Evaluating Habitats and Feeding Habits Through Ecomorphological Features in Glyptodonts (Mammalia, Xenarthra)

Abstract

The goal of this study is to evaluate ecomorphological variables in glyptodonts from different ages (Miocene to early Holocene), clades, and sizes, with the purpose of inferring their habitats and feeding habits. The analyses included estimation of body masses and three variables: relative muzzle width (RMW), hypsodonty index (HI), and dental occlusal surface area (OSA). RMW allows the distinction of two main groups: the small-sized early Miocene propalaehoplophorids were selective feeders, while the larger post-Miocene forms were more bulk feeders. The Pleistocene Glyptodon appears as an exception, implying a reversion to a selective feeding behavior. The relation between RMW and HI allows discriminating feeding niche partitioning in glyptodonts. Among the early Miocene propalaehoplophorids, Eucinepeltus would have been a highly selective feeder in relatively closed environments, Propalaehoplophorus a highly selective feeder in moderately open habitats, and Cochlops a less selective feeder in moderately open habitats. Among the large Pliocene and Pleistocene taxa, cf. Neuryurus and Neosclerocalyptus were probably bulk feeders in relatively open environments, while Panochthus and Doedicurus were bulk feeders in open environments. Alternative interpretations can be assessed for Glyptodon: it was a more selective feeder in closer habitats, or had a different feeding behavior, browsing on specific plants at higher levels from the ground, and/or a specialized physiology. The late Miocene and Pliocene forms (Hoplophractus, Eosclerocalyptus, and Urotherium) were probably intermediate between the Miocene and the Pleistocene ones.

Full text

305
EVALUATING HABITATS AND FEEDING HABITS
THROUGH ECOMORPHOLOGICAL FEATURES
IN GLYPTODONTS MAMMALIA, XENARTHRA
1
División Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque s/n, B1900FWA, La Plata, Argentina. CONICET - CIC. vizcaino@fcnym.unlp.edu.ar,
msbargo@fcnym.unlp.edu.ar, gcassini@fcnym.unlp.edu.ar
2
División Paleontología, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Av. Ángel Gallardo 470, C1450DJR Buenos Aires, Argentina. jctano@macn.gov.ar
3
Universidad Nacional de Luján, Departamento de Ciencias Básicas, Ruta Nacional 5 y Av. Constitución, 6700 Luján, Argentina.
AMEGHINIANA, Tomo 48 (3): 305 - 319 ISSN 0002-7014
SERGIO F. VIZCAÍNO
1
, GUILLERMO H. CASSINI
1,3
, JUAN C. FERNICOLA
2,3
 M. SUSANA BARGO
1
AMGHB2-0002-7014/11$00.00+.50
Abstract. e goal of this study is to evaluate ecomorphological variables in glyptodonts from dierent ages (Miocene to early Holocene),
clades, and sizes, with the purpose of inferring their habitats and feeding habits. e analyses included estimation of body masses and three
variables: relative muzzle width (RMW), hypsodonty index (HI), and dental occlusal surface area (OSA). RMW allows the distinction of
two main groups: the small-sized early Miocene propalaehoplophorids were selective feeders, while the larger post-Miocene forms were more
bulk feeders. e Pleistocene Glyptodon appears as an exception, implying a reversion to a selective feeding behavior. e relation between
RMW and HI allows discriminating feeding niche partitioning in glyptodonts. Among the early Miocene propalaehoplophorids, Eucinepel-
tus would have been a highly selective feeder in relatively closed environments, Propalaehoplophorus a highly selective feeder in moderately
open habitats, and Cochlops a less selective feeder in moderately open habitats. Among the large Pliocene and Pleistocene taxa, cf. Neuryurus
and Neosclerocalyptus were probably bulk feeders in relatively open environments, while Panochthus and Doedicurus were bulk feeders in
open environments. Alternative interpretations can be assessed for Glyptodon: it was a more selective feeder in closer habitats, or had a dif-
ferent feeding behavior, browsing on specic plants at higher levels from the ground, and/or a specialized physiology. e late Miocene and
Pliocene forms (Hoplophractus, Eosclerocalyptus, and Urotherium) were probably intermediate between the Miocene and the Pleistocene ones.
Keywords. Mammalia. Cingulata. Masticatory apparatus. Paleoecology.
Resumen: EVALUANDO HÁBITAT Y HÁBITOS ALIMENTARIOS EN GLIPTODONTES (MAMMALIA, XENARTHRA) A
TRAVÉS DE VARIABLES ECOMORFOLÓGICAS. El objetivo de este estudio es evaluar variables ecomorfológicas en gliptodontes de
diferentes edades (Mioceno al Holoceno temprano), clados y tamaños, con el propósito de inferir sus hábitats y hábitos alimentario. El
análisis incluyó una estimación de las masas corporales y tres variables: ancho relativo del hocico (RMW), índice de hipsodoncia (HI) y área
de la supercie oclusal dentaria (OSA). RMW permite distinguir dos grupos principales: los pequeños propalaehoplofóridos del Mioceno
temprano se alimentaban de forma selectiva, mientras que las formas más grandes post-miocenas se alimentaban al bulto. Glyptodon del
Pleistoceno aparece como una excepción, que implica una reversión a una conducta de alimentación selectiva. La relación entre RMW y HI
permite discriminar la partición del nicho alimentario en gliptodontes. Entre los propaleoplofóridos del Mioceno temprano, Eucinepeltus
habría tenido una alimentación altamente selectiva en ambientes relativamente cerrados, Propalaehoplophorus una alimentación muy selectiva
en hábitats moderadamente abiertos y Cochlops una alimentación menos selectiva en hábitats moderadamente abiertos. Entre los taxones
grandes del Plioceno y Pleistoceno, cf. Neuryurus y Neosclerocalyptus probablemente se alimentaban al bulto en ambientes relativamente abi-
ertos, mientras que Panochthus y Doedicurus se alimentaban al bulto en entornos abiertos. Se pueden proponer interpretaciones alternativas
para Glyptodon: tenía una alimentación más selectiva en hábitats más cerrados o una conducta de alimentación diferente, alimentándose de
plantas especícas en los niveles más superiores, y/o una siología especializada. Las formas del Mioceno tardío y del Plioceno (Hoplophractus,
Eosclerocalyptus y Urotherium) probablemente tuvieron hábitos intermedios entre las del Mioceno temprano y las del Pleistoceno.
Palabras clave. Mammalia. Cingulata. Aparato masticatorio. Paleoecología.
 are very peculiar extinct mammals of the New
World that vanished as part of the great end-Pleistocene ex-
tinction of continental mammals. ey comprise some 65
named genera, according to the last all-inclusive revision of
the fossil mammals (McKenna and Bell, 1997), and more
than 150 named species according to Mones (1986).
In recent years several authors have either synonymized
several names (e.g., Perea, 2005; Zurita et al., 2008, 2009),
excluded taxa (e.g., Porpino et al., 2009), named new spe-
cies, or validated previously abandoned names (e.g., Carlini
et al., 2008; Fernicola, 2008; Porpino and Bergqvist, 2002;
Porpino et al., 2010; Zurita and Ferrero, 2009; Zurita et al.,
2008). However, the taxonomic status of numerous genera
and species is still in ux.
e bony carapace of glyptodonts clearly relates them to
the armadillos (including dasypodids —extant armadillos—
and extinct pampatheres; see Hostetter, 1958, 1982; Pat-
terson and Pascual, 1968, 1972; Engelmann, 1985; Gaudin

