are ( i ) belief 

 in the close relationship of the Lance fauna to that of the Morrison, (2) recognition of 

 the great diflFerence between the known Lance and Puerco faunas, and (3) the assump- 

 tion that this difiEerence is due not to migrations or changes of facies but to evolution in 

 place during a long period unknown by mammalian fossils. 



This view of Marsh was strongly combated by Osborn who disagreed with 

 Marsh on every point. In 1893 Osborn stated that, "The fact is, these Laramie mam- 

 mals are surprisingly near those of the Puerco, and in some cases almost identical with 

 them ; in other cases they are of a somewhat older type. Therefore, the greatest gaf to 

 be jilled by future discovery is between this Laramie fauna and the Jurassic} For this 

 Laramie fauna is separated from the Puerco about as widely as the Puerco is from the 

 Wahsatch, but no more widely ; whereas it is separated by a profound gap from the 

 Jurassic fauna . . . [The Lance Cimolomys (referred to Ptilodus by Osborn) and the 

 Torrejon (upper Puerco of Cope and Osborn) Ptilodus^ are in substantially the same 

 stage of dental evolution, and so nearly alike that the writer was for a long time 

 tempted to believe that the Laramie and Puerco faunae were contemporaneous. . . . 

 Two features make the Laramie ["trituberculate"] fauna appear more ancient than 

 the Puerco : first, the non-development of an internal cingulum, which is common in 

 the Puerco ; second, the entire absence of the hypocone, which is quite strong in some 

 Puerco mammals. On the other hand [certain of the] upper and lower molars . . . are 

 analogous to Ectoconus, Dissacus, Diacodon, and Hafloconus of the Puerco [or To- 

 rrejon]." 



Thus Osborn's chief conclusions were ( i ) that there is a profound gap between 

 the Lance and the Morrison, (2) that the Lance and Puerco faunas are closely related 

 and in a comparable stage of evolution so that they were close to each other in time, or 



^ Italics Osborn's. 



