Reconstructed masticatory biomechanics of Peligrotherium tropicalis, a non-therian mammal from the Paleocene of Argentina

Abstract

The large, bunodont, mammal Peligrotherium tropicalis is an enigmatic member of the earliest Paleocene fauna of Punta
Peligro, Argentina. While being a contemporary of many of the earliest large-bodied “archaic ungulates” in the Northern
Hemisphere, P. tropicalis is a remnant of an endemic Mesozoic non-therian lineage. The interpretation of P. tropicalis as
an omnivore/herbivore has therefore been difficult to evaluate, given its phylogenetic placement outside of the therian
clade, and lack of many of the molar characteristics thought to be essential for the forms of mastication seen in mar�supials and placentals. Here we present a three-dimensional generalization of the classical “bifulcral” biomechanical
model of bite force and joint force estimation, which is capable of accommodating the wide range of mediolateral force
orientations generated by the muscles of mastication, as estimated by the geometry of their rigid attachment surfaces.
Using this analysis, we demonstrate that P. tropicalis is more herbivorously adapted (viz. shows a greater Group 2 relative
to Group 1 jaw adductor advantage for producing postcanine orthal bite forces) than even the hypocarnivorous carniv�orans Procyon lotor and Ursus arctos, and is similar to the ungulates Sus scrofa and Diceros bicornis. This similarity
also extends to the mediolateral distribution of relative muscle group advantage, with Group 1 muscles (responsible
for effecting the initial adduction of the working-side hemimandible into centric occlusion) having greater orthal bite
forces labially; and Group 2 muscles (those responsible for producing occlusal grinding motions) being more powerful
lingually. Finally, we show that P. tropicalis preserves relatively little of its orthal bite force magnitude at high gape,
suggesting that large-object durophagy would not have been a likely feeding strategy.

Full text

Acta Palaeontol. Pol. 67 (1): 177–201, 2022 https://doi.org/10.4202/app.00912.2021
Reconstructed masticatory biomechanics of
Peligrotherium tropicalis, a non-therian mammal
from the Paleocene of Argentina
TONY HARPER, CALEB F. ADKINS, and GUILLERMO W. ROUGIER
Harper, T., Adkins, C.F., and Rougier, G.W. 2022. Reconstructed masticatory biomechanics of Peligrotherium tropicalis,
a non-therian mammal from the Paleocene of Argentina. Acta Palaeontologica Polonica 67 (1): 177–201.
The large, bunodont, mammal Peligrotherium tropicalis is an enigmatic member of the earliest Paleocene fauna of Punta
Peligro, Argentina. While being a contemporary of many of the earliest large-bodied “archaic ungulates” in the Northern
Hemisphere, P. tropicalis is a remnant of an endemic Mesozoic non-therian lineage. The interpretation of P. tropicalis as
an omnivore/herbivore has therefore been difficult to evaluate, given its phylogenetic placement outside of the therian
clade, and lack of many of the molar characteristics thought to be essential for the forms of mastication seen in mar-
supials and placentals. Here we present a three-dimensional generalization of the classical “bifulcral” biomechanical
model of bite force and joint force estimation, which is capable of accommodating the wide range of mediolateral force
orientations generated by the muscles of mastication, as estimated by the geometry of their rigid attachment surfaces.
Using this analysis, we demonstrate that P. tropicalis is more herbivorously adapted (viz. shows a greater Group 2 relative
to Group 1 jaw adductor advantage for producing postcanine orthal bite forces) than even the hypocarnivorous carniv-
orans Procyon lotor and Ursus arctos, and is similar to the ungulates Sus scrofa and Diceros bicornis. This similarity
also extends to the mediolateral distribution of relative muscle group advantage, with Group 1 muscles (responsible
for effecting the initial adduction of the working-side hemimandible into centric occlusion) having greater orthal bite
forces labially; and Group 2 muscles (those responsible for producing occlusal grinding motions) being more powerful
lingually. Finally, we show that P. tropicalis preserves relatively little of its orthal bite force magnitude at high gape,
suggesting that large-object durophagy would not have been a likely feeding strategy.
Ke y w o r ds : Mammalia, Meridiolestida, Peligrotherium, bifulcral, mastication, vertical kinematic phase.
Tony Harper [anthony.harper@lmunet.edu] and Caleb F. Adkins [caleb.adkins@lmunet.edu], Lincoln Memorial Uni-
versity, DeBusk College of Osteopathic Medicine, 9737 Cogdill Rd., Knoxville, TN 37932, USA.
Guillermo W. Rougier [guillermo.rougier@louisville.edu] (corresponding author), University of Louisville, Depart-
ment of Anatomical Sciences and Neurobiology, 511 S Floyd St, Louisville, KY 40202, USA.
Received 29 May 2021, accepted 24 January 2022, available online 30 March 2022.
Copyright © 2022 T. Harper et al. This is an open-access article distributed under the terms of the Creative Commons
Attribution License (for details please see http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original author and source are credited.
Introduction
The rigid structure of the mammalian jaw has been sculpted
over evolutionary time by the selective obligation to fa-
cilitate biting, ingestion, mastication, and other depascent
behaviors. At the time of their initial divergence from the
sauropsid amniotes, ancestral forms of inertial feeding were
likely the sole mode of mechanical processing available to
the early synapsids for the preparation of ingested material.
From this plesiomorphic condition, selection pressures for
increasingly dynamic and efficient use of low-gape behav-
iors, and the distal elements of the marginal dentition, pro-
moted the wide variety of postcanine crown morphologies
and mandibular geometries that we see in mammals and
their closest relatives (Bramble and Wake 1985).
The fossil record of later synapsids provides many ex-
amples of skeletal feeding adaptations, such as the sim-
plification and strengthening of the facial region and each
hemimandible, lateral flaring of the zygomatic arches, re-
positioning of the mandibular condyle relative to the lower
postcanine tooth-row, and the presence of mediolaterally
flaring pterygoid and masseteric flanges on the dentary
bone (Lillegraven et al. 1979; Kielan-Jaworowska et al.
2005). These transformations seem to suggest that rotation
and mediolaterally extensive translation of the mandible
(relative to the skull) became increasingly important within
this lineage (Grossnickle 2017, 2020). Prior investigations
(Bramble and Wake 1985) have also suggested that the bran-
chiomeric trigeminal motor pathways driving the feeding
apparatus have also been evolutionarily conservative, and

178 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022
liable to be exapted towards complex and possibly conver-
gent sequences of muscular activation during mastication
(Crompton 1995; Gould and Vrba 1982).
This paper presents evidence for an unsuspectedly de-
rived example of transversely elaborated mastication in a
non-therian crown mammal, the early Paleocene Peligro the-
rium tropicalis Bonaparte, Van Valen, and Kramarz, 1993,
from southern Argentina (see Figs. 1 and 2). Because of the
geometrical nature of this evidence, a novel three-dimen-
sional biomechanical method for comparing estimates of rel-
ative bite and joint forces is also presented here, allowing for
size-standardized comparisons of masticatory performance
between P. tropicalis and a representative sample of extant
therian species. Given the phylogenetic location of P. tropica-
lis outside of the clade of extant tooth-bearing mammals (the
therian mammals), these comparisons with modern therians
should only signify functional similarities and differences be-
tween the various autecological modes of mammalian feed-
ing, and not the recency of common ancestry among the taxa
analyzed. The method described here is a three-dimensional
generalization of the classic bifulcral analysis originally
presented by Bramble (1978), which models each hemiman-
dible’s transmission of attached muscle tension (treated as
a concentrated input force) into compressive bite and joint
forces using two simultaneous lever equations. While the
Archimedean lever equation is used to solve a fundamentally
planar problem, modeling hemimandibular rotation and all
relevant forces within a single or several mutually parallel
planes, our method is the first to not require that each indi-
vidual lever equation involved with the mechanical parame-
terization of the jaw be coplanar (or even mutually parallel) to
each other or with any particular standard anatomical plane.
Peligrotherium tropicalis (Figs. 1 and 2) is currently
the sole genus recognized within its monotypic family
Peligrotheriidae, and is the largest (large- dog sized) and most
recent known representative of the larger South American
endemic lineage of cladotherian mammals termed the
Mesungulatomorpha (Rougier et al. 2011), along with the
shrew-sized Reigi theri idae (Bonaparte 1990; Harper et al.
2019; Rougier et al. 2021) and the cat-sized Mesungulatidae
(Bonaparte 1986; Rougier et al. 2009). All mesungulato-
morphs are characterized by a reduction in postcanine den-
tal formula, extreme development of bunodonty, and well
developed lower molar cingulids. Along with the likely re-
lated Reigitherium, Peligrotherium also shows the develop-
ment of labially extensive exodaenodont lobes on all three
lower molar crowns, which are embellished with several
neomorphic accessory cuspulids (although see Martinelli et
al. 2021 for an alternative interpretation). Most importantly
these two taxa have lost all capacity for embrasure shearing
(Paéz-Arango 2008), the ancestral mode of oral mechani-
cal digestion in the majority of (presumably insectivorous)
Jurassic and Cretaceous cladotherian mammals. However,
this shearing capacity was likely present in the more plesio-
morphic members of the Meridiolestida (Rougier et al. 2011,
2012), the larger clade encompassing P. tropicalis and its
closest relatives.
Based on the well-preserved material described in Paéz-
Arango (2008), the postcanine dental formula in P. tropi-
calis is inferred to include three upper and lower premo-
lars and three upper and lower molars, thus matching the
postcanine dental formula seen in many modern eutherian
groups. As also mentioned by Harper et al. (2019), several
“high-level” topographic features of the lower second molar
of P. tropicalis are also broadly comparable to those seen
in stratigraphically younger therian mammals (in this case
the herbivorously adapted polydolopomorph marsupials). In
particular, Harper et al. (2019) found that P. tropicalis shows
closer similarity to these Paleogene South American marsu-
pials in the dental topographic metrics Relief Index (RFI;
Boyer 2008), and Dirichlet Normal Energy (DNE; Bunn
et al. 2011), than do even the most herbivorously-adapted
eutherians known before the latest Cretaceous. While these
broad similarities do provide some prima facie justification
for extrapolating the mapping of form-function relation-
ships found in the vast literature on mammalian feeding
(based on modern therians), the large amount of apical wear
seen on the cusp apices and elevated cingula/cingulids in
P. tropicalis may also indicate its limitation to a more plesi-
omorphic (strictly orthal) mode of mastication.
Institutional abbreviations.— IMNH, Idaho Museum of
Natu ral History, Idaho State University, Pocatello, USA;
ISM, Illinois State Museum, Springfield, USA.
Other abbreviations.—For list of anatomical and biome-
chanical abbreviations used in text see Table 1.
Geological setting
Peligrotherium tropicalis comes from the wester n exposures
of the “banco negro inferior” within the Hansen Member of
the greater Salamanca Formation. This sedimentary facies
represents a brief episode of brackish, shallow water sedi-
mentation in the wider history of the Salamancan Sea, the
first marine transgression recorded in the San Jorge basin
of southern Patagonia, Argentina (Andreis et al. 1977). The
paleofauna of Punta Peligro, an irregular peninsula which
has yielded specimens of P. tropicalis and a wide variety of
other vertebrate taxa, is based on several localities located
approximately 27 km northeast of Comodora Rivadavia in
southern Chubut Province, Argentina. This region has been
commemorated as the stratotype for the Peligran South
American Land Mammal Age (SALMA) by Bonaparte et
al. (1993), and considered to represent later early Paleocene
time (postdating the Tiupampan and preceding Carodnia
Zone beds). Later authors (Marshall et al. 1997) have pro-
posed an earliest Paleocene age for the Peligran SALMA,
somewhat older than the Tiupampan, but for the following

HARPER ET AL.—MASTICATION IN PALEOCENE MERIDIOLESTIDAN MAMMAL FROM ARGENTINA 179
discussion the original age attribution for P. tropicalis and
its associated paleofauna is assumed here.
Being a large-bodied, early Cenozoic, bunodont mam-
mal, the original taxonomic attribution of P. tropicalis by
Bonaparte et al. (1993) to an aberrant condylarth lineage
with close affinities to periptychids is very understandable.
Only with the detailed morphological analyses provided
by Gelfo and Pascual (2001), Paéz-Arango (2008), and
Rougier et al. (2009), has it become clear that P. tropi-
calis (along with the Miocene Necrolestes patagonensis;
Rougier et al. 2012; Wible and Rougier 2017) represents one
of the latest known surviving lineages of the non-therian
South American group Meridiolestida. While the precise
derivation of Meridiolestida from the cosmopolitan, mainly
Jurassic and Cretaceous, clade Dryolestoidea has recently
been called into question (Averianov et al. 2013), the meridi-
olestidans can be placed uncontroversially within the crown
mammalian clade Trechnotheria, and within Trechnotheria
are most likely members of Cladotheria, a group that prior
authors (e.g., Crompton et al. 1994; Schultz and Martin 2014;
Grossnickle 2017, 2020) have suggested to be characterized
by the apomorphic capacity for increased mediolateral man-
dibular excursion and rotation relative to more rootward
stem-therian plesions.
Table 1. Listing of anatomical and biomechanical abbreviations used in text
Abbreviation Name Description
au arbitrary unit arbitrary unit of magnitude, corresponding to 100 units of total muscle
category input force
BF bite force magnitude magnitude in arbitrary units
BP-W/B bite plane; working-side/balancing-side
BJ balancing-side joint BS hemimandibular center of motion (condylare)
BS balancing-side of skull and mandible
B-WR ratio of balancing-side to working-side muscle activation here set to 0.33 for all muscle categories
BV bite vertex a point on working-side lower postcanine toothrow and fulcrum for
motion in each BP-W/B
CG closed gape approximately at centric relation
CP condylar plane
DM-W/B deep masseter MC; working-side/
balancing-side
DMR direction of maxillary resistance the occlusal up direction at closed gape
G1 Group 1 MCs muscles effecting Phase I motion
G2 Group 2 MCs muscles effecting Phase II motion
ILBP-W/B in-lever in bite plane; working-side/balancing-side
ILCP-W/B in-lever in condylar plane; working-side/balancing-side
LOA line of action direction of pull, intersecting attachment centroids for a particular
muscle category
MBG maximal bony gape as defined in Fricano and Perry 2019
MC muscle category one of eight jaw adductor muscle groupings, defined in text
MDL mesiodistal location ratio indicating each point’s mesiodistal location in postcanine too-
throw
MP-W/B medial pterygoid MC; working-side/balancing-side
OG open gape approximately maximal bony gape, but see text description
OLBP-W/B out-lever bite plane; working-side/balancing-side
OLCP-W/B out-lever condylar plane; working-side/balancing-side
P1 Phase I Phase I of the masticatory power stroke
P2 Phase II Phase II of the masticatory power stroke
PCSA physiological cross section area
PD distal point point defining the distal end of the working-side postcanine toothrow
PM mesial point point defining the mesial end of the working-side postcanine toothrow
PT-W/B posterior temporalis MC;
working-side/balancing-side
SCF static correction factor multiplier for output forces to ensure force equilibrium
at each bite vertex
SF stretch factor
SM-W/B superficial masseter MC; working-side/balancing-side
TR working-side lower postcanine toothrow lower crown surfaces between PM and PD
WJ working-side joint working-side hemimandibular center of motion (condylare)
WS working-side working-side of skull and mandible