306
AMEGHINIANA, Tomo 48 (3): 305 - 319
and Wible, 2006). Glyptodonts and armadillos together
constitute the clade Cingulata within the Xenarthra, but
glyptodont evolutionary history has been independent from
that of armadillos at least since the middle Eocene Mustersan
Age (Scillato-Yané, 1986), i.e., about 45–48 million years
ago (Flynn and Swisher, 1995). Although phylogenetic re-
lationships among cingulates have not been fully resolved,
it is clear that glyptodonts represent the most derived forms
among them (e.g., Gaudin and Wible, 2006).
Recent phylogenetic analyses (Fig. 1) by Fernicola (2008)
and Porpino et al. (2010) focused on cranial and postcra-
nial characters of the endoskeleton, while previous analy-
ses on the subject relied mostly on features of the exoskel-
eton (e.g., Ameghino, 1889; Castellanos, 1932; Hostetter,
1958). Since endoskeletal remains are much rarer than the
osteoderms constituting the exoskeleton (carapace, cephal-
ic shield, and caudal sheath), the phylogenies of Fernicola
(2008) and Porpino et al. (2010) used a limited number of
Figure 1. Phylogeny and chronological distribution (thick line) of the
genera of Glyptodontia (based on Fernicola, 2008 and Porpino et al.,
2010) / logenia y distribución cronológica (línea gruesa) de los géneros
de Glyptodontia (basadas en Fernicola, 2008 y Porpino et al., 2010)
taxa, although this sample represented several of the main
groups considered by earlier authors.
Fernicola (2008) and Porpino et al. (2010) strongly sup-
ported the monophyly of Glyptodontia, rejecting several
of the historical groupings (Ameghino, 1889; Castellanos,
1931, 1932; Hostetter, 1958; Paula Couto, 1979) and
reinstated other groups that had been proposed previously,
and subsequently ignored. While these previous phyloge-
netic proposals had implied Propalaehoplophorus Ameghino,
1887a, and allied genera were paraphyletic, Fernicola (2008)
and Porpino et al. (2010) supported the existence of a basal
dichotomy with Propalaehoplophoridae as the sister group
of Glyptodontoinei including the remaining glyptodonts.
Following these authors, Hoplophorinae (sensu Hostetter,
1958), Hoplophorini, and Plohophorini are considered to be
paraphyletic. e late Miocene “Plohophorini” Stromaphorus
Castellanos, 1926, and Pseudoplohophorus Castellanos, 1926,
and the early and late Pleistocene “Hoplophorini” Hoplo-
phractus Cabrera, 1939, and Eosclerocalyptus C. Ameghino,
1919, were placed basally in a successive and intercalated ar-
rangement (Fig. 1). e monophyly of the last two tribes, as
well as the Hoplophorinae, was contradicted by the derived
allocation of the “Hoplophorini” Neosclerocalyptus Paula
Couto, 1957, and Hoplophorus Lund, 1839, which form a
monophyletic group with the Panochthinae. is monophy-
letic group is named Panochthidae (sensu Fernicola, 2008).
Also the “Plohophorini” Plohophorus Ameghino, 1887b,
is in a derived position and forms a monophyletic group
with the Glyptodontinae (sensu Fernicola, 2008) Doedicu-
rus Burmeister, 1874, and Glyptodon Owen, 1839. Finally,
the “Hoplophorinae” Lomaphorini Urotherium Castellanos,
1926, was interpreted by Porpino et al. (2010) as the sister
group of the clade including Plohophorus, Doedicurus, and
Glyptodon. e Neuryurini (sensu Hostetter, 1958), with
the genus Neuryurus Ameghino, 1889, were not included in
the phylogentic analysis by Fernicola (2008) and Porpino et
al. (2010), and were considered only as Glyptodontia incer-
tae sedis (Fernicola, 2008).
e morphology of the glyptodont skull (Fig. 2) in gen-
eral, and the masticatory apparatus in particular, is probably
more bizarre than even the carapace. Fariña and Vizcaíno
(2001) provided a basic description that we summarize here.
Compared with that of dasypodid armadillos, a shorten-
ing of the snout is apparent, along with a reduction in the
premaxillae. Fariña and Vizcaíno (2001) conrmed a previ-
ous hypothesis by Fariña and Parietti (1983) on its unique
geometry among mammals. It suggested that the skull of

