Muzzle of South American Pleistocene Ground Sloths
(Xenarthra, Tardigrada)
M. Susana Bargo,1* Néstor Toledo,2 and Sergio F. Vizcaı́no1

1División Paleontologı́a Vertebrados, Museo de La Plata, and CIC-CONICET La Plata, Argentina
2Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, 122 y 60, 1900 La Plata, Argentina

ABSTRACT Sloths are among the most characteristic
elements of the Cainozoic of South America and are rep-
resented, during the Pleistocene, by approximately nine
genera of gigantic ground sloths (Megatheriidae and My-
lodontidae). A few contributions have described their mas-
ticatory apparatus, but almost no attention has been paid
to the reconstruction of the muzzle, an important feature
to consider in relation to food intake, and particularly
relevant in sloths because of the edentulous nature of the
muzzle and its varied morphology. The relationship be-
tween dietary habits and shape and width of the muzzle is
well documented in living herbivores and has been con-
sidered an important feature for the inference of alimen-
tary styles in fossils, providing an interesting methodolog-
ical tool that deserves to be considered for xenarthrans.
The goal of this study was to examine models of food
intake by reconstructing the appearance and shape of the
muzzle in five species of Pleistocene ground sloths
(Megatherium americanum, Glossotherium robustum,
Lestodon armatus, Mylodon darwini, and Scelidotherium
leptocephalum) using reconstructions of the nasal carti-
lages and facial muscles involved in food intake. The pres-
ervation of the nasal septum, and the scars for muscular
attachment in the rostral part of the skulls, allow making
a conservative reconstruction of muzzle anatomy in fossil
sloths. Wide-muzzled ground sloths (Glossotherium and
Lestodon) had a square, nonprehensile upper lip and were
mostly bulk-feeders. The lips, coupled with the tongue,
were used to pull out grass and herbaceous plants.
Narrow-muzzled sloths (Mylodon, Scelidotherium, and
Megatherium) had a cone-shaped and prehensile lip and
were mixed or selective feeders. The prehensile lip was
used to select particular plants or plant parts. J. Morphol.
267:248–263, 2006. © 2005 Wiley-Liss, Inc.

KEY WORDS: ground sloths; Xenarthra; Tardigrada;
Pleistocene; muzzle; feeding

Tardigrada (sloths) are the most diverse group
within the Xenarthra, but at present it is repre-
sented only by two genera, the tree sloths Bradypus
(Linné, 1758) and Choloepus (Illiger, 1811). There is
general consensus that they cluster together with
the Vermilingua (anteaters) in a natural group
called Pilosa (Gaudin, 2004). Sloths are among the
most characteristic elements of the Cainozoic of
South America, but are also well represented in
Central and North America (more than 80 genera
have been named; see McKenna and Bell, 1997).

Gaudin’s (2004) analysis corroborates the mono-
phyly of the four ground sloths families (Megatheri-
idae, Megalonychidae, Nothrotheriidae, and Myl-
odontidae), and the diphyly of the living tree sloths,
with Bradypus as the sister-taxon to all other sloths
and Choloepus allied with members of Megalonychi-
dae (Fig. 1). During the Pleistocene of South Amer-
ica approximately nine genera of gigantic ground
sloths (Megatheriidae and Mylodontidae) are re-
corded. The most conspicuous and better known spe-
cies are the megatheriid Megatherium americanum
and the mylodontids Glossotherium robustum, Lest-
odon armatus, Mylodon darwini, and Scelidoth-
erium leptocephalum (Fig. 2).

Extensive descriptions of the skull and mandible
of these ground sloths were given by Owen (1842,
1856, 1857), Lydekker (1894), Ameghino (1889),
Kraglievich (1922, 1923, 1928, 1934), and more re-
cently by McDonald (1987), De Iuliis (1996), Esteban
(1996), and Bargo (2001a,b). The most characteristic
features of the masticatory apparatus of extant xe-
narthrans, and possibly all extinct taxa, are a lack of
enamel in the adult dentition, a deciduous dentition,
and the cuspation pattern observed in other mam-
mals. Teeth are strongly reduced in number (5/4 in
sloths, except in Mylodon with 4/4 and the Plio-
Pleistocene nothrotheres with 4/3), composed of os-
teodentine, always hypselodont, and although they
can be lobate or bear lophs (mylodontids and
megatheriids, respectively), they are usually simple
and separated by short diastemata. A predental
space (predental spout) is particularly well devel-
oped in sloths.

With the exception of the proposition that at least
one may have been occasionally carnivorous (Fa-
riña, 1996; Fariña and Blanco, 1996), ground sloths
have been largely considered herbivorous, primarily
by analogy with living tree sloths. Despite the com-

*Correspondence to: M. Susana Bargo, División Paleontologı́a Ver-
tebrados, Museo de La Plata, Paseo del Bosque s/n, B1900FWA La
Plata, Argentina. E-mail: msbargo@fcnym.unlp.edu.ar

Published online 28 November 2005 in
Wiley InterScience (www.interscience.wiley.com)
DOI: 10.1002/jmor.10399

JOURNAL OF MORPHOLOGY 267:248–263 (2006)

© 2005 WILEY-LISS, INC.



mon features mentioned above, the great variation
in skull and dental morphology, body size, and pro-
portions among ground sloths suggest that they had
diversified to fill a variety of niches (Bargo, 2001a).
There are some fairly detailed morphofunctional
analyses of the masticatory apparatus of both North
American and South American Pleistocene ground
sloths (McDonald, 1987, 1995, 1997; Naples 1987,
1989; Bargo, 2001a,b). However, almost no attention
has been paid to the reconstruction of the muzzle.
The latter is an important feature to consider in
relation to food intake, and is particularly relevant
in sloths, as well as in almost remaining xenar-
thrans, because of the edentulous nature of the muz-
zle and its varied morphology.

Several authors have studied the relationship be-
tween dietary habits and shape and width of the
muzzle in herbivores, particularly in ungulates (see
Janis and Ehrhardt, 1988, and references therein).
This feature has been considered important for the
inference of alimentary styles in fossil herbivores
(Solounias et al., 1988; Solounias and Moelleken,
1993a,b; Janis, 1995), providing an interesting
methodological tool that deserves to be considered
for xenarthrans.

Here we examine models of food intake by recon-
structing the appearance and shape of the muzzle in
the five South American Pleistocene giant ground
sloths: Megatherium americanum, Glossotherium

robustum, Lestodon armatus, Mylodon darwini, and
Scelidotherium leptocephalum.

Muzzle Musculature and Nasal Cartilages
in Living Pilosa

Very few works have dealt with the anatomy of
the facial musculature of living Pilosa. Windle and
Parson (1899) and Uekermann (1912) speculated on
the supposedly primitive condition of the facial mus-
culature in sloths and vermilinguas. Saban (1971)
described or mentioned some specific muscles in dif-
ferent xenarthrans. The most complete contribution
on facial musculature in both tardigrades (Bradypus
and Choloepus) and vermilinguas (Myrmecophaga,
Tamandua, and Cyclopes) is that by Naples (1985),
who made detailed descriptions of the muscles
through dissections of adult specimens.

Recently, a series of insightful contributions con-
sidered the narial anatomy of living ungulates (Wit-
mer et al., 1999; Clifford and Witmer, 2004a,b) and
have been very useful in the reconstruction of fossil
taxa (e.g., Mihlbachler and Solounias, 2004). Wit-
mer et al. (1999) and Clifford and Witmer (2004a,b)
standardized the nomenclature of the anatomical
structures following veterinary anatomy texts and
the Nomima Anatomica Veterinaria (NAV, 1994).

The following is a summary of the muscles de-
scribed in living Pilosa by Naples (1985), who fol-

Fig. 1. Phylogeny of Tardigrada
(modified from Gaudin, 2004).