180 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022
Historical background
The fact that the non-tribosphenic mammal Peligro therium
tropicalis has so closely approximated the size, robustness,
and gestalt dental morphology of paracontemporaneous
therian taxa (see Fig. 1) naturally presents the hypothesis
that some or all of the behavioral and mechanical char-
acteristics of the elaborate mastication seen (or inferred)
in therian omni vores and herbivores may also have been
present in Peligrotherium as well (due either to convergence
or synapomorphy). Prelimi nary reports on phylogenetically
wide-scale correlations between mandibular morphology
and electromyographic activity in therians (Vinyard et al.
2011; also see Hylander and Johnson 1994) have suggested
that increased surface area of the mandibular symphysis
in particular is a fossilizable character correlated with de-
creased working-side (WS) relative to balancing-side (BS)
activity ratios of the deep masseter muscle (Vinyard et al.
2011), and especially with increasing time delays of the peak
activity between the WS and BS deep masseter (Vinyard
et al. 2011; Williams et al. 2008; Crompton et al. 2010).
The wide, rugose (but unfused) mandibular symphysis in
P. tropicalis therefore provides at least prima facie evi-
dence for a type of unilateral, asymmetric, and temporally
prolonged rhythmic chewing pattern characteristic of mod-
ern omnivores and herbivores in the “transverse chewing”
group of modern therians (Vinyard et al. 2011). The goal of
this report is to describe additional independent evidence
for transverse chewing capabilities in P. tropicalis based on
the geometrical characteristics and general robusticity of the
craniomandibular morphology apparent in the fragmentary
material known for this taxon (Fig. 2). We also discuss the
likelihood of these adaptations being the elaboration of be-
havioral symplesiomorphies retained from the cladotherian
common ancestor, convergence in mandibular function pro-
moted by similar ecological habit, or some combination of
both. Both the modern therian and “pre-mammalian” syn-
apsid feeding apparatus have been the focus of many biome-
chanical studies modeling the relative motion of the skull
and mandible as an inertially dampened third-class lever
system (i.e., where parameters such as mass and velocity are
considered negligible and input muscular forces are applied
between their fulcrum and resistance; Greaves 2012). While
mechanically simplistic, this model has concisely explained
many seemingly paradoxical evolutionary trends and ar-
rangements of craniomandibular geometry in fossil taxa,
such as the concurrent enlargement of the jaw adductor
musculature and diminution of the jaw joint(s) and postden-
tary elements in advanced cynodonts, and the unexpectedly
weak bite forces generated by saber- toothed cats (Crompton
and Hylander 1986; McHenry et al. 2007). These studies
have also had considerable success in discovering dietarily
relevant patterns of force distribution within the tooth-rows
of their respective focal taxa, but have had a limited capacity
to comment on longer-term trends in the evolution of the
mammalian jaw. This is because of the lack of such studies
on non-therian crown mammals (such as monotremes and
many Mesozoic lineages; although see Wall and Krause
1992 and Grossnickle 2017), and the restricted estimation
of lever-arm-lengths and vectors in a reduced subset of the
three orthogonal anatomical planes through the cranium.
However, experimental kinematic and myographic results
from a wide range of extant therians suggest that jaw mo-
tions (and their causal forces) directed obliquely in the trans-
verse and coronal planes may have been emphasized in even
very plesiomorphic stem therian lineages (Crompton 1995;
Grossnickle 2017, 2020; Jäger et al. 2020).
From these prior leverage analyses several phylogeneti-
cally widespread geometric adaptions (having the effect of
increasing the mechanical advantage of the jaw apparatus)
have become apparent (Greaves 1995, 2012). These include
(i) increasing the relative distance between the muscular ef-
fort resultant(s) and the mandibular fulcrum (condyle); and
(ii) decreasing the distance of food resistance points (viz.
the tooth crown surface) from this same fulcrum. However,
the wide variety of oblique directions with which the mus-
cles of mastication can impart motion to an adducting man-
dible suggest that there is an equally wide evolutionary
scope for the modulation of lever-arms, bite forces, and joint
forces, through the mediolateral reorientation of muscle ef-
Fig. 1. Reconstruction of cranial eidonomy and osteology of the meridio-
lesti dan mammal Peligrotherium tropicalis Bonaparte, Van Valen, and
Kramarz, 1993, Punta Peligro, Argentina, Early Paleocene. A. Illustrated
life reconstruction (courtesy of Amy Bishop). B. Digitized skull and man-
dible reconstructions produced by Paéz-Arango (2008).

HARPER ET AL.—MASTICATION IN PALEOCENE MERIDIOLESTIDAN MAMMAL FROM ARGENTINA 181
fort vectors. The ability to measure lever-equation param-
eters within a natural three-dimensional context therefore
seems especially important for understanding the evolution-
ary trends within many Mesozoic crown mammalian lin-
eages. For example, many Mesozoic groups independently
reduce the mechanical linkages between the dentary bone
and middle ear elements, thereby gaining greater capaci-
ties for novel relative motions of the mandible (e.g. roll and
yaw), and for asymmetrical muscular recruitment during
rhythmic chewing (Crompton and Hiiemae 1970; Hiiemae
and Crompton 1985; Crompton 2011; Grossnickle 2017,
2020; Jäger et al. 2020). Because the Paleocene P. tropi-
calis is a large but fairly plesiomorphic representative of
one of these non-therian crown mammalian lineages (with
detached postdentary elements and a complex postcanine
dentition) the biomechanical approach used in this report
is to analyze reconstructed jaw adductor leverage in its full
three-dimensional context, accommodating for differential
recruitment of working- and balancing-side musculature,
and hypothetical Phase I (P1) vs. Phase II (P2) activity pat-
terns of the masticatory power stroke where appropriate
(Ross and Iriarte-Diaz 2019).
Although side-view (norma lateralis) photographs of as-
sociated skulls and mandibles have historically been the
fundamental data for static mechanical studies of jaw func-
tion (see Fig. 2A), triangulated three-dimensional surfaces
provide a more informative and geometrically unbiased ba-
sis with which to study the masticatory apparatus (Davis
et al. 2010; Santana et al. 2010; Lautenschlager 2013). The
widening availability of research quality three-dimensional
surface files and image stacks accessioned in online repos-
itories such as Morphosource (www.morphosource.org) has
also made this type of information increasingly available for
a wide variety of purposes.
Material and methods
Comparative sample.—For this report, digital image stacks
and surface meshes for a sample of twelve comparative the-
rian specimens (see Figs. 3 and 4 for examples) were down-
loaded from previously published projects accessioned on
Morphosource. The citations for these original publications,
and for previously published anatomical depictions of the
muscles of mastication in these and related taxa, are listed in
the following paragraphs.
Surface data for Didelphis marsupialis (Fig. 3) was pro-
vided by Martín-Serra and Benson (2020), and the approxi-
mate muscle maps for Didelphis are inferred from Hiiemae
and Jenkins (1969), and more explicitly Turnbull (1970).
Similarly surface data for Erinaceus europaeus was pro-
vided by IMNH, with muscle attachments inferred based
on the related Echinosorex gymnura ( Tu r nb u l l 19 70).
Unpublished image data for the skull of a juvenile Tupaia
sp. was also provided for unrestricted download by the
Evolutionary Anthropology Department of Duke University,
and corresponding myological photographs and illustrations
were used from a recent dissertation by Krisjohnson (2019).
The suoid omnivores Sus scrofa and Tayassu pecari
were provided by Alexander Prucha at George Washington
University, and IMNH, respectively. Myological informa-
tion for suids was taken from Herring and Scapino (1973)
and Kneepkens and MacDonald (2010); and the muscle
maps figured for Pecari tajacu in Woodburne (1968) were
used as a guide for Tayassu pecari. Image stacks of two
large herbivorous perissodactyls, the quagga (Equus quagga
quagga) and black rhinoceros (Diceros bicornis, Fig. 4C, D),
were acquired from Zhou et al. (2019); with muscle dia-
Fig. 2. Reconstructed skull and hemimandibles of meridiolestidan mammal
Peligrotherium tropicalis Bonaparte, Van Valen, and Kramarz, 1993(digi-
tized from models produced by Paéz-Arango 2008). A. Skull and mandibles
articulated into closed gape position and showing the attachments of several
major muscle groups. B. Right-inferior oblique view of lower left hemi-
mandible and skull in open gape position. Here the left side of the skull is
assumed to be the working-side (WS), and the right side the balancing-side
(BS); see Tables 1 and 2. Colored tubes are the lines-of-action of their re-
spective muscle category; matching colored surfaces represent correspond-
ing estimated origin and insertion areas; and black spheres show the loca-
tions of corresponding origin or insertion centroids. Abbreviations: DM-W,
deep masseter working-side; MP-W/B, medial pterygoid working-side/
balancing-side; PT-W, posterior temporalis muscle category working-side;
SM-W, superficial masseter working-side.

182 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022
grams provided by Turnbull (1970) for Equus ferus caballus
(although see below). Additionally, Beddard and Treves
(1889, for Dicerorhinus sumatrensis) and Bressou (1961, for
Acrocodia indica) were taken as guides for muscular attach-
ment in both the sampled perissodactyls.
It should be mentioned at this point that the sampled
therians chosen to represent “herbivores” in this study are
all colloquially categorized as members of a “transverse
chewing” group (Vinyard et al. 2011), a categorization that,
in terms of overall diversity, the majority of herbivorous
mammal species throughout time would not correspond to.
The incredible diversity of clades that emphasize proal (e.g.,
many rodents and elephantids) and palinal (e.g., the extinct
multituberculates) movements during mastication clearly
demonstrates that unilateral horizontal motions of the man-
dible are not prerequisite for a successful plant-based feed-
ing strategy. We have left these clades out of consideration,
because P. tropicalis lacks distinctive features of the skull
and dental wear that characterize proal or palinal mastica-
tion in extant therians.
The hypercarnivorous feliform carnivorans Puma con-
color and Crocuta crocuta were provided by the ISM, and
IMNH, respectively. Muscle attachment information was
estimated from the description of Felis in Turnbull (1970)
and the large cats in Hartstone-Rose et al. (2012) for Puma
concolor, and from Buckland-Wright (1969) for Crocuta
crocuta. Finally, surface files for the three hypocarnivorous
carnivorian taxa Canis familiaris, Procyon lotor, and Ursus
arctos were provided by Tseng et al. (2016), with associated
myological illustrations taken from Evans and De Lahunta
(2016) for Canis familiaris, Gorniak (1986) for Procyon
lotor, and Davis (1955, 1964), and Endo et al. (2003) for
Tremarctos ornatus, Ailuropoda melanoleuca, and Ursus
thibetanus.
The reconstructed skull and associated hemimandibles
of P. tropicalis (Figs. 1B and 2) were created as part of the
Fig. 3. Comparison of 2D side-view and fully 3D definitions of parameters for the bifulcral model of mandibular leverage, shown using Didelphis marsu-
pialis Linnaeus, 1758 as an example. A. Left side-view of mandible in closed gape position, showing the locations of the working-side condylar fulcrum
(green) and bite point fulcrum (blue). The example temporalis force vector (red) drives rotation about both of these fulcra, and produces output force
vectors that are tangential to circles centered on their respective fulcra (dashed arcs). B. Oblique lingual view showing important points and lever arms cor-
responding to the working-side medial pterygoid force (MP-W; shown as a red arrow) using a fully three-dimensional model of bifulcral mandibular lever-
age. The very medially directed line-of-action for the medial pterygoid demonstrates the large differences in orientation between the condylar plane (CP;
shown with a green transparent plane), bite plane (BP; shown with a blue transparent plane), and a parasagittal (side-view) plane. The three-dimensional
bifulcral model calculates bite forces and joint forces by projecting load points into their respective planes, as distances perpendicular to these projections
do not affect leverage calculations. Definition of numbered points: 1, mesial postcanine point (PM); 2, distal postcanine point (PD); 3, centroid of insertion
area for working-side medial pterygoid (MP-W); 4, centroid of origin surface for MP-W on skull; 5, location of working-side joint (WJ); 6, projection
of WJ into BP; 7, projection of an example bite vertex on the third lower molar into the CP. Abbreviations: BF, bite force; ILBP-W, in-lever of the bite
plane on the working-side; ILCP-W, in-lever of the condylar plane on the working-side; JF-W, joint force on the working-side; MP-W, medial pterygoid
working-side; OLBP-W, out-lever of the bite plane on the working-side; OLCP-W, out-lever of the condylar plane on the working-side. Not to scale.