307
VIZCAÍNO et al.: ECOMORPHOLOGY OF GLYPTODONTS
of processing, and that these dierences in capacity might
reect competitive exclusion by niche partitioning among
sympatric species. Further observations by Fariña and Viz-
caíno (2001) on the stout architecture of the masticatory
apparatus and the very hypsodont teeth with occlusal ridges
of hard dentine indicated that glyptodonts were probably
grazers. However, in that study glyptodonts were treated as
a whole and no comparisons among them were provided.
One persistent problem when inferring dietary habits in
extinct xenarthrans is that dietary labels tend to be vague
and ambiguous. For example, Bargo and Vizcaíno (2008)
considered many uses of the terms “browser” and “grazer”
to be ambiguous. eir review of the literature available on
living herbivores demonstrates that the terms 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 and/or to eating dicotyledonous material.
Moreover, “grazing” denotes grass eating, but is also used to
mean consuming forbs. Consequently, when applied to ex-
tinct species it is not always clear whether the terms refer
to a mode of food acquisition where food is selected by size
and shape (thus, independent of the plant taxonomy), to the
selection of specic taxa (independent of the size and shape),
or to some unspecied combination of both selection crite-
ria. ese authors indicated that the problem with trying to
classify mammals as either browsers or grazers is that these
terms are based on dierent criteria. While browsing reects
capacity for searching on a variety of vegetation independent
of plant taxonomy, grazing implies taxonomic and structural
constraints. After further considerations, Bargo and Vizcaí-
no (2008) advocated analyzing morphology based on how it
deals with physical properties (hardness, resistance to wear,
size) and using dietary categories based on the main physical
properties of the food, inferred from the morphology of the
feeding apparatus.
e goal of this study is to apply some ecomorphologi-
cal variables from recent studies of other fossil xenarthrans
(particularly the Pleistocene ground sloths; Bargo and Viz-
caíno, 2008) to glyptodonts from dierent ages (Miocene
to Holocene), clades, and sizes, for the inference of feeding
habits. Ultimately, this will lead us to specic paleoecological
interpretations for these enigmatic glyptodonts.
MATERIALS AND METHODS
Acronyms
MACN A, Museo Argentino de Ciencias Naturales “Ber-
nardino Rivadavia”, Colección Nacional Florentino Ameghi-
glyptodonts has undergone a very peculiar process of tele-
scoping, in which the braincase is positioned dorsal to the
posterior half of the tooth row. Another conspicuous feature
is the huge descending process of the anterior portion of the
zygomatic arch, formed by the maxillary and jugal bones.
e glenoid fossa —bordered posteriorly by the paraoccipi-
tal process— is transversely elliptical and rather at, directed
backward, and slightly downward and outward, lying well
above the tooth row.
e jaw also has the distinctive feature of a highly devel-
oped angle. In general, the mandibular symphysis is com-
pletely fused. e mandibular condyle is transversely elon-
gated and dorsoventrally convex. Its articular facet is directed
forward, nearly at a right angle respect of the arrangement
normally observed in mammalian herbivores. Each hemi-
mandible possesses a row of eight cheek-teeth that lies almost
perfectly parallel to the sagittal plane and that is sinusoid in
lateral view. e teeth (Fig. 3) have a curious morphology
lacking the tribosphenic pattern generally present in other
mammals (see Vizcaíno et al., 2004; Vizcaíno, 2009, and
references therein). ey are three-lobed, with the exception
of some groups (e.g., propalaehoplophorids), in which the
rst two teeth are usually reduced and oval shaped. Teeth
are hypselodont (ever-growing and open-rooted), and true
enamel is entirely absent in glyptodont teeth (and in most,
if not all, other xenarthrans). Glyptodonts show a three-lay-
ered tooth construction with an inner, often branching core
of vascularized osteodentine, and an outer layer of ortho-
dentine of which the outermost part is especially hardened
(Ferigolo, 1985; Kaltho, 2011).
Despite that the last thirty years —but especially the last
fteen— have seen an increasing number of studies on the
masticatory apparatus and its relationships with feeding hab-
its in other xenarthrans such as armadillos, pampatheres, and
sloths (for overviews see Vizcaíno et al., 2004; Bargo and
Vizcaíno, 2008; Vizcaíno et al., 2008; Vizcaíno, 2009), yet
glyptodonts remain much less understood. Gillette and Ray
(1981) proposed that they were probably browsers belong-
ing to Turnbull’s (1970) “rodent-gnawing” type. However,
an analysis of the craniomandibular joint (CMJ) by Fariña
and Vizcaíno (2001) indicated that the vertical articular sur-
face of the CMJ implies mostly vertical sliding, while in ro-
dents sliding is mostly horizontal. e CMJ of glyptodonts
also allowed —albeit limited— side-to-side movements of
the jaw. In a preliminary study Vizcaíno (2000) proposed
that the main dietary dierences among cingulates lied in
the degree of coarseness of the vegetation they were capable

308
no, Buenos Aires, Argentina; MACN Pv, Museo Argentino
de Ciencias Naturales “Bernardino Rivadavia”, Colección
Nacional de Paleovertebrados, Buenos Aires, Argentina;
MLP, Museo de La Plata, La Plata, Argentina; MPM-PV,
Museo Regional Provincial “Padre M. J. Molina”, Paleon-
tología Vertebrados, Río Gallegos, Argentina.
Materials
Propalaehoplophoridae cf. Asterostemma depressa Ame-
ghino, 1889. MACN A-7663, partial cranium; locality: Ca-
ñadón Jack Harvey, Santa Cruz Province, Argentina; stratig-
raphy: Santa Cruz Formation (latest Early Miocene).
Cochlops muricatus Ameghino, 1889. MPM-PV 3420,
cranium, partial skeleton, and carapace; locality: Puesto Es-
tancia La Costa (= Corriguen Aike), Santa Cruz Province,
Argentina; stratigraphy: Fossiliferous Level (FL) 5.3 (Tauber,
1997), Santa Cruz Formation (latest Early Miocene). MPM-
PV 3432, partial cranium (including maxilla with dentition,
and left hemimandible), fragment of carapace; locality: An-
teatro, Santa Cruz Province, Argentina; stratigraphy: Santa
Cruz Formation (latest Early Miocene).
Eucinepeltus petesatus Ameghino, 1891. MACN A-4758,
complete cranium and mandible; locality: Monte Observa-
ción, Santa Cruz Province, Argentina; stratigraphy: Santa
Cruz Formation (latest Early Miocene). MACN A-4760:
left hemimandible; locality: Monte Observación, Santa Cruz
Province, Argentina; stratigraphy: Santa Cruz Formation
(latest Early Miocene).
Propaleohoplophorus incisivus Ameghino, 1887a. MACN
A-7655, cranium; locality: Corriguen Aike, Santa Cruz
Province, Argentina; stratigraphy: Santa Cruz Formation
(latest Early Miocene). MACN A-7656: partial cranium;
locality: Corriguen Aike, Santa Cruz Province, Argentina;
stratigraphy: Santa Cruz Formation (latest Early Miocene).
Propalaehoplophorus minus Ameghino, 1891. MACN
A-4757, right hemimandible; locality: Corriguen Aike, San-
ta Cruz Province, Argentina; stratigraphy: Santa Cruz For-
mation (latest Early Miocene).
Propalaehoplophorus australis Ameghino, 1887a. MLP
16-15, cranium, mandible, and skeleton; locality: Santa
Cruz Province, Argentina; stratigraphy: Santa Cruz Forma-
tion (latest Early Miocene); gured in Lydekker (1894, 3,
Pl. 32).
Propalaehoplophorus sp. MACN-A 4754, cranium; lo-
cality: Corriguen Aike, Santa Cruz Province, Argentina;
stratigraphy: Santa Cruz Formation (latest Early Miocene).
AMEGHINIANA, Tomo 48 (3): 305 - 319
Figure 2. Skulls in lateral and frontal views of some specimens of the glyptodonts studied/ cráneos en vista lateral y frontal de algunos especimenes
de los gliptodontes estudiados. 1, MLP 16-15 Propalaehoplophorus australis. 2, MLP 24 Neosclerocalyptus sp. 3, MLP 16-29 Panochthus tuberculatus. 4,
MLP 16-24 Doedicurus clavicaudatus. 5, MLP 16-41 Glyptodon sp. (taken from Lydekker, 1894). Escala/scale bar: 200 mm
1 2 3 4 5