249MUZZLE RECONSTRUCTION OF FOSSIL GROUND SLOTHS

Journal of Morphology DOI 10.1002/jmor



lowed the terminology of Rinker (1954). In this sec-
tion we mention only some muscles of the pars
intermedia and oris of the sphincter colli profundus
in living sloths that are relevant for the reconstruc-
tion of muzzle anatomy in fossil sloths. When possi-

ble, references to other xenarthrans (Dasypodidae)
are provided. We include in parentheses the ana-
tomical equivalent in living ungulates following Wit-
mer et al. (1999) and Clifford and Witmer (2004a)
(see Table 1).

Fig. 2. A: Skull of Megath-
erium americanum in lateral
view, and (B) muzzle frontolateral
view. C: Skull of Glossotherium
robustum in lateral view and (D)
muzzle frontal view. E: Skull of
Lestodon armatus in lateral view
and (F) muzzle frontal view.
G: Skull of Mylodon darwini in
lateral view and (H) muzzle fron-
tolateral view. I: Skull of Scelido-
therium leptocephalum in lateral
view and (J) muzzle frontolateral
view.

250 M.S. BARGO ET AL.

Journal of Morphology DOI 10.1002/jmor



1. Pars Intermedia
M. zygomaticolabialis (� zygomaticus). It is

present only in Choloepus as a thin sheet of fibers,
arising on the fascia anterior of the ear, inserting on
to the fascial covering of the posterior projection of
the zygomatic arch.

M. nasolabialis (� levator naso-labialis). It is
present in Bradypus, Choloepus, Myrmecophaga,
and Tamandua, but absent in Cyclopes. In sloths it
arises from the supraorbital ridge anterior to the
postorbital prominence and inserts into the fibrous
pad on the lateral surface of the maxilla. It elevates
the upper lip in its caudal portion.

2. Pars Oris
M. maxillo-labialis (� caninus). Although Na-

ples (1985) indicates that it is absent in sloths and
anteaters, Saban (1971) mentions its presence in
Bradypus, Tamandua, and the dasypodids Dasypus
and Euphractus, comprising dorsal and ventral fas-
cicles. It attaches on the base of the zygomatic arch
and inserts on the upper lip and outline of the naris,
topographically equivalent to the nasolabialis pro-
fundus pars maxillaris superficialis and pars max-
illaris profundus of Naples (1985) (see below). This
muscle retracts the nostrils caudally.

M. dilatator nasi (�levator labii superioris).
It is present in all genera of sloths and anteaters. In
Choloepus and Bradypus it arises from the ventral
portion of the anterior border of the zygomatic plate,
deep to the origin of the m. nasolabialis profundus
pars maxillaris superficialis. The anterior half is
composed of a flat tendon that inserts into the dor-
solateral border of the rhinarium. In vermilinguas
the origin and insertion follow the same pattern as
in sloths. Saban (1971) also describes it for Dasypus.
This muscle elevates and everts the upper lip.

Nasolabialis profundus (� lateralis nasi).
Following Naples (1985) this muscle is reduced or
simplified in Pilosa, including three parts in sloths,

and five parts in vermilinguas. Some of them do not
have bony attachments.

i) Pars interna (� dilatator naris apicalis?, trans-
versus nasi). It is present only in Tamandua and
Myrmecophaga (Saban, 1971; Naples, 1985). It
arises deep to the skin over the nasal bones, and
inserts into the notch at the anterior corner of the
naris. Some fibers pass laterally and ventrally to
insert dorsal to the fat pad above the upper lip.

ii) Pars media superioris. It is absent in sloths but
present in vermilinguas. It arises from the bridge of
the rostrum with fibers passing ventrally, deep to
the tendon of m. dilatator nasi and inserting into the
fibers of m. orbicularis oris.

iii) Pars anterior. It is absent in Bradypus and
Myrmecophaga, but present in Choloepus, Taman-
dua, and Cyclopes. In Choloepus it arises from the
nasal cartilage, on the dorsolateral margin of the
naris and inserts into the fat pad and connective
tissue of the upper lip.

iv) Pars anterior profunda. It is only present in
Tamandua.

v) Pars maxillaris (� caninus?). It is divided into
pars superficialis and pars profunda, and is present
in the five genera of Pilosa. In Choloepus the pars
superficialis arises via a short aponeurosis on the
ventral portion of the anterior border of the zygo-
matic plate, but does not extend onto the maxillary
bone. It inserts by a tendon, which gives off a num-
ber of branches into the fibers of m. nasolabialis
pars orbicularis oris on the upper lip. The condition
is similar in Bradypus. The pars profunda arises
from a thin aponeurosis on the anterior side of the
zygomatic plate, dorsally to the origin of pars max-
illaris superficialis.

M. buccinator (� buccinator). It is present in
all five genera. In sloths it arises from the lateral
surface of the maxilla above the row of cheek teeth,
terminating directly posterior to the last tooth. It
forms the cheek, as in other mammals. This muscle

TABLE 1. Muscle nomenclature and homologies, and facial muscles described for the living sloths by Naples (1985)

Naples, 1985 Boas and Paulli, 1908

Witmer et al., 1999;
Clifford and Witmer,

2004a, b Choloepus Bradypus

Zygomaticolabialis Zygomaticus platismatis Zygomaticus � –
Nasolabialis Nasolabialis Levator nasolabialis � �
Dilatator nasi Maxilolabialis superior Levator labii superioris � �
Nasolabialis profundus Nasalis Lateralis nasi?

Pars interna Lateralis nasi Dilatator naris apicalis? – –
Pars media superior — — – –
Pars media inferior — — – –
Pars mediana Transversus nasi Dilatator naris apicalis? – –
Pars anterior — — � –
Pars maxillaris superficialis — — � �
Pars maxillaris profunda — — � �

Maxillolabialis Maxillolabialis inferior Caninus – –
Buccinator Buccinatorius Buccinator � �

Pars orbicularis oris Pars rimana Orbicularis oris � �
Pars intermaxillaris Pars supralabialis Incisivus superior � �

�, present and –, absent.

251MUZZLE RECONSTRUCTION OF FOSSIL GROUND SLOTHS

Journal of Morphology DOI 10.1002/jmor



inserts into the lateral surface of the mandible, dor-
sal to the roots of the cheek teeth. Following Naples
(1985), the pars orbicularis oris and pars intermax-
illaris are part of the buccinator. The first acts as a
sphincter of the mouth. The second is not clearly
differentiated from other parts of the m. buccinator
in sloths, but it could be a synonym of the incisivus
superior of Clifford and Witmer (2004a) (see Discus-
sion), the principal action of which is to elevate and
retract the upper lip.

Although a detailed reconstruction of the nasal
cartilages is not the goal of this work, some general
aspects of these structures, necessary for the muzzle
reconstruction, can be inferred from features of the
nasal cavity. The nasal capsule of the chondrocra-
nium of Choloepus and Tamandua was described
and illustrated by Zeller et al. (1993). The chondro-
cranial anatomy of the rostrum nasi (the anterior
end of the nasal capsule) of Choloepus resembles
that of a generalized eutherian more than that of
Tamandua. In Choloepus, the tectum nasi is broadly
continuous with the cartilago cupularis. A processus
cupularis (� processus alaris inferioris) extends lat-
erally from the rostroventral edge of the septum
nasi. Laterally, the paries nasi is separated from the
tectum by an expanded fenestra nasi superior. A
processus lateralis ventralis completes the capsule
ventrally. A processus alaris superioris protects the
external nasal aperture. A septomaxilla lies super-
ficially at the posterior corner of the fenestra narina.
Although Wegner (1950) reported this bone for some
specimens, Zeller et al. (1993) did not find it in
macerated skulls of either Tamandua or Choloepus.

MATERIALS AND METHODS
Acronyms

CN: Zoological Museum, Copenhagen, Denmark. MACN:
Museo Argentino de Ciencias Naturales, Buenos Aires, Argen-
tina. MLP: Museo de La Plata, La Plata, Argentina. MMP: Museo
Municipal de Ciencias Naturales, Mar del Plata, Argentina.
MNHN-BOL: Museo Nacional de Historia Natural, La Paz, Bo-
livia. MNHNP: Muséum National d’ Histoire Naturelle, Paris,
France.

