Ground sloth vertebrae identification through multivariate analysis

ABSTRACT

Extinct sloths (Xenarthra, Folivora) are morphologically diverse, despite some similarities among some Pleistocene genera. Cranial and diagnostic postcranial elements (especially limb bones) are taxonomically informative but the axial postcranial skeleton can prove difficult to classify, as in cases with only vertebral remains or when closely related taxa are found together. Here, 24 linear and angular measurements of presacral axial skeletons from eight genera of Pleistocene ground sloths were analysed through multivariate methods for assigning their vertebrae to a genus and to a position along the vertebral column. Both isolated vertebrae and vertebrae associated with partial and complete skeletons of each genus were included. Principal components and linear discriminant analyses show a high percentage (over 90%) of correct taxonomic reclassification. For the position of vertebrae, the accuracy increased significantly when cervical, thoracic and lumbar regions were separately considered instead of the whole column, misclassifying the position of the vertebrae only by a few positions in most cases. Our database is useful for confidently assigning isolated vertebrae of extinct sloths to a specific genus and position, showing that beyond the apparent homogeneity in the axial postcranial skeleton of ground sloths, quantitative studies allow the evaluation of diversity and variation between groups.

KEYWORDS:

• Xenarthra
• ground sloths
• axial skeleton
• morphology
• Pleistocene

Acknowledgments

We are grateful to Andrés Rinderknecht (Museo Nacional de Historia Natural), Alejandro Kramarz (Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”), Marcelo Reguero (Museo de La Plata) and Amanda Milhouse (National Museum of Natural History) for access to the collections under their care. We also thank Luciano Varela for very useful discussions in previous versions of this manuscript, and Mariana Di Giacomo for improving the usage of English. Thanks to Robert K. McAfee, Eli Amson, and an anonymous reviewer for critical suggestions and corrections that greatly improved the final manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article can be accessed here.

Additional information

Funding

This work was funded by Agencia Nacional de Investigación e Innovación (ANII, grants: FCE_1_2014_1_104572, POS_NAC_2014_1_102504) and National Geographic (Grant number 178431).