text: ? oryzomys pliocaenicus ? oryzomys pliocaenicus is a fossil rodent from the ( late miocene ) of kansas , central united states . it is known from a single mandible ( lower jaw ) with the back part missing . all three molars are present , but very worn . together , the molars are 3 . 6 mm long . the fossil was discovered in 1935 and described in 1939 as a possible species of oryzomys ( in open nomenclature ) . later authors doubted this allocation and suggested that it may instead belong in bensonomys or jacobsomys , but the material may not allow a definite identification . discovery and context the only known specimen of ? oryzomys pliocaenicus is a mandible ( lower jaw ) found in the spring of 1935 by david dunkle in edson quarry , sherman county , kansas . it is in the collections of the museum of comparative zoology at harvard university as specimen mcz . edson quarry is in the ogallala formation and the north american land mammal age . claude w. hibbard described the mandible as ? oryzomys pliocaenicus in a 1939 paper . hibbard wrote that the fauna was middle pliocene , but it is now considered miocene . the edson quarry fauna contains a diversity of other fossils , including mammals , birds , reptiles , and amphibians . description ? oryzomys pliocaenicus is known from a single mandible with the incisor and three molars in it . much of the back of the jaw is missing , including the angular , condyloid , and coronoid processes . the mental foramen , a foramen ( opening ) at the front of the jaw , in the diastema between the incisor and molars , opens upwards , a little more so than in the living marsh rice rat ( oryzomys palustris ) . the masseteric ridges , which anchor some of the chewing muscles and are located on the outer surface of the mandible , are similar to those of oryzomys . the molars are very worn , so that only traces of the cusps remain ; no accessory small cusps are visible . each of the teeth has two roots . the length of the toothrow is 3 . 6 mm and the depth of the mandible below the first molar is also 3 . 6 mm . interpretations hibbard wrote that the condition of the mental foramen and masseteric ridges excluded the specimen from , peromyscus , , and and that in these features and in its depth and size , the fossil more closely resembled oryzomys ; therefore , he placed it in that genus with a query . oryzomys is a living genus that has occurred in the united states since at least 300 , 000 years ago . in 1966 , philip hershkovitz wrote that hibbard had reconsidered his opinion after re - examining ? o. pliocaenicus in 1952 ; he no longer thought that it was an oryzomys and instead believed it may be a bensonomys . the latter genus occurs in the late miocene and early pliocene of north america and has been variously interpreted as a close relative of the south american calomys or merely as an evolutionarily convergent member of the north american subfamily . jon baskin mentioned the animal in 1978 and 1986 , asserting that it cannot be identified to genus level , but may be bensonomys . although some continue to list it under oryzomys , it is now usually excluded from the genus . in 2008 , everett lindsay listed ? o. pliocaenicus as a questionable species of jacobsomys , a pliocene north american genus which he said may be ancestral to oryzomys .