Skulls of the following specimens were analyzed and measured
for this study: Megatherium americanum Cuvier, 1796. MACN
5002, MACN 15154, MACN 6 P, MACN 2832, MLP 2-64, MNHN
PAM 276. Glossotherium robustum (Owen, 1842). MACN 11769,
MLP 3-136, MLP 3-137, MLP 3-138, MLP 3-140, MMP 1489-M,
MMP 1490-M. Mylodon darwini Owen, 1839. CN 43, MACN
15348, MLP 3-762, MLP 3-764, MNHN-BOL-V 006470. Lestodon
armatus Gervais, 1855. MLP 3-3, MLP 3-29, MLP 3-30, MMP
47-S. Scelidotherium leptocephalum Owen, 1840. MLP 3-671,
MLP 3-401, MLP 3-402, MMP 9-S, MMP 31-S, MMP 127-S, MMP
157-S, MMP 458-S, MMP 549-S, MMP 614-M, MMP 1155-M.

Reconstruction of Musculature and Nasal
Cartilages

The attachment sites of the musculature are usually well indi-
cated in mammals by features of the skull and jaws, such as
roughened surfaces, scar lines, ridges, and crests. These features
are usually very conspicuous in fossils, but it depends on the size
and the degree of preservation. The areas of origin and insertion

of the musculature were reconstructed based on features of the
skull, the patterns of musculature in modern mammals (Maynard
Smith and Savage, 1959; Turnbull, 1970), particularly those in
tree sloths Bradypus and Choloepus (Macalister, 1869; Windle
and Parson, 1899; Edgeworth, 1935; Sicher, 1944; Naples, 1982,
1985), and the narial anatomy of Tapirus (Perissodactyla, Tapiri-
dae), Alces (Artiodactyla, Cervidae), and Saiga (Artiodactyla, Bo-
vidae) (Witmer et al., 1999; Clifford and Witmer, 2004a,b, respec-
tively). The nomenclature follows Naples’ (1985) scheme with
changes and additions after Witmer et al. (1999) and Clifford and
Witmer (2004a).

Uncertainty regarding the interpretation of the shape and de-
velopment of the nasal cartilages is greater than that for the
muscular reconstruction. For the fossils, we made the most con-
servative graphic reconstruction based on the described anatomy
of immature specimens of the closest living relative, the tree sloth
Choloepus by Zeller et al. (1993). We looked for evidence on the
septum nasi (when preserved), general morphology of the area,
and the scars that could not be attributed to muscular or ligament
attachments, and could correspond topographically to cartilages.

Relative Width and Shape of the Muzzle

As mentioned above, there is a relationship between dietary
habit, shape, and width of the muzzle in ungulates. Following
Janis and Ehrhardt (1988), the width of the palate could repre-
sent a measure of the rate of food ingestion, which reflects a high
correlation with body mass. These authors calculated the relative
muzzle width as palatal width divided by muzzle width. High
values of this ratio indicate selective feeders with narrow muz-
zles. The muzzle width in ungulates was measured at the
premaxillo-maxillary suture, and the palatal width is the dis-
tance between the M2 protocones (Janis and Ehrhardt, 1988;
Janis, 1990).

Because the premaxillae are reduced in sloths, the maximum
muzzle width (MMW) is generally on the maxilla and these mea-
surements were conveniently modified for the ground sloths. The
palatal width (PW) was measured as a mean of the anterior and
posterior width, since in some mylodontids the palate is wider
anteriorly. Therefore, the index of relative muzzle width for the
ground sloths is given as RMW � PW/MMW. The values obtained
cannot be directly compared with those for ungulates, but provide
a framework for comparison among ground sloths.

Dietary categories (browsers, grazers, and mixed feeders) were
differentiated also in extant ruminants through analysis of pre-
maxillary shape (Solounias et al., 1988) and applied to their
extinct relatives (Solounias and Moelleken, 1993a). Following
this approach, browsers have pointed premaxillae, grazers have
square premaxillae, and mixed feeders have an intermediate
condition between the first two. As the premaxillae are reduced in
ground sloths, the muzzle is formed mainly by the maxillae, and
includes an edentulous predental space formed by the premaxil-
lae and maxillae (Fig. 3). The shape of the muzzles was evaluated
graphically, superimposing outlines of the premaxillae and ante-
rior part of the maxilla, standardized at the width at the first
molariform.

The feeding categories mentioned above are based on field
observations, analyses of stomach contents and structure, and
fecal studies (see Hofmann and Stewart, 1972; Hofmann, 1973,
1989; Jarman, 1974). However, these terms are imprecise, and
have been used to refer to the mode of food acquisition as well as
the type of food ingested, i.e., browsing may refer to selective
feeding of any food type, as well as eating dicot material, whereas
grazing denotes grass eating, but is used to mean eating of forbs
as well, or may refer to a nonselective feeding. As it is clear that
the form (i.e., shape and size) of the muzzle imposes physical
constraints to the size and amount of food that can be taken with
each bite, we adopt a morphofunctional criterion classifying di-
etary habits in a gradation from highly selective to highly non-
selective or bulk feeders.

252 M.S. BARGO ET AL.

Journal of Morphology DOI 10.1002/jmor



RESULTS
Muzzle Muscular Attachments and Nasal
Cartilages

The facial muscles reconstructed in ground sloths
are listed in Table 2, which also includes those de-
scribed for the tree sloths. As mentioned above, the
features of the skull and jaws (roughened surfaces,
scar lines, ridges, crests) are very conspicuous in
fossil ground sloths. On the contrary, the attach-
ment areas of the muscles of living sloths can be
observed through dissections, but do not always

show rugose surfaces or scars in the corresponding
bones. Such difference must be certainly explained
by allometry, since the fossils are two or three orders
of magnitude larger than the living ones. Differ-
ences are also expected because the different habits
of the fossil forms, which require stronger muscles.

Megatherium americanum (Fig. 4). Most
skulls of Megatherium americanum either lack the
septum nasi or preserve only its proximal portion.
Nevertheless, the preserved proximal portion of the
septum indicates that this structure is quite robust.
An exceptional specimen exhibited at the MNHN
(MNHN PAM 276) completely preserves a well-
developed and ossified septum nasi (see Fig. 2A,B),
which allows us to confirm the robustness of this
structure and to infer some morphological features.
The septum shows two deep grooves that run on both
sides of its dorsal edge, continuing ventrally until
the level of the premaxilla. These grooves present
lateral flanges in their proximal portions, adjacent
to the nasoincisive notch, which could be interpreted
as insertion areas for the nasal cartilage.

The distal edges of the maxilla, forming the narial
aperture are very rugose, possibly for insertion of
nasal cartilages. The distal ends of the premaxillae
show very distinctive, concave, roughened surfaces
that are slightly ventrally directed. They could be
interpreted as the origin area of the bilateral retrac-
tor muscle of the upper lip (M. buccinator pars in-
termaxillaris � incisivus superior). Above the distal
end of the premaxillae there is a robust ascending
process, the area of insertion for the septum nasi.
This process is rugose anteriorly, and could also be
part of the origin area of the incisivus superior.

The anteromedial border of the orbit, dorsal to the
infraorbital foramen, has a roughened and de-
pressed area that could indicate the origin of m.

Fig. 3. Muzzle palatal views of the five species of ground
sloths. A: Lestodon armatus. B: Glossotherium robustum. C: My-
lodon darwini. D: Scelidotherium leptocephalum. E: Megath-
erium americanum.

TABLE 2. Facial muscles described for the ground sloths

Facial muscles Choloepus Bradypus Megatherium Scelidotherium Mylodon Glossotherium Lestodon

Zygomaticolabialis
(�zygomaticus) � – NF NF NF NF NF

Nasolabialis (�levator
nasolabialis) � � P P P P P

Dilatator nasi (�levator labii
superioris) � � P NF P P NF

Nasolabialis profundus
Pars interna – – NF NF NF NF NF
Pars media superior – – NF NF NF NF NF
Pars media inferior – – NF NF NF NF NF
Pars mediana – – NF NF NF NF NF
Pars anterior � – NB NB NB NB NB
Pars maxillaris superficialis � � NF NF NF NF NF
Pars maxillaris profunda � � NF NF NF NF NF

Maxillolabialis (�caninus) – – P NF P P NF
Buccinator � �

Pars orbicularis oris
(�orbicularis oris) � � NB NB NB NB NB

Pars intermaxillaris
(�incisivus superior) � P P P P P

P, present; NF, not found or absent; NB, no bony attachment; �, present, –, absent.