309
MPM-PV 3419, complete mandible and partial carapace;
locality: Puesto Estancia La Costa (= Corriguen Aike), Santa
Cruz Province, Argentina; stratigraphy: Fossiliferous Level
(FL) 6 (Tauber, 1997), Santa Cruz Formation (latest early
Miocene). MPM-PV 3422, partial cranium (anterior part)
with dentition and complete right mandible, humerus, par-
tial scapula, other fragments of skeleton, and isolated os-
teoderms; locality: Puesto Estancia La Costa (= Corriguen
Aike), Santa Cruz Province, Argentina; stratigraphy: Fossil-
iferous Level (FL) 5.3 (Tauber, 1997), Santa Cruz Formation
(latest early Miocene).
Glyptodon reticulatus Owen, 1845. MACN-Pv 10153,
cranium and mandible of a sub-adult specimen; locality:
Río Luján, Buenos Aires Province; stratigraphy: “Pampeano”
Formation (Pleistocene).
Glyptodon munizi Ameghino, 1881. MACN-Pv 8706,
complete cranium and mandible; locality: Cauce del Río de
La Plata, Vicente López, Buenos Aires Province, Argentina;
stratigraphy: Ensenada Formation (early Pleistocene).
Glyptodon sp. MACN Pv-2908, complete cranium and
mandible; locality: unknown; stratigraphy: “Pampeano”
Formation (Pleistocene). MLP 16-41, cranium, mandible,
and skeleton; locality: Olivera, Buenos Aires Province; stra-
tigraphy: “Pampeano” Formation (Pleistocene); gured in
Lydekker (1894, 3, Pl. 5).
Eosclerocalyptus planus (Rovereto, 1914). MACN-Pv
4853, cranium; locality: La Hoyada, Catamarca Province, Ar-
gentina; stratigraphy: “Araucanian” (late Miocene–Pliocene).
Urotherium antiquum Castellanos, 1926. MACN A-229,
partial cranium and partial mandible; locality: Monte Her-
moso clis, Buenos Aires Province, Argentina; stratigraphy:
Monte Hermoso Formation (early Pliocene).
Hoplophractus proximus (Moreno and Mercerat 1891).
MLP 31-X-12-19, cranium; locality: Corral Quemado,
Catamarca Province, Argentina; stratigraphy: “Araucanian”
(late Miocene–Pliocene).
Figure 3. Upper tooth series of glyptodonts / series molares superiores de gliptodontes. 1, Cochlops muricatus (MPM-PV 3432). 2, Neosclerocalyptus
sp. MACN Pv-8579. 3, Glyptodon reticulatus (MACN-Pv 10153).
1 2 3
VIZCAÍNO et al.: ECOMORPHOLOGY OF GLYPTODONTS

310
Neosclerocalyptus paskoensis Zurita 2002. MACN
18107, cranium and mandible; locality: Carhué, Buenos Ai-
res Province, Argentina; stratigraphy: “Pleistocene”.
Neosclerocalyptus sp. MACN Pv-8579, complete cranium;
locality: “Toscas” del Río de la Plata, Olivos, Buenos Aires
Province, Argentina; stratigraphy: Ensenada Formation (ear-
ly Pleistocene). MACN Pv-8091, complete cranium; local-
ity: Mar del Plata, Buenos Aires Province, Argentina; stra-
tigraphy: Ensenadan (early Pleistocene). MACN Pv-15151,
cranium; locality: unknown; stratigraphy: unknown.
Doedicurus sp. MACN Pv-2762, complete cranium and
mandible; locality: Río Salado, Buenos Aires Province, Ar-
gentina; stratigraphy: Lujanian (late Pleistocene–early Ho-
locene). MACN Pv-15153, complete cranium; locality: Ar-
royo Tapalqué, Olavarría, Buenos Aires Province, Argentina;
stratigraphy: “Pampeano” Formation (Pleistocene).
Panochthus tuberculatus Owen, 1839. MLP 16-29, cra-
nium and mandible, skeleton, carapace, and caudal tube;
locality: Tapalqué, Buenos Aires Province, Argentina; stra-
tigraphy: “Pampeano” Formation (Pleistocene); gured in
Lydekker (1894, 3, pls. 22–23).
Panochthus intermedius Lydekker, 1894. MLP 16-37,
cranium, mandible, skeleton and caudal tube; locality: un-
known; stratigraphy: unknown.
Panochthus sp. MLP P-1, cranium and mandible; locality:
unknown; stratigraphy: unknown.
Neuryurini cf. Neuryurus sp. MLP 16-151, cranium and
mandible; locality: Monte Hermoso, Buenos Aires Province,
Argentina; stratigraphy: Monte Hermoso Formation (early
Pliocene); gured in Lydekker (1894, 3, pl. 8, g. 2).
is sample represents a wide size and temporal range of
glyptodonts, at least with respect to the skull and mandible.
METHODS
Following previous research in Pleistocene ground sloths
(Bargo et al., 2006a, b; Vizcaíno et al., 2006; Bargo and
Vizcaíno, 2008), the ecomorphological analyses performed
here include estimation of body mass, relative muzzle-width
index (RMW), hypsodonty index (HI), and dental occlusal
surface area (OSA).
Body masses (BM) were assigned by the following crite-
ria. For each specimen, a published estimate was preferred
when available. When published estimates were not available
for a particular specimen, a body size estimate was obtained
assuming a geometric similarity among dierent sizes, where
overall size was based on the total length of the upper tooth
row. However, if there was a published body mass estimate
for a close relative (either at the species or genus level), geo-
metric similarity was calculated in comparison with the spec-
imen used for the published mass estimations. For specimens
where there is no body mass estimation published even for
close relatives (Urotherium, Hoplopractus and Eosclerocalyp-
tus), we used an average mass value obtained by assuming
geometric similarity among specimens belonging to every
genus with published mass estimations (Tab. 1).
e RMW was calculated following Janis and Ehrhardt
(1988) for ungulates, as palatal width (PW) divided by max-
imum muzzle width (MMW). Following these authors, the
width of the palate is correlated with the rate of food inges-
tion. High values of this ratio indicate selective feeders with
narrow muzzles. In ungulates, MMW was measured at the
premaxillo-maxillary suture and PW is the distance between
the M2 protocones (Janis and Ehrhardt, 1988; Janis, 1990).
Because the premaxillae are reduced in glyptodonts, as in
ground sloths, MMW is generally on the maxilla only, and
this variable was measured following Bargo et al. (2006b) as
the distance between labial alveolar margins of rst molari-
forms. PW was measured between the lingual alveolar mar-
gins of central lobe of the fourth molariform, where the pal-
ate is narrowest in glyptodonts. e values obtained cannot
be compared directly with those for ungulates, because the
obviously dierent phylogenetic constraints in each group
and the lack of clear homology in the landmarks used; how-
ever, they provide a framework for comparison among xen-
arthrans, especially glyptodonts.
e HI was calculated as depth of mandible (DM) di-
vided by length of molariform tooth row (LTR), as used by
Bargo et al. (2006a) for ground sloths. Xenarthran specialists
have agreed that the relative increase in depth of the jaw in
sloths reects increased hypsodonty (Bargo et al., 2006a and
references therein); the same applies for glyptodonts. In this
case, depth of the mandible was measured at the sixth molari-
form, where this variable presents its maximum value, instead
of at the third molariform as used by Bargo et al. (2006a).
e OSA was evaluated following Vizcaíno et al. (2006)
as the total cheek-tooth occlusal surface area, considering the
infolding contour of the tooth. Vizcaíno et al. (2006) ap-
proached this problem considering OSA as the two-dimen-
sional projection of a three-dimensional structure, using dig-
ital photos of the occlusal surfaces of the upper cheek-tooth
rows. Herein we use a Microscribe G2 to digitize the three-
dimensional contour and inner osteodentine crests of each
upper molariform. e data-points obtained were converted
to a mesh-surface to calculate the area with Rhinoceros 4.0
AMEGHINIANA, Tomo 48 (3): 305 - 319

