The South American mice referred to Microryzomys and Thallomyscus

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ZOOLOGICAL SERIES

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FIELD MUSEUM OF NATURAL HISTORY

Volume XX CHICAGO, DECEMBER 11, 1933 Pages 1-8

THE SOUTH AMERICAN MICE REFERRED TO
MICRORYZOMYS AND THALLOMYSCUS

BY WILFRED H. OSGOOD

CURATOR, DEPARTMENT OF ZOOLOGY

In reporting on a collection of Peruvian mammals in 1914 (Field
Mus. Nat. Hist., Zool. Ser., 10, p. 158, 1914), I pointed out
some of the characters which distinguish the small forest mouse
then known as Oryzomys dryas. At that time, the resemblance of
dryas to Rhipidomys was noted and a close ally of dryas was referred
to minutus, the determination being based largely on comparison
by Oldfield Thomas of the original types and specimens sent to
London for that purpose. Some years later, Thomas (1917) gave
the generic name Microryzomys to the group which I had previously
defined and, at this time, he designated Oryzomys minutus as the
type. Still later, the same author (1926) renounced Microryzomys
and proposed Thallomyscus with dryas as type, in the belief that
minutus and dryas were not congeneric and that his first identification
of minutus had been erroneous. He gave no grounds for this change
of opinion further than to state that the toothrow in the type of
minutus "measures 3.1 mm. in length, a dimension never equalled
in Thallomyscus."

That one so acute as Thomas should thus reverse himself is
sufficient evidence that the case is one of considerable difficulty
not lightly to be entered into again. Nevertheless, after study of
all the material which was accessible to Thomas and much more
which was not, I am convinced that the types of minutus and dryas
are one and the same species, collected at the same place, at nearly
the same time, by the same collector. This, therefore, means another
right-about-face by which Microryzomys regains its position as the
generic or subgeneric term for the species in question and
Thallomyscus becomes the synonym.

The entire matter depends upon the identification of the type
of minutus. This type is quite young and the skin is one of those
unreliable preparations "from spirit." However, so also is that
of dryas and by external characters the two are quite indistinguishable.

No. 319

2 FIELD MUSEUM OF NATURAL HISTORY — ZOOLOGY, VOL. XX

The skull of the type of minutus is in fragments of which only those
including the upper molars and one infraorbital plate have any
comparative value. The molars are quite unworn, the last one
only partly erupted and still below the level of the others. The
toothrow in this condition, as stated by Thomas, is 3.1 mm. in
length, but contrary to his statement, this is not an excessive length
for the toothrow in this species. It should be said that Thomas
must have made his comparisons with aurillus, for of dryas itself
(exclusive of the type of minutus} the British Museum still possesses
only the type specimen in which the molars are worn nearly to the
roots and unquestionably shortened. In four specimens of aurillus
at hand from northern Peru, the measurements of the toothrow
are 3, 3.2, 3.2, 3. In the type of aurillus the measurement is 3
and in the type of dryas also 3. A recent specimen from Ecuador
measures 3.1. Therefore it is evident that the length of the tooth-
row gives no evidence to distinguish the type of minutus from that
of dryas. The same is true of the infraorbital plate which is of
the non-projecting form and in the type of minutus its antero-
posterior dimension is 1.55. In the type of dryas it is 1.6. In north
Peruvian specimens the measurements are 1.8, 1.6, 1.6, 1.7. In
two random examples of "stolzmanni" the toothrows are 3.5, 3.4
and the zygomatic plates 2, 2.1. Therefore, so far as measurements
are concerned all the evidence favors the identity of minutus and
dryas. The same is true of external characters and nothing remains
upon which to base a distinction.

A somewhat cursory review of the entire group to which minutus
belongs and a further examination of the Peruvian and Ecuadorean
allies of Oryzomys longicaudatus (i.e. destructor, stolzmanni, et al.)
indicates that the two groups are quite distinct, one departing from
typical Oryzomys in the direction of Thomasomys and the other having
the principal characters of the very slightly differentiated but wide-
spread group known as Oligoryzomys. As stated in 1914, what may
now be called Microryzomys is far better distinguished from Oryzomys
than is Oligoryzomys, although the two have been somewhat confused
because both include species of very small size. As contrasted with
any of the smaller Oligoryzomys, the skull in Microryzomys has a
recognizable general form produced by the slender rostrum, the
narrow non-projecting infraorbital plate, and the short, broad, and
more rounded braincase. The cheekteeth are relatively smaller
and the first upper molar has its anterior elements, the parastyle
and protoconule, definitely and nearly evenly divided. Also, the

1933 SOUTH AMERICAN MICE — OSGOOD 3

protoconule is not greatly out of line with the protocone and hypo-
cone. These characters of the teeth, it may be said, are clearly
shown in the type of minutus. All the characters above mentioned
are repeated in Thomasomys rather than Oryzomys and, if it were
not for the rather long palate and marked lateral pits, there could
be no objection to regarding minutus as a diminutive Thomasomys.
Its thick, soft pelage and its general external appearance also point
to affinity with Thomasomys.

