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Hind Limb Bone Proportions Reveal Unexpected Morphofunctional Diversification in Xenarthrans

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Abstract

Divergence in forelimb morphology is often associated with functional habits exhibited within the Xenarthra, ranging from terrestrial-digging in armadillos to arboreal-suspension in sloths. We hypothesized that quantitative differences in hind limb form also will be predictive of the diverse lifestyles observed in this small clade. A total of 26 morphofunctional indices were calculated from 42 raw measurements of bone length/width/depth in a sample of N = 76 skeletal specimens (18 species). Index data for each species were categorized by substrate preference and use and then evaluated using a combination of stepwise Discriminant Function Analysis (DFA) and Principal Component Analysis (PCA) to determine significant osteological correlates (traits) among extant taxa. Additionally, character states of the morphometric data were inferred using a recent hypothesis of xenarthran phylogeny. DFA determined 14 distinct morphofunctional indices relating to femur robustness, hip/ankle/limb mechanical advantage, and foot and claw length as the most discriminating features. PCA clearly separated armadillos from sloths in morphospace based on overall robustness versus gracility, as well as proximal versus distal lengths of skeletal elements (including the claws), whereas these characteristics were intermediate in the hind limbs of anteaters and selected armadillos having either a larger greater trochanter or modified foot/claw proportions. Two-toed and three-toed sloths showed further separation from each other in morphospace primarily driven by proportions of their tibia and hind feet despite evidence of convergence for numerous functional traits. Moreover, the majority of the traits measured had significant phylogenetic signal and several of these indicated clear patterns of convergent and divergent evolution in xenarthrans by evaluation of their tip states. Our assessments expand functional interpretations of xenarthran limb form and identify potentially conserved and secondarily modified traits related to fossoriality across taxa, including in three-toed sloths, demonstrate possible morphological trade-offs between digging and climbing habits, and suggest derived traits adapted for arboreal lifestyle and suspensory function.

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Fig. 1

taken from the a femur, b proximal femur, c tibio-fibula, d tibia, e hind foot, f calcaneus, and g claw. The measurements shown here were used for index calculations: FL, femur length; FMW, femur mid-shaft width; FMD, femur mid-shaft depth; FHL, femur head length; FHW, femur head width; FHD, femur head depth; PFL, proximal femur length; GTL, greater trochanter length; GTW, greater trochanter width; FCW, femur condylar width; FCL-M, femur condylar length-medial; FCD-M, femur condylar depth-medial; FCL-L, femur condylar length-lateral; FBL, fibula length; TL, tibia length; TMW, tibia mid-shaft width; TMD, tibia mid-shaft depth; TPEW, tibia proximal end width; TDED, tibia distal end depth; TDEW, tibia distal end width; MML, medial malleolus length; MMW, medial malleolus width; MT3L, third metatarsal length; MT3W, third metatarsal width; PP3L, proximal phalanx length of digit III; PP3W, proximal phalanx width of digit III; CL, calcaneus length; PC3L, pes claw length of digit III. Greater trochanter depth and femur condylar depth (lateral) are not illustrated. Selected measurements adapted from Salton and Sargis (2009)

Fig. 2

Adapted from Gibb et al. (2016)

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Data Availability

Data generated for this study are included in this published article in adherence with disclosure policy of the journal. The authors also share condensed osteological and index data as a supplement. Additional raw data is available upon reasonable request.

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Acknowledgments

We sincerely thank Darren Lunde (NMNH), Eileen Westwig (AMNH), and Bruce Patterson and Lauren Smith (FMNH) for coordinating access to museum collections. We thank The Sloth Sanctuary of Costa Rica for the opportunity to harvest bones from frozen sloth specimens. Thanks to the Instituto de Medicina y Biología Experimental de Cuyo, Mendoza, Argentina for access to rare armadillo specimens. Special thanks to Mykaela Wagner, Brooke Copland, Amber Landsman, Lindsey Moon, Chris Riwniak, Jacob Aiello, and Jessica Yeager for assistance with data collection and data entry. Portions of the work were submitted as a Masters Thesis by S.K.M.. The YSU Department of Biological Sciences and College of STEM are also gratefully acknowledged.

Funding

This work was supported by a Journal of Experimental Biology Travelling Fellowship to S.K.M. award number JEBTF-170817.

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Authors and Affiliations

  1. Department of Biological Sciences, Youngstown State University, Youngstown, OH, USA

    Sarah K. Marshall, Kyle B. Spainhower, Thomas P. Diggins & Michael T. Butcher

  2. Department of Biology and Earth Science, Otterbein University, Westerville, OH, USA

    Brandon T. Sinn

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  1. Sarah K. Marshall
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  2. Kyle B. Spainhower
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Contributions

S.K.M. developed the concepts and experimental approach, collected and analyzed data, and drafted and edited the manuscript; K.B.S collected data and revised the manuscript; B.T.S. developed the analytical approach, analyzed data, and edited the manuscript; T.P.D. developed the analytical approach, analyzed data, and edited the manuscript; M.T.B. developed the concepts and approach, collected and analyzed data, and drafted and revised the manuscript.