311
VIZCAÍNO et al.: ECOMORPHOLOGY OF GLYPTODONTS
Catalogue number Taxa RMW HI OSA Body mass Source body mass
MACN-A 7663 cf. Asterostemma depressa 0.726 375.9335 78.13 Vizcaíno et al., 2006
MPM-PV 3420 Cochlops muricatus 0.655 340.3753 79.27 GS- MLP 16-15
MPM-PV 3432 Cochlops muricatus 0.655 0.434 303.4343 86.71 GS-MLP 16-15
MACN-A 4758 Eucinepeltus petesatus 0.710 0.362 457.4104 115.00 Croft, 2000
MACN-A 4760 Eucinepeltus petesatus 0.362
MPM-PV 3419 Propalaehoplophorus sp. 0.398
MPM-PV 3422* Propalaehoplophorus sp. 0.778 0.403
MACN-A 7655 Propalaehoplophorus incisivus 0.752 314.1836 74.49 GS-MLP 16-15
MACN-A 7656 Propalaehoplophorus sp. 0.820 332.1412 77.98 GS-MLP 16-15
MACN-A 4757 Propalaehoplophorus minus 0.361
MACN-A 4754 Propalaehoplophorus australis 365.0685 93.48 GS-MLP 16-15
MLP 16-15 Propalaehoplophorus australis 0.745 0.427 330.1710 81.10 De Esteban-Trivigno et al., 2008
MACN-A 10153 Glyptodon reticulatus 0.714 0.415 1590.3930 819.04 GS-MACN 200**
MACN-A 8706 Glyptodon munizi 0.664 0.438 1950.1597 1150.05 GS-MACN 200
MACN-A 2908 Glyptodon sp. 0.631 0.384 1886.9831 968.03 GS-MACN 200
MLP 16-41 Glyptodon sp. 0.656 0.406 1590.5900 831.90 GS-MACN 200
MACN-A 4853 Eosclerocalyptus lilloi 0.540 621.6683 185.73 Mean GS
MACN-A 229 Urotherium antiquus 0.564 833.2613 326.41 Mean GS
MLP 31-X-12-19 Hoplophractus proximus 0.577 769.9640 221.83 Mean GS
MACN-A 18107 Neosclerocalyptus paskoensis 0.491 0.422 1302.1728 574.53 GS-MLP 16-28
MACN-A 8579 Neosclerocalyptus sp. 0.583 859.8465 383.99 GS-MLP 16-28
MACN-A 8091 Neosclerocalyptus sp. 0.485 1092.6327 485.40 GS-MLP 16-28
MACN-A 15151 Neosclerocalyptus sp. 1292.6393 507.44 GS-MLP 16-28
MLP 16-28 Neosclerocalyptus ornatus 1276.1900 598.30 Vizcaíno et al., 2006
MACN-A 2762 Doedicurus sp. 0.522 0.530 1812.2203 1547.20 GS-MLP 16-24***
MACN-A 15153 Doedicurus sp. 0.509 1759.2136 1215.54 GS-MLP 16-24
MLP P-1 Panochthus sp. 0.538 0.428 1101.63 GS-MLP 16-29
MLP 16-29 Panochthus tuberculatus 0.527 0.488 2578.5900 1061.00 Fariña et al., 1998
MLP 16-37 Panochthus intermedius 0.488 0.427 1157.40 GS-MLP 16-29
MLP 16-151 cf. Neuryurus sp. 0.484 0.447 1119.1010 311.00 Vizcaíno et al., 2006
* Some specimens do not have body mass estimate because the upper dentition is incomplete or they are only mandibles / Algunos especímenes
no poseen estimaciones de masa corporal porque la dentición superior está incompleta o son solo mandíbulas.
**MACN-A 200 Glyptodon reticulatus (862.00 kg, Fariña et al., 1998).
*** MLP 16-24 Doedicurus clavicaudatus (1468.00 kg, Fariña et al., 1998).
GS-catalogue number: geometric similarity with a fossil taxon with body mass estimate /similitud geométrica con un taxón fósil que posee esti-
mación de masa corporal.
Mean-GS: mean of body mass, obtained assuming geometric similarity to specimens belonging to every genus with published mass estimates /
media de la masa corporal obtenida asumiendo similitud geométrica con especímenes de cada género con estimación de masa édita.
Table 1. Relative muzzle width index (RMW), hypsodonty index (HI), tooth occlusal surface area (OSA), and body mass estimates and their
sources / índice del ancho relativo del hocico (RMW), índice de hipsodoncia (HI), área de la supercie oclusal dentaria (OSA) y estimaciones
de masa corporal y sus fuentes.

