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GEOLOGICAL SERIES

OF
FIELD MUSEUM OF NATURAL HISTORY

Volume VI Chicago, May 26, 1937 No. 17

MOUNTED SKELETON OF HOMALODOTHERIUM

By Elmer S. Riggs

Curator op Paleontology

The skeleton of Homalodotherium figured on page 235 has been
reconstructed from a specimen in Field Museum previously described
by Professor William B. Scott. Since the publication of Scott's
monograph (Field Mus. Nat. Hist., Geol. Mem., 1, No. 1) the specimen
has been carefully worked over by James H. Quinn, Assistant in
Paleontology, and a number of smaller bones, not before described,
have been recovered. Much restoration has been found neces-
sary in mounting the skeleton; this restoration has been done by
careful comparison with specimens of other related animals. While
the vertebral formula cannot be determined from the specimen in
hand, and the number of ribs is therefore conjectural, most other
features of the skeleton may be taken as definitely determined.

The specimen of Homalodotherium, Field Museum No. P13092,
was collected by the First Marshall Field Paleontological Expedition
to Argentina and Bolivia in 1923. It was found by the present writer
in blocks of tufaceous sandstone at tide level of the Atlantic coast,
some twenty miles north of Cape Fairweather, on the sheep farm
of Don Pedro Montes. The blocks containing the specimen had
fallen from the face of the sea-wall, apparently some eighty feet
above the level of high tide. This would place the horizon near the
middle of the Santa Cruz beds.

The parent ledge was not approachable by ordinary means
because of the steepness of the cliff. On the beach the waves had
worn the fallen mass into two rounded blocks, with sections of lower
jaws, leg bones, and ribs appearing at the surface. The action of
the waves was thus fast destroying a specimen which probably had
consisted of an entire skeleton. A single femur of Nesodon associated
with the specimen led to its being at first recorded under that name.

The specimen was removed from the matrix by the late J. B.
Abbott, veteran preparator and member of the expedition which
collected it. The unusual structure of foot and leg was recognized

No. 385 233

**** ffistory Survey
— Library

234 Field Museum of Natural History — Geology, Vol. VI

as soon as they were brought to light. By mutual arrangement the
specimen v/as sent to Professor Scott at Princeton University for
study, and his splendid monograph resulted. The skull, being quite
well known, was not submitted to Professor Scott for study; likewise,
the left half of the pelvis was not sent to him.

Six years after Scott's monograph had been published it was de-
cided to restore the skeleton and to mount it. The former preparator
having meantime died, the specimen was placed in the hands of
Mr. Quinn. The fragments remaining, after the first preparation,
were again carefully worked over. In this process the trapezium,
the pisiform, and a vestigial first metacarpal were recovered. These
were not seen by Scott and have remained undescribed until now.

A detailed list of the bones of Homalodotherium now known to
belong to skeleton No. P13092 are as follows:

Skull: abraded in the facial region so that the nasals, the frontals and the

superior portion of the parietals are lost; the palatal surface, including

the maxillaries, with molar-premolar dentition, the basi-cranial region

with condyles, the supraoccipital, and parts of the zygomatic arches are

preserved.
Mandibles: posterior halves of both rami, including the second and third

molars, the condyles and the coronoid processes are preserved from one

side or the other.
Cervical vertebrae: the seventh only.

Dorsal vertebrae: Nos. 1, 3, 4, 5, 6, 11, 12, 13, 14, 15 — ten in all.
Sacral vertebrae: none.
Caudal vertebrae: none.
Ribs: eight rib-heads and parts of three shafts from the right side; eight

rib-heads, part of one shaft from the left.
Scapulae: right, articular surface, section of lamina, including posterior margin;

left, spine and parts of superior and inferior lamina.
Clavicle: left, entire.
Pelvis: right half almost entire; left ilium lacking part of crest, pubis entire;

ischium, posterior section from tubercle to margin of obturator foramen.
Humerus: left, distal half.

Radii: left, entire; right, lacking the distal epiphysis.
Ulnae: left, entire.

Carpus: left series entire save trapezium; right, distal row only.
Metacarpals: right, 1, 3, 4; left, 3, 4.
Phalanges: right fore foot, digit III, 2, 3; left fore foot, digit III, 1, digit IV,

2, 3.
Femora: right distal end and shaft as far as the margin of the digittal fossa;

left, head and trochanter lacking crest and one-third of shaft, including

digital fossa, entire.
Patellae: none.
Tibia: right, entire.

