The Mylagaulidæ; an extinct family of sciuromorph rodents

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Field Columbian Museum

Publication 34.

Geological Series. Vol. I, No. 4.

THE MYLAGAULID^:

AN EXTINCT FAMILY OF SCIUROMORPH

RODENTS

BY

Elmer S. Riggs, A.M.,

ASSISTANT CURATOR, DEPARTMENT OF GEOLOGY.

Oliver Cummings Farrington, Ph.D.,

CURATOR, DEPARTMENT OF GEOLOGY.

Chicago, U. S. A.
March, 1899.

THE MYLAGAULID/E; AN EXTINCT FAMILY OF SCIUROMORPH

RODENTS.

The genus Mylagaulus*, Cope, was originally based upon a sin-
gle lower premolar tooth, but later it was supported by the descrip-
tion of a fairly complete mandible, f From this material two species
were determined, M. Monodon and M. sesquipedalis. Later the fam-
ily name, Mylagau/idce, was proposed for these forms. To this
group it is now proposed to add the two genera described below.

Lateral view of mandible of Mesogaulus ballensis X \.

Dentition of same as seen from above X \.

Mesogaulus ballensis, gen. et sp. nov-

A rodent mandible from the Deep River escarpments near White
Sulphur Springs, Montana, and now in the Museum collection, shows
interesting characters hitherto undescribed. As suggested by Pro-
fessor Scott, its nearest affinities are with Mylagaulus, Cope, though it is
a more primitive form and evidently stands in an ancestral relation to
that group. The name Mesogaulus ballensis is proposed for it in
honor of Mr. S. H. Ball, the collector. The characters distinguish-
ing this genus from Mylagaulus are: First molar elongate antero-pos-
teriorly with enamel slightly inflected externally ; the third molar
rather stronger than the second and rotund in outline. The type
specimen was found in the alluvium at the base of an escarpment, so
that its horizon cannot be exactly determined. However, it certainly
belongs to the upper beds.^

•Bulletin of the U. S. Geological and Geographical Survey, vol. iv., No. 2, May, 1878.
tAmerican Naturalist, Jan. 1883.

JThe Mammalia of the Deep River Beds. Am. Phil. Soc, Oct. 1893.
I8l

182 Field Columbian Museum — Geology, Vol. i.

The specimen consists of a single mandible from which the angle,
condyle, part of the coronoid process and the tip of the incisor have
been lost. The remainder is in good preservation and shows a per-
fect molar dentition. The teeth are deeply set in the alveolus and
their well-worn crowns indicate an animal fully adult. The molars
are ranged in a line parallel to the plane of the symphysis, but the
axes of the teeth themselves are inclined well inward at the crowns.
As in Mylagaulus, the angle of the mandible is within the plane of the
incisive alveolus, but the condyle would fall somewhat without it.
The coronoid process is strong at the base and is directed outward,
leaving a broad fossa between its mesial surface and the posterior
molars. At the hinder margin of this fossa the dental foramen opens.
Almost continuous with the anterior margin of the coronoid process,
arising just below the posterior third of the premolar alveolus and
extending downward toward the angle, is a strong masseteric ridge.
The symphysis extends far back beneath the premolar alveolus where
its termination forms a prominent angle in the inferior outline of the
mandible. The ramus is concave on the mesial surface throughout,
and posteriorly bears evidence of a deep fossa. The incisor is cov-
ered anteriorly with a thin orange-colored enamel. At the alveolus,
where it is broken, the section is roughly sub-triangular. The ante-
rior surface meets the median one in a sharp angle ; with the lateral
surface it forms a rounded one ; while posteriorly the faces converge
to form a single rounded surface. The premolar, as before men-
tioned, has its crown much elongated antero-posteriorly and the
enamel slightly inflected on the lateral margin. A series of three
enamel fossettes are arranged in the median line of the crown, with a
trace of a fourth lying lateral to them. The anterior surface is
comparatively narrow, the crown reaching its greatest width oppo-
site the third fossette, while opposite the middle one occurs the exter-
nal enamel inflection. The second molar is a small tooth, com-
pressed antero-posteriorly, and like the third, entirely devoid of fossae
or enamel inflections. In both, the enamel is absent from the mesial
surface and in the first it is noticeably thinner. All are encased in
thick sheaths of cement. The last molar is grooved on the mesial
surface throughout its length, indicating the not very remote coales-
cing of roots. The other molars I have not examined below the alve-
olus. A possible trace of a fourth alveolus remains, but the space
between the third molar and the dental foramen is so reduced that a
fourth tooth could not have developed in this individual.