Whether or not more than one bona fide species is referable to
Microryzomys is doubtful. Various names have been given but the
distinctions drawn, so far as verified, prove to be either non-existent
or so slight as to indicate no more than subspecific importance.
In referring to Microryzomys as a "group," therefore, we are probably
referring to a group of subspecies rather than a group of species.
As a species, minutus is easily recognizable, but it has a mixture
of characters heretofore regarded as diagnostic of considerable
assemblages of species, thus making its generic status difficult.
Such species are all too numerous among South American rodents
and the combination of characters they offer runs from one extreme
to another so it is clear that no ultimate generic and subgeneric
classification will be possible until all these species are thoroughly
understood. There are species which seem to connect Oryzomys
with Thomasomys, others which connect Thomasomys with
Rhipidomys and still others which through Oecomys return the connec-
tion, back to Oryzomys. Some of these have received special generic
names and others have not. Some, like Oryzomys albigularis, Have
only slight leanings away from the group to which they have been
assigned, but the direction of these leanings is fairly obvious. Under
these conditions, sound judgment is impossible without considering
the whole subject with ample material, and this no one has yet
been able to do. For the present, therefore, it may be best to retain
approximately the present status and recognize Microryzomys sub-
generically under Oryzomys. To give it full generic rank or to transfer
it to Thomasomys unquestionably would be premature.

Material for a wholly satisfactory revision of the forms closely
allied to minutus has not been assembled, but they may be sum-
marized to the following extent.

Oryzomys (Microryzomys) minutus Tomes.

Hesperomys minutus Tomes, Proc. Zool. Soc. Lond., p. 125, 1860 — Ecuador,
probably near Pallatanga. Type in British Museum. Collected by Fraser,
1859.

4 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. XX

Oryzomys dryas Thomas, Ann. & Mag. Nat. Hist., (7), 7, p. 267, 1898 — near
Pallatanga, Ecuador. Type in British Museum. Collected by Fraser,
Dec. 1858.

Oryzomys (Microryzomys) minutus Thomas, Smiths. Misc. Coll., 68, No. 4,
p. 1, April 10, 1917; Proc. U. S. Nat. Mus., 58, p. 229, 1920— subgenus
proposed with notes on first examination of type of minutus.

Thallomyscus dryas Thomas, Ann. & Mag. Nat. Hist., (9), 17, p. 613, May
1926 — Thallomyscus proposed as genus with dryas as type after second
examination of type minutus.

Besides the types of minutus and dryas in the British Museum,
I have examined one specimen from Molleturo, western Ecuador
(alt. 7,600 ft.) kindly loaned by the. American Museum of Natural
History. This had previously been submitted to Thomas and bears
his notation "Agrees, both as to skin and skull, with type of dryas."
To this I fully subscribe, but two specimens from Pallatanga (alt.
4,400 ft.), also from the American Museum, do not agree and are
plainly referable to the longicaudatus group, perhaps to 0. I. stolz-
manni (or balneator). It is more than likely, therefore, that Eraser's
types were not collected at the level of Pallatanga itself but on higher
ground of which there is no lack within a short distance. That the
minutus group usually occupies a higher zone than longicaudatus
and allies I am able to testify from personal experience with them
in Peru and Venezuela.

The scanty material representing typical minutus indicates that
it may perhaps be distinguished from both aurillus and humilior
by somewhat richer, more saturate color. In size and cranial
characters it agrees with aurillus rather than with humilior.

Oryzomys (Microryzomys) minutus aurillus Thomas.

Oryzomys dryas Osgood, Field Mus. Nat. Hist., Zool. Ser., 10, p. 158, 1914.
Oryzomys (Microryzomys) aurillus Thomas, Smiths. Misc. Coll., 68, No. 4,

p. 1, 1917— Torontoy, Peru.

Microryzomys aurillus Thomas, Ann. & Mag. Nat. Hist., (9), 17, p. 314, 1926.
Thallomyscus aurillus Thomas, Ann. & Mag. Nat. Hist., (9), 17, p. 612, 1926;

ibid, 18, pp. 160, 1926.

Besides a series from the type locality, Thomas has recorded
this form from several localities in northern Peru. The type speci-
men, kindly loaned by Gerrit S. Miller of the United States National
Museum, is a richly colored example agreeing with others from
various parts of Peru but somewhat paler than existing specimens of
minutus from Ecuador. Although it occurs at fairly high altitudes,
it appears to be a forest animal and mainly or wholly confined to
the eastern and more humid ranges of the Andes.

1933 SOUTH AMERICAN MICE — OSGOOD 5

Oryzomys (Microryzomys) minutus humilior Thomas.

Oryzomys dryas humilior Thomas, Ann. & Mag. Nat. Hist., (7), 2, p. 268,
1898— Bogota region (alt. 8,750 ft.), Colombia.

This is slightly paler than minutus and therefore much the same
in color as aurillus. It is a little smaller than either and is also
distinguished by having a smaller, narrower skull with a less expanded
cranium. It is represented in Field Museum by a large series (20)
from the mountains of Merida, by two specimens from Macotama,
Santa Marta, and by two from an altitude of 6,000 feet on Mount
Turumiquire in eastern Venezuela. The last of these are quite
removed geographically from the others, but do not show any
differentiating characters although a larger number might do so.