Fibula: right, entire; left, distal half only.
Tarsus: right, complete except calcaneum; left, all save cuboid and cuneiforms

1 and 3.
Metatarsals: right, II, III, IV, V; left, II, III, IV, V.
Phalanges: right, digit V, 1, 2; left, digit V, 2.
Sternebrae: five segments, including manubrium and ziphisternum.

,

235

236 Field Museum of Natural History— Geology, Vol. VI

The specimen is an important one, not only as the unique skeleton
of its kind, but as a highly specialized member of a rare family.
For that reason it has appeared desirable to mount the skeleton,
even though important sections of the vertebral column are missing
from it and had to be reconstructed. The vertebral column as
restored has fifteen dorsal and seven lumbar vertebrae, according
to the formula given Tomashuxleya by Simpson. The sacrum is
restored by comparison with an undescribed specimen of Pliocene
Zotodon (P14440) in this Museum. The caudal series is restored
according to the general toxodont type. The missing and broken
ribs are reconstructed by comparison with others in the same series.
The only other parts remaining in doubt are the proximal half of
the humerus, the second metacarpal, the first metatarsal, the patella,
and the unguals of the pes.

The skull, as explained above, is abraded in the frontal and
parietal regions, but beautifully preserved in the palatal aspect. It
has recently been figured by Patterson from this view (The Internal
Structure of the Ear in Some Notoungulates, Field Mus. Nat. Hist.,
Geol. Ser., 6, No. 15, 1936). From the lack of reduction in the
first premolar tooth, and from comparative measurements, this
specimen has been referred to the species H. cunninghami Flower.
A series of measurements is given below.

Measurements of Dentition mm

P 1 , a-p diameter 18.0

P 1 , trans, diameter 20.0

P*, a-p diameter 20.0

P x , trans, diameter 27.0

pa, a-p diameter 22.0

P a , trans, diameter 29.0

P±, a-p diameter 21.5

P-, trans, diameter 33.0

M 1 , a-p diameter along ectoloph 33.5

M 1 , a-p diameter at center of tooth 29.0

M 1 , trans, diameter 36.0

M 2 , a-p diameter along ectoloph 43.5

M-, a-p diameter at center of tooth 36.0

M 2 , trans, diameter 41.5

M a , a-p diameter along ectoloph 43.5

M 1 , a-p diameter at center of tooth 33.0

M a , trans, diameter 38.0

Length, P A -M a along ectolophs 185.5

Length, P x -M a across centers of teeth 177.0

Length, P*-* 81.0

Length, M A - a along ectolophs 111.0

Length, M A - a across centers of teeth 97.0

Mj, trans, diameter 22.0

Mi, a-p diameter 48.5

Ms, trans, diameter 20.0

Mounted Skeleton of Homalodotherium 237

Measurements of Skull mm

Skull, basilar length 411.0

Cranium, length from condyle at anterior border

of orbit 263.0

Width of exoccipitals 127.0

Length of zygomatic arch, from posterior border

of glenoid cavity 160.0

Width of cranium at postorbital constriction .... 62.5

Width of face at anterior border of orbits 141.0

Palate, length in median line 233.0

Palate, width at P* 56.5

Palate, width at M 1 64.0

Mandible, depth at anterior border of M 5) external 61.5

Mandible, height of condyle 164.0

Mandible, height of coronoid 235.0

Mandible, length from angle to My 138.5

Sternum. — Some question was raised by Scott with regard to
the sternal series, particularly the presternum. In mounting the
skeleton the segment designated by him as the presternum has
been reversed so as to place the rounded extremity forward and
the broad flattened end in contact with the next segment, which
also is quite broad. In this position the difficulty disappears. The
pits for the first pair of costal cartilages are near the anterior end
of the presternum; the rounded end of the clavicle, with some
allowance for interclavicular cartilage, fits the rounded anterior
end of the presternum. While this arrangement leaves a rather
wide space between the first and second pairs of costal cartilages,
it appears to be the natural articulation of these elements. The
bones can be made to articulate reasonably well in no other way.
The five sternal segments preserved may or may not constitute the
entire number, but they have been mounted in series.