Corresponding author

Correspondence to Michael T. Butcher.

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Appendix 1

Appendix 1

List of skeletal specimens and museum collection or origin. Museum acronyms are as follows: NMNH, National Museum of Natural History (Washington D.C. USA); AMNH, American Museum of Natural History (New York, NY USA); FMNH, Field Museum of Natural History (Chicago, IL USA).

Species

Museum

Specimen

Number

Tolypeutes matacus

NMNH

583,927

Tolypeutes matacus

NMNH

291,935

Tolypeutes matacus

NMNH

598,002

Tolypeutes matacus

AMNH

248,394

Priodontes maximus

NMNH

261,024

Priodontes maximus

NMNH

270,373

Priodontes maximus

NMNH

299,630

Chaetophractus villosus

NMNH

396,655

Chaetophractus villosus

NMNH

543,430

Chaetophractus villosus

NMNH

155,411

Chaetophractus villosus

FMNH

153,772

Chaetophractus villosus

FMNH

134,611

Chaetophractus villosus

FMNH

60,467

Chaetophractus vellerosus

–

Cve1a

Chlamyphorus truncatus

–

Ct1a

Zaedyus pichiy

FMNH

153,782

Zaedyus pichiy

FMNH

104,817

Zaedyus pichiy

FMNH

23,809

Zaedyus pichiy

FMNH

15,626

Cabassous centralis

FMNH

121,224

Cabassous centralis

FMNH

134,458

Cabassous unicinctus

AMNH

209,943

Cabassous unicinctus

AMNH

133,314

Cabassous unicinctus

AMNH

133,317

Cabassous unicinctus

AMNH

23,441

Euphractus sexcinctus

NMNH

256,115

Euphractus sexcinctus

NMNH

258,603

Euphractus sexcinctus

NMNH

257,968

Dasypus septemcinctus*

AMNH

133,258

Dasypus hybridus

AMNH

205,708

Dasypus hybridus

AMNH

205,707

Dasypus novemcinctus

–

Dn1b

Dasypus novemcinctus

–

Dn2b

Dasypus novemcinctus

–

Dn3b

Dasypus novemcinctus

–

Dn4b

Dasypus novemcinctus

NMNH

053,321

Dasypus novemcinctus

NMNH

240,091

Dasypus novemcinctus

NMNH

A49398

Cyclopes didactylus

NMNH

304,941

Cyclopes didactylus

NMNH

283,876

Cyclopes didactylus

NMNH

012,097

Cyclopes didactylus

NMNH

200,353

Cyclopes didactylus

AMNH

139,228

Cyclopes didactylus

AMNH

130,107

Cyclopes didactylus

AMNH

97,317

Cyclopes didactylus

FMNH

51,889

Tamandua tetradactyla

NMNH

589,602

Tamandua tetradactyla

NMNH

172,999

Tamandua tetradactyla

NMNH

339,663

Tamandua tetradactyla

NMNH

21,658

Tamandua tetradactyla

AMNH

211,662

Tamandua tetradactyla

AMNH

96,258

Tamandua tetradactyla

AMNH

150,733

Tamandua tetradactyla

FMNH

256,759

Bradypus variegatus

–

Bv1c

Bradypus variegatus

–

Bv2c

Bradypus variegatus*

–

Bv3c

Bradypus variegatus

–

Bv4c

Bradypus variegatus

–

Bv5c

Bradypus variegatus*

–

Bv6c

Bradypus variegatus

–

Bv7c

Bradypus variegatus

–

Bv8c

Bradypus tridactylus

FMNH

93,296

Bradypus tridactylusd

AMNH

130,106

Bradypus tridactylus

AMNH

74,136

Choloepus hoffmanni

–

Ch3-Rc

Choloepus hoffmanni

–

Ch3-Lc

Choloepus hoffmanni

–

Ch5c

Choloepus hoffmanni

–

Ch7c

Choloepus hoffmanni

–

Ch8c

Choloepus hoffmanni

–

Ch9c

  1. *Juvenile specimen
  2. Specimen sources:
  3. aInstituto de Medicina y Biología Experimental de Cuyo Mendoza, Argentina
  4. bPrevious study (Copploe et al. 2015)
  5. cThe Sloth Sanctuary, Costa Rica; bone specimens stored in PI freezer R-L, right and left
  6. dSub-adult specimen

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Marshall, S.K., Spainhower, K.B., Sinn, B.T. et al. Hind Limb Bone Proportions Reveal Unexpected Morphofunctional Diversification in Xenarthrans. J Mammal Evol 28, 599–619 (2021). https://doi.org/10.1007/s10914-021-09537-w

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  • DOI: https://doi.org/10.1007/s10914-021-09537-w

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