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Full text of "A new genus of aquatic rodents from Abyssinia"

THE UNIVERSITY 

OF ILLINOIS 

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FIELD MUSEUM OF NATURAL HISTORY 

FOUNDED BY MARSHALL FIELD, 1893 

PUBLICATION No. 250 
ZOOLOGICAL SERIES VOL. XII, No. 15 




T;; L;:.'.ABY OF USE 

WILFRED H. OSGOOD nr n i A aoc/ 

PC.U A- 1 |g0 

Curator, Department of Zoology 

UNIVERSITY OF ILLINOIS 




CHICAGO, U. S. A. 

November 21, 1928 



PRINTED IN THE UNITED STATES OF AMERICA 
BY FIELD MUSEUM PRESS 



* A NEW GENUS OF AQUATIC RODENTS 



FROM ABYSSINIA 

O- BY WILFRED H. OSGOOD 

The recent Field Museum Chicago Daily News Abyssinian Ex- 
, r pedition obtained important collections of small and medium-sized 
<r mammals upon which a full report cannot be made at the present 
time. However, there is one outstanding novelty which is so distinct 
from known forms that a preliminary description may be given at 
once. This is a murine rodent with rather pronounced aquatic 
modifications which was found in a small mountain stream near the 
source of the Little Abbai or Blue Nile. Rodents with similar exter- 
nal modifications are known from other parts of the world, but 
heretofore the African continent has furnished nothing so nearly 
analogous to them as the animal here described. Its coloration as 
well as its thick soft pelage and its large hind feet are suggestive of 
the South American Ichthyomys, but its adaptations for aquatic life 
seem not to have proceeded quite so far as in that form. These 
adaptations are mainly in the character of the pelage, the reduction 
of the external ears and the enlargement of the hind feet. The skull 
shows certain interesting similarities to Ichthyomys and Hydromys, 
but otherwise is not greatly modified and may indicate derivation at 
no very remote period from some of the common types widely dis- 
tributed in central Africa. The only other African rodent with 
aquatic adaptations is Dasymys, but this shows no especial affinity 
to the present genus, and doubtless the two had independent origins. 

Nilopegamys plumbeus gen. et sp. nov. 

Type from small stream tributary to the Little Abbai, between 
Sakalla and Njabarra, Gojam, Abyssinia. Altitude 8500 ft. No. 
28633 Field Museum of Natural History. Adult male. Collected 
Mar. 20, 1927 by Wilfred H. Osgood. Original No. 6401. 

Generic characters. External form not especially unusual, the 
size (head and body 148 mm.) and proportions of head, body, and 
tail about as in various other African rodents; external ears much 
reduced, but projecting somewhat above the surrounding pelage; 
hind feet large and broad, equaling in length about . 2 7 of the length 

185 



1 86 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XII. 

of the head and body; pelage very soft and dense. Skull with general 
resemblance to that of Mastomys; braincase large, smooth, and in- 
flated in appearance; interorbital region abruptly depressed. First 
upper molar tooth four-rooted, one large antero-external root, one 
postero-external, and two smaller internal roots; first upper molar 
occupying about half the length of the toothrow; last upper molar 
small and comprised of only two elements, a small antero-internal 
cusp and a larger main cusp. 

Color. Upperparts nearly uniform blackish slate, the underlying 
color a slightly paler slate gray; underparts approaching pure white, 
this extending throughout to the roots of the hairs; color of the 
upperparts reaching on the outer side of the arm to the carpal joint 
where it terminates broadly and abruptly; white of underparts car- 
ried forward rather broadly above the upper lip to the base of the 
whiskers and to the muzzle which is hairy except for the very small 
and deeply cleft rhinarium; about half the whiskers white, the rest 
blackish; ears blackish, their lower edges narrowly white; a large 
whitish subauricular spot; upper side of tail blackish, the hairs 
short and not concealing the scaly annulations (13 to cm.); under 
side of tail dull whitish except for a sharp median black line, slightly 
interrupted proximally and becoming confluent with the upper color 
terminally; forefeet and wrists white, the digits rather thinly haired; 
hind feet and metatarsal joint dull whitish, the hairs short and thinly 
distributed. 