Clavicle. — The element designated as the clavicle of this speci-
men is a short bone having no distinct articular ends; the proximal
end is expanded, round and rugose at its extremity, evidently for
cartilaginous connections. The opposite extremity is broad and
flattened, with one surface oblique, somewhat rugose, and likewise
fitted for cartilaginous attachment. The shaft is quite rounded and
moderately curved. It bears on its surface an oblique ridge which
extends from the margin of the flattened extremity along the convex
surface of the shaft and disappears at a point well past its middle.
The bone is clavicle-like in its form and proportions; it has the
texture and appearance of the other bones of this specimen. There
is no other element of the skeleton for which it might be mistaken;
neither is there a reasonable probability that it might belong with
the solitary femur of Nesodon, which was found in association with

238 Field Museum of Natural History — Geology, Vol. VI

this specimen. It may, therefore, be determined as a clavicle and
as belonging to the specimen under consideration. The presence
of such an element in Homalodotherium is consistent with the use
of the claw-bearing fore foot as observed in this specimen and in
other specimens of this genus. It is likewise consistent with the

Fig. 56. Posterior view of pelvis and hind legs of H. cunninghami. F.M. No. P13092. X 1/12.
Drawing by C. F. Gronemann.

fossorial habits attributed by Lydekker and by Scott to Homalodo-
therium, and concurred in by this writer.

Pelvis (fig. 56). — The pelvis is broad across the ilia and encloses a
wide pelvic aperture. The articulation with the sacrum is of limited
extent and apparently admitted of some freedom of movement.
The ischiatic protuberances are unusually prominent.

Femora. — The structure of the femur in this specimen has been
well described by Scott (op. cit.), but doubt was left as to the
exact length of this element. Although both femora are incomplete,

Mounted Skeleton of Homalodotherium

239

the shaft of the right bone is preserved, as far as the inferior margin
of the digital fossa, and the proximal end of the left bone includes
the entire margin of the same. This gives a common point in the
right and left bones of the same animal from which the length of this
element has been computed with full assurance of accuracy.

Fig. 57. Fore foot of H. eunninghami, showing entire carpal series and relative size of
Metacarpal I. sc., scaphoid; I., lunar; cu., cuneiform; Id., trapezoid; m., magnum; «., unciform; I, II,
III, IV, V, metacarpals. F.M. No. P13092. Xl/2. Drawing by C. F. Gronemann.

Fore foot (fig. 57). — While the fore foot of Homalodotherium has
been variously figured by Ameghino and by Scott, none of the figures

240 Field Museum of Natural History — Geology, Vol. VI

published have been entirely correct ; neither is the foot-structure of
any member of this genus entirely known from a single individual
at this time. 1

The most important features in the composition of the carpus
of H. cunninghami are: The scaphoid and lunar articulate closely;

Fig. 58. Trapezium. A, lateral view, showing facets for articulation, (a) with the trapezoid,
(6) with Metacarpal II, (c) with Metacarpal I; B, mesial view showing articular facets; C, D, Meta-
carpal I, mesial and anterior views. F. M. No. P13092. Xl/1. Drawing by C. F. Gronemann.

their proximal surfaces, as seen from the anterior view, form a con-
tinuous, convex outline. The trapezoid and magnum are similar
in size and from the front view their outlines are almost symmetrical

1 The figure of Ameghino (Rev. del Jardin Zool. de Buenos Aires, T. II,
1894), while poorly drawn and lacking the scaphoid entirely, shows the lunar,
the cuneiform, pisiform, magnum and unciform essentially the same as in the
Field Museum specimen. The trapezium and trapezoid are shown so much
in profile that their outline and relations to other elements can not be well deter-
mined. The first metacarpal, restored as bearing a functional digit, is entirely
inconsistent with the specimen under study. The figure by Scott (Princeton
Expeditions, 6, PI. XXX, fig. 1), though beautifully lithographed, does not
consistently represent the lunar, magnum, trapezium, trapezoid, or first meta-
carpal. The later figure by Scott (Field Mus. Nat. Hist., Geol. Mem., 1, No. 1,
PI. Ill, fig. 12) has corrected all of the former errors except that in Metacarpal I,
but does not figure the trapezium or the pisiform. The scaphoid and the trapezoid
have been allowed to drop down out of position.

