FINITE ELEMENTS ANALYSIS OF SLOTH'S MANDIBLES SHOWS DIFFERENCES IN ECOMORPHOLOGICAL ADAPTATIONS AMONG TAXA

1. FINITE ELEMENTS ANALYSIS OF SLOTHS’
MANDIBLES SHOWS DIFFERENCES IN
ECOMORPHOLOGICAL ADAPTATIONS
AMONG TAXA
L. Varela1,2* , P.S. Tambusso1,2, J.M. Pérez Zerpa3,
R.K. McAfee4, and R.A. Fariña1,2
1Departamento de Paleontología, Facultad de Ciencias, Universidad de la República,
Iguá 4225, 11400, Montevideo, Uruguay
2Servicio Académico Universitario y Centro de Estudios Paleontológicos (SAUCE-P),
Universidad de la República, Departamento de Canelones, Santa Isabel s/n, 91500,
Sauce, Uruguay
3Instituto de Estructuras y Transporte, Facultad de Ingeniería, Universidad de la
República
4Department of Biomedical Sciences, Philadelphia College of Osteopathic Medicine -
Georgia Campus, Suwanee, Georgia, USA
*luciano.lvr@gmail.com

2. The fossil record of
sloths
Sloths are a group of
xenarthrans represented
today only by two distantly-
related extant genera of
arboreal and folivorous
small mammals, Choloepus
and Bradypus.
However, the fossil record of
the clade is composed of
many more taxa, with a
much more diverse
morphology, including giant
terrestrial forms with no
clear modern analogs.
Extant sloth Choloepus.
Extinct sloth Lestodon, showing the large size attained by some members
of the clade and the clearly different locomotory habits of the fossil forms.
2
Genera diversity curve obtained from data in
Paleobiology Database.

3. The dentition of
sloths
Xenarthrans differ from the rest of the mammalian clades by the
individual morphology and number of teeth (McDonald 2003).
When present, teeth in most adult xenarthrans lack enamel and
are usually homodont, hypselodont, tubular, and primarily
composed of orthodentine and vasodentine, which makes it
difficult to identify homologies with the teeth and cusps of other
mammals (Vizcaíno 2009, Hautier et al. 2016). In sloths, the
dentition is reduced to a maximum of five upper and four lower
teeth, with caniniforms (cf), teeth resembling canines, present in
some taxa.
3
Mandibles showing the large size caniniform (cf) present in
some taxa and the rest of the teeth with molariform (mf)
morphology.

4. The study of fossil
sloths dietary
preferences
In this context, several approaches have been
implemented to explore the ecological adaptations of
fossil taxa and, in particular, their dietary preferences.
Some examples are shown on the right.
In this work we used Finite Elements Analysis (FEA) in
order to explore the ecomorphology of sloths and the
possible differences among taxa related to dietary
adaptations.
Most sloths are interpreted to have been herbivorous
animals. However, some taxa have been previously
interpreted as selective browsers, while others have
been suggested to be grazers (Bargo & Vizcaíno 2008).
Furthermore, some fossil sloths have been proposed to
be opportunistic omnivores, potentially feeding on
animal matter, perhaps as carrion-eaters (Fariña 1996;
Tejada et al. 2021).
Jaw biomechanics
(e.g., Bargo & Vizcaíno 2008)
Coprolites analysis
(e.g., van Geel et al. 2021)
Microwear analysis of teeth
surface
(e.g., Kalthoff & Green 2018)
Stable Isotopes analysis
(e.g., Larmon et al. 2019)
4

5. 3D data
acquisition
• 3D models of the mandibles of 11 taxa were
obtained (two extant, Choloepus and Bradypus, and
nine extinct, Acratocnus, Neocnus, Megatherium,
Megalonyx, Scelidotherium, Mylodon, Paramylodon,
Glossotherium, and Lestodon) representing members
of all the major clades within the group.
• The models represent 3D surface data and were acquired
using a combination of structured light and
photogrammetry techniques. The resulting surface meshes
where then processed in Meshlab and transformed in
tetrahedral solid meshes using TetGen. The resulting
meshes were composed of ~300k elements.
• For specimens with missing teeth or parts of the
mandible, we reconstructed missing parts using
bilateral counterparts when available. In a small
number of cases, missing teeth were reconstructed
based on photographs of other specimens.
5
3D surface model of the mandible of Lestodon showing
missing right tooth and left coronoid, which were
reconstructed in Meshlab using bilateral counterparts.