HARPER ET AL.—MASTICATION IN PALEOCENE MERIDIOLESTIDAN MAMMAL FROM ARGENTINA 183
original description of specimens in Paéz-Arango (2008:
fig. 22). This reconstruction (a series of plastic casts) was
surface scanned using a HDI Advance white light surface
scanner located at The Johns Hopkins University Center for
Functional Anatomy and Evolution. Because of the lack of
prior myological estimations for P. tropicalis, attachment
areas for the muscles of mastication were approximated
based on preserved local topography and using the anatomy
of Didelphis and Lautenschlager et al. (2017) as guides.
For all of the modern comparative taxa with only im-
age Z-stack data archived on Morphosource, surface files
(STLs) were generated using python wrapper utilities pro-
vided through the free and open-source graphical library
Visualization Tool Kit (VTK; Schroeder et al. 2006), in
particular the function vtkMarchingCubes. All downloaded
and algorithmically generated surfaces and the scanned sur-
face of Peligrotherium, were further remeshed to ensure
manifold topology in the regions analyzed, and to evenly re-
distribute and reduce triangle count, using the free mesh ed-
itors Meshmixer (Autodesk, California) and Blender (www.
blender.org; Sutton et al. 2016).
Group 1 vs. Group 2 analysis.—Using this surface data, a
novel biomechanical analysis is presented here as a three-di-
mensional generalization of the bifulcral model of man-
dibular mechanics pioneered by Bramble (1978) (see also
Greaves 2012; and Fig. 3A). The novel characteristic of our
bifulcral method is the use of two lever equations which
exchange the positions of the load point and fulcrum across
either side of a single input effort vector (Fig. 3B: the red
tubular arrow corresponding to MP-W), and allow output
forces to act in separate planes (the blue and green planes
in Fig. 3B). The working-side (WS) hemimandible is there-
fore modeled as two third-class levers simultaneously. For
each particular muscular input force this model then defines
two simultaneous lever equations which can be analytically
solved for output forces (a bite force and a joint force; Fig. 3B:
BF, JF-W) corresponding to arbitrary positions of the bite
point (tooth-food contact point), center of rotation during
adduction (hypomochlion; Fig. 3B: point 5), and point of
application for muscular tension (Fig. 3B: point 3). For any
given geometric relationship between these three points and
effort vector (magnitude and orientation) a corresponding
bite force magnitude can be calculated using the classical
equation modeling the hemimandibular center of rotation as
a fulcrum, and bite point (a location on the lower postcanine
tooth-row, referred to as a BV for “bite vertex” as in Fig. 4B)
as the point of application of the occlusal bite force. For this
first lever equation, pertinent lever-arms are parallel to the
plane containing both the muscular input vector and condy-
lar fulcral point (here called the condylar plane, CP, shown
as a transparent green triangular plane in Fig. 3B). So, for
instance the “in” lever- arm of the condylar plane (Fig. 3B:
ILCP-W) is defined as the perpendicular distance between
the muscular effort vector and condylar fulcrum, whereas
the “out” lever-arm in this plane (OLCP in Fig. 3B) is de-
fined as the distance from the condylar fulcrum to the pro-
jection of the bite point (BV) into the CP. The perpendicular
distance of the BV to the CP (the distance between point
2 and the base of the BF vector) is inconsequential with
respect the calculation of its leverage. The orientation of
the output bite force vector can also be determined as being
perpendicular to (a cross product of) a vector representing
the OLCP and the normal vector of the CP.
The second lever equation corresponding to a particular
muscular input in the bifulcral model treats the BV as the
fulcrum of a third-class lever which imparts an output joint
force (JF; as in Figs. 3B and 4A) at the corresponding condy-
lar center of motion (therefore, opposite the situation in the
first lever). The input muscular effort vector and its point of
application within the hemimandible remain the same as in
the first lever equation described in the previous paragraph
(the red arrow, corresponding to MP-W, and point 3, respec-
tively in Fig. 3B; and the red arrow in Fig. 4A). However,
for this lever equation the CP is not the appropriate plane
within which to measure input and output lever-arm lengths
relevant for the production of JF; and a second plane (here
called the bite plane, shown as a transparent blue triangular
plane in Fig. 3B, and in light green and pink for Diceros
bicornis in Fig. 4B) is defined as the plane containing the
BV fulcrum and muscular line of action. The BP is used
for the calculation of lever-arm distances and output vector
orientations relevant for the estimation of joint force. This
differentiation of CP and BP represents our method’s main
point of departure from the classical bifulcral model pre-
sented in Bramble (1978), Greaves (2012), Hartstone-Rose
et al. (2012), inter alia, as in these analyses the estimation of
all lever-arm distances and vector orientations were calcu-
lated within a single, parasagittal (lateral- view) plane (e.g.,
compare the mechanical models of Fig. 3A and B). The
reliance on photographic data with a standard orientation
has required that many prior analyses prescind potentially
important geometric variation inherent in the mediolateral
positioning of muscular attachments and their correspond-
ing effort vectors. The three-dimensional surfaces used here
allow for each muscular effort vector (categorized into eight
major jaw adductor groups, described below) to define the
orientations within which parameters of the relevant lever
equations should be calculated, and do not restrict these
vectors to a parasagittal orientation (e.g., the oblique ori-
entations of the CP and BP seen for Diceros bicornis in
Fig. 4B). Since a muscular line-of-action (LOA; for exam-
ple, between points 3 and 4 in Figs. 3B, 4) defines the
single line in 3D space which is shared by both the CP and
BP, the orientation of each muscular effort vector has the
potential to drastically modify the lengths of all four of its
corresponding lever arms and magnitudes of its two output
forces. This mediolateral modulation acts in addition to the
variability exhibited by the relative heights of the bite point,
muscular insertion centroid and mandibular condyle, and the
rostrocaudal component of the corresponding muscle resul-
tants focused on in prior analyses (Grossnickle et al. 2021).

184 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022
The fact that neither the WS or BS condyle are generally
located within the BP also requires these points to be pro-
jected into the BP (e.g., Fig. 3B: point 6) for calculation of
the relevant bifulcral “out” lever-arm distance (OLBP-W for
the WS as seen in Fig. 3A; and OLBP-B for the BS), similar
to the calculations for the CP lever equation. Also, as in the
CP lever equation, the orientation of each joint force vector
(vectors corresponding to JF-W and JF-B for the WS and
BS, respectively) is calculated by taking the cross product
of the normal vector of the BP and a vector representing the
OLBP-W or OLBP-B, for either the working or balancing-
side (see WS joint forces illustrated in Fig. 4A).
Fig. 4. Example 3D renderings of the bifulcral mandibular leverage parameters used in this analysis. A. Example of bifulcral lever arms corresponding
to the working-side superficial masseter (SM-W) shown in a semi-transparent hemimandible of Canis familiaris Linnaeus, 1758; the muscular resultant
vector is shown as a red arrow, with corresponding bite force vectors (yellow arrows) and working-side joint force vectors (purple arrows) shown for three
positions on the lower postcanine tooth row. As can be seen, the in-lever lengths around the bite point fulcra (used in the calculation of joint force) increase
significantly mesially. In labial (A1) and oblique (A2) views. B. Example of the varying condylar planes (CP) and bite planes (BP) used in the 3D bifulcral
analysis, shown in Diceros bicornis (Linnaeus, 1758) for a bite point on the lower first molar. The mandible is shown in semi-transparent gray, while the
working- and balancing-side origin surfaces for the deep masseter muscles on the skull are shown bilaterally in solid pink; similarly, the isolated origin
surfaces of the medial pterygoids are shown in solid green. The muscular lines of action (LOA) for these four muscles are also shown as solid tubes, while
transparent triangles demonstrate the orientations of the CP and BP for the corresponding muscle. In superior (B1) and oblique (B2) views. Not to scale.
Abbreviations: BS, balancing-side; BV, bite vertex; ILBP-W, in-lever of the bite plane on the working-side; SM-W, superficial masseter working-side;
WS, working-side.

HARPER ET AL.—MASTICATION IN PALEOCENE MERIDIOLESTIDAN MAMMAL FROM ARGENTINA 185
In the models used here, muscular effort during mastica-
tion and jaw adduction at wide-gape is categorized into four
partitions per side, here termed muscle categories (MCs).
These are the superficial masseter (SM), deep masseter
(DM), medial pterygoid (MP), and posterior temporalis
(PT) categories for both the WS and BS of the cranium (SM-
W, DM-W, MP-W, PT-W, and SM-B, DM-B, MP-B, PT-B;
making eight forces in total). While SM-W/B, and MP-W/B
forces correspond to the force vectors elicited from homog-
enous activation of motor units within the muscles of the
same name (Druzinsky et al. 2011), DM-W/B and PT-W/B
are groupings of potentially several separate homologous
muscular bellies, which must be considered in aggregate
because of our inability to resolve the discrete attachment
areas of the component muscles of these MCs using only
surface models of the skull and mandible. All these MCs
correspond to the composition of Group 1 (G1) and Group 2
(G2) chewing muscles as described by Crompton (2011) for
Didelphis. These “Groups” are conceptual extensions of the
original functional segregation of the muscles of mastica-
tion into “Triplets” defined by Weijs (1994). The differen-
tial activation of these “Triplets” produces the dorsolateral
and dorsomedial motions of the working-side hemimandible
during the “power stroke” vertical kinematic phase of a
generalized mammalian rhythmic chewing cycle (Ross and
Irarte-Diaz 2019). As mentioned above, the treatment of the
DM and PT MCs used here differs from the original formu-
lation of Crompton (2011) in that DM incorporates all zygo-
matic arch muscles which are not the superficial masseter or
a subdivision of the temporalis (most importantly we include
zygomaticomandibularis in DM). Additionally, our inability
to reliably distinguish mutually exclusive origination areas
for SM-W/B and DM-W/B in the ventral zygomatic arch
requires that the same area of masseteric origination be used
as the origin area for both of these MCs.
To estimate the attachment of the “posterior temporalis”
mentioned by Crompton (2011) for Didelphis virginiana, we
have developed a “temporalis cutter” algorithm which iso-
lates a posterior subdivision of the area of muscular origina-
tion for all temporalis muscles (such as superficial tempora-
lis, deep temporalis, and zygomatic temporalis) by clipping
the total temporalis origination area with a plane running
through the coronoid processes. We therefore operationally
define the “posterior temporalis” origin as the section of all
subdivisions of the temporalis originating posterior to this
plane. This plane is also inclined craniocaudally to follow
the average inclination of the vectors connecting the cen-
troid of the temporalis insertion area to the apical tip of each
coronoid process, on both the WS and BS. The apical tips
of the WS and BS coronoid process are therefore landmarks
which need to be placed by hand on the surface of each
hemimandible. Averaging the coronoid-centroid vectors
from both sides removes the mediolateral (yaw) orientation
of this clipping plane (for example see Fig. 2A: PT-W).
The protocol for producing standardized estimates of
the three-dimensional orientation and magnitude for each
of four MC input forces, on both the WS and BS, utilized
the mesh-editing and scripting capacities provided by the
free and open-source graphics program Blender (version
2.8+; see above). In particular, the capacities for computa-
tion on the geometry and connectivity of mesh objects pro-
vided by the blender-python API (Application Programming
Interface; in particular the modules bpy and bmesh) and
efficient mathematical utilities for scalar and array variables
provided by base packages in python (e.g., numpy; www.py-
thon.org), make Blender an ideal environment within which
to conduct biomechanical and morphometric analyses on
the geometry of digitized surfaces of all kinds.
For P. tropicalis and all 12 comparative therian taxa,
associated surface files of the skull and both hemimandibles
were imported into Blender and manipulated into a “closed
gape” (CG) relationship simulating the “intercuspital phase”
of mastication (i.e., near centric relation; Ross and Irarte-
Diaz 2019). Once the associated skull and jaw surfaces were
translated and rotated into this natural CG position, changes
were applied and a python script was used to scale all sur-
faces to a standard size based on the geometric mean of the
skull’s maximum dimensions (all points in the skull and jaw
surfaces were scaled by the reciprocal of the cube root of the
product of the skull’s maximum length, mediolateral width,
and dorsoventral depth). Changes were applied before and
after this operation, and in general it is important to make
sure that all transformations are applied to Blender mesh ob-
jects before proceeding with the analyses described below.
The geometric-mean size-scaling is necessitated by the
current lack of published information on the Physiological
Cross-Sectional Area (PCSA) in the muscles of mastication
for the taxa analyzed (especially the extinct Peligrotherium),
and our consequent inability to work with absolute estimates
of muscle force (given the empirical average of ~0.3 N per
square millimeter) of skeletal muscle PCSA in mammals
(Close 1972). For comparative purposes, our operational
solution to this problem is to assign a standard 100 au of
muscle force per size-standardized taxon, distributed to each
respective MC proportional to its relative combined surface
area (origin + insertion) and a balancing- to working-side
muscle recruitment ratio (B-WR). For this analysis B-WR
is uniformly assumed to be 0.33 for all sampled taxa (e.g., a
3.0 WS-BS ratio or 0.75 WS/total force ratio). In the absence
of taxon-specific values for B-WR, this value was chosen to
represent an estimate of W-BR in an “unspecialized” living
therian, (and in particular based on the “ratio of working to
balancing-side muscle force” during isometric biting at the
postcanine dentition, as estimated by Hylander (1979) for
Otolemur crassicaudatus, through a combination of geome-
tric, bite force, and bone-strain analyses). The current lack
of published measurements or even rough approximations
of B-WR for most therian species makes the uniform ap-
plication of B-WR = 0.33 a conservative assumption, as
choosing more extreme values (e.g., B-WR ~1 or 0) for more
derived taxa (having either elaborate asynchronous chewing
kinematics, or no recruitment of balancing-side musculature