Mounted Skeleton of Homalodotherium

241

to the line of their inter-articulation. The scaphoid is supported
entirely by the trapezoid, the lunar rests between the magnum and
the unciform. The magnum presents only a narrow facet to the
second metacarpal. The first metacarpal is reduced to a vestige
and bears no distal articulation to support phalanges.

Trapezium (fig. 58). — The trapezium of this specimen has not
been previously described. It is a strong bone subpentagonal in its
mesial aspect and bears three relatively small facets for articulation
and a prominent tubercle at the posterior margin. The facet for
the trapezoid is triangular in outline and oblique to the long axis

Fig. 59.
proximal view.

Pisiform bone of H. cunninghami.
Drawing by C. F. Gronemann.

F.M. No. P15242. Xl/1. A, inferior view; B,

of the bone. A small D-shaped facet for articulation with the second
metacarpal appears at the antero-superior margin and forms an
obtuse angle with the facet for the trapezoid. A small oval facet
at the distal extremity gives articulation for the vestigial first
metacarpal.

Pisiform (fig. 59). — This is a strong bone almost as wide as
it is long. It presents a broad, convex facet to the cuneiform and
a minor facet to the ulna. The lateral tubercle is smaller by half
than the distal end. The latter bears a strong rugosity which extends
over one-third of the inferior surface.

Metacarpal I (fig. 57). — This element was figured by Ameghino
from the proximal half of a specimen and was assumed to indicate
a functional first digit. It now appears probable that the bone so
figured was introduced and did not belong to Homalodotherium at

242 Field Museum of Natural History — Geology, Vol. VI

all. While perpetuating this error in the published drawing, Scott's
description of the Field Museum specimen explained that no speci-
men with first metacarpal had been found at that time. The
specimen in hand is barely two-ninths as long as Metacarpal III,
as shown in fig. 57. It has no distal articular surface and bears
the usual marks of a vestigial metacarpal. It articulates with the
trapezium by a small oval facet at the distal margin of that bone.
The entire lateral surface is rugose, indicating a ligamentary attach-
ment with the second metacarpal.

GENERIC CHARACTERS
The following distinguishing characters of the genus may be
given: Dentition, If, Ci, Pi, Ml; canines little differentiated from
incisors; molar and premolar dentition, brachyodont and inter-
grading; molar crowns trapezoidal with cingulum. Clavicle present;
fore leg capable of rotation; fore foot digitigrade and claw-bearing;
hind foot plantigrade. Digits IV-IV with vestiged Mc. I and Mt. I.

Measurements of Skeleton mm

Skeleton, height as mounted 1090

Skeleton, length in projection 1900 <-

Femur, computed length 480

Pisiform, greatest length 51

Pisiform, greatest breadth 46

Trapezium, greatest length 38

Trapezium, greatest breadth 32

Metacarpal I, length 36

PROBABLE HABITS OF HOMALODOTHERIUM
This animal had the proportions of a huge bear, uplifted in front
by the digitigrade position of the fore feet. Great strength in the
shoulders is evidenced by the proportionate size of the humerus
and the extension and prominence of the deltoid area. Freedom of
movement is apparent in the structure of fore arm and fore foot.
These characters, taken with the presence of a functional clavicle
and a claw-bearing foot, indicate a prehensile member capable
of digging in quest of food or of pulling down branches of trees in
order to feed upon the fruit or foliage. Such habits are usually
attributed to the ponderous gravigrade sloths, but in Homalodo-
therium we have an animal much more slender than they and one
having an arm and hand modeled in a much higher type of structure.
The hind legs are relatively short and stout, longer in the thigh
and shorter in the lower leg. Armed with plantigrade feet capable
of being firmly planted, these members are well calculated for

Mounted Skeleton of Homalodotherium 243

supporting the body, either in the badger-like act of digging or in
an upright posture, in which the fore legs would be given entire
freedom in gathering fruits or foliage from trees. The prominent
ischiatic projection offers a broad anchorage for the adductor muscles,
which play an important part in supporting the body in the upright
position. From its size, as well as from structural characteristics,
Homalodotherium is not to be considered as a burrowing animal,
but, like many of the edentates of South America, and like the oft-
cited Moropus of North America, this animal appears to have been
fossorial as well as arboreal in its feeding habits.