253MUZZLE RECONSTRUCTION OF FOSSIL GROUND SLOTHS

Journal of Morphology DOI 10.1002/jmor



maxillo-labialis (� caninus) and/or m. dilatator nasi
(� levator labii superioris). The prominent postor-
bital processes, with obvious scarlines on their dor-
solateral surfaces, indicate the origin area for the
postorbital ligament closing the orbit caudally. On
both sides of the skull the area of the fronto-naso-
maxillary suture shows a surface with fine scarlines
that could be the origin area for m. nasolabialis.

Glossotherium robustum (Fig. 5). The septum
nasi is incomplete, as in most of the ground sloths
skulls analyzed, but based on the small preserved

parts it seems to be thin, in contrast to Megatherium
americanum and Mylodon darwini (see below).

Distally, the end of the premaxillae form a slightly
rugose flange, indicating the origin of m. incisivus
superior. The dorsal surface of each premaxilla
bears a short ascending process that fuses at the
midline with that of the opposite side to form a
small, triangular, anterodorsally oriented projec-
tion. Posterior to the projection there is a roughened,
depressed, triangular, and well-marked area, con-
tinuing inside of the nasal cavity. It could be the

Fig. 4. Megatherium america-
num. Skull showing (A) main at-
tachments areas of muscles and car-
tilages, (B) reconstruction of nasal
cartilages, (C) reconstruction of
muscles, and (D) reconstruction of
external appearance.

254 M.S. BARGO ET AL.

Journal of Morphology DOI 10.1002/jmor



attachment area for the cartilaginous floor of the
nares (processus lateralis ventralis?). The borders of
the nasal opening (maxillae and nasals) are rugose,
but this feature is more accentuated at the edge of
nasals.

In the anteromedial edge of the orbit, dorsal to the
infraorbital foramen, there is a rugose, depressed,
and well-defined area indicating the origin of m.
maxillo-labialis (� caninus) and/or m. dilatator nasi
(� levator labii superioris). At the fronto-naso-
maxillary suture, on both sides of the skull, there is

slightly depressed surface, which could be the origin
of m. nasolabialis.

Lestodon armatus. Although clearly larger than
Glossotherium robustum, the shape of the muzzle of
this species clearly constitutes a common morpho-
type with it, so we refer to Figure 5 for the muscle
reconstructions. The septum nasi is lost in the spec-
imens studied. Some remains in the floor and roof of
the nasal cavity indicate that the septum was thin
and weak, as in G. robustum. The broad anterior
edges of the nasals are very rugose, probably indi-

Fig. 5. Glossotherium robustum.
Skull showing (A) main attach-
ments areas of muscles and carti-
lages, (B) reconstruction of nasal
cartilages, (C) reconstruction of
muscles, and (D) reconstruction of
external appearance.

255MUZZLE RECONSTRUCTION OF FOSSIL GROUND SLOTHS

Journal of Morphology DOI 10.1002/jmor



cating a robust cartilago cupularis. As in the other
ground sloths, the premaxillae have their anterome-
dial edge broadly roughened, showing the origin of
m. incisivus superior.

The origin areas of m. maxillo-labialis (� caninus)
and m. dilatator nasi (� levator labii superioris) are
not visible, and that of m. nasolabialis is not well
defined.

Mylodon darwini (Fig. 6). The septum nasi is
broken in the skulls of Mylodon darwini examined,
but the preserved proximal part indicates that it
was robust. In the collections of the CN there is an
excellent skull (CN 43), with the complete nasal
arch preserved (Fig. 2G,H) that was figured by Re-

indhart (1879). The end of the premaxillae form a
thick, slightly roughened flange. A robust ascending
process fuses to the nasals, forming a nasal arch.
This arch is nearly complete in two or three speci-
mens. The lower part of the anterior face of the arch
is also rugose. The retractor muscle of the upper lip
(incisivus superior) probably originated in this area,
i.e., end of the premaxillae and ascending process, as
in Megatherium americanum.

On the anterior edge of the orbit, dorsal to the
infraorbital foramen, there is a depressed rugose
area which could be the origin for m. dilatator nasi
(� levator labii superioris) and/or m. maxillo labialis
(� caninus), as observed in Megatherium america-

Fig. 6. Mylodon darwini. Skull
showing (A) main attachments areas
of muscles and cartilages, (B) recon-
struction of nasal cartilages, (C) re-
construction of muscles, and (D) re-
construction of external appearance.

256 M.S. BARGO ET AL.

Journal of Morphology DOI 10.1002/jmor



num. Ventral and anterior to the infraorbital fora-
men there is an oval and depressed area that ex-
tends anteriorly and ventrally to the ventral edge of
the maxilla at the level of the nasal opening. It could
be interpreted as the attachment area for the m.
buccinator. The distal edges of the maxillae are not
rugose. The nasal opening shows a constriction pro-
ducing a figure-8 outline in frontal view; the anterior
borders of the nasals are clearly roughened.

The descending process of the zygomatic arch has
a smooth depression on its lateral surface that could
be the origin area of m. nasolabialis profundus pars
maxillaris or m. zygomatico labialis (� zygomati-

cus). Above the orbits, on the fronto-naso-maxillary
suture area, there are two depressed and well-
delimited surfaces, indicating the origin of m. naso-
labialis.

Scelidotherium leptocephalum (Fig. 7). Most
of the Scelidotherium leptocephalum specimens ob-
served lack the septum nasi. The distal ends of the
premaxillae are slightly rugose, not as accentuated
as in Megatherium americanum, indicating the pos-
sible origin areas for the retractor muscle of the
upper lip (m. incisivus superior). The dorsal faces of
the premaxillae present a short ascending process,
indicating the distal end of the septum nasi. The

Fig. 7. Scelidotherium lepto-
cephalum. Skull showing (A) main
attachments areas of muscles and
cartilages, (B) reconstruction of na-
sal cartilages, (C) reconstruction of
muscles, and (D) reconstruction of
external appearance.

257MUZZLE RECONSTRUCTION OF FOSSIL GROUND SLOTHS

Journal of Morphology DOI 10.1002/jmor



distal ends of the nasal bones are roughened, prob-
ably for insertion of the nasal cartilage.

The attachment area for the origin of m. maxillo-
labialis (� caninus) and/or m. dilatator nasi (� le-
vator labii superioris) in the anteromedial edge of
the orbit is not as marked as in Megatherium ameri-
canum. The area on the nasomaxillary suture pos-
sesses two slightly depressed surfaces, anterior to
the orbits, that would indicate the origin of m. na-
solabialis.

Some remarks about the skulls of the five species
of ground sloth that summarize the results pre-
sented above:

1) The origin areas for m. nasolabialis, m. maxillo-
labialis, m. dilatator nasi, and m. incisivus superior
are clearly defined.

2) The rugose surfaces observed in the borders of
the nasal opening could be related to the attachment
of soft tissues forming the outer nasal cartilages.
Alternatively, they could represent the origin of the
m. nasolabialis profundus pars interna.

3) The rugose distal ends of premaxillae in most
ground sloths could indicate the origin of an individ-
ual muscle of the upper lip, the m. buccinator pars
intermaxillaris (� m. incisivus superior), that com-
plements the function of m. dilatator nasi and m.
nasolabialis retracting the lip.

Relative Width and Shape of the Muzzle
Table 3 includes values of relative muzzle width

(RMW � PW/MMW) for different specimens and
mean values for each species. The figures have a
comparative value among xenarthrans and are not
quantitatively comparable with ungulates. In ungu-
lates the palate is generally broader than the muz-
zle; thus, the relative width value is larger than 1. In
ground sloths the palate is always narrower than
the muzzle, and the relative muzzle width is always
less than 1. Lestodon armatus shows the lowest
value and Megatherium americanum the highest.
Within the mylodontines, Glossotherium robustum
has the second lowest value after Lestodon armatus,
and Mylodon darwini has the highest. The scelido-
therine Scelidotherium leptocephalum shows inter-
mediate values between M. darwini and G. robus-
tum.