186 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022
at all) could artificially bias differences among these forms.
Conversely our uniform application of an empirical B-WR
estimate from a relatively unspecialized, omnivorous, pla-
cental mammal is an approximation that will, in aggregate,
minimize the deviations of each individual sampled taxon’s
unknown B-WR with the operationally assumed value of
0.33. Our reasoning for making this operational assumption
is based on our desire not to (i) include an MC-specific bal-
ancing-working ratio in our model, and (ii) not wanting to
speculate on individual values of B-WR for each sampled
taxon that has not been the subject of a detailed feeding
study (the vast majority of our N = 12 comparative taxa,
and P. tropicalis). This would necessitate the introduction
of four wholly unknown parameter values for each taxon
(taxon-and-MC-specific balancing-working ratios), as op-
posed to the reasonable approximation of just one value for
our whole sample. In the case of Sus scrofa and Didelphis
marsupialis it is likely that justifiable aggregate B-WR val-
ues could be calculated through a weighted sum involving
the ratios of empirically recorded electromyographic volt-
ages in individual muscles of mastication observed during
rhythmic chewing, given the long and rigorous literature on
the physiology of feeding in these genera (e.g., Crompton
and Hiiemae 1970; Herring and Scapino 1973). Given the
exploratory nature of our analysis overall, we are hesitant to
introduce a novel method for the estimation of one or more
balancing-working ratios, or to introduce systematically bi-
ased taxon-specific balancing-working ratios into our model
to accommodate estimates for just two sampled taxa. We
hope that empirical values of B-WR will soon be available
in the literature for a wide range of therian species, allowing
us to incorporate taxon-specific data of this type in future
analyses.
The use of arbitrary standard units for the strength of
muscle contraction also requires that the measurement of
lengths used in the calculation of force moments (a force
magnitude multiplied by a corresponding distance to a ful-
crum perpendicular to it) also be standardized to values
which are comparable across all taxa sampled. Therefore,
the standardization and scaling protocol used here produces
results which allow for comparisons of mechanical advan-
tage (a unit-less quantity) and the distribution of relative bite
force and joint force orientations and magnitudes across
taxa, gapes (open and closed), and P1 vs. P2 muscle recruit-
ment scenarios, for varying BV points along the lower work-
ing-side postcanine tooth-row (TR; e.g., between points 1
and 2 in Fig. 3B). However, because all pertinent lever equa-
tions are parameterized in terms of these arbitrary standard
magnitudes, estimates of derivative magnitude and torque
values cannot be related to their absolute physical units
(newtons and newton-meters, respectively). Additionally,
because of the wide variation in mandibular shape (pre-
sumably with geometrical functional implications), linear
distance measurements of the mandible were not incorpo-
rated into the calculation of the size-scaling factor used to
standardize the size of the cranial meshes studied.
The LOA for each muscular force vector was approxi-
mated by a line connecting the centroids of the origin and in-
sertion attachment areas for each MC (e.g., Fig. 3B: points 3
and 4). These attachment areas were approximated using
the face selection tool in Blender’s edit mode to “paint” the
areas corresponding to each MC’s rigid enthesis area. This
operation was done by hand using the comparative and my-
ological sources for each taxon (listed above) and local bony
topography as a guide in delimiting each MC’s attachment
area. The ability to freely paint measurable areas on to digi-
tized meshes of the skull and mandible is both a strength and
weakness of this three-dimensional approach, as the lack of
practical limitations on the ability to designate bony surfaces
as attachment areas will likely introduce some amount of
inter-observer variation into the shape and extent of each MC
attachment area. However, only the relative muscle magni-
tude (a ratio between 0.0–1.0), and the 3D position of each at-
tachment’s centroid (an average of many vertex coordinates
included in an attachment surface) are directly influenced
by the extent of “painted” surface assigned to a MC’s attach-
ment area. This is an important consideration because in-
ter-observer error introduced by moderately imprecise mesh
editing, especially along boundaries, cannot dramatically
alter the estimation of these values and downstream calcula-
tions dependent on them. For instance, since the centroid of
an MC attachment area is the average of the coordinates of
many surface vertices, inter-observer variation in the exact
inclusion or exclusion of particular vertices at the margins
of the attachment area will have only a minor effect on the
location of the area’s centroid, unless those variably included
vertices encompass an exorbitant amount of area or are lo-
cated inordinately far from the other vertices.
Another, more severe, limitation of the method presented
here is its current inability to account for (the likely exten-
sive) attachment of the jaw adductor musculature to the dense
connective tissues investing the skull and mandible in vivo.
There are many known forms of such tendinous jaw muscle
attachments, such as those seen in the temporal fascia and
bodenaponeurosis (Crompton and Hylander 1986; Werneburg
2013), or in the internal tendinous ultrastructure of the mas-
seter and medial pterygoid (Druzinsky et al. 2011; most im-
portant for this analysis would probably be the “tendinous
bar” within the masseteric insertion of Equus described by
Turnbull 1970). While this is obviously an undesirable situa-
tion, it is a disadvantage shared with all prior 2D and 3D geo-
metrical analyses of mammalian masticatory leverage; and
unlike prior analyses, the graphical approach presented here
can very conceivably be extended in the future to incorporate
information on the soft anatomy of tendinous structures in
the feeding apparatus using the mesh editing capabilities of
Blender. While the neglect of tendinous attachments has an
unknown effect on estimates of muscle vector orientation, the
results produced here are at least comparable in this way to
those reported in prior literature.
The orientation of the LOA for each of the eight MCs
was used to create unit vectors following the “point load”

HARPER ET AL.—MASTICATION IN PALEOCENE MERIDIOLESTIDAN MAMMAL FROM ARGENTINA 187
method described above (also see Davis et al. 2010). The
magnitude of these unit vectors was then scaled (multi-
plied) by several coefficients to be proportional to (i) the
total available muscle magnitude (100au), (ii) the relative
magnitude of each MC, and (iii) the B-WR, thus ensuring
that the total amount of combined MC muscle magnitude
input equaled 100 au at CG. Each MC vector is therefore
treated as a concentrated force by modeling the centroid
of each MC’s insertion area as the point of application for
its corresponding force vector. Because of the assumption
of static (instantaneous or motionless) equilibrium attained
between each MC input force and its elicited reaction forces
(corresponding to BF and either JF-W or JF-B, depending
on side), a static correction factor (SCF) is introduced as
an additional linear coefficient for each MC’s BF and JF to
ensure static “force-equilibrium” with their input muscular
force. This correction is similar to the “linear correction fac-
tor” used by Davis et al. (2010) to ensure that the integrated
input stresses distributed over each muscular attachment
area add up to the measured total muscular input force.
In addition to these MC forces, four manually-placed
landmarks corresponding to the WS dentary-squamo-
sal joint center of motion (WJ; Fig. 3B: point 5), BS den-
tary-squamosal joint center of motion (BJ), a point demar-
cating the mesial-most point of the working-side postcanine
tooth- row (PM; Fig. 3B: point 1), and a point demarcating
the distal-most point of the working-side postcanine tooth-
row (PD; Fig. 3B: point 2), were added for each specimen
as small icosphere objects on the CG hemimandible sur-
faces in Blender (using the 3D cursor tool). Both WJ and
BJ were placed on an approximation of the caudal-most
point of the articular surface of the mandibular condyle
(condylare), as this point represents the assumed center of
motion for the mandible in prior biomechanical studies of
mandibular function such as Hartstone-Rose et al. (2012).
The landmarks demarcating PM and PD were placed on
the dorsal surface of the working-side dentary bone in the
CG position. These two points are used to define a single,
straight line running along the TR, thereby allowing for BVs
(bite vertices, that is vertices comprising the lower work-
ing-side postcanine tooth crown mesh) to be projected to
corresponding nearest positions within this line. The points
PM and PD therefore define a variable ratio, here termed the
mesiodistal location (MDL), which describes the ratio of the
length of the line segment connecting the mesial point (PM)
to a particular projection of a BV, to the total length of the
line connecting PM to PD. Because of the need to compare
postcanine vertex locations in a consistent way among spec-
imens, the variable MDL (used as the abscissa in Figs. 5C,
6C, 8, 9) must take values ranging between 0.0–1.0 in all
taxa analyzed, thereby necessitating that PM and PD be
placed just mesial to the first postcanine tooth crown, and
just distal to the last postcanine tooth crown, respectively.
The MC vectors (derived from highlighted MC at-
tachment areas), the joint landmarks WJ and BJ, and the
landmarks defining the mesial and distal ends of the work-
ing-side postcanine tooth-row (PM and PD) are all the user
input required for the calculation of output forces from dif-
ferential muscle recruitment scenarios (e.g., G1 vs. G2 ac-
tivation) during mastication (viz. at CG; see left-hand side
subplots in Fig. 7). For example, intermediate coordinates
Fig. 5. Results of Group 1 (G1) minus Group 2 (G2) analysis of orthal
bite force (BF) for Peligrotherium tropicalis Bonaparte, Van Valen, and
Kramarz, 1993. A. Lower left working-side hemimandible of P. tropicalis
with postcanine crowns colorized by relative G1 vs. G2 advantage (yellow
shows areas where G1 produces higher orthal BF, while bluer colors corre-
spond to higher G2 BF values; white areas are where G1 and G2 forces are
sub-equal). B. Violin boxplot showing distribution of total GP1 minus GP2
BF and its orthal component marginal over all locations in the postcanine
tooth-row (TR). C. 2D histogram plot showing distribution of estimated
bifulcral force magnitudes for G1 and G2 muscle recruitment regimes, as a
function of mesiodistal location (MDL). JF-W/B, working-/balancing-side
joint force.

188 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022
and vectors, such as the vector corresponding to the “oc-
clusal-up” direction can be solved for by defining a line
connecting the points of closest approach of the two skew
lines representing the TR (between PM and PD) and the
“bicondylar line” (between WJ and BJ). This occlusal-up
vector for the closed gape position is also used to define the
“direction of maxillary resistance” (DMR) for the postca-
nine tooth-row in the open gape (OG) position.
Open vs. closed gape analysis.—For the comparison of out-
put forces between CG and OG mandibular positions (see
right-hand side subplots in Fig. 7), additional user input to
define the geometry of the lower mandible at OG is required.
To generate this information, the meshes corresponding to
both hemimandibles, the MC insertion areas, PM, and PD
in the CG position were duplicated and re-positioned as a
group (together) into a position approximating the “maximal
bony gape” for each taxon (MBG; Fricano and Perry 2019).
However, because WJ and BJ are assumed to be located on
stationary axes of rotation for both adducting hemimandibles,
the distal extent of the articular surface of the OG mandibular
condyles were not translated to the anterior-most extent of the
glenoid fossa (as they are for Fricano and Perry 2019; also see
Crompton et al. 2006). The points WJ and BJ are therefore
operationally assumed to not glide anteroposteriorly during
the gape cycle. The position of OG for Diceros bicornis,
Equus quagga, Erinaceus Europaeus, Sus scrofa, and Tupaia
sp. were also estimated based on photographic evidence of
living species, because of the exorbitant level of mandibular
abduction permissible using only bony landmarks.
Once the duplicate mandibular meshes were placed into
the OG position in Blender and all changes were applied, es-
timations of the MC force vectors and all intermediate vari-
ables (such as the “occlusal up vector”) are estimated for the
OG position using the same methods as for the CG position
described above. In order to generate the plots comparing
OG and CG output forces between corresponding BVs, a
short R script was used to identify matching vertices on the
OG and CG meshes based on their MDL value (see SOM,
Supplementary Online Material available at http://app.pan.
pl/SOM/app67-Har per_etal_SOM.pdf).
The distances between each MC origin centroid and in-
sertion centroid at CG are also compared to their correspond-
ing wider distances at OG, allowing for the calculation of a
stretch factor (SF; here estimated as the ratio of the distance
between MC origin and insertion centroids at OG divided
Fig. 6. Results of closed gape minus open gape analysis of orthal bite force
(BF) for Peligrotherium tropicalis Bonaparte, Van Valen, and Kramarz,
1993. A. Lower left working-side hemimandible of P. tropicalis shown in
closed gape (CG) and open gape (OG) position, postcanine crown surfaces
are colorized by relative orthal BF (warmer colors represent higher rela-
tive orthal BF, and correspond among the taxa seen in Fig. 12). B. Violin
boxplot showing distribution of total CG minus OG BF and its orthal com-
ponent marginal over all locations in the woking-side postcanine tooth-
row. C. 2D histogram plot showing distribution of estimated bifulcral force
magnitudes for CG and OG, as a function of mesiodistal location (MDL).
JF-W/B, working-/balancing-side joint force.

HARPER ET AL.—MASTICATION IN PALEOCENE MERIDIOLESTIDAN MAMMAL FROM ARGENTINA 189
by this same distance at CG). These SFs (see Table 2) are
used to decrement the strength of MC contraction at open
gape according to the empirical formula for whole-muscle
length-tension curves reported for the “masseter” of the
miniature domestic pig (Anapol and Herring 1989). These
length-tension curves show a characteristic offset between
fascicle length at closed gape (with teeth in occlusion)
and a somewhat longer optimum length representing the
length of muscular extension corresponding with maximal
whole-muscle tension (i.e., maximal MC force magnitude).
The value of this offset shown for the masseter in Anapol
and Herring (1989), is ~5% based on the distance between
the gonial angle (equivalent to the angular process) and the
central region of the zygomatic arch. However, because the
categorization of MCs used in this report combines several
of the discrete muscular bellies making up the compos-
ite “masseter” and other muscles analyzed by Anapol and
Herring (1989), and because of our use of different distances
to define SF in this report (between attachment centroids),
the 5% value of the closed-maximal offset produces unre-
alistic multipliers for maximal MC force magnitude. This
is likely a product of the shorter distances between muscle
attachment centroids for most MCs (where, for instance on
concave surfaces, the centroid is located hovering some dis-
tance off of the surface of its bony attachment). To decrease
this methodological mismatch, the method we consistently
apply for all taxa and MCs is to decrement the magnitude
of MC force at OG following the “descending limb” of
the isometric length-tension relationship of the masseter
reported by Anapol and Herring (1989), assuming a 30%
offset between optimal length and length at closed gape for
all taxa. The optimal magnitude of MC contraction is also
Fig. 7. Violin boxplots showing distribution of Group 1 minus Group 2 values (on left) and closed gape minus open gape values (on right) for N = 12
representative extant therians. A. Canis familiaris (domestic dog). B. Crocuta crocuta (spotted hyaena). C. Diceros bicornis (black rhino). D. Didephis
marsupialis (opossum). E. Equus quagga (quagga). F. Erinaceus europaeus (European hedgehog). G. Procyon lotor (racoon). H. Puma concolor (moun-
tain lion). I. Sus scrofa (domestic pig). J. Tayassu pecari (white-lipped peccary). K. Tupaia sp. (treeshrew). L. Ursus arctos (brown bear). Abbreviations:
T, total bite force; O, orthal bite force. Vertical axes represent magnitude values in au.