312
AMEGHINIANA, Tomo 48 (3): 305 - 319
software. When OSA values obtained with this method are
contrasted with those obtained on the same specimens by
Vizcaíno et al. (2006), the absolute value (modulus) of the
mean dierence obtained is of only 38 mm
2
(i.e., less than
5%). us, we consider our results to be comparable with
those of Vizcaíno et al. (2006).
e allometric relationship between OSA and BM was
assessed following Vizcaíno et al. (2006), calculating the re-
gression lines by the least-squares method, using log trans-
formed OSA as the dependent variable and log transformed
BM as the independent variable. Deviation of the observed
slope from the predicted slope of isometry and from the pre-
dicted slope given by Kleiber’s (1932) law (0.66 and 0.75
respectively) was tested by two-tailed Student’s t statistic.
Residuals from the regression (size independent OSA) was
plotted against both HI and RMW.
RESULTS
Body mass estimates RMW, HI, and OSA values are
summarized in Table 1. Figure 4 shows RMW and HI for
the specimens studied. Values of RMW close to 1 or 0 indi-
cate more selective-feeding or more bulk-feeding specializa-
tion respectively. Using 0.6 as a threshold value of RMW,
two main groups can be dened. Propalaehoplophorids plus
Glyptodon have values above 0.6, with Propalaehoplophorus
showing the highest values. e remaining genera all fall be-
low 0.6, with cf. Neuryurus and two Neosclerocalyptus speci-
mens showing the lowest values. Higher values of HI indicate
greater hypsodonty. Although there is no clear threshold,
0.44 is a tentative boundary between high and low hypso-
donty. In our sample there were no mandibles of cf. Aster-
ostemma, Urotherium, and Hoplophractus complete enough
to obtain HI. For the remaining genera there was only one
mandible for each, except for Glyptodon. While this fact lim-
its the extent of the predictions to be made, some facts can
be addressed. Propalaehoplophorids, Glyptodon, and Neo-
sclerocalyptus show values lower than 0.44, with Eucinepeltus
showing the lowest and Glyptodon values ranging from 0.38
to 0.44. Among the high values of cf. Neuryurus, Doedicu-
rus, and Panocthus, the highest are those of Doedicurus (0.53).
HI discards by itself a potential size-eect on the degree of
hypsodonty, a detail readily evident by the fact that the large
Glyptodon again clusters with the propalaehoplophorids.
When RMW was plotted against HI (Fig. 5), a general
pattern emerges: glyptodonts with low RMW have high
values of HI and vice versa. Among the glyptodonts with
RMW below 0.6, Doedicurus has the highest HI followed
by one specimen of Panochthus. e remaining specimens of
the latter show moderate HI (between 0.42 and 0.44), over-
lapping with some narrow-muzzled glyptodonts (Glyptodon,
Cochlops, and Propalaehoplophorus). Among the narrow-
muzzled glyptodonts, Eucinepeltus has the lowest HI.
Figure 6 shows the distribution of taxa around the re-
gression line of OSA against body mass and the residuals of
regression analysis plotted against RMW and HI. Statistical
parameters for the new regression equation for log OSA and
body mass are shown in Table 2. e observed slope is not
signicantly dierent from the isometry slope, but it is dif-
ferent from that expected based on Kleiber’s (1932) law of
metabolism. Most taxa fall on or close the regression line.
Panochthus and cf. Neuryurus lie above the regression line
Figure 4. Graphic distribution of relative muzzle width index (RMW)
and hypsodonty index (HI) of the specimens studied /distribución
gráca del índice del ancho relativo del hocico (RMW) y el índice de hip-
sodoncia (HI) de los especímenes estudiados.

313
whereas Doedicurus lies below, indicating higher and lower
values than expected for their body masses respectively. e
residuals show that the genera that depart from expected val-
ues of OSA (i.e., those for which the residual modulus is
greater than 0.06) have the lowest RMW and the highest
HI. e only exception is Cochlops, which has intermediate
values.
DISCUSSION
is ecomorphological approach, previously applied to
other xenarthrans (Bargo and Vizcaíno, 2008), allows eluci-
dating of paleobiological hypotheses and provides a interest-
ing preliminary insight to the evolution of the masticatory
apparatus in glyptodonts.
It seems quite evident that the RMW by itself allows
distinguishing two main groups of glyptodonts based on
their inferred feeding behavior. In general, the small-sized
early Miocene propalaehoplophorids are narrow-muzzled
forms, while the larger post-Miocene forms developed wider
muzzles. e fact that smaller glyptodonts have lower RMW
than larger ones relies on the allometric relationships between
the two variables (MMW and PW) involved in the index
and body mass. On the one hand, decimal log-transformed
PW shows a positive correlation with log transformed body
mass (R
2
=0.89, F(1, 19)= 152.67), , with an isometric trend
(slope=0.2994) not signicantly dierent from theoretical
0.333 for isometry (t(19)=1.3992 p=0.089). On the other
hand, log-transformed MMW shows signicant positive
correlation with log-transformed body mass (R2=0.97, F(1,
19)=649.41), with a positive allometric trend (slope=0.38397)
signicantly higher than the theoretical 0.333 (t(19)=3.3605,
p=0.0016).
Even more, considering only MMW, the positive allo-
metric trend indicates that larger glyptodonts have propor-
VIZCAÍNO et al.: ECOMORPHOLOGY OF GLYPTODONTS
Figure 5. Plot of relative muzzle width index (RMW) against hypsodonty index (HI) / gráca de índice del ancho relativo del hocico (RMW) y el índice
de hipsodoncia (HI)