Following Janis and Ehrhardt (1988), a relatively
narrow muzzle is important for those species that
feed selectively on certain plants or parts of plants.
In contrast, bulk-feeders have relatively wider muz-
zles. Application of this pattern to the ground sloths
suggests that the wide-muzzled Lestodon armatus
and Glossotherium robustum have morphologies
consistent with bulk-feeding, while the muzzles of
Scelidotherium leptocephalum and Mylodon darwini
are more consistent with selective-feeding. The
megatheriine Megatherium americanum, with the
narrowest muzzle, would represent the most
selective-feeder.

Our analyses of muzzle shape following Solounias
et al. (1988) and Solounias and Moelleken (1993a)
complement the information given above on the pos-
sible dietary categories (Fig. 8). Lestodon armatus
has the widest muzzle, followed by Glossotherium
robustum, suggestive of less selective feeding ecolo-
gies. With their narrower muzzles, Scelidotherium
leptocephalum and Mylodon darwini are more likely
to have been selective feeders. With the narrowest
muzzle of all, Megatherium americanum may have
been the most selective feeder.

DISCUSSION
Muzzle Reconstruction

The homology of some of the muscles studied is
not clear. One case is that of the M. buccinator.
Naples (1985) based her nomenclature on the work
of Rinker (1954), who in turn used that of Meinertz
(1935), among others. Saban (1971) considered that
Meinertz’s M. buccinator pars intermaxillaris was a

TABLE 3. Relative muzzle width of ground sloths

Taxa PW MMW RMW

Glossotherium robustum
MLP 3-136 59.5 148 0.40
MLP 3-137 70.5 166 0.42
MLP 3-138 54 160 0.34
MLP 3-140 76 175 0.43
MACN 11769 79 157 0.50
MMP 1489-M 84.5 170 0.49
MMP 1490-M 70.5 175 0.40

X � 0.42
Lestodon armatus
MLP 3-29 64 210 0.30
MLP 3-30 69 250 0.27
MLP 3-3 73 250 0.29
MMP 47-S 62 245 0.25

X � 0.27
Mylodon darwini
MLP 3-764 79 157 0.50
MLP 3-762 85 122 0.69
MACN 15348 44 124 0.35
MNHN-BOL-V 006470 68 89 0.76

X � 0.57
Scelidotherium leptocephalum
MLP 3-671 33.5 65.5 0.51
MMP 9-S 26 55 0.47
MMP 31-S 36.5 81 0.45
MMP 127-S 36 69 0.52
MMP 157-S 33 64 0.52
MMP 458-S 26 54 0.48
MMP 549-S 39 67 0.58
MMP 614-M 32 65.5 0.49
MMP 1155-M 33 79 0.42

X � 0.49
Megatherium americanum
MLP 2-64 60 89 0.67
MACN 5002 54.5 60 0.90
MACN 15154 60 65 0.92
MACN 6 P 52.5 54 0.97
MACN 2832 61 80 0.76

X � 0.84

PW, palatal width. MMW, maximum muzzle width. RMW, rela-
tive muzzle width. X, mean.

258 M.S. BARGO ET AL.

Journal of Morphology DOI 10.1002/jmor



synonym of the M. buccinator pars supralabialis of
Boas and Paulli (1908). On the other hand, Clifford
and Witmer (2004a) consider the m. incisivus superior
as homologous with the M. buccinator pars suprala-
bialis of Boas and Paulli (1908), indicating homology
with Naples’ M. buccinator pars intermaxillaris, a
muscle she could not identify in living sloths, probably
because they lack it as a specialization. Following Clif-
ford and Witmer (2004a,b), in ungulates the m. incisi-
vus superior is a fan-like muscle arising from the ros-
tral part of the premaxillae, inserting on the upper lip
and on the underside of the rhinarium that acts in
elevating and/or everting the upper lip and retracting
the muzzle. The homology of some other muscles de-
scribed for ungulates by Witmer et al. (1999) and Clif-
ford and Witmer (2004a,b), such as the malaris,
depresor labii superioris and nasalis, remains uncer-
tain to us, and they could not be reconstructed follow-
ing Naples’ (1985) descriptions. Another example is
the nasolabialis profundus (� lateralis nasi?), which
following Naples (1985) is reduced or simplified in
Pilosa, including three parts in sloths and five parts in
vermilinguas, but is described by Witmer et al. (1999)
and Clifford and Witmer (2004a,b) as a single muscle.
Other muscles described by Naples (1985) as absent in
living sloths, but present in living vermilinguas (m.
nasolabialis profundus pars interna and pars media
superior) cannot be reconstructed in ground sloths
with confidence. The reconstruction of some muscles
with no bony attachments such as the m. orbicularis
oris is highly speculative. The m. nasolabialis profun-
dus pars anterior arises from cartilages and inserts on
soft tissues.

The contributions on the masticatory apparatus of
two fossil ground sloths of North America Nothroth-
eriops shastense and Paramylodon harlani (see Na-
ples, 1987, 1989, respectively) paid partial attention
to the muzzle reconstruction. That author proposed
the presence of large nasal cartilages and abundant

soft tissue extending beyond the premaxillae, as
suggested by the large nasal openings and the long
predental spout, and the presence of a flexible upper
lip in both ground sloths to aid in food manipulation,
but did not consider the relevant musculature.

The shape and size of the narial aperture in
ground sloths are limited by the cartilago cupularis,
processus cupularis, and processus alaris superior.
These elements usually extend beyond the bony na-
sal cavity and their reconstructions are very specu-
lative. However, our interpretations were conserva-
tive; i.e., they do not surpass the projected outlines
of the distal margins of the nasals, maxillae, and
premaxillae (see Figs. 4B, 5B, 6B, 7B).

A phylogenetically important feature of the xen-
arthran muzzle is the septomaxilla. Whether or not
this bone is homologous with that of monotremes,
and other Mesozoic mammals (see Wible et al., 1990,
Zeller et al., 1993, for further discussion), its pres-
ence in both Tamandua and Choloepus suggests
that it was present in the common ancestor of Tar-
digrada and Vermilingua, and that its absence in
Bradypus, the sister taxon of all the remaining Tar-
digrada, is derived within Xenarthra (Zeller et al.,
1993; Gaudin, 2004). Consequently, it would be ex-
pected to be present in all the taxa studied here.
However, it is unknown in the few well-preserved
fossil Tardigrada and Vermilingua skulls known,
probably due to the small size of this bone and the
apparent lack of sutures with other bones (Zeller et
al., 1993). Moreover, its function (if any) is un-
known, and its reconstruction would be not only
speculative but also irrelevant.

Muzzle and Feeding Behavior in
Pleistocene Ground Sloths

Some authors have inferred the feeding habits for
South American fossil tardigrades, including some

Fig. 8. A: Dietary categories of ungulates as inferred from the shape of the muzzle following Solounias and Moelleken (1993a, fig.
1). B: Shape of the muzzles of ground sloths.