Oryzomys (Microryzomys) minutus fulvirostris Allen.

Oryzomys (OKgoryzomys) fulvirostris Allen, Bull. Am. Mus. Nat. Hist., 31,
p. 86, 1912 — Munchique, Cauca, Colombia.

Nothing in the original description of this indicates any means
by which it can be distinguished from 0. m. humilior. Four years
after it was described its author apparently discovered its true
relationship, for he then said (I.e., 35, pp. 526-527, 1916): "0.
humilior Thomas (1898) is the first described member of a widely
distributed South American group to which is referable 0. fulvirostris
Allen (1912), representing in the Western and Central Andes 0.
humilior of the Eastern Andes, the Santa Marta region and the
Merida Andes." Its distinction from humilior is very doubtful.

Oryzomys (Microryzomys) minutus altissimus subsp. nov.

Oryzomys minutus Osgood, Field Mus. Nat. Hist., Zool. Ser., 10, p. 158,
April 1914.

Type from La Quinua, mountains north of Cerro de Pasco, Peru.
Alt. 11,600 ft. No. 24,699 Field Museum of Natural History. Adoles-
cent female. Collected May 14, 1922, by Edmund Heller. Orig.
No. 6062.

Diagnosis. — Similar in size and cranial characters to 0. minutus
and 0. m. aurillus, but entire coloration much paler, the upper
and under parts well distinguished, the tail broadly and completely
bicolor, the feet wholly whitish without dusky markings. Upper
parts with dominant color Ochraceous-Buff rather than Ochraceous-
Tawny; under parts Warm Buff to Light Buff rather than Ochra-
ceous-Tawny.

6 FIELD MUSEUM OF NATURAL HISTORY— ZOOLOGY, VOL. XX

Measurements. — Average of ten adults measured by the collector:
total length 196.4 (190-207); tail 114.5 (110-123); hind foot 22.1
(21.5-23). Skull of type which is somewhat immature (selected to
show unworn dentition) and a normal adult with well-worn teeth:
greatest length 22, 24.1; zygomatic width 11.3, 12.6; width of
braincase 11.1, 11.3; interorbital constriction 3.5, 3.4; interparietal
8.3 x 2.5, 8.4 x 2.4; nasals 7.5, 9.4; length of infraorbital plate 1.8,
2.1; palatine foramina 3.9, 4.4; diastema 5, 5.7; bony palate from
back of anterior foramina 3.7, 3.8; cheekteeth 3.3, 3.3.

Remarks. — Although characterized only by color, this is a very
distinct form possibly entitled to specific rather than subspecific
rank. It is found at high altitudes in the puna zone of the less
humid western and central cordilleras of northwestern Peru and
either extends unchanged into Ecuador or is represented there by
a slightly darker variety. Owing to its lighter color, it is super-
ficially much more similar to 0. I. destructor than are the richly
tawny minutus and aurillus. Its softer pelage, smaller hind foot,
and non-projecting infraorbital plate, however, are sufficient to
indicate its real affinity.

Two specimens of this mouse from mountains near Otuzco, Peru,
have been in Field Museum since 1914 when they were referred to
minutus on the somewhat qualified advice of Oldfield Thomas.
Their distinction from dryas was clear enough at that time, but
Thomas then refused to admit the identity of dryas and minutus.
My subsequent examination of the two types finds them both
rich-colored and quite indistinguishable except as to age, one being
quite old and the other immature. Therefore, the name minutus
cannot be applied to the present form.

In a collection made in 1922-23 by Edmund Heller and J. T.
Zimmer in mountains near the headwaters of the Huallaga River,
there is now in Field Museum a considerable series in which both
aurillus and altissimus are well represented. At La Quinua and
Chiquirin, Heller obtained thirty-two specimens typical of altissimus.
Somewhat farther east in mountains near Huanuco, Zimmer obtained
eighteen, of which fourteen are aurillus and four are altissimus.
Reference to Zimmer 's notes shows these were from two different
stations in the same mountains, one at the upper edge of the forest
and the other above it on the open puna. At the lower station,
where both forest and puna were accessible, he took the entire
series of aurillus and three examples of altissimus. At the upper
station, altissimus was taken, but not aurillus. This corresponds

1933 SOUTH AMERICAN MICE — OSGOOD 7

with my own experience in northern Peru where I collected altis-
simus in open puna near Otuzco and aurillus only in heavy humid
forest in the central and western cordilleras. The two forms, or
representatives, also occur under similar circumstances in Ecuador,
but farther north in Colombia and Venezuela only one of them
appears to have a counterpart in humilior.

Clear evidence of intergradation between the two forms is lacking
in present material, but it is suggested in several specimens from the
Huanuco Mountains, in one from mountains east of Balsas (Caja-
marca district), and in one or two from Ecuador.

In the British Museum, at the time my examination of the types
of minutus and dryas was made, only three examples of altissimus
were encountered. These were collected by Russell W. Hendee at
Yana Mayu, Rio Tarma, Peru, at an altitude of 8,500 feet.

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