314
tionately wider muzzles than the smaller ones. Consequently,
propalaehoplophorids were probably more selective-feeders
while the larger post-Miocene forms were bulk-feeders to a
greater degree. A remarkable exception is the large Pleistocene
Glyptodon, whose muzzle is relatively as narrow as in propal-
aehoplophorids. Propalaehoplophorids are considered a basal
clade and Glyptodon a more derived taxon (Fernicola, 2008;
Porpino et al., 2010); this implies a reversion to selective feed-
ing behavior.
e validity of HI as an indicator of feeding habits has
been discussed recently, especially for ungulates, and there
seems to be a consensus that hypsodonty correlates to forag-
ing in open habitats more than indicating the sort of food
items eaten (Mendoza and Palmqvist, 2008, and references
therein). An increase in total masticatory eort has been also
evoked as a cause for increasing tooth wear (and increasing
hypsodonty) in hypsodont mammals (Williams and Kay,
2001; Billet et al., 2009), associated not only with the inges-
tion of abrasive elements, but also with food of low nutri-
tive value that must be ingested in large quantities (Williams
and Kay, 2001; Sanson, 2006; Strömberg, 2006) and tougher
plants or plant parts that require investing more energy in
chewing (Pérez-Barbería and Gordon, 1998). Janis (1988)
and Mendoza and Palmqvist (2008) demonstrated that high-
crowned teeth in ungulates represent an adaptation to resist
tooth wear resulting from the airborne grit and dust accumu-
lated on herbaceous plants in open environments.
Regarding xenarthrans, the use of HI was applied by Mc-
Donald (1995) for North American mylodontine ground
sloths and more recently to South American ground sloths by
Bargo et al. (2006a). e latter extensively discussed the sub-
ject and suggested that dierences in crown height in fossil
sloths may be explained by a combination of variables (rather
than any single one), including dietary preferences (e.g., food
quality), habitat (closed or open, temperate or tropical) and
behavior (feeding at ground level or higher, digging). In the
case of glyptodonts, it is also dicult to interpret the indi-
vidual dierences in HI.
e relation between RMW and HI allows us to infer
feeding niche-partitioning among glyptodonts. is method
improves upon the preliminary hypothesis proposed by Viz-
caíno (2000) for Pleistocene cingulates. Interestingly, these
variables also allowed us to discriminate feeding niches be-
tween Pleistocene ground sloths although in a dierent way:
bulk-feeders (Lestodon Gervais, 1855, and Glossotherium
Owen, 1839) are less hypsodont than the selective-feeders
(Megatherium Cuvier, 1796, and Scelidotherium Owen, 1839)
(Bargo and Vizcaíno, 2008). is is probably related to dier-
ences in feeding physiology (see below).
When RMW and HI are interpreted together in our
analysis (Fig. 5), a general relationship between these traits
becomes apparent. is suggests that selective-feeders and
bulk-feeders have a lesser and a greater degree of hypsodonty
respectively. e small latest early Miocene propalaehoplo-
phorids would have been selective feeders that required a
moderate chewing eort; among them, Eucinepeltus and
Propalaehoplophorus are inferred to have been more selective
in their feeding behavior than Cochlops was. e latter two
genera likely preferred more open environments than the rst
one. Consequently, Eucinepeltus would have been a highly
selective-feeder in relatively closed environments, Propalae-
hoplophorus a highly selective-feeder in moderately open habi-
Table 2. Results of simple linear regression of OSA vs body mass. Coecient of determination (R
2
) and estimators of regression coecients are
in bold. Dierences between the observed slopes and the predicted slope of isometry (2/3) were not signicant in both regressions (p < 0.05).
Dierences between the observed slopes and the predicted slope given by Kleiber´s law (0.75) were signicant except in the regression tagged
by an asterisk (p = 0.05) / Resultados de las regresiones lineales simples de OSA vs. masa corproral. Coeciente de determinación (R
2
) y
estimadores de los coecientes de regresión indicados en negrita. Las diferencias entre las pendientes observadas y la esperada para
isometría (2/3) fueron no signicativas en ambas regresiones (p < 0.05). Las diferencias entre las pendientes observadas y las esperadas a
partir de la ley de Kleiber (0.75) fueron signicativas, excepto en la regresión marcada por un asterisco (p = 0.05).
Taxa Range of n Nº of R2 Std. error Intercept Std. error Slope Std. error Isometry Kleiber’s
W (kg) genera estimate at W=1 kg t(n-2) law t(n-2)
Glyptodontia
Vizcaíno et al., 73.40 - 1061 10 7 0.955 0.0788 1.182 0.14276 0.729 0.05626 1.1116 -0.3697
(2006) ± 0.1113
Glyptodontia
This article 74.49 - 1547 24 12 0.966 0.058 1.291 0.06606 0.6563 0.02611 -0.3966 -3.5887*
± 0.0541
AMEGHINIANA, Tomo 48 (3): 305 - 319

315
tats, and Cochlops a less selective-feeder in moderately open
habitats. Pliocene and Pleistocene forms seem to reverse this
pattern, the smallest forms like cf. Neuryurus and Neosclero-
calyptus having narrower muzzles and being less hypsodont
than the largest forms, suggesting bulk-feeding in relatively
open environments. e largest forms Panochthus and Doedi-
curus are more hypsodont, suggesting bulk-feeding in open
environments, and probably required a high masticatory ef-
fort. e narrow muzzle and low hypsodonty of Glyptodon
suggest more selective-feeding and a better performance in
closed habitats. Interestingly, with the exception of Glypto-
therium Osborn 1903, recently recorded in Venezuela as a
possible migrant from North America (Carlini et al., 2008),
Glyptodon is the only large South American glyptodont rep-
resented in northern South America (see Carlini et al., 2008,
and references therein). erefore, it is possible to hypoth-
esize that the extensive closed environments present during
the Plio-Pleistocene in at least a great part of northern South
America (Clapperton, 1993; Cione et al., 2003) did not act
as a geographical barrier for its dispersal. However, because
Glyptodon is frequently recorded in the same formations as
the others, an alternative interpretation is that it had a dif-
ferent feeding behavior, browsing on specic plants or plant
parts higher o the ground; perhaps it had a more specialized
physiology. We could not calculate HI for the late Miocene
and Pliocene glyptodonts in our sample, Hoplophractus, Eo-
sclerocalyptus and Urotherium. However, assuming the above
mentioned relationship between RMW and HI, these forms
are interpreted as bulk-feeding inhabitants of moderately
open habitats —occupying a position intermediate between
the groups constituted by cf. Neuryurus, Neosclerocalyptus,
Panochthus, and Doedicurus, on the one hand, and the propa-
laehoplophorids and Glyptodon on the other.
Phylogeny is ambiguous (see Fig. 1) in establishing the
“ancestral habitat” of the studied glyptodonts. However, it
seems plausible to assume that it was not frankly open, if the
habitat inferred for the oldest forms —the propalaehoplo-
phorids— is considered. If our assumption about the habitat
of Hoplophractus, Eosclerocalyptus, and Urotherium is correct,
it would lead us to reconstruct the “ancestral habitat” for
Panochthus and Doedicurus as a moderately open one. How-
ever, as they do not share a close phylogenetic relationship, we
must infer “habitat convergence” in the ancestors of Doedi-
curus and Panochthus. e previous assumption also implies
a moderately open “ancestral habitat” for Glyptodon, which
leads us to infer for this genus a “habitat reversion” to more
closed conditions similar to the habitat of the propalaehoplo-
phorids. Alternatively, as mentioned above, it may have had
specic dierent feeding behavior or physiology.
ese conclusions about glyptodont habitats are consis-
tent with the general picture of environmental evolution dur-
ing the Cenozoic in South America. For instance, Pascual and
Ortiz-Jaureguizar (1990) stated that during the latest Ceno-
zoic, many xenarthrans (in addition to ungulates and various
gigantic native rodents) followed a similar dental modica-
tion pattern, from low-crowned to high-crowned teeth. is
instance of dental evolution was likely a response to a shift
from predominantly closed-forested, warm and wet habitats
to open temperate grasslands, followed by a shift toward hot
deserts or cold habitats. More recently, Ortiz-Jaureguizar and
Cladera (2006) summarized paleoenvironmental changes in
southern South America (SSA) during the Cenozoic. ese
changes were inferred mainly from dental adaptations in
mammalian faunas, but also from evidence drawn from oral
VIZCAÍNO et al.: ECOMORPHOLOGY OF GLYPTODONTS
Figure 6. Regression of occlusal surface area (OSA) against body mass,
and the residuals of regression analysis plotted against RMW and HI /
regresión del área de la supercie oclusal (OSA) contra la masa corporal y
residuos del análisis de regresión gracado contra RMW y HI.