259MUZZLE RECONSTRUCTION OF FOSSIL GROUND SLOTHS

Journal of Morphology DOI 10.1002/jmor



of the species considered here. Based on the mor-
phology of the calcaneum, Scillato-Yané (1977) sug-
gested that grasses were the basis of the diet of both
Megatheriinae and Mylodontidae, but without any
morphofunctional or ecomorphological analysis of
the masticatory apparatus. The only direct evidence
is from late Pleistocene fossil dung preserved with
skeletal and skin remains at Mylodon Cave, South-
ern Chile. From this it has been inferred that the
diet of Mylodon darwini at that specific time, geo-
graphic region, and season the remains were depos-
ited comprised mostly grasses (Gramineae) and
sedges (Cyperaceae) (Moore, 1978). McDonald
(1987) indicated that the Plio-Pleistocene Scelidoth-
eriinae were selective feeders because the long and
narrow muzzle was appropriate for selecting plant
parts. He proposed that Scelidotherium leptoceph-
alum probably searched for underground food with
the help of its forelimbs, although feeding on other
plant material above, but near the ground level, was
also possible. Biomechanical analysis of the fore-
limbs of this species by Bargo et al. (2000) and
Vizcaı́no et al. (2001) indicates clear adaptations to
digging, and the degree of hypsodonty suggests an
abundance of grit in the food (Bargo et al., in press),
which is in accordance with its provenance from
below or near ground level in relatively open envi-
ronments. Bargo (2001a) concluded that Glossoth-
erium robustum and Lestodon armatus were bulk
feeders, Mylodon darwini was a mixed feeder, and
Scelidotherium leptocephalum was a selective feeder
specialized on succulent plant material, e.g., fruits,
buds, and tubers, although it could also browse on
bushes and grasses.

Fariña (1996) proposed a singular hypothesis on
the basis of a study of the trophic relationships
between the South American Lujanian and North
American Rancholabrean (both ages being Late
Pleistocene/Early Holocene) megamammals, on the
basis of body size and its ecological implications.
That author proposed that ground sloths, especially
Megatherium, were opportunistically carrion eaters.
Biomechanical and morphofunctional evidence pro-
vided by Bargo (2001b) indicates that M. america-
num would have been a generalized selective feeder,
capable of consuming tough items, i.e., browsing for
small branches, leaves, and fruits, while meat (car-
rion?) cannot be discounted. Wroe et al. (2004) sug-
gested that there was an apparent lack of large
mammalian omnivores in both Australia and pre-
GABI South America, which might indirectly sup-
port the contention that some xenarthrans were
more opportunistic feeders than previously sup-
posed.

The reconstruction of the muzzle in fossil mam-
mals can be crucial in understanding food intake
styles, and hence insightful in interpreting feeding
habits and defining niches in a paleoecologic context.
However, this subject has rarely been considered in
paleobiological reconstruction. Only a few articles

have devoted attention to the muzzle reconstruction
in fossil mammals, using phylogenetically closely
related living forms as models for comparison (e.g.,
Solounias et al., 1988; Solounias and Moelleken,
1993a,b). This sort of approach may be particularly
important when dealing with extinct animals that
have no clear analogs among living relatives, as is
the case for the ground sloths.

As mentioned above, although data on muzzle
shape and width in ungulates cannot be directly
compared with values obtained for ground sloths,
they provide a framework for tentatively assigning
them to certain dietary categories. Following the
idea proposed by Owen-Smith (1982), and applied to
living and extinct ungulates (Solounias et al., 1988;
Solounias and Moelleken, 1993a), that narrow muz-
zles are important for animals that feed selectively
on certain plants or parts of plants, in each lineage
narrower muzzles could be related to selective feed-
ing, and wider muzzles with bulk feeding. An anal-
ogy occurs even among ungulates: equids are less
selective feeders than grazing ruminants; however,
their relative muzzle width is smaller, suggesting
that direct comparisons must be limited to within
lineages (Janis and Ehrhardt, 1988). The same con-
cept has been applied to sloths (see Vizcaı́no and De
Iuliis, 2003; Vizcaı́no et al., in press, for a discussion
on phylogenetic constraints in xenarthrans). Based
on the shape and proportions of the predental spout
of the mandible and premaxillae, McDonald (1997)
proposed that Early Miocene sloths were browsers,
and that wider muzzles begin to develop in the post-
Miocene mylodontids, reaching its maximum ex-
pression with the Pleistocene ground sloths, in rela-
tion to a change to more grazer forms and/or more
open environments. Following this criterion, and ac-
cording with our analysis, within the mylodontid
ground sloths Glossotherium robustum and Lest-
odon armatus would be bulk-feeders, while Scelido-
therum leptocephalum and Mylodon darwini would
be mixed or selective feeders. The megatheriid
Megatherium americanum would be the most selec-
tive feeder ground sloth.

The shape of the muzzle of ground sloths is also
reflected in the shape of the predental spout (sym-
physis) of the mandible, so even an isolated jaw
could be used to identify dietary categories. The
mandibular symphysis forms a rigid and stout struc-
ture in adult ground sloths (it fuses early during
ontogeny), so it should be considered when analyz-
ing the modes of food procurement (see below).

In the wide-muzzled ground sloths, Glossotherium
robustum and Lestodon armatus, the premaxillae
are weakly articulated to the maxillae, so they are
usually lost, and there is no trace of the presence of
an arch or ossified nasal cartilage. In the narrow-
muzzled Mylodon darwini, Scelidotherium lepto-
cephalum, and Megatherium americanum, the pre-
maxillae are completely fused to the maxillae. Even
more, in M. darwini the premaxillae form a strong

260 M.S. BARGO ET AL.

Journal of Morphology DOI 10.1002/jmor



anterior ascending process that fused with a de-
scending process of the nasals, forming an unusual
complete arch, which was extensively described by
Kraglievich (1934). Scelidotherium leptocephalum
also possesses an ascending process on the premax-
illae, apparently supporting the nasal cartilage. In
one specimen the nasal cartilage was ossified, form-
ing a structure analogous to that of M. darwini
(McDonald, 1987). A similar condition occurs in aged
individuals of M. americanum, in which the distal
ends of the premaxillae develop ascending pro-
cesses, but which do not reach the nasals.

These different morphologies may be related to
differences in the way food was taken into the oral
cavity. In narrow-muzzled forms, like Mylodon dar-
wini and, to a lesser extent Scelidotherium lepto-
cephalum, the muzzle constituted a robust structure
that, combined with the fused symphysis, is consis-
tent with adaptation to resist considerable stress,
suggesting that the processing of favored plant ma-
terials required significant effort. In M. darwini at
least, the degree of fusion of the premaxilla and
maxilla, the stout nasal arch, and the loss of the first
upper tooth suggest the presence of a horny struc-
ture on the premaxilla, analogous to the premaxil-
lary pads in bovids, that would aid in clipping or
tearing off the food. The muzzle of Megatherium
americanum is even narrower, particularly com-
pared with the remaining parts of the skull, and
very robust. The symphysis is also very stout and
the tip is directed downward instead of upward, as
in mylodontids. The long predental space suggests
that the presence of a strong and movable upper lip,
as inferred from the muscle reconstruction, was in-
volved in food intake. The buccinator pars intermax-
illaris (� incisivus superior), a muscle that retracts
the upper lip in different ways, depending on the
position of the fibers applied, appears to be well
developed in the narrow-muzzled ground sloths
(Figs. 4C, 6C, 7C), especially in M. americanum.
This fact could indicate the presence of a thick,
cone-shaped, and prehensile upper lip, useful for
food intake, as in the black rhinoceros. The prehen-
sile lip of this species enables it to selectively take
leaves/twigs, whereas the broad nonprehensile lip of
the white rhinoceros is used to crop short grasses
and herbaceous plants.

Kraglievich (1921) suggested that, because of the
extreme width of their muzzles and shovel-shaped
mandibular symphyses, mylodontines (Glossoth-
eriun and Lestodon) first removed soil with their
forelimbs in order to efficiently access grasses and
roots. Our analysis clearly indicates that the wide
muzzles of G. robustum and L. armatus allowed
them to procure great amounts of food, and demon-
strates that, regarding muzzle morphology, they
were the best-adapted of all ground sloths to a graz-
ing niche. The upper lip, formed by the buccinator
pars intermaxillaris (� incisivus superior), was
probably square-shaped and not prehensile (Fig.

5C), as in the white rhinoceros. This fact, coupled
with the absence of incisors, indicates that G. robus-
tum and L. armatus simply used the upper lip to
grasp grass against the lower lip, pulling up to clip
it.

Finally, the action of the tongue cannot be ne-
glected when considering food intake. Studies in
progress on the hyoid apparatus of several ground
sloths reveal that the tongue of forms like Glossoth-
erium would have been quite movable and protrac-
tile, helping to pull out plants in the manner of cows.
This capacity must have been reduced in Scelidoth-
erium and, especially, in Megatherium. We are un-
familiar with the hyoids of Lestodon and Mylodon,
but judging from the shape of the mandible these are
most similar to Glossotherium and Scelidoterium,
respectively.