190 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022
Fig. 8. 2D histograms showing distribution of estimated force magnitudes for Group 1 (G1) and Group 2 (G2) muscle recruitment scenarios (on ordinate),
as a function of mesiodistal location (MDL; on abscissa). A. Canis familiaris. B. Crocuta crocuta. C. Diceros bicornis. D. Didephis marsupialis. E. Equus
quagga. F. Erinaceus europaeus. G. Procyon lotor. H. Puma concolor. I. Sus scrofa. J. Tayassu pecari. K. Tupaia sp. L. Ursus arctos. Abbreviations:
BF, bite force; JF-W/B, working-/balancing-side joint force.

HARPER ET AL.—MASTICATION IN PALEOCENE MERIDIOLESTIDAN MAMMAL FROM ARGENTINA 191
Fig. 9. 2D histograms showing distribution of estimated force magnitudes for closed gape (CG) and open gape (OG) mandible positions (on ordinate), as
a function of mesiodistal location (MDL; on abscissa). A. Canis familiaris. B. Crocuta crocuta. C. Diceros bicornis. D. Didephis marsupialis. E. Equus
quagga. F. Erinaceus europaeus. G. Procyon lotor. H. Puma concolor. I. Sus scrofa. J. Tayassu pecari. K. Tupaia sp. L. Ursus arctos. Abbreviations:
BF, bite force; JF-W/B, working-/balancing-side joint force.

192 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022
assumed to be negligibly different from the force of MC
contraction at the closed gape position (described above),
which is equivalent to assuming an approximately horizon-
tal “ascending limb” of the log length-tension curve for each
MC. Additionally, the component of muscle tension contrib-
uted by passive tension generated from the parallel elasticity
of the apomorphic intramuscular tendinous sheets in the pig
masseter is here assumed to be negligible for S. scrofa and
all other sampled taxa as well.
Significance tests.—The above analyses, implemented as
a sequentially executed group of scripts for the Blender-
Python API, outputs estimates of the relative magnitude of
BF, orthal BF, JF-W, and JF-B, generated under G1 vs. G2
muscle recruitment scenarios, and (as a separate analysis)
at CG vs. OG (Figs. 5B, 6B, and 7). Plots of these relative
magnitudes as a function of MDL are shown in Figs. 5C, 6C
for P. tropicalis, and Figs. 8, 9 for the N = 12 comparative
therians. These results (described below) are clearly not
normally distributed, and significance tests for differences
in BF characteristics cannot be based on the assumption
of their being distributed normally. The pairwise Monte
Carlo randomization tests implemented here are based on
testing for significant differences in the mean and variance
of per-vertex G1 minus G2 BF and orthal BF relative mag-
nitudes; and the mean and variance of per-vertex CG minus
OG BF and orthal BF (parallel to the DMR) relative magni-
tudes. Because of the many thousands of relative magnitude
estimates measured in a lower tooth-row mesh, there is a
strong tendency for these parameters to present significant
differences in pairwise tests between taxa driven solely by
the large sample size (not effect size). The large number
of measured vertices per taxon also would demand exor-
bitant computational resources to perform a large number
Fig. 10. Connectivity graphs summarizing the results of pairwise randomization tests performed on the per-vertex values of orthal bite force (BF) dif-
ferences. Lines connect taxa that are not found to be significantly different, and gray-scale value of individual nodes are proportional to the value of the
parameter tested (e.g., dark tones are lower in value, and higher values are closer to white). A. Graph summarizing pairwise significance tests of mean
Group 1 (G1) minus Group 2 (G2) orthal BF. B. Graph summarizing pairwise significance tests of F-value (variance ratio) of G1 minus G2 orthal BF.
C. Graph summarizing pairwise significance tests of mean closed gape minus open gape orthal BF. D. Graph summarizing pairwise significance tests of
F-value (variance ratio) of closed gape minus open gape orthal BF.

HARPER ET AL.—MASTICATION IN PALEOCENE MERIDIOLESTIDAN MAMMAL FROM ARGENTINA 193
of randomized relabelings for all measured vertices in all
taxa studied. The methods used here perform 1000 random-
ized relabelings for each pairwise significance test, which
randomly subsample 100 vertices from each of the lower
molar meshes for each permuted relabeling. The observed
value for the difference in mean G1 minus G2 magnitudes,
and CG minus OG magnitudes, were then pooled and com-
pared with this randomized sample using a two-tailed (the
rejection region includes 2.5% of permutations greater than
|x| and less than -|x|) significance criterion. For the vari-
ance of per-vertex G1 minus G2 magnitudes, and CG minus
OG magnitudes, the F parameter (a sample size adjusted
ratio of variances) was calculated for each pair of taxa,
and this in turn was tested with a 2-tailed criterion for the
F-distribution: the rejection region included 2.5% of permu-
tations greater than max(x, 1/x) and 2.5% less than min(x,
1/x). The results of these pairwise significance tests are tab-
ulated in the final section of the supplementary information
document, and are summarized graphically as the network
plots in Fig. 10. In these plots, lines connect statistically
indifferentiable taxa, with empirical p >0.05.
Results
From Fig. 7 it is apparent that there is considerable within-
tooth-row variation in BF and orthal BF. Figures 5C, 6C, 8,
and 9 further show that BF and the orthal component of BF
(that is, directed toward the DMR) show a near-monotonic
increase for all leverage scenarios, while JF-W and JF-B
similarly show a near-monotonic decrease running distally
along the TR in all cases (that is running from MDL val-
ues of 0.0 to 1.0 along the x-axis). The sensitivity of these
output force magnitudes to the precise location of a BV
within a particular crown surface also seems to vary among
taxa, with Peligrotherium tropicalis, Tupaia sp., the suoids,
Crocuta crocuta, and the hypocarnivores showing much
wider within-tooth-crown variability than the other sampled
taxa. Tayassu pecari and Tupaia sp. in particular (Figs. 7, 8,
and 9) seem to show the most variability in output force mag-
nitudes both marginally and especially at distal MDL values.
Group 1 vs. Group 2 analysis.—Only Didelphis marsu-
pialis shows broadly subequal BF values for both G1 and
G2 muscles across the entire length of the TR. However,
Peligrotherium tropicalis, Sus scrofa, and Diceros bicor-
nis show non-parallel G1 and G2 BF distributions along
the TR which coalesce or intersect mesially (Figs. 5C, 8C,
I) . Only the derived obligatory grazer Equus shows BF
values for G2 muscles markedly higher than G1 muscles
everywhere. When just the orthal component of G1 or G2
bite force is considered, only P. tropicalis, Didephis mar-
supialis, Erinaceus eruopaeus, and Equus quagga show a
marked difference in magnitude distribution compared with
total BF magnitude (Figs. 5C, 8D–F). This a product of the
naturally very orthal direction of G1 and G2 postcanine bite
forces in Tupaia sp. and the other carnivores and ungulates.
The orthal component of BF in Didephis marsupialis is
everywhere higher for G2 muscles than G1. This can be ex-
plained as a reflection of the emphasis that opossums place
on P2 during mastication and the reduction of ingested ma-
terial, which require particles to be compressed in the orthal
direction against the upper dentition (Fig. 8D; Crompton
and Hiiemae 1970). As with total BF, Equus quagga also
shows much higher orthal G2 BF magnitudes compared
with G1 (Fig. 8E). The pattern of orthal BF distribution
seen in P. tropicalis, S. scrofa, Tayassu pecari, and Diceros
bicornis is less parallel than in the other sampled taxa and
converges mesially, much as with total BF. For Tayassu pe-
Table 2. Estimated relative force contributions and stretch factors measured in sampled taxa. Abbreviations: B, balancing-side; DM, deep masseter;
MP, medial pterygoid; PT, posterior temporalis; R, relative contribution to total jaw adductor attachment area; SF, stretch factor; SM, superficial
masseter; W, working-side.
Canis
familiaris
Crocuta
crocuta
Diceros
bicornis
Didelphis
marsupialis
Equus
quagga
Erinaceus
eruopaeus
Peligrotherium
tropicalis
Procyon
lotor
Puma
concolor
Sus
scrofa
Tayassu
pecari
Tupaia sp.
Ursus
arctos
R SM 0.11 0.1 0.32 0.19 0.29 0.16 0.15 0.12 0.1 0.28 0.16 0.17 0.12
R DM 0.18 0.17 0.21 0.22 0.23 0.2 0.19 0.18 0.21 0.23 0.2 0.21 0.18
R MP 0.1 0.06 0.11 0.05 0.25 0.09 0.1 0.07 0.06 0.22 0.19 0.09 0.07
R PT 0.6 0.68 0.37 0.53 0.23 0.55 0.55 0.62 0.64 0.27 0.45 0.54 0.63
SF SM-W 1.23 1.14 1.24 1.41 1.17 1.34 1.27 1.18 1.15 1.32 1.26 1.31 1.2
SF SM-B 1.2 1.18 1.25 1.44 1.16 1.38 1.23 1.21 1.23 1.3 1.28 1.24 1.2
SF DM-W 1.32 1.3 1.13 1.87 1.27 1.45 1.57 1.38 1.28 1.38 1.33 1.53 1.47
SF DM-B 1.3 1.32 1.17 1.86 1.29 1.52 1.38 1.37 1.37 1.35 1.32 1.39 1.47
SF MP-W 1.2 1.12 1.19 1.23 1.13 1.22 1.29 1.12 1.18 1.2 1.1 1.26 1.13
SF MP-B 1.19 1.14 1.17 1.28 1.12 1.21 1.39 1.13 1.14 1.22 1.1 1.28 1.13
SF PT-W 1.29 1.28 1.23 1.24 1.09 1.2 1.17 1.26 1.22 1.14 1.17 1.15 1.31
SF PT-B 1.28 1.29 1.23 1.23 1.07 1.21 1.22 1.24 1.2 1.12 1.16 1.14 1.33

194 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022
cari this geometric arrangement of forces causes the relative
magnitude of G1 to G2 orthal force to increase distally and
labially in the molar region. However, even at the mesial
most extremity of the TR, orthal G2 magnitudes are not
appreciably greater than G1 (Fig. 8J). This relative pattern
is accentuated in P. tropicalis, Diceros bicornis, and S.
scrofa, where the trends of G1 and G2 orthal BF cross in the
distal premolar region (Figs. 5C, 8C, I). This causes orthal
G2 forces to be markedly greater mesially in the TR and
also lingually within the molar row. Importantly, within the
molars, the regions with greater G2 relative to G1 forces
are located lingually, within the trigonid (and talonid for S.
scrofa and Diceros bicornis) of each molar crown. Areas
on the molar surface with appreciably greater G1 orthal BF
correspond to the exodaenodont lobes of P. tropicalis (labial
regions of the molar crown which lack occlusal contact with
the upper dentition; Fig. 5A). Of the taxa sampled for this re-
port, P. tropicalis, S. scrofa, and Diceros bicornis are unique
in showing a switch from greater G1 to greater G2 orthal
forces within the lower postcanine surface. All other taxa
can be fairly cleanly separated into those with consistently
greater orthal G1 to G2 BFs (Canis familiaris, Crocuta cro-
cuta, Erinaceus europaeus, Procyon lotor, Puma concolor,
Tayassu pecari, Tupaia sp., and U. arctos; Fig. 8A, B, F–H,
J–L) vs. those with consistently greater orthal G2 relative to
G1 forces (Didelphis marsupialis and Equus quagga; and
only Equus quagga shows this relationship with total BF
as well; Fig. 8D, E). As opposed to Didephis marsupialis
(where relative orthal G2 advantage remains approximately
constant) and Equus (where the value of orthal G1 BF minus
orthal G2 BF becomes more negative distally), the pattern of
relative G2 advantage in P. tropicalis, S. scrofa, and Diceros
bicornis follows a lingually directed gradient.
The randomization tests for pairwise differences in mean
TR G1–G2 values (see section “Group 1 minus Group 2 or-
thal BF pairwise test results” in the SOM) differentiate the
sampled taxa into broad feeding categories, with the carniv-
orans U. arctos, Crocuta crocuta, and Procyon lotor having
the highest G1–G2 orthal BF magnitudes (G1–G2 orthal BF
values ~12 au). This is a product of the much greater orthal BF
values under G1 muscle recruitment compared with G2 mus-
cle recruitment scenarios. At the opposite extreme is Equus
quagga, which shows the highest orthal BF magnitudes under
G2 muscle recruitment (mean G1–G2 orthal BF is ~-4.4 au).
The fossil P. tropicalis shows greater similarity to Equus in
that its TR average G1–G2 orthal BF value is 0.95, and there-
fore closer in value to Equus quagga (and statistically indif-
ferentiable from S. scrofa and Diceros bicornis) even though
having a positive mean G1–G2 orthal BF value.
The pairwise randomization tests of standardized vari-
ance ratios (F-values) find fewer significant differences be-
tween taxa than the mean difference tests, the most salient
result from these tests being the much greater TR standard
deviation of G1–G2 orthal BF values in Tayassu pecari rel-
ative to every other sampled taxon (SD = 4.04). At the other
extreme, both Didephis marsupialis and Puma concolor
show significantly lower intra-tooth-row standard deviations
in G1–G2 orthal BF than the remaining taxa (SD ~0.7 au).
The interpretation of these tests is discussed further below.
Open vs. closed gape analysi s.—In Crocuta crocuta, Diceros
bicornis, Sus scrofa, Equus quagga, and Peligrotherium
tropicalis, total BF is broadly similar between CG and OG
throughout the whole length of the TR (Figs. 6C, 9B, C, E,
I). It is also noteworthy that the magnitudes of CG and OG
BF also converge mesially within the tooth-row of Tayassu
pecari (Fig. 9J), as they approach the location of its enlarged
vertical canine. In the case of Diceros bicornis, S. scrofa,
Equus quagga, and Tayassu pecari, the similarity of CG and
OG BF is likely a product of the extremely narrow maximal
gape attainable by these taxa (in some locations even elicit-
ing greater BF at OG than at CG; Fig. 7C, E, I, J). However,
in the case of the durophagous Crocuta crocuta, this pres-
ervation of high-force at high gape should be considered a
major geometrical adaptation. Whether these results might
also be taken as evidence of a similarly durophagous diet in
P. tro picalis could therefore be informed by the orthal com-
ponent of its OG bite force magnitudes (Fig. 6).
When only the orthal component of BF (parallel to the
DMR) is considered across gapes, several taxa show much
greater magnitudes of orthal BF at CG compared to OG.
From the violin plots in Fig. 7 it is apparent that no taxon
has higher orthal BF at OG than CG, but several of the taxa
with high OG bite forces lose a large proportion of that mag-
nitude when only its orthal component is considered. This
is especially true of S. scrofa and P. tropicalis (Figs. 9I, 6B)
which show an average drop of ~5.1 au and 9.4 au of bite
force magnitude, respectively (compared to Crocuta with
only a 3.2 au decrease; Fig. 9B).
The magnitudes of JF-W are distinctly higher for CG
compared with OG for most of the length of the TR in all
taxa except Erinaceus eruopaeus (Fig. 9; in several others
OG JF-W exceeds CG JF-W only far distally). The CG and
OG values of JF-B are low and broadly similar for all taxa.
The randomization tests for pairwise differences in mean
TR orthal CG-OG values (see section “Open minus closed
gape orthal BF pairwise test results” in the SOM) show most
taxa being significantly different from all others. At one ex-
treme, Ursus shows the least ability to maintain high orthal
forces at high gape (Fig. 9L), while Crocuta crocuta and the
perissodactyls listed above show the greatest amount of OG
orthal BF preservation (Fig. 9B, C, E). The pairwise ran-
domization tests of standardized variance ratios (F-values)
of CG-OG orthal BF values find much fewer significant
differences, and as with the G1–G2 variance randomiza-
tion tests Ta y a s s u pecari shows a much higher variance of
CG-OG values compared with the other sampled taxa (Figs.
7J, 9J). The conspicuously larger variance in estimated man-
dibular leverage parameters for Tayassu pecari, compared
with all other sampled taxa, is a reflection of its wide range
(Fig. 7J) and steep mesiodistal gradient (Fig. 9J) of mea-
sured BF and orthal BF at CG. The distributions of these