316
successions, diastrophism, and sedimentology. ey pointed
out that the record of land-mammals in SSA suggests a large-
scale succession of climates: from the early Paleocene to the late
Pleistocene, the overall trend of environmental change moved
from more equable to less equable conditions. In other words,
climates changed from warm, wet, and non-seasonal, to cold,
dry, and seasonal. Concomitantly, biomes transformed from
tropical forests to steppes, with the following intermediate
stages: subtropical forest, woodland-savanna, park-savanna,
and grassland-savanna. During the Quaternary, glacial cycles
caused cold, dry climates interrupted by warmer and wetter
intervals. Accordingly, several pulses of expansion and retrac-
tion of steppes (and, concomitantly, advances and retreats of
the northern tropical forests) have been recorded. is cyclic
pattern of changes produced the provincialism that character-
ized the South American biota from the early Pleistocene to
the present day.
Another aspect to consider is that despite some ability
to adopt bipedal postures proposed for glyptodonts (Fariña,
1995; Vizcaíno et al., 2011), all glyptodonts would have need-
ed to forage mostly near the ground because of their body
form and fusion of the cervical vertebrate. Hence, they may
have been conned to a very narrow feeding-zone regarding
access to vegetation. is would result in greater competition
among glyptodonts than among other xenarthrans such as
ground sloths. e latter would have had access to a greater
variety of vegetation at dierent levels o the ground. Also,
it may explain glyptodont diversity in a fauna with only one
type of glyptodont per ecological niche.
e relation between OSA and body mass agrees in gen-
eral with the results of Vizcaíno et al. (2006) stating that
glyptodonts, like other xenarthrans, have less OSA avail-
able for triturating food than other placentals of similar size.
However, probably because of the small sample available in
Vizcaíno et al. (2006), slope values for both dasypodids and
glyptodontids had a wide 95% condence interval that in-
cludes the predicted slopes of isometry (0.67) or of Kleiber´s
law (0.75). The results of our more inclusive sample allow us
to reject a slope of 0.75 for glyptodonts. Vizcaíno et al. (2006)
related the smaller OSA to the low basal metabolic rates char-
acteristic of living xenarthrans, which fall between 40 and 60
percent of the rates expected from mass in Kleiber’s (1932)
relation for placental mammals (McNab, 1985). is sug-
gests that xenarthrans have lower energetic requirements than
other placental mammals and, therefore fewer requirements
for a specic type of food; they can survive with lower intake
rates than other placental herbivorous mammals of similar
body mass. Vizcaíno et al. (2006) also found that, within xen-
arthrans, cingulates have higher OSA values than some tardi-
grades, suggesting a greater food processing in the oral cavity.
Particularly, the extremely low OSA of mylodontid ground
sloths was interpreted as indicating low eciency in oral food
processing, probably compensated by high fermentation in
the digestive tract (Vizcaíno et al., 2006). is may explain
the lower HI of wide-muzzled sloths mentioned above.
e major deviations from the correlation between OSA
and body mass are recorded in post-Miocene glyptodonts,
particularly the Pleistocene mega-mammals (i.e., adult body
mass above 1000 kg., see discussion in Vizcaíno et al., in
press). While Glyptodon specimens fall on the regression line,
meaning that they have the OSA expected for glyptodonts of
their size, Panochthus lies well above the regression line and
Doedicurus lies well below, indicating higher and lower values
than expected for their body masses respectively. is sug-
gests that these two forms followed divergent evolutionary
pathways.
Vizcaíno et al. (2006) suggested that a reduction in OSA
can be compensated by an increase in tooth lobation or
the extension of hard osteodentine crests. In general, pam-
patheres seem to have higher OSAs, less complex lobation,
fewer ridges, and less shearing surfaces than glyptodonts. e
relationships among these three variables among glyptodonts
remain to be tested.
CONCLUSIONS
e results provided by this ecomorphological approach
allow us to infer habitat and dietary habits of several taxa
of glyptodonts, improving the preliminary hypothesis pro-
posed by Vizcaíno (2000). Additionally, it is possible to infer
possible niche dierentiation among the taxa that lived in
the same general habitat.
e Relative Muzzle Width Index (RMW) allows distin-
guishing two main groups among glyptodonts based on their
feeding behavior, i.e., the small-sized early Miocene propa-
laehoplophorids were selective-feeders, whereas the larger
post-Miocene forms were more like bulk-feeders. e large
Pleistocene Glyptodon, with a relatively narrow muzzle as in
propalaehoplophorids, appears to be an exception implying
—from a phylogenetic point of view— a reversion to a selec-
tive feeding behavior.
e dierences recorded in the Hypsodonty Index (HI)
between the studied specimens are dicult to interpret when
the index is considered alone. However, together with RMW
it did allow us to infer feeding niche partitioning. Among
AMEGHINIANA, Tomo 48 (3): 305 - 319

317
the early Miocene propalaehoplophorids, Eucinepeltus would
have been a highly selective-feeder of relatively closed en-
vironments, Propalaehoplophorus a highly selective-feeder in
moderately open habitats, and Cochlops a less selective-feeder
in moderately open habitats. Among the larger Pliocene and
Pleistocene taxa, the smallest forms (cf. Neuryurus and Neo-
sclerocalyptus) were probably bulk-feeders in relatively open
environments, while the largest (Panochthus and Doedicurus)
were clearly bulk-feeders in open environments.
Two alternative interpretations can be oered in the
case of Glyptodon, i.e., it was a more selective-feeder in more
closed habitats, or it had a dierent feeding behavior, brows-
ing on specic plants or plant parts higher above the ground,
or perhaps it had unknown physiological specializations. e
late Miocene and Pliocene forms (Hoplophractus, Eoscleroca-
lyptus, and Urotherium) were probably intermediate between
the Miocene and the Pleistocene ones.
Finally, the relation between tooth Occlusal Surface Area
(OSA) and body mass agrees with the results of Vizcaíno et
al. (2006) stating that glyptodonts, like other xenarthrans,
have lower energetic requirements than other placental
mammals and, therefore, can survive with lower intake rates
than other placental herbivores of similar body mass.
ACKNOWLEDGMENTS
e authors acknowledge the following persons for access to collections: M.
Reguero (Museo de La Plata), A. Kramarz (Museo Argentino de Ciencias
Naturales “Bernardino Rivadavia”, Buenos Aires), and A. Tauber (Museo
Regional Provincial P.M.J. Molina, Río Gallegos). To J. M. Perry for a criti-
cal review of the manuscript. To the reviewers, Greg McDonald and Rich-
ard Fariña, for their interesting suggestions that enriched the discussion.
is is a contribution to projects PIP-CONICET 1054, PICT 0143, and
UNLP N 647.
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doi: 10.5710/AMGH.v48i2(364)
Recibido: 23 de febrero de 2010
Aceptado: 26 de noviembre de 2010