CONCLUSIONS

1. The preservation of the nasal septum and scars
for muscular attachment in the rostral part of the
skull allows conservative reconstruction of the muz-
zle anatomy in fossil sloths.

2. The most conspicuous feature is the muscular
attachment of the buccinator pars intermaxillaris (�
incisivus superior). The complex function of this
muscle complements the action of the m. dilatator
nasi and m. nasolabialis in retracting the lip. In
wide-muzzled sloths (Glossotherium and Lestodon)
it forms a squared, nonprehensile upper lip (Fig.
5D), while in the narrow-muzzled (Megatherium,
Mylodon, and Scelidotherim) it forms a cone-shaped
and prehensile lip (Figs. 4D, 6D, 7D).

3. Our analysis of muzzle anatomy complements
previous studies of other oral data, and on these
bases we postulate the following models of feeding
behavior in fossil sloths:

i) Wide-muzzled sloths (Glossotherium and Lest-
odon) were mostly bulk-feeders, and the lips coupled
with the tongue were used to pull out grass and
herbaceous plants.

ii) Narrow-muzzled sloths (Mylodon, Scelidoth-
erium, and Megatherium) were mixed or selective
feeders, with a prehensile lip that was used to select
particular plants or plant parts.

ACKNOWLEDGMENTS

We thank S. Wroe and N. Solounias for comments
on an early version and two anonymous reviewers
for valuable comments and suggestions on the arti-
cle. We thank C. De Muizon (Museúm national
d’Histoire naturelle, Paris, France), A. Kramarz
(Museo Argentino de Ciencias Naturales, Buenos
Aires, Argentina), and O. Scaglia (Museo Municipal
de Ciencias Naturales, Mar del Plata, Argentina) for
access to the collections under their care, and Per
Christiansen (Zoological Museum, University of

261MUZZLE RECONSTRUCTION OF FOSSIL GROUND SLOTHS

Journal of Morphology DOI 10.1002/jmor



Copenhagen, Denmark) for providing pictures of the
Mylodon skull. This article is a contribution to the
projects Universidad Nacional de La Plata N336 and
CONICET-PIP 5240.

LITERATURE CITED

Ameghino F. 1889. Contribución al conocimiento de los mamı́f-
eros fósiles de la República Argentina. Anales Academia Nacio-
nal de Ciencias de Córdoba 6:1–1027.

Bargo MS. 2001a. El aparato masticatorio de los perezosos ter-
restres (Xenarthra, Tardigrada) del Pleistoceno de la Argen-
tina. Morfometrı́a y biomecánica. PhD dissertation. Univer-
sidad Nacional de La Plata, La Plata, Argentina.

Bargo MS. 2001b. The ground sloth Megatherium americanum:
skull shape, bite forces, and diet. In: Vizcaı́no SF, Fariña RA,
Janis C editors. Biomechanics and paleobiology of vertebrates.
Acta Palaeontol Pol 46:173–192.

Bargo MS, Vizcaı́no SF, Archuby FM, Blanco RE. 2000. Limb
bone proportions, strength and digging in some Lujanian (Late
Pleistocene-Early Holocene) mylodontid ground sloths (Mam-
malia, Xenarthra). J Vert Paleontol 20:601–610.

Bargo MS, De Iuliis G, Vizcaı́no SF. 2006. Hypsodonty in Pleis-
tocene ground sloths (Xenarthra, Tardigrada). Acta Paleontol
Pol 51(1).

Boas JEV, Paulli S. 1908. The elephant’s head: studies in the
comparative anatomy of the organs of the head of the Indian
elephant and other mammals. Part I. Copenhagen: Folio,
Gustav Fisher.

Clifford AB, Witmer LM. 2004a. Case studies in novel narial
anatomy. 2. The enigmatic nose of moose (Artiodactyla: Cervi-
dae: Alces alces). J Zool Lond 262:339–360.

Clifford AB, Witmer LM. 2004b. Case studies in novel narial
anatomy. 3. Structure and function of the nasal cavity of saiga
(Artiodactyla: Bovidae: Saiga tatarica). J Zool Lond 264:217–
230.

De Iuliis G. 1996. A systematic review of the Megatheriinae
(Mammalia: Xenarthra: Megatheriidae). PhD dissertation. Uni-
versity of Toronto, Canada.

Edgeworth FH. 1935. The cranial muscles of the vertebrates.
Cambridge, UK: Cambridge University Press.

Esteban G. 1996. Revisión de los Mylodontinae cuaternarios
(Edentata, Tardigrada) de Argentina, Bolivia y Uruguay. Sis-
temática, Filogenia, Paleobiologı́a, Paleozoogeografı́a y Paleo-
ecologı́a. PhD dissertation. Facultad de Ciencias Naturales e
Instituto Miguel Lillo, Universidad Nacional de Tucumán.

Fariña RA. 1996. Trophic relationships among Lujanian mam-
mals. Evol Theory 11:125–134.

Fariña RA, Blanco RE. 1996. Megatherium, the stabber. Proc R
Soc Lond B Biol 263:1725–1729.

Gaudin TJ. 2004. Phylogenetic relationships among sloths (Mam-
malia, Xenarthra, tardigrada): the craniodental evidence. Zool
J Linn Soc Lond 140:255–305.

Hofmann RR. 1973. The ruminant stomach. East African mono-
graphs in biology. Nairobi, Kenya: East African Literature Bu-
reau.

Hofmann RR. 1989. Evolutionary steps of ecophysiological adap-
tation and diversification of ruminants: a comparative view of
their digestive system. Oecologia 78:443–456.

Hofmann RR, Stewart DRM. 1972. Grazer or browser: a classifi-
cation based on the stomach-structure and feeding habits of
East African ruminants. Mammalia 36:226–240.

Janis CM. 1990. Correlation of cranial and dental variables with
dietary preferences: a comparison of macropodoid and ungulate
mammals. Mem Queens Mus 28:349–366.

Janis CM. 1995. Correlations between craniodental morphology
and feeding behavior in ungulates: reciprocal illumination be-
tween living and fossil taxa. In: Thomason J, editor. Functional
morphology in vertebrate palaeontology. Cambridge, UK: Cam-
bridge University Press. p 76–98.

Janis CM, Ehrhardt D. 1988. Correlation of the muzzle width and
relative incisor width with dietary preference in ungulates. Zool
J Linn Soc Lond 92:267–284.

Jarman PJ. 1974. The social organization of antilopes in relation
to their ecology. Behaviour 58:215–267.

Kraglievich L. 1921. Estudio sobre los Mylodontinae. Descripción
del cráneo y mandı́bula del Pseudolestodon myloides Galleni, n.
sbsp. Anal Mus Nac Hist Nat Buenos Aires 31:119–134.

Kraglievich L. 1922. Estudio sobre los Mylodontinae. Análisis
comparado de los valores craneométricos de los milodontinos de
Norte y Sud América. Anal Mus Nac Hist Nat Buenos Aires
31:457–464.

Kraglievich L. 1923. Descripción comparada de los cráneos de
Scelidodon rothi Ameghino y Scelidotherium parodii n. sp. Anal
Mus Nac Hist Nat Buenos Aires 33:57–103.

Kraglievich L. 1928. “Mylodon darwini” Owen es la especie
genotipo de “Mylodon” Owen. Rectificación de la nomenclatura
genérica de los milodontes. Physis 9:169–185.

Kraglievich L. 1934. Contribución al conocimiento de Mylodon
darwini Owen y especies afines. Rev Mus La Plata 34:255–292.

Lydekker R. 1894. Contributions to the knowledge of the fossil
vertebrates of Argentina. II. The extinct edentates of Argen-
tina. Anal Museo de La Plata 3:1–118.

Macalister A. 1869. On the myology of Bradypus tridactylus; with
remarks on the general anatomy of the Edentata. Annu Mag
Nat Hist 4:51–67.