HARPER ET AL.—MASTICATION IN PALEOCENE MERIDIOLESTIDAN MAMMAL FROM ARGENTINA 195
BF estimates are in turn a consequence of the unique com-
bination of mandibular characteristics seen only in Tayassu
pecari in our sample. These unique characteristics are a re-
sult of combining (i) a very mesiodistally elongate jaw, with
(ii) a mandibular condyle positioned nearly in-line with the
dorsoventral height of the postcanine tooth-row.
Discussion
With the methods and comparative samples described above,
several significant and autecologically meaningful patterns
are apparent concerning the use of G1 vs. G2 muscles and
CG vs. OG behaviors in Peligrotherium tropicalis. While
the tables in the supplementary information document pres-
ent these results explicitly, it is helpful to visualize the 676
pairwise randomization tests (169 each for the variables
G1 minus G2 and CG minus OG, mean and variance, re-
spectively) in aggregate using network graphics (generated
using the igraph package for R; Csardi and Nepusz 2006).
Figure 10 displays the patterning of these results among
our sampled taxa by placing a line connecting taxa that
are not found to be significantly different in the parameter
tested (p >0.05). Conversely, taxa that are not directly con-
nected by a line are found to be significantly different. As
can be seen in Fig. 10A, the patterning of mean G1 minus
G2 orthal BF magnitudes places P. tropicalis in a statisti-
cally mutually indistinguishable cluster with the ungulates
Diceros bicornis and S. scrofa, based on their relatively
high G2 advantage. Similarly, mean G1 minus G2 orthal BF
also clusters the carnivorans Crocuta crocuta, U. arctos,
and Procyon lotor, in a “high G1 advantage” group, and
Puma, Canis, and Erinaceus based on their intermediate
G1 minus G2 values. While it is surprising that the hyper-
car nivore Puma concolor has not been placed in the highest
G1 advantage group, these clusters of taxa with generally
similar feeding strategies provide some confidence that the
placement of P. tropicalis with the omnivore S. scrofa and
the obligate browser Diceros indicates some level of her-
bivory in its feeding strategy. It is important to realize that
this inference is not based on P. tropicalis grouping into a
homogeneous, or cohesive, feeding category with these two
ungulates; and inferences should only be based on its being
significantly different from all other sampled taxa in mean
G1 minus G2 orthal BF. For example, P. tropicalis has a
ratio of G1 to G2 advantage that is definitively dissimilar to
the plesiomorphic therians Didephis marsupialis, Erinaceus
europaeus, and Tu p a ia sp., the hypo-and-hypercarnivo-
rous carnivorans, and the derived ungulates Equus quagga
and Tayassu pecari. As such, P. tropicalis, S. scrofa, and
Diceros bicornis may (and actually do) have unique masti-
catory behaviors that are effected by the subtle geometrical
differences in their mechanical feeding apparatuses, but are
together statistically differentiated from all other mammals
sampled. Figure 11 provides a graphical comparison of the
Fig. 11. Comparison of orthal Group 1 (G1) vs. orthal Group 2 (G2) bite force (BF) across the lower postcanine tooth-row. Warmer colors correspond to
higher G1 muscle advantage, and cooler colors correspond to higher G2 advantage. A. Canis familiaris Linnaeus, 1758. B. Sus scrofa Linnaeus, 1758.
C. Equus quagga Boddaert, 1785. Note that Sus scrofa matches Peligrotherium tropicalis most closely, in having greater G1 advantage disto-buccally
and greater G2 advantage mesio-lingually. Color scale is based on range of G1 minus G2 orthal BF magnitudes scaled by the value of single sample
(within-taxon) standard deviation in this value.

196 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022
distribution of relative G1 minus G2 orthal BF among three
of the sampled therians.
These results therefore demonstrate that mandibular
geometry can be added to the list of similarities, such as
size (large), and gestalt molar condition (bunodont) shared
between P. tropicalis and many therian omnivores and her-
bivores. Unlike body mass (which is ambiguous) and dental
form and formula (associated with the statistics of dietary
material properties; Kay and Hiiemae 1974; Lucas 2004),
the convergent emphasis on “Group 2” geometry in the
feeding apparatus of P. tropicalis, S. scrofa, and Diceros
bicornis, suggests that the “grinding forces” (sensu Kay
and Hieemae 1974) transmitted by the dentition and driven
by the muscles of mastication in these forms, are in turn
coordinated by fundamentally similar motor programs.
The dynamic behaviors effecting mastication in these her-
bivores-omnivores are therefore plausibly more similar to
each other than they would be to an ancestral therian mam-
mal or even plesiomorphic extant therians such as Didelphis
marsupialis, Erinaceus europaeus, or Tupaia sp.
It is also important to note that the results of this analysis
do not support a single cohesive feeding geometry of “primi-
tive therians”, clustering Didelphis marsupialis, Erinaceus
europaeus, and Tupaia sp. This is opposite to what would be
expected if the earliest, or least derived, therians and earlier
mammaliaforms were characterized by a single stereotyped
mode of branchiomotor muscle recruitment during mastica-
tion (see Gill et al. 2014 for further biomechanical evidence
for the early feeding diversity of mammaliaforms gener-
ally). The clusters of taxa shown in Fig. 10A therefore do not
reflect or imply phylogenetic patterns of jaw conformation.
The above considerations answer the question of whether
the clustering of P. tropicalis, S. scrofa, and Diceros bicor-
nis in terms of G1 and G2 orthal BF could represent some
shared ancestral motor pattern; as such a synapomorphic
motor pattern would presumably have to include the more
plesiomorphic therians sampled as well. The geometric ev-
idence presented here strongly suggests a geometrical con-
vergence of the feeding apparatus between P. tropicalis ( Fig.
5A), S. scrofa (Fig. 11B), and Diceros bicornis (Fig. 4B), as
opposed to a retained synapomorphy inherited from a likely
Jurassic common ancestor. However, the fact that the feed-
ing apparatus in Peligrotherium morphologically associates
more closely to some large therian herbivores/omnivores
than all the sampled therians associate among themselves
supports several prior hypotheses for the evolution of masti-
catory capacities of mammals generally.
The electromyographic and biomechanical analyses re-
ported by Crompton and Hylander (1986) have suggested
that, commencing with the development of a dentary-squa-
mosal jaw articulation, at least the structural capacity for
unilateral mastication could have been feasible among ad-
vanced cynodonts. Whether this capacity was realized by
mammaliaforms generally, or was restricted in distribu-
tion only to therians, or convergently found in more inclu-
sive mammaliaform sub-clades showing a detachment of
Meckel’s element and postdentary elements from the den-
tary, is still unresolved (Meng et al. 2003). Cladotheria is
one such clade of Jurassic and later mammaliaforms which
includes P. tropicalis and extant therians, and has been
hypothesized to show evidence of extensive medio-lateral
excursions during mastication (Grossnickle 2017). Features
such as wider upper molars relative to corresponding low-
ers, the presence (or possibly reappearance) of a true an-
gular process and the widespread (but homoplastic) loss of
Meckel’s groove within Cladotheria have been interpreted
as indicating a “primitive tribosphenic” mode of unilateral
mastication effected by a stereotyped asymmetrical regime
of chewing muscle recruitment (Grossnickle 2020). While
the evidence presented here does not associate the feeding
geometry of Peligrotherium with any of the plesiomorphic
therians listed above, the fact that P. tropicalis does show
a high level of “Group 2 advantage” does lend support to
the hypothesis that the common ancestor of Peligrotherium
and therians would have shown at least an incipient form
of rhythmic chewing with a power stroke subdivided into
P1 and P2 vertical kinematic phases. This incipient pattern
of cladotherian mastication was likely then exapted and
exaggerated independently in the meridiolestidan ancestors
of Peligrotherium, the “dichobunoid” artiodactyl ancestors
of Sus, and the stem-perissodactylan ancestors of Diceros
(Gould and Vrba 1982). This convergence is also seen in the
mediolateral gradient of increasing G2 advantage lingually
(Figs. 5C, 11B), where grinding motions would be more
emphasized. Conversely, the labial portions of the postca-
nine tooth-row in these three taxa are where G1 muscles
have a greater advantage, suggesting that times of labial
molar-crown food contact, with either the exodaenodont
lobes (in P. tropicalis) or the labial upper molar ectolophs
(in Diceros), occur during P1 of the masticatory power
stroke.
The significance tests summarized by the connectivity
graphs for mean CG minus OG orthal bite force (Fig. 10C)
and for the variance of both G1 minus G2 and CG minus
OG orthal bite force values (Fig. 10B, D) are more difficult
to interpret ecologically. As mentioned above, the tested
parameter “CG minus OG orthal bite force magnitude”
conflates taxa that are genuinely adapted for wide-gape
behaviors (most obviously Crocuta bicornis) with strongly
herbivorous taxa that, because of their very limited maxi-
mal gapes, do not show a large loss of orthal bite force at
their widest gapes. We have not attempted to resolve this
issue here, because many of the obvious solutions to this
problem (such as dividing the CG minus OG orthal bite
force magnitude value by some standard measurement of
gape-distance or gape-angle) either are extremely sensitive
to how size and linear distances are standardized among
sampled taxa, or are arbitrary and difficult to apply across a
large range of cranial morphologies, or both. The mean CG
minus OG orthal BF significance tests do however suggest
that P. tropicalis does not fit in in either of these conflated
feeding categories (wide-gape carnivore, limited-gape her-

HARPER ET AL.—MASTICATION IN PALEOCENE MERIDIOLESTIDAN MAMMAL FROM ARGENTINA 197
bivore), and place Peligrotherium among a large cluster
of the more plesiomorphic and/or generalized extant taxa
sampled (Figs. 10C, 12). Similarly, the significance tests
for differences in F-value (variance) for both G1 minus
G2 and CG minus OG values place P. tropicalis among
large and ecologically heterogeneous clusters of extant taxa
(Fig. 10B, D).
Validation and criticism of the new method.—The paucity
of reported correlations between average empirical bite
forces (such as measured through in vivo bone-strain and
pressure transducer experiments) and the maximal theoret-
ical bite force estimates produced by biomechanical stud-
ies such as this one prevent us from making a definitive
statement on whether our novel Blender method presents
a more realistic model of mandibular function compared
to earlier approaches (see Ellis et al. 2008 and Davis et al.
2010 for important correlative studies however). Despite
this lack of empirical cross validation from these different
approaches, our method’s explicit use of vectors in their
natural three-dimensional context provides a unique, and
previously unavailable, capacity to answer questions regard-
ing the relative significance of the mediolateral positioning
and orientation of muscular effort vectors. This potential is
all the more important for investigations of the relatively
understudied non-tribosphenic crown mammals, given the
novel evolution of several translational and angular degrees
of freedom derived in the early mammalian feeding appa-
ratus (Grossnickle 2017, 2020; Bhullar et al. 2019; Jäger et
al. 2020).
This analysis also requires a smaller number of mechan-
ical assumptions regarding the generation and transmission
of jaw adductor tension compared with similar approaches,
and is likely robust to several of the major forms of un-mea-
surable error inherent to these methods (see also Ellis et al.
2008). Typical operational assumptions required by recent
jaw biomechanical analyses include: the equal and homoge-
nous activation of both working and balancing-side muscles
of mastication (Reed et al. 2016); a priori knowledge on the
orientation of the axes of rotation of the adducting mandi-
ble (Perry et al. 2011; Reed et al. 2016; Grossnickle 2017);
conjoint (identical) motion of both hemimandibles (Ellis et
al. 2008; Reed et al. 2016); and the restriction of output bite
vectors to the “occlusal-up” direction (Lautenschlager 2013;
Reed et al. 2016). Perhaps more importantly, this meth-
od’s use of digitized meshes as a fundamental data type
allows for the (condylar plane and bite plane) lever equation
parameter “output lever arm length” to be modeled as a
densely sampled independent variable. The capacity to com-
pute output forces corresponding to all vertices within the
lower tooth-row allows subtle contours within the general
monotonic increase in BF and decrease in JF distally to be
qualitatively and statistically contrasted between species
(e.g., Figs. 8, 9). From these results it is apparent that the
relationship between BF and JF-W/B estimates produced by
our method generally agrees with the theoretical predictions
of force distribution by Greaves (2012) in his treatment of
the jaw as a statically-loaded two-dimensional lever (see
Greaves 2012: fig. 1.5). Also matching these expectations,
Fig. 12. Comparison of orthal bite force (BF) distribution across lower tooth-row at closed gape (CG) and open gape (OG). A. Didelphis marsupialis
Linnaeus, 1758, a generalized mammal showing relatively low orthal BF magnitudes at both CG and OG. B. Ursus arctos Linnaeus, 1758, which shows
very little ability to preserve high orthal BF at high gape among the therians sampled, but has very high orthal BF at CG. C. Crocuta crocuta (Erxleben,
1777), a taxon capable of preserving a large amount of orthal BF at high gape, and showing similar orthal BF values at CG and OG. Color bar at right
is scaled to units of the combined (across-taxa) sample standard deviation in estimated orthal bite force values (generated using all muscle categories,
see text). Therefore, colors within the postcanine toothrow correspond to matching, size-scaled, orthal BF magnitudes across all sampled taxa (and in
Peligrotherium tropicalis as seen in Fig. 6A).

