Morphology of the Bony Labyrinth of Select Species of the Superorder Xenarthra
Christopher T Wilkinson | Joe Sertich PhD
MSMHA MS Candidate 2015 | DMNS
Background & Rationale
Figure 1. The currently accepted phylogenetic
relationships among Xenarthra. The genera
Choloepus and Bradypus are highlighted in
pink. Other genera looked at in this study are
highlighted in green. Figure adapted from
Gaudin, 2003
• Sloths, Anteaters, and Armadillos are all part of the superorder
Xenarthra
• There are only two extant tree sloth genera (Bradypus and
Choloepus)
• Distantly related, (Fig. 1) but share many traits:
• Appearance
• Shoulder morphology7
• Habitat9
• Inner Ear structure2
• Previous studies have established adaptations specific to the inner ear
of extant tree sloths1,2
• However, they don’t resemble the inner ear of other extant
Xenarthrans
• A recent study looked at Megalonyx jeffersoni (an extinct ground
sloth) to examine whether the tree sloth inner ear morphology was
shared with extinct ground sloths1
• The inner ear of Megalonyx jeffersoni more resembles the inner
ear of an anteater than that of a tree sloth (either genera)1
• Goals of this study:
• Increase the number of models of Xenarthran inner ears
• Take a closer look at the inner ear morphology of supposed ground
sloths
• Hypothesis- The inner ear morphology of tree sloths is unique
among Xenarthrans.
Project Approach
• Obtained CT scans of Xenarthran skulls and
isolated temporal bones currently in collections at
the Denver Museum of Nature and Science
• Modeled 46 bony labyrinths
• The Half Maximum Height method was used to
estimate the border between air and bone3,5,8
• Used these HMH values to segment out the bony
labyrinth in Avizo4
• Fig. 2 illustrates landmarks as they appear in
coronal slices
• All bony labyrinths were modeled by the same
observer
• Measurements were made in Geomagic Design
X 64*
• A first PCA was conducted looking at all
measured traits
• A second PCA was conducted looking at
• the relationship between ampulla and canal
diameter (3 measurements)
• the relationship between canal size and total
inner ear size (3 measurements)
*All measurements can be found in the electronic
supplementary materials along with additional
graphics displaying how measurements were made in
Geomagic
Figure 2. (left) This figure displays select slices from a sequence of scans of the right
inner ear of a Tamandua mexicana. The number in the lower right corner of each screen
capture is the slice number and the labels correspond to landmarks as they appear in CT
cross sections. All of these images are taken from coronal sections and the full bony
labyrinth in this specimen was more than 250 slices. IAM-Internal acoustic meatus. SSC
superior semicircular canal. SSCa-Superior semicircular canal ampulla. LSC lateral
semicircular canal. LSCa lateral semicircular canal ampulla.PSC-posterior semicircular
canal. PSCa-posterior semicircular canal ampulla.
Results
Results
• 26 CT scans
Scans of the temporal region were taken for each
specimen(Fig. 3)
• 46 inner ear models
Both inner ears were modeled whenever possible (Fig. 3)
• A visual comparison shows the specimens’ bony labyrinths
falling in one of two categories (Fig. 4)
• Tree sloth ear type
• Canals that vary greatly from an orthogonal orientation
• Canals whose cross sectional diameter is close to the cross
sectional diameter of their ampulla
• Canals whose overall size is small relative to the inner ear
size (RATIO).
• Anteater ear type
• Canals that are roughly orthogonal
• Canals that are much smaller in cross sectional diameter
then their corresponding ampulla
• Canals whose overall size is large relative to the total inner
ear size.
• The first PCA (Fig. 7 A) was run using
twelve factors (SSCR, PSCR, LSCR, LSC/
PSC angle, LSC/SSC angle, SSC/PSC angle,
LSC c/a ratio, PSC c/a ratio, SSC c/a ratio,
IEH, Coc turns, and CocR)
• Divided the specimens into four groups
• tree sloths
• ground sloths
• anteaters
• armadillos
• The tree sloths separated from the other
Xenarthrans with only a slight overlap
• The second PCA (Fig. 7 B) used only six
factors, looking specifically at the size of the
semicircular canals (LSCR, SSCR, PSCR,
LSC c/a ratio, PSC c/a ratio, and SSC c/a
ratio).
• This separates the tree sloths even more
clearly
• The three leftmost points within the ground
sloth grouping are all Acratocnus
B
Figure 3. (Right )The bony labyrinth of a Choloepus hoffmani within a transparent cranium.
Only half of the skull was scanned to obtain the highest resolution possible. The top picture
shows posterior view of the cranium and the bottom picture shows a right lateral view.
A
D
A
Figure 7. Part A shows the PCA using twelve factors while part B looks
specifically at factors related to SSC size
Conclusions
• The tree sloth inner ear is represented by a distinct set of adaptations
• Acratocnus (classified as a ground sloth) shares these adaptations
• Acratocnus likely exhibits a locomotor repertoire similar to extant tree sloths
References & Acknowledgements
B
1) Billet G, Germain D, Ruf I, de Muizon C, and Hautier L. 2013. The inner ear of Megatherium and the evolution of the vestibular system in sloths. J. Anat. 557-567.
2) Billet G, Hautier L, Asher R. 2012. High morphological variation of vestibular system accompanies slow and infrequent locomotion in three-toed sloths. Proc Biol
Sci 279: 3932-3939.
E
C
3) Coleman M and Colbert M. 2007. Technical note: CT thresholding protocols for taking measurements on three-dimensional models. Am J Phys Anthropol 133:
723-725.
4) Ekdale E. 2013. Comparative anatomy of the bony labyrinth (inner ear) of placental mammals. PLoS ONE 8: 1-100.
5) Fajardo R, Ryan T, and Kappelman J. 2002. Assessing the accuracy of high-resolution X-ray commuted tomography of primate trabecular bone in comparisons with
histological sections. Am J Phys Anthropol 118: 1-10.
6) Gaudin T. 2003. Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence. Zool J Linn Soc 255-305.
7) Nyakatura J, and Fischer M. 2011. Functional morphology of the muscular sling at the pectoral girdle in tree sloths: convergent morphological solutions to new
functional demands. J. Anat 219: 360-374.
8) Spoor C, Zonneveld F, and Macho G. 1993. Linear measurements of cortical bone and dental enamel by computed tomography: applications and problems. Am J
Phys Anthropol 91: 469-484.
9) Superina M. 2006. Biology of Xenarthra. Edentata 7:59-60.
Figure 4. This figure depicts the bony labyrinth models created for five different genera. The specimens grouped on the left have the tree sloth characteristics, those on the
right hand side have the anteater type characteristics. The species depicted are: Choloepus hoffmani (A), Bradypus variegatus (B), Acratocnus (C), Megalonyx jeffersoni
(D), and Tamandua Mexicana (E).
The authors thank Kendra Huber and Ramesh Karki for assisting with data collection and analysis.
Support by NIH AG090000 to M.C. McGwire.