The Mylagailid*: — Rigcs. 1:83

Ml AM Kl MINTS. M.

Length from posterior incisive alveolus to posterior molar surface 023

Length of inferior molar series 013

Length of diastema 010

Anteroposterior diameter of premolar 009

( Ireatest lateral breadth of premolar 0042

Greatest lateral breadth of last molar 0035

Depth of mandible at anterior molar alveolus 017

Protogaulus hippodus gen. nov. (sp. Cope.)

In view of the later described relationship of Meniscomys hip-
podus* to the mylagaulid phylum, as well as its differences from
other members of the genus, it seems desirable for the sake of
natural grouping to transfer the species to the Mylagaulida. Upon
these grounds it is proposed to create for it a new genus, retain-
ing the specific name. The generic characters thus distinguish-
ing Protogaulus from Meniscomys are: " Superior molars short-
rooted with external face plane ; inferior molar with a prominent
median transverse crest ;"t premolar much larger than molars
and having the crescents separated by a deep external fissure ;
inferior incisor with a wide groove on its external face.

MEASUREMENTS.! M.

Length of superior molar series

Length from base of first superior molar to base of incisor.

Width between bases of first molars

Length of first inferior molar

Depth of ramus at second molar

008
0065
002
0033

0050

Relationship : The relations of Mylagaulus were for some time
left in doubt, but eventually Professor Cope proposed for these
forms the family Mylagaulidce. As pointed out by him, the
marked development of the lower premolar and the characters
of its crown present striking similarities to the dentition of cer-
tain species of Hystrix, especially H. refossa. But the complete
absence of roots, even in the more primitive form, Mesogaulus,
together with the dissimilar origin of the angular portion of the
mandible, render the suggested relationship improbable. All of
these characters are essentially sciuromorph and to that group the
Mylagaulidce certainly belong.

The presence of but three molar teeth and the complete isola-
tion of the enamel fossettes were offered as grounds for the erec-
tion of the family Mylagaulidtc If these characters were pos-

♦Proceedings American Philosophical Society, 1878 (-79), p. 67.
tCope's Tertiary Vertebrata, p. 829.

184 Field Columbian Museum — Geology, Vol. i.

sessed by no other forms they might be regarded as of more than
generic importance, but in point of fact, the dental formula is
the same as that of Eucastor Leidy* and the later described Sig-
mogomphius Merriam,f both of which have true castoroid affinities.
As to the second feature, the isolated enamel lacunae are inde-
pendently developed in several forms and are doubtless depend-
ent upon the amount of wear to which the tooth is subjected.
Therefore this group has not been generally recognized by later
writers, and the genus has been provisionally included under the
Castoridce.

Nevertheless the Mylagaulida appear to be a unique, and, until
now, isolated, form. The one prominent feature is the unusual
development of the premolar to the exclusion of the posterior-
lying teeth. Associated with this is the great strength and
shortness of the mandible, the prominence and anterior position
of the masseteric ridge and the depth of the ramus from alveolus
to angle. These tell an unmistakable story: Unusual capacity
for crushing or grinding and the attendant specialization of the
premolar to perform the function laid upon it. Just as in the
Carnivora, the lower first molar, lying immediately anterior to the
insertion of the masseter muscles, has developed into the princi-
pal shearing tooth ; so in these forms, the last premolar, stand-
ing in the same relation, has fitted itself for a crushing imple-
ment which, in M. Monodon, has reached the highest degree of
specialization known to Rodentia. It would be most interesting
to know the food habits of these unique animals. Such a ma-
choire might well have fitted them for nut-cracking or the crush-
ing of hard-shelled seeds ; but the abraded, crowns of all the
molars, together with their hyposodont character of growth,
makes it probable that the great tooth with its enamel pits was
used largely as a grinding surface.