Skull and teeth. Skull with large full braincase, depressed inter- 
orbital region, and straightened rostrum; no prominent ridges or 
angularities; parietals large and extended well forward; f rentals de- 
pressed anteriorly and the interorbital edges slightly elevated; nasals 
convex and semi-tubular anteriorly, extended posteriorly into the 
interorbital depression and ending well behind the very small dorsal 
exposure of the lacrymals; zygomata slender, the so-called zygomatic 
plate with its anterior edge rising vertically to a rounded dorsal 
border which is not especially prominent when viewed from above; 
under side of skull much as in Mastomys ; palatine slits ending about 
on the level of the back of the anterior root of first upper molar; 
palate rather wide and not definitely channeled; audital bullae of 
medium size; ramus of mandible rather slender; coronoid process 
long and slender. Teeth similar in general to those of Mastomys; 
first upper molar with four roots, a large antero-external root, a 
smaller postero-external, and two still smaller internal ones; first 



NEW GENUS OF RODENTS OSGOOD. 



187 



upper molar with the usual three median tubercles and three external 
and two internal ones; lateral tubercles not separated from median 
ones by deep sulci; first upper molar relatively large, occupying 
about half the length of the toothrow; last upper molar small and 
including merely a rounded central part and a smaller antero-inter- 
nal tubercle ; relations of incisors and cheekteeth somewhat modified 
so that a line projected forward from the level of the grinding surfaces 
of the cheekteeth crosses the lower half of the incisors instead of the 
vicinity of the gnathion as in most related forms. 




Fig. i. Skull of type of Nilopegamys plumbeus x i 1/9 

Measurements. Adult male (type), measured in flesh. Total 
length 328; head and body 148; tail 180; hind foot with claw 40; 
ear from notch (dry) 13. Skull of type: Greatest length 35.1 ; basilar 
length 29.4; zygomatic breadth 17.8; mastoid breadth 14.7; nasals 
r 3-7 x 3-5J depth of braincase 10.8; breadth of braincase 15.4; least 
interorbital breadth 5.2; breadth of zygomatic plate 3.3; length of 
palate from gnathion 17; palatal foramina 6.8 x 2.4; diastema 10; 
upper toothrow 5.8; crown of first molar upper 2.9. 

Remarks. The external characters of this genus distinguish 
it at once from all other African rodents. The very dense soft 
pelage, the small eyes and ears, the large broad swimming hind feet 



1 88 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XII. 

and the strikingly contrasted color pattern together furnish a 
combination which is unique, at least in Africa. The cranial char- 
acters also are unique, but in most cases their relation to the animal's 
habits are by no means so apparent. Of especial interest is the de- 
pression of the dorsal outline of the skull above the orbits and the 
accompanying tendency to dorsal flexure of the rostrum and ant- 
orbital part of the skull. The recurrence of these characters in this 
form and in the widely separated but similarly aquatic forms, Hy- 
dromys and Ichthyomys, leads to the suspicion that they bear some 
rather definite relation to life in the water. The suggestion may be 
ventured, therefore, that this relation may be in connection with 
the act of swimming, in which the head is thrown back and the 
nostrils kept elevated while other parts of the animal are submerged. 

The general cranial characters of Nilopegamys seem to indicate 
fairly close genetic relationship with the common rodents of central 
Africa, especially Mastomys and Stenocephalomys, the former widely 
distributed and the latter, like Nilopegamys, confined to the higher 
mountains of Abyssinia. The teeth are quite similar to those of 
Mastomys, but the first upper molar has four roots instead of three, 
that is, there are two closely connected internal roots occupying the 
position of the single internal root of Mastomys. However, in Steno- 
cephalomys, this internal root is divided, although not quite so de- 
finitely as in Nilopegamys. Only a few species of Mastomys have 
been examined with respect to this character and it is possible that 
it may be subject to some variation within the group. Unfortunately 
no female of Nilopegamys is available and the mammary formula 
cannot be stated. The principal superspecific groups of African 
rodents have been carefully defined recently by Oldfield Thomas 
(Ann. & Mag. Nat. Hist., (9), 17, pp. 174-179, 1926) and his findings 
regarding the number and arrangement of roots in the first upper 
molar seem to indicate that the condition in Nilopegamys is not a 
common one. In most groups having the tooth four-rooted, as 
Aethomys and Dephomys, the fourth root is in median external 
position. In Nilopegamys this median root is absent, but the inter- 
nal root is divided, making the same total of four roots. Dasymys, with 
six and even seven roots, appears to need no consideration in this 
connection. 

The type and only specimen of this interesting water rat was 
taken in a small clear stream, probably nameless but tributary to 
the Little Abbai not far from its source. The trap which caught it 
was set in a little runway leading from the water across a tiny islet, 



NEW GENUS OF RODENTS OSGOOD. 189 

scarcely more than a weed-bordered stepping-stone. Unfortunately, 
no further opportunity was afforded for trapping in similar situa- 
tions, so additional specimens were not obtained. 



T8E L13SARY GF Tlfc 
DEC 14 J928 

UNIVERSITY OF ILLINOIS 



UNIVERSITY OF ILLINOIS-URBANA