198 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022
the magnitude of each muscle category’s input force elicits
a subequal magnitude from the combined BF and JF (of
the corresponding side). However, it is not known if these
estimates of relative magnitude as a function of MDL would
allow reasonably approximate estimates of cumulative BF
and JF pressure by integrating areas under these empirical
MDL-magnitude curves (see Greaves 2012).
The geometric similarity between P. tropicalis and two
of the more omnivorous/herbivorous extant therians sampled
here strongly implies a functional similarity in the muscu-
lar activation and leverage produced during mastication in
these forms. Given the additional similarities in size and de-
gree of bunodonty between Peligrotherium, Sus, and many
Paleogene archaic ungulates, our results are unlikely to be the
product of random error. However, as with any novel method
the possibility of several forms of systematic errors in this
protocol do have to be considered. Large systematic errors
can be byproducts of overly simplistic model assumptions,
and for this report, the major assumption is that all non-geo-
metric aspects of jaw adductor myology correspond to a the-
rian (and in particular a “generalized therian”) pattern.
For instance, the locations of muscle attachments for the
superficial masseter, deep masseter, posterior temporalis,
and medial pterygoid muscle categories in P. tropicalis are
based on topological indicators influenced by our experi-
ence with extant therian mammals (especially Didelphis),
which is tantamount to assuming that P. tropicalis had a
distribution of masticatory muscles segregated into some-
thing approximating the major therian muscle groups. We
feel that this is a reasonable assumption because of the pre-
viously stated similarity between Peligrotherium and many
extant and Paleogene eutherian taxa (in which the inferred
presence of the major temporalis, medial pterygoid, and
masseter muscle groups are noncontroversial). This assump-
tion would have to be reconsidered if future evidence sup-
ports a more monotreme-like (Griffiths 1978) or otherwise
incomparable subdivision of the chewing musculature in
Peligrotherium and other stem-therian mammals.
These operational assumptions also extend to all sam-
pled extant taxa, because of the above-mentioned standard-
ized muscle categorizations (posterior temporalis, medial
pterygoid, superficial masseter, and deep masseter) and
B-WR imposed on all taxa. Our necessity of modeling the
masticatory apparatus using the simplest terms and most
convenient definitions is a byproduct of prior research into
the vertical kinematic phases of masticatory function being
phrased in this way, and the lack of pertinent EMG and
myological data availability for most extant mammalian
species. Additionally, the taxa with the most complex ten-
dinous subdivision of the masseter muscle mass (Equus
quagga and S. scrofa; Turnbull 1970; Herring and Scapino
1973) also show greater similarity with P. tropicalis using
our method. What effect the future incorporation of empir-
ical information of the internal pinnation of these muscles
would have on our similarity measures and significance
tests is not known.
Conclusions
The above caveats being mentioned, the results summarized
above still represent strong evidence for the Cretaceous–
Paleogene adaptation of non-therian mammals in South
America along similar trajectories to their therian con-
temporaries in the northern landmasses. Additionally, the
new 3D generalization of the bifulcral model provides a
widely applicable method for the estimation of masticatory
forces that involves fewer physical constraints and opera-
tional assumptions than previously published approaches.
We believe that this method will be particularly important
for elucidating the functional consequences of the increas-
ingly mediolateral orientations of muscular resultant vectors
during the evolution of mammalian mastication.
Finally, while in the above discussion we have used the
historically prevalent phrase “bite force” and similar terms,
we recommend that future studies in masticatory biome-
chanics use more disciplined terminology in discussions
of the mammalian postcanine dentition. In particular the
behavioral distinction between “biting” and “chewing”
(mastication) should be respected, and terms referencing
“bite force” or “biting force” should be reserved solely for
contact points within the canine and incisor tooth-row.
Mastication, unlike biting, for the most part is a process
which occurs unilaterally, within a specialized anatomical
region (the postcanine tooth-row), and is produced by dif-
fering, time-dependent, patterns of muscular recruitment.
In the future, terms like “occlusal force” or even “Group 2
occlusal force” will likely better reflect the actual phenom-
ena being studied, and reduce confusion.
Acknowledgements
As with the other contributions in this series, we would like to sin-
cerely thank Richard Cifelli (Oklahoma Museum of Natural History,
Norman, USA) for his years of research on many South American and
Mesozoic mammalian lineages (Cifelli 1985 being just one such ex-
ample). So much of his work has proven foundational to our field that
we would have a hard time finding parts of this report that are not
influenced by it. We are also extremely grateful for the time and talent
donated for this project by Amy Bishop (Lincoln Memorial University
DeBusk College of Osteopathic Medicine, Knoxville, USA), who pro-
duced the detailed pen and ink reconstruction of Peligrotherium seen
in Fig. 1A. TH would also like to thank the many vertebrate paleon-
tologists/functional morphologists who have donated their time and
attention to the conceptual/computational aspects of this project, es-
pecially Jonathan Perry (Johns Hopkins University, Baltimore, USA),
Calum Ross (Uni versity of Chicago, USA), Brian Davis (University of
Louisville, USA), Jillian Davis (West Virginia University, Morgantown,
USA). Roger Benson and Roberto Portela Miguez (both Museum of
Natural History, London, UK), Marco Ansón (Paleoart, Madrid, Spain),
Joseph Frederickson (Weis Earth Science Museum, University of
Wisconsin, Menasha, USA), and Matt Wedel (Western University of
Health Sciences, Ponoma, USA) have also provided invaluable data
and expertise in the production of this report. The Idaho Museum of
Natural History provided access to these data; the collection of which
was funded by Rick Carron Foundation.

HARPER ET AL.—MASTICATION IN PALEOCENE MERIDIOLESTIDAN MAMMAL FROM ARGENTINA 199
Collecting, processing, and curating early mammals is a demanding
task that would have not been possible without the help of numerous
other colleagues and students. In particular we want to thank Leandro A.
Canessa (Museo Egidio Feruglio, Trelew, Argentina) who found the best
specimens of Peligrotherium tropicalis and Pablo Puerta (Museo Egidio
Feruglio), Analia Forasiepi (Conicet CRICYT, Mendoza, Argentina),
Agustín Martinelli (Conicet, Museo Argentino de Ciencias Naturales,
Buenos Aires, Argentina), and Raul Gomez (Conicet, UBA, Buenos
Aires, Argentina) for additional help in collecting the material, curating
it and organizing the field work. We thank Eduardo Ruigómez and the
rest of the staff at the Museo Egidio Feruglio as well, for their help with
the collection over the years. Finally, we would like to sincerely thank
Adam Hartstone-Rose (North Carolina State University, Raleigh, USA),
David Grossnickle (University of Washington, Seattle, USA), and
Olivier Lambert (Royal Belgian Institute of Natural Sciences, Brussels,
Belgium) for their time and insight while reviewing this manuscript.
Partial funding for this project was provided by the Department of
Anatomical Sciences and Neurobiology, University of Louisville sup-
ported and PICT 2016 2682 GWR.
References
Anapol, F. and Herring, S.W. 1989. Length-tension relationships of masse-
ter and digastric muscles of miniature swine during ontogeny. Journal
of Experimental Biology 143: 1–16.
Andreis, R.R. 1977. Geología del área de Cañadón Hondo, Departamento
Escalante, Provincia del Chubut, República Argentina. Obra del Cen-
tenario del Museo de La Plata 4: 77–102.
Averianov, A.O., Martin, T., and Lopatin, A.V. 2013. A new phylogeny for
basal Trechnotheria and Cladotheria and affinities of South American
endemic Late Cretaceous mammals. Naturwissenschaften 100: 311–326.
Beddard, F.E. and Treves, F. 1889. On the anatomy of Rhinoceros suma-
trensis. Proceedings of the Zoological Society of London 57: 7–25.
Bhullar, B.S., Manafzadeh, A.R., Miyamae, J.A, Hoffman E.A., Brainerd,
E.L., Musinsky, C., and Crompton, A.W. 2019. Rolling of the jaw is
essential for mammalian chewing and tribosphenic molar function.
Nature 566: 528–532.
Boddaert, P. 1785. Elenchus Animalium, Vol. 1. 174 pp. Rotterodami Apud
C.R. Hake, Rotterdam. [available online, https://doi.org/10.5962/bhl.
title.40283]
Bonaparte, J.F. 1986. Sobre Mesungulatum houssayi y nuevo mamiferos
Cretácicos de Patagonia, Argentina. Actas IV Congreso Argentino de
Paleontología y Biostratigraphia 2: 48–61.
Bonaparte, J.F. 1990. New Late Cretaceous mammals from the Los Alam-
itos Formation, northern Patagonia. National Geographic Research 6:
63–83.
Bonaparte, J.F., Van Valen, L.M., and Kramarz, A. 1993. La fauna local de
Punta Peligro, Paleoceno inferior, de la provincia del Chubut, Patago-
nia, Argentina. Evolutionary Monographs14: 1–61.
Boyer, D.M. 2008. Relief index of second mandibular molars is a correlate
of diet among prosimian primates and other euarchontan mammals.
Journal of Human Evolution 55: 1118–1137.
Bramble, D.M. 1978. Origin of the mammalian feeding complex: models
and mechanisms. Paleobiology 4: 271–301.
Bramble, D.M. and Wake, D.B. 1985. Feeding mechanisms of lower tetra-
pods. In: M. Hilderbrand, D.M. Bramble, K.F. Liem, and D.B. Wake
(eds.), Functional Vertebrate Morphology, 230–261. Harvard Univer-
sity Press, Cambridge.
Bressou, C. 1961. La myologie du tapir (Tapirus indicus L.). Mammalia
25: 358–400.
Buckland-Wright, J.C. 1969. Craniological observations on Hyaena and
Crocuta (Mammalia). Journal of Zoology 159: 17–29.
Bunn, J.M., Boyer, D.M., Lipman, Y., St. Clair, E.M., Jernvall, J., and
Daubechies, I. 2011. Comparing Dirichlet normal surface energy of
tooth crowns, a new technique of molar shape quantification for di-
etary inference, with previous methods in isolation and in combina-
tion. American Journal of Physical Anthropology 145: 247–261.
Cifelli, R.L. 1985. South American ungulate evolution and extinction.
In: F.G. Stehli and D. Webb (eds.) The Great American Biotic Inter-
change, 249–266. Springer, Boston.
Close, R.I. 1972. Dynamic properties of mammalian skeletal muscles.
Physiological reviews 52: 129–197.
Crompton, A.W. 1995. Masticatory function in nonmammalian cynodonts
and early mammals. In: J.J. Thomason (ed.), Functional Morphology
in Vertebrate Paleontology, 55–75. Cambridge University Press, Cam-
bridge.
Crompton, A.W. 2011. Masticatory motor programs in Australian herbiv-
orous mammals: Diprotodontia. Integrative and Comparative Biology
51: 271–281.
Crompton, A.W. and Hiiemae, K. 1970. Molar occlusion and mandibu-
lar movements during occlusion in the American opossum, Didelphis
marsupialis L. Zoological Journal of the Linnean Society 49: 21–47.
Crompton, A.W. and Hylander, W.L. 1986. Changes in mandibular func-
tion following the acquisition of a dentary-squamosal jaw articulation.
In: N. Hotton, P.D. MacLean, J.J. Roth, and E.C. Roth (eds.), The
Ecology and Biology of Mammal-like Reptiles, 263–282. Smithsonian
Institution Press, Washington.
Crompton, A.W., Lieberman, D.E., and Aboelela, S. 2006. Tooth orienta-
tion during occlusion and the functional significance of condylar trans-
lation in primates and herbivores. In: M.T. Carrano, T.J. Gaudin, RW.
Blob, and J.R. Wible (eds.), Amniote Paleobiology: Perspectives on
the Evolution of Mammals, Birds, and Reptiles, 367–388. The Univer-
sity of Chicago Press, Chicago.
Crompton, A.W., Owerkowicz, T., and Skinner, J. 2010. Masticatory mo-
tor pattern in the koala (Phascolarctos cinereus): a comparison of
jaw movements in marsupial and placental herbivores. Journal of
Experimen tal Zoology Part A: Ecological Genetics and Physiology
313: 564–578.
Crompton, A.W., Wood, C.B., and Stern, D.N. 1994. Differential wear of
enamel: A mechanism for maintaining sharp cutting edges. In: V.L.
Bels, M. Chardon, and P. Vandewalle (eds.), Advances in Comparative
and Environmental Physiology 18, 321–346. Springer-Verlag, Berlin.
Csardi, G. and Nepusz, T. 2006. The igraph software package for complex
network research. Complex Systems 1695: 1–9.
Davis, D.D. 1955. Masticatory apparatus in the spectacled bear, Tremarc-
tos ornatus. Fieldiana: Zoology 37: 25–46.
Davis, D.D. 1964. The Giant Panda: A morphological study of evolution-
ary mechanisms. Fieldiana Zoology Memoirs 3: 1–339.
Davis, J.L., Santana, S.E., Dumont, E.R., and Grosse, I.R. 2010. Predicting
bite force in mammals: two-dimensional versus three-dimensional le-
ver models. Journal of Experimental Biology 213: 1844–1851.
Druzinsky, R.E., Doherty, A.H., and De Vree, F.L. 2011. Mammalian mas-
ticatory muscles: Homology, nomenclature, and diversification. Inte-
grative and Comparative Biology 51: 224–234.
Ellis, J.L., Thomason, J.J., Kebreab, E., and France, J. 2008. Calibration of
estimated biting forces in domestic canids: comparison of post‐mor-
tem and in vivo measurements. Journal of Anatomy 212: 769–780.
Endo, H., Taru, H., Yamamoto, M., Arishima, K., and Sasaki, M. 2003.
Comparative morphology of the muscles of mastication in the giant
panda and the asiatic black bear. Annals of Anatomy–Anatomischer
Anzeiger 185: 287–292.
Erxleben, J.C.P. 1777. Systema regni animalis per classes, ordines, genera,
species, varietates cvm synonymia et historia animalivm. Classis I.
Mammalia. 754 pp. Impensis Weygandianis, Leipzig.
Evans, H.E. and De Lahunta, A. 2013. Miller’s Anatomy of The Dog,
Fourth Edition. 850 pp. Saunders, China.
Fricano, E.E. and Perry, J.M. 2019. Maximum Bony Gape in Primates. The
Anatomical Record 302: 215–225.
Gelfo, J.N. and Pascual, R. 2001. Peligrotherium tropicalis (Mammalia,
Dryolestida) from the early Paleocene of Patagonia, a survival from a
Mesozoic Gondwanan radiation. Geodiversitas 23: 369–379.
Gill, P.G., Purnell, M.A., Crumpton, N., Brown, K.R., Gostling, N.J.,
Stampanoni, M., and Rayfield, E.J. 2014. Dietary specializations and

