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Full text of "The South American mice referred to Microryzomys and Thallomyscus"

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ZOOLOGICAL SERIES 

OF 
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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