Maynard Smith J, Savage RJ. 1959. The mechanics of mamma-
lian jaws. School Sc Rev 141:289–301.

McDonald HG. 1987. A systematic review of the Plio-Pleistocene
scelidothere ground sloths (Mammalia: Xenarthra, Mylodonti-
dae). Ph.D. dissertation, University of Toronto, Toronto, Can-
ada.

McDonald HG. 1995. Gravigrade Xenarthrans from the Early
Pleistocene Leisey Shell Pit 1A, Hillsborough County, Florida.
B Fl Mus Nat Hist 37:345–373.

McDonald HG. 1997. Xenarthrans: Pilosans. In: Kay RF, Madden
RH, Cifelli RL, Flynn JJ, editors. Vertebrate paleontology in
the Neotropics. The Miocene fauna of La Venta, Colombia.
Washington, DC: Smithsonian Institution Press. p 233–245.

McKenna MC, Bell SK. 1997. Classification of mammals above
the species level. New York: Columbia University Press.

Meinertz T. 1935. Die Hautmuskulatur der Säugetiere mit be-
sonderer Rücksicht auf das superfizielle Facial gebiet, II Das
Kaninchen: 1. Cutaneus maximus, Sphincter colli superficialis,
samt Platysma und seine Derivate beim Kaninchen. Morphol
Jahrb 75:15–61.

Mihlbachler MC, Solounias N. 2004. Anatomy and evolution of
the bizarre “battering ram” of the Brontothere, Embolotherium.
J Morphol 260:313.

Moore DM. 1978. Post-glacial vegetation in the South Patagonian
territory of the giant ground sloth, Mylodon. Bot J Linn Soc
77:177–202.

Naples VL. 1982. Cranial osteology and function in the tree
sloths, Bradypus and Choloepus. Am Mus Novit 2739:1–41.

Naples VL. 1985. The superficial facial musculature in sloths and
vermilinguas (anteaters). In: Montgomery GG, editor. The evo-
lution and ecology of the armadillos, sloths and vermilinguas
(Mammalia, Xenarthra�Edentata). Washington, DC: Smithso-
nian Institution Press. p 173–190.

Naples VL. 1987. Reconstruction of cranial morphology and anal-
ysis of function in Nothrotheriops shastensis. Contrib Sci Los
Angeles Co Mus Nat Hist 389:1–21.

Naples VL. 1989. The feeding mechanism in the Pleistocene
ground sloth, Glossotherium. Contrib Sci Los Angeles Co Mus
Nat Hist 415:1–23.

Nomina Anatomica Veterinaria. 1994. Ithaca, NY: World Associ-
ation of Veterinary Anatomy.

Owen R. 1842. Description of the skeleton of an extinct gigantic
sloth, Mylodon robustus, Owen, with observations on the oste-
ology, natural affinities, and probable habits of the megatheri-
oid quadruped in general. London: R. & J. E. Taylor.

262 M.S. BARGO ET AL.

Journal of Morphology DOI 10.1002/jmor



Owen R. 1856. On the Megatherium (Megatherium americanum
Cuvier and Blumenbach). III. The skull. Philos Trans R Soc
Lond B 146:571–589.

Owen R. 1857. On the Scelidothere (Scelidotherium leptoceph-
alum) Owen. Philos Trans R Soc Lond B 147:101–110.

Owen-Smith N. 1982. Factors influencing the consumption of
plant products by large herbivores. In: Hundey BJ, Walker BH,
editors. The ecology of tropical savannas. Berlin: Springer. p
359–404.

Reindhart J. 1879. Beskrivelse af Hovedskallen af et Kaempe-
dorendyr, Grypotherium darwini, fra La Plata-Landenes plejs-
tocene Dannelser, en Del Kongelige Danske Videnskabernes
Selskabs Schriftor, Raekke, Naturv. og mathem. Afdeling XII:
353–380.

Rinker GC. 1954. The comparative myology of the mammalian
genera Sigmodon, Oryzomys, Neotoma, and Peromyscus (Crice-
tinae), with remarks on their intergeneric relationships. Publ
Mus Zool Univ Michigan 83:1–124.

Saban R. 1971. Peauciers de la tête et du cou. In: Grassé P-P,
editor. Traité de Zoologie. Tome XVI, Fasc. 3 Musculature des
membres, muscul. peaucière, des monotrèmes, Arthrologie. p
480–625.

Scillato-Yané GJ. 1977. Octomylodontinae: nueva subfamilia de
Mylodontidae (Edentata,Tardigrada). Descripción del cráneo y
mandı́bula de Octomylodon robertoscagliai n. sp., procedentes
de la formación Arroyo Chasicó (Edad Chasiquense, Plioceno
temprano) del sur de la Provincia de Buenos Aires (Argentina).
Algunas consideraciones filogenéticas y sistemáticas sobre los
Mylodontoidea. Publ Mus Mun Cienc Nat Mar del Plata 2:123–
140.

Sicher H. 1944. Masticatory apparatus of the sloths. Field Zool
29:161–168.

Solounias N, Dawson-Saunders B. 1988. Dietary adaptations and
palaeoecology of the late Miocene ruminants from Pikermi and
Samos in Greece. Palaeogeog Palaeoclim Palaeoecol 65:149–
172.

Solounias N, Moelleken SM. 1993a. Dietary adaptation of some
extinct ruminants determined by premaxillary shape. J Mam-
mal 74:1059–1071.

Solounias N, Moelleken SM. 1993b. Tooth microwear and pre-
maxillary shape of an archaic antelope. Lethaia 26:261–268.

Solounias N, Teaford M, Walker A. 1988. Interpreting the diet of
extinct ruminants: the case of a non-browsing giraffid. Paleobi-
ology 14:287–300.

Turnbull WD. 1970. Mammalian masticatory apparatus. Field
Geol 18:49–356.

Uekermann A. 1912. Untersuchungen ubre die Gesichtzmuzku-
latur der Xenarthra. Zeit Wis Zool 102:337–424.

Vizcaı́no SF, De Iuliis G. 2003. Evidence for advanced carnivory
in fossil armadillos. Paleobiology 29:123–138.

Vizcaı́no SF, Zárate M, Bargo MS, Dondas A. 2001. Pleistocene
large burrows in the Mar del Plata area (Buenos Aires Province,
Argentina) and their probable builders. In: Vizcaı́no SF, Fariña
RA, Janis C, editors. Biomechanics and paleobiology of verte-
brates. Acta Palaeontol Pol 46:157–169.

Vizcaı́no SF, Bargo MS, Cassini GH. 2005. Dental occlusal sur-
face area in relation to body mass, food habits and other biologic
features in fossil xenarthrans. Ameghiniana 42(4).

Wegner RN. 1950. Unterschiede der Nasenlochgestaltungund des
Os nariale bei den Säugetieren (Choloepus) und den Bauri-
amorphen. Verh Anat Ges 28:104–111.

Wible JR, Miao D, Hopson JA. 1990. The septomaxilla of fossil
and recent synapsids and the problem of the septomaxilla of
monotremes and armadillos. Zool J Linn Soc Lond 98:203–228.

Windle BCA, Parsons FG. 1899. On the myology of Edentata.
Proc Zool Soc Lond 1:314–339.

Witmer LM, Sampson SD, Solounias N. 1999. The proboscis of
tapirs (Mammalia: Perissodactyla): a case study in novel narial
anatomy. J Zool Lond 249:249–267.

Wroe S, Argot C, Crowther M, Dickman C. 2004. On the rarity of
big fierce carnivores and the primacy of isolation and landmass
area. Proc R Soc Lond B Biol 271:1203–1211.

Zeller U, Wible JR, Elsner M. 1993. New ontogenetic evidence on
the septomaxilla of Tamandua and Choloepus (Mammalia, Xe-
narthra), with a reevaluation of the homology of the mamma-
lian septomaxilla. J Mammal Evol 1:31–46.

263MUZZLE RECONSTRUCTION OF FOSSIL GROUND SLOTHS

Journal of Morphology DOI 10.1002/jmor