no. 2077. AN EXTINCT MARSUPIAL FROM FORT UNION— OIDLEY. 401 



at this early period (Paleocene) there is no evidence of a close simi- 

 larity to a didelphid type of dentition, noris there a suggestion of any 

 particularly close affinity to the dasyurids, which are considered i>y 

 Bensley to be direct derivatives of the early didelphids, as represented 

 by Peratherium. It is obvious that if the Myrmecobidae had a begin- 

 ning so very much earlier, as is indicated by the Fort Union specimen, 

 so likewise must the Dasyuridae and probably all (certainly some) of 

 the other living families of marsupials. 



This conclusion also is not in accordance with the view expressed by 

 Osborn 1 regarding the origin of marsupials of Australia, which he 

 suggests were derived from the " introduction into Australia of some 

 small arboreal opossum of Didelphys-]ike form as the source of the 

 wonderful adaptive radiation of the marsupials of this continent." 

 The fossil evidence at present available, as I interpret it, does not 

 apparently support either Osborn's or Bensley's view concerning the 

 origin of the modern families of marsupials, nor in reality does it give 

 more than a small part of the life history of this great order of mam- 

 mals. In fact, it seems reasonable to assume that at present nothing 

 is definitely known regarding the ancestry of several of the living 

 marsupial families, including probably all the diprotodonts, 2 because 

 they are not represented in our collections from beds older than the 

 Pleistocene, and that in the known fossils we have only an incomplete 

 and indefinite history of the origin and development of a part only of 

 the polyprotodonts. Thus it seems from the paleontological evi- 

 dence we are at present not justified in assuming more than that the 

 Didelphidae only are represented in such forms as Peratherium 

 (Oligocene), Proteodidelphus (Paleocene?), and possibly Didelphops 

 of the Lance formation. The Myrmecobidae are presumably repre- 

 sented by Myrmecoboides of the Fort Union (Paleocene), while 

 Dasyurus and possibly the Peramelidae may have been derived with 

 the Didelphidae from differing forms of the Didelphops (Lance) group. 

 The Thylacinidae and Caenolestidae are apparently not known from 

 fossils older than the Miocene where they had reached almost their 

 present state of specialization. 



Such a view, I am aware, assumes a vastly more ancient origin for 

 all the living families of marsupials than has hitherto been held for 

 them while it must be conceded that the greater part of their evolu- 

 tionary development remains practically unknown. 



In the early attempts of vertebrate paleontologists to read the life- 

 history of the globe as recorded by the fossil animal remains, it appears 

 to have been too often assumed that the known fossils of a few widely 

 scattered localities told the greater part of the whole story of the 



i The Age of Mammals, 1910, p. 78. 



2 The Caenolestidae have been placod in this great group apparently on the diprotodont-like develop- 

 ment of the lower jaw. However, this may be an entirely independently acquired character. This family 

 more probably belongs with the Polyprotodonts. 



50758°— ProcN.M. vol.48— 14 26 