200 ACTA PALAEONTOLOGICA POLONICA 67 (1), 2022
diversity in feeding ecology of the earliest stem mammals. Nature 512:
303–305.
Greaves, W.S. 1995. Functional predictions from theoretical models of the
skull and jaws in reptiles and mammals. In: J.J. Thomason (ed.), Func-
tional Morphology in Vertebrate Paleontology, 99–115. Cambridge
University Press, Cambridge.
Greaves, W.S. 2012. The Mammalian Jaw: A Mechanical Analysis. 114 pp.
Cambridge University Press, Cambridge.
Griffiths, M. 1978. The Biology of The Monotremes. 367 pp. Academic
Press, New York.
Grossnickle, D.M. 2017. The evolutionary origin of jaw yaw in mammals.
Scientific reports 7: 1–13.
Grossnickle, D.M. 2020. Jaw roll and jaw yaw in early mammals. Nature
582: E6–E8.
Grossnickle, D.M., Weaver, L.N., Jäger, K.R.K., and Schultz, J.A. 2021.
The evolution of anteriorly directed molar occlusion in mammals. Zo-
ological Journal of the Linnean Society [published online https://doi.
org/10.1093/zoolinnean/zlab039].
Gorniak, G.C. 1986. Architecture of the masticatory apparatus in eastern
rac coons (Procyon lotor lotor). American Journal of Anatomy 176:
333–351.
Gould, S.J. and Vrba, E.S. 1982. Exaptation–a missing term in the science
of form. Paleobiology 8: 4–15.
Harper, T., Parras, A., and Rougier, G.W. 2019. Reigitherium (Meridioles-
tida, Mesungulatoidea) an enigmatic Late Cretaceous mammal from
Patagonia, Argentina: Morphology, affinities, and dental evolution.
Journal of Mammalian Evolution 26: 447–478.
Hartstone‐Rose, A., Perry, J.M., and Morrow, C.J. 2012. Bite force estima-
tion and the fiber architecture of felid masticatory muscles. The Ana-
tomical Record 295: 1336–1351.
Herring, S.W. and Scapino, R.P. 1973. Physiology of feeding in miniature
pigs. Journal of Morphology 141: 427–460.
Hiiemae, K. and Jenkins, F.A. 1969. The anatomy and internal architecture
of the muscles of mastication in Didelphis marsupialis. Postilla 140:
1–49.
Hiiemae, K.M. and Crompton, A.W. 1985. Mastication, food transport and
swallowing. In: M. Hilderbrand, D.M. Bramble, K.F. Liem, and D.B.
Wake (eds.), Functional Vertebrate Morphology, 262–290. Harvard
University Press, Cambridge.
Hylander, W.L. 1979. The functional significance of primate mandibular
form. Journal of Morphology 160: 223–239.
Hylander, W.L. and Johnson, K.R. 1994. Jaw muscle function and wish-
boning of the mandible during mastication in macaques and baboons.
American Journal of Physical Anthropology 94: 523–547.
Jäger, K.R.K., Cifelli, R.L., and Martin T. 2020. Molar occlusion and jaw
roll in early crown mammals. Scientific Reports 10: 1–12.
Kay, R.F. and Hiiemae, K.M. 1974. Jaw movement and tooth use in recent
and fossil primates. American Journal of Physical Anthropology 40:
227–256.
Kielan-Jaworowska, Z., Cifelli, R.L., and Luo, Z.X. 2005. Mammals From
The Age of Dinosaurs: Origins, Evolution, and Structure. 630 pp. Co-
lumbia University Press, New York.
Kneepkens, A.F.L.M. and MacDonald, A.A. 2010. Cranial muscles of the
Sulawesi Babirusa (Babyrousa celebensis). Anatomia Histologia Em-
bryologia: Journal of Veterinary Medicine 39: 120–137.
Kristjanson, H.L. 2019. Tupaiid Masticatory Anatomy and The Applica-
tion of Extant Analogs to Reconstructing Plesiadapiform Jaw Ad-
ductors. 331 pp. Doctoral dissertation, Johns Hopkins University,
Baltimore.
Lautenschlager, S. 2013. Cranial myology and bite force performance of
Erlikosaurus andrewsi: a novel approach for digital muscle recon-
structions. Journal of Anatomy 222: 260–272.
Lautenschlager, S., Gill, P., Luo, Z.X., Fagan, M.J., and Rayfield, E.J.
2017. Morphological evolution of the mammalian jaw adductor com-
plex. Biological Reviews 92: 1910–1940.
Lillegraven, J.A., Kielan-Jaworowska, Z., and Clemens, W.A. (eds.) 1979.
Mesozoic Mammals: the First Two-thirds of Mammalian History 311
pp. University of California Press, Berkeley.
Link, H.F. 1795. Ueber die Lebenskräfte in naturhistorischer Rücksicht
und die Classification der Säugethiere. 127 pp. Stiller, Rostock.
Linnaeus, C. 1758. Systema naturæ per regna tria naturæ, secundum
classes, ordines, genera, species, cum characteribus, differentiis, syn-
onymis, locis. Tomus I. Editio decima, reformata. 824 pp. Laurentius
Salvius, Stockholm.
Lucas, P.W. 2004. Dental Functional Morphology: How Teeth Work. 355 pp.
Cambridge University Press, Cambridge.
Marshall, L.G., Sempere, T., and Butler, R.F. 1997. Chronostratigraphy of
the mammal-bearing Paleocene of South America. Journal of South
American Earth Sciences 10: 49–70.
Martinelli, A.G., Soto-Acuña, S., Goin, F.J., Kaluza, J., Bostelmann, E.,
Fonseca, P.H.M., Reguero, M.A., Leppe, M., and Vargas, A.O. 2021.
New cladotherian mammal from southern Chile and the evolution of
mesungulatid meridiolestidans at the dusk of the Mesozoic era. Scien-
tific Reports 11: 1–18.
Martín-Serra, A. and Benson, R.B.J. 2020. Developmental constraints do
not influence long-term phenotypic evolution of marsupial forelimbs
as revealed by interspecific disparity and integration patterns. The
American Naturalist 195: 547–560.
McHenry, C.R., Wroe, S., Clausen, P.D., Moreno, K., and Cunningham, E.
2007. Supermodeled sabercat, predatory behavior in Smilodon fatalis
revealed by high-resolution 3D computer simulation. Proceedings of
the National Academy of Sciences 104: 16010–16015.
Meng, J., Hu, Y., Wang, Y., Li, C. 2003. The ossified Meckel’s cartilage and
internal groove in Mesozoic mammaliaforms: Implications to origin of
the definitive mammalian middle ear. Zoological Journal of the Linnean
Society 138: 431–448.
Paéz-Arango, N.P. 2008. Dental and Craniomandibular Anatomy of
Peligrotherium tropicalis: The Evolutionary Radiation of South Amer-
ican Dryolestoid Mammals. 116 pp. Master’s Thesis, University of
Louisville, Louisville.
Perry, J.M., Hartstone-Rose, A., and Logan, R.L. 2011. The jaw adductor
resultant and estimated bite force in primates. Anatomy Research In-
ternational 2011: 1–11.
Reed, D.A., Iriarte‐Diaz, J., and Diekwisch, T.G.H. 2016. A three dimen-
sional free body analysis describing variation in the musculoskeletal
configuration of the cynodont lower jaw. Evolution & Development
18: 41–53.
Ross, C.F., and Iriarte-Diaz, J. 2019. Evolution, constraint, and optimality
in primate feeding systems. In: V. Bels and I.Q. Whishaw (eds.) Feed-
ing in Vertebrates, 787–829. Springer, Cham.
Rougier, G.W., Apesteguía, S., and Gaetano, L.C. 2011. Highly specialized
mammalian skulls from the Late Cretaceous of South America. Nature
479: 98–102.
Rougier, G.W., Forasiepi, A.M., Hill, R.V., and Novacek, M. 2009. New
mammalian remains from the Late Cretaceous La Colonia Formation,
Patagonia, Argentina. Acta Palaeontologica Polonica 54: 195–212.
Rougier, G.W., Turrazzinni G.F., Cardozo, M.S., Harper, T., Lires, A.I.,
and Canessa, L.A. 2021. New specimens of Reigitherium bunodontum
from the La Colonia Formation, Patagonia, Argentina and meridioles-
tidan diversity in South America. Journal of Mammalian Evolution
28: 1051–1081.
Rougier, G.W., Wible, J.R., Beck, R.M., and Apesteguía, S. 2012. The
Miocene mammal Necrolestes demonstrate the survival of a Mesozoic
nontherian lineage into the late Cenozoic of South America. Proceed-
ings of the National Academy of Science 109: 20053–20058.
Santana, S.E., Dumont, E.R., and Davis, J.L. 2010. Mechanics of bite force
production and its relationship to diet in bats. Functional Ecology 24:
776–784.
Schroeder, W.J., Martin, K., and Lorensen, B. 2006. The Visualization
Toolkit: An Object–Oriented Approach to 3D Graphics: 4th Edition.
512 pp. Kitware, Clifton Park.
Schultz, J.A. and Martin, T. 2014. Function of pretribosphenic and tribos-
phenic mammaloian molars inferred from 3D animation. Naturwissen-
schaften 101: 771–781.
Sutton, M.D., Rahman, I.A., and Garwood, R. 2016. Virtual paleontolo-
gy—an overview. Paleontological Society Papers 22: 1–20.
Tseng, Z.J., Grohé, C., and Flynn, J.J. 2016. A unique feeding strategy of

HARPER ET AL.—MASTICATION IN PALEOCENE MERIDIOLESTIDAN MAMMAL FROM ARGENTINA 201
the extinct marine mammal Kolponomos: convergence on sabretooths
and sea otters. Proceedings of the Royal Society B: Biological Sciences
283: 1–8.
Turnbull, W.D. 1970. Mammalian masticatory apparatus. Fieldiana: Geo-
logy 18: 149–356.
Vinyard, C.J., Williams, S.H., Wall, C.E., Doherty, A.H., Crompton, A.W.,
and Hylander, W.L. 2011. A preliminary analysis of correlations be-
tween chewing motor patterns and mandibular morphology across
mammals. Integrative and Comparative Biology 51: 260–270
Wall, C.E. and Krause, D.W. 1992. A biomechanical analysis of the masti-
catory apparatus of Ptilodus (Multituberculata). Journal of Vertebrate
Paleontology 12: 172–187.
Weijs, W.A. 1994. Evolutionary approach of masticatory motor patterns in
mammals. In: V.L. Bels, M. Chardon, and P. Vandewalle (eds.), Biome-
chanics of Feeding in Vertebrates, 281–320. Springer, Berlin.
Werneburg, I. 2013. The tendinous framework in the temporal skull region
of turtles and considerations about its morphological implications in
amniotes: a review. Zoological Science 30: 141–153.
Wible, J.R. and Rougier, G.W. 2017. Craniomandibular anatomy of the
subterranean meridiolestidan Necrolestes patagonensis Ameghino,
1891 (Mammalia, Cladotheria) from the early Miocene of Patagonia.
Annals of Carnegie Museum 84: 183–252.
Williams, S.H., Wall, C.E., Vinyard, C.J., and Hylander, W.L. 2008. Sym-
physeal fusion in selenodont artiodactyls: New insights from in vivo
and comparative data. In: C. Vinyard, M.J. Ravosa, and C. Wall (eds.),
Primate Craniofacial Function and Biology, 39–61. Springer, Boston.
Woodburne, M.O. 1968. The cranial myology and osteology of Dicotyles
tajacu, The Collared Peccary, and its bearing on classification. Mem-
oirs of The Southern California Academy of Sciences 7: 1–48.
Zhou, Z., Winkler, D.E., Fortuny, J., Kaiser, T.M., and Marcé-Nogué, J.
2019. Why ruminating ungulates chew sloppily: Biomechanics discern
a phylogenetic pattern. PloS One 14: 1–21.