Evidently, the animals of this series were extremely rare or absent in the areas where the known South American deposits of the Pleistocene and Pliocene were laid down, for there is a very long hiatus in their history from the Recent to the Santa Cruz, during which none has yet been found, except one genus (†Zygolestes) in the Paraná. In the Santa Cruz, however, there was a great abundance of these little marsupials, to which various generic names have been given and which displayed considerable variety in the forms of the teeth. Some (e.g. †Garzonia) agreed with Cænolestes in having no trenchant shearing teeth; behind the large, procumbent lower incisor, followed four or five very minute teeth, which must have been nearly or quite functionless, succeeded by the well-developed molars. Other genera (e.g. †Abderites) had a similar dentition, with the important exception that the last upper premolar and first lower molar were enlarged and trenchant, together forming a shearing pair; these teeth were vertically fluted or ribbed in very characteristic fashion. The Australian phalangers have very similar trenchant and fluted teeth, but in that family the lower one of the pair is the last premolar, not the first molar. Marsupials of this type have not been found in formations older than the Deseado.
The relationship of these South American genera to the Australian phalangers is a very interesting question from the standpoint of mammalian distribution, but is not likely to receive a positive answer until something is learned regarding the history of the Australian family.
Suborder †Allotheria
Fig. 303.—Skull of Paleocene †allothere (†Ptilodus gracilis), enlarged, Fort Union stage. (After Gidley.)
This extinct suborder is still very imperfectly understood, for it is known almost exclusively from jaws and teeth; so far, the skull of one genus and most of that of another have been obtained, but hardly anything of the skeleton. The †Allotheria were small or minute marsupials, herbivorous or omnivorous, which had lost all trace of the canines and had one pair of incisors above and below, which grew from persistent pulps and had a scalpriform, rodent-like character. The molars were composed of numerous tubercles (whence the name “†Multituberculata,” often applied to the group) arranged in two or three longitudinal rows, and the premolars were either like the molars, but of simpler pattern, or compressed, sharp-edged and trenchant. The †Allotheria were among the most ancient of mammals and have been found in the Triassic of Europe, the Jurassic of Europe and South Africa, the Jurassic and Cretaceous of North America and the Paleocene of both northern continents, while the South American Eocene (Casa Mayor) had certain problematical genera (†Polydolopidæ), which may be referable to the †Allotheria or to the Cænolestes series. The suborder was thus preëminently a Mesozoic one and, with the doubtful exception of South America, it is not known to have passed beyond the limits of the Paleocene. There is not the least likelihood that any existing mammals were derived from the †Allotheria.
Fig. 304.—Head of †Ptilodus gracilis, about natural size. Restored from a skull in the United States National Museum.
While the †Allotheria have an antiquity at least equal to that of any other mammals known, there were other groups in the Jurassic and Cretaceous, which, so far as may be judged from teeth alone, would seem to have been ancestral to the other marsupials and to the placentals. It would serve no useful purpose to describe these minute creatures, which are so very incompletely known, though to the specialist they are of the highest interest. The genera found in the Triassic of North Carolina may or may not represent the primitive mammalian stock.