A new genus of aquatic rodents from Abyssinia

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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 nrn i A «aoc/

PC.U A-1 |g£0

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
r3-7 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

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