6. Finite Elements
Models
• We modeled the three major muscles involved in mastication (masseter, temporalis,
and pterygoid), Insertion areas of the muscles were determined based on
observation on fossil materials and published research. The force input was
calculated by multiplying the muscles insertion areas and the maximum tension
produced by mammalian muscle fibers (0.3 N/mm2; Wroe et al. 2005). We simulated
unilateral mastication (the balancing-side muscle forces were adjusted to 60%).
• Since material properties are not know for the modeled taxa, we used commonly
used values for mammalian bone in the literature (Young’s modulus = 19 GPa;
Poisson ratio = 0.3). Considering that we are interested in the general response
of the mandible, we modeled it as homogeneous, not considering teeth as
separate structures with different properties. Also, since sloths present a
completely fused symphysis, this part was not given different properties.
• For the finite elements model, we used the open
software FEBio (Maas et al. 2012).
• We set the following boundary conditions using three nodal constraints: left and right temporomandibular joints (TMJ;
preventing all translational movement except in the axis of the joint “z” on the balancing side) and bite point
(preventing translational movement int the vertical “y” axis). We modeled four different conditions, one for each tooth
along the toothrow (Bt1, Bt2, Bt3, and Bt4).
6
Coordinate system and boundary conditions used in
the models.

7. Qualitative results
- The results were analyzed
qualitatively regarding the
distribution of von Mises
stress (vMs).
- The phylogeny represents
the more recent results
obtained using molecular
data (Delsuc et al. 2019).
Filogenia con vMs
7

8. Quantitative results
Average vMs
Adjusted Strain Energy (SE)
vs.
Mechanical Efficiency (ME)
8
• The Adjusted Strain Energy (SE) was obtained following Dumont et al. (2009), using
Choloepus as standard, as:
SEx,adjusted = (Volumex/VolumeCholoepus)1/3(InputLoadCholoepus/InputLoadx)2*SEx

9. Dietary adaptations
Considerable differences in the distribution of
high-vMs areas and SE values were found
among taxa
 Mylodontid taxa generally considered to be grazers, like
Glossotherium and Lestodon, showed smaller high-vMs
areas and lower average vMs and SE values when biting with
posterior teeth, consistent with the requirements of grass
processing.
 Other mylodontids, such as Mylodon, Paramylodon, and,
especially, Scelidotherium, showed higher vMs and SE
values, consistent with the consumption of less hard foods
and more browser habits.
 The two extant sloths showed results consistent with their known
diets. Bradypus showed less stiff (higher SE) mandibles, consistent
with a diet based almost completely on leaves. On the other hand,
Choloepus showed a stiffer (lower SE) mandible, which could
correspond with the more variable diet known for this sloth.
 Great Antilles taxa, Acratocnus and Neocnus, and Megalonyx
showed results consistent with the processing of hard food objects
(lower vMs and SE values), which does not corresponds with
previous findings indicating mostly browsing habits. However, our
results could indicate the processing of harder objects like roots
and tubers, something that was previously suggested by Antúnez &
Suárez (2013) based on coprolites analyses.
 Megatherium showed the highest vMs and SE values among the
studied taxa, indicating a mandible less efficient for the processing
of hard food objects. These results would be consistent with diets
composed of fleshy food, like succulent plants and fruits, but also
with previous predictions of meat consumption.
9

10. Caniniform function
We observed considerably higher vMs and SE values in taxa
with caniniform, which could indicate its involvement in
sexual display rather than in food processing or more
strenuous activities
 Taxa with caniniform showed
much higher SE values when
biting with the caniniform than
when biting with molariforms.
 These results show that, when
biting with the large caniniform
present in some taxa, the
mandible showed higher SE
values, which would render it
less efficient for biting hard
objects.
 These results show that, besides
diet, the presence of such
attributes as potentially sexually
selected large caniniforms (Varela
et al. 2021) in some taxa, might
also be an influence on the
mechanical response of the
mandible when biting.
10

11. Summary
• We found considerable differences in
the distribution of high-vMs areas
and SE values, which could be
related to dietary adaptations.
• We made 3D FEA models for several
fossil sloths.
• We observed considerably higher
vMs and SE values when biting with
the caniniform (in taxa with
caniniform), which indicate less
efficiency to process food objects or
perform strenuous activities with
these structures.
The diagram shows a displacement magnitude plot,
for a Lestodon mesh formed by 1500 nodes. The
constitutive behavior assumed is nonlinear elastic,
while a few nodes are fixed and one node is loaded.
• Ongoing and future work
• Sensitivity analyses
• Phylogenetic Comparative Methods
• Adding more taxa
• Modeling soft tissues
• Implementation on a FEA software developed in Uruguay
11
A preliminary analysis was done
using ONSAS (www.onsas.org),
an Open Nonlinear Structural
Analysis Solver developed by
researchers at the School of
Engineering in Universidad de la
República, with collaborators
from universities in Europe and
South America.

12. Thank you
Luciano Varela
luciano.lvr@gmail.com