PHYLOGENY OF THE MYLAGAULIDiE.

Upon the problem of phylogeny the limited material thus far
known throws comparative little light. An ancestor of Mesogau-
lus would be expected to have a less strongly developed premo-
lar with at least one lateral enamel inflection, to show traces of a
third lower molar, and perhaps retain further evidences of molar
roots. These conditions are largely fulfilled in the proposed
genus Protogaulus hippodus. In this form the mandible has the

♦American Naturalist, July, 1881, p. 586.

tBulletin of the Department of Geology, Univ. of California, vol. 1, No. 13.

The Mylagaulid.* — Riggs.

185

premolar considerably enlarged, though the crown is little elon-
gate antero-posteriorly. The third molar is retained, but is
somewhat smaller than the second, and short roots are present
throughout. Cope describes the mandibular dentition as fol-
lows:* " The crowns of the inferior molars, in the unworn con-
dition, terminate in two crescents; that is, in elevated, anterior,
external and posterior borders, with a transverse ridge equally
dividing the space thus marked out and joining the notch in the
external crest. This pattern resembles somewhat that of Anchi-
theriiim. The first inferior molar (premolar) differs from the
others in its superior size and its having the crescents more
widely separated by a deeper external emargination. On attri-
tion the spaces bounded by the enamel crests are enclosed
by the junction of the extremities of these crests on the
inner side of the crown. Further attrition results in three
lakes within the crown and one notch of the external bor-
der and two notches of the internal border. The anterior
molar has two lakes in its posterior area and one large one
in its anterior area. In old teeth there are successively one
and no lakes left to interrupt the dentine. The inferior incisor
has a wide shallow groove or concavity on its external face."

(a) Lower dentition of Mesogaulus X 2. (b) The same of Protogaulus (Mettiscomys, Cope) X 4.
(c) Upper dentition of Hystrix refassa X 2.

From this the affinities with Mesogaulus at once appear. In
that genus the lateral emargination of the premolar has almost
disappeared, and the cleft is represented by the deeper middle
fossette. The large anterior lake described in Proiogaulus is
replaced by a smaller one in the later form; the deep postero-in-

♦Copes Tertiary Vertebra, p. 828.

186 Field Columbian Museum — Geology, Vol. i.

ternal cleft is traceable in the elongate third fossa, which is barely
separated from the enamel border. A mere trace of a fourth
fossette lying lateral to, and between the second and third, doubt-
less represents the posterior one in the ancestral form. The true
molars have lost their important characteristics in the process of
reduction, but even in the earlier form the first molar shows the
antero-posterior compression so noticeable in Mesogaulus, while
in that form the second molar retains something of the diagonal
elongation seen in the other, and also a trace of its median inter-
nal flexure.

The following table will be of service in showing the distribu-
tion, both in time and locality, of the extinct sciuromorph rodents.
It is based upon Zittel's classification, except in instances where
later discoveries have thrown additional light upon the subject:

The Mvlagaulid^e— Riggs.

187

DISTRIBUTION OF SCIUROMORPH RODENTS
IN NORTH AMERICA.

Oligo-
cene.

MIOCENE.

Plio-
cene.

Pleisto-
cene.

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Geomyidae :

Gymnoptychus montanus

X

" trilophus

X

Heliscomys vetus.

X

Pleuro liens leptophrys .

X

Entoptychus crassiramis

X

" pianifrous .

X

Castoridae :

Steneofiber nebrascensis

" peninsulatus

X

" pansus

X

" gradatus

X

" montanus

X

Eucastor tortus .

X

Castor fiber

X

X

" pelicedens

X

" rosina

X

Sciuridae:

A rctomys vetus .

X

" monax

X

" fossilis

X

Sciurus relictis .

X

" vottmani

X

" calycinus

X

" panolius

X

Tantius lavideus .

X

Meniscomys liolophus

X

" cavatus

X

" niteus

X

Mylagaulidse :

Protogaulus hippodus ....

X

Mesogaulus ballensis ....

X

Mylagaulus sesquipedalis .

X

•' monodon ....

X