Does cortical bone thickness in the last sacral vertebra differ among tail types in primates?

Area of Science:

• Primate paleontology
• Comparative anatomy
• Paleoanthropology

Background:

• External sacral morphology informs primate tail type but internal morphology remains understudied.
• Internal sacral morphology offers functional insights, especially for incomplete fossil remains.
• Cortical bone thickness of the sacrum is a potential indicator of tail characteristics.

Purpose of the Study:

• To investigate if sacral vertebral body cortical bone thickness differs among primate tail types.
• To determine if this thickness can reconstruct tail types in extinct primates.

Main Methods:

• CT scans of sacra from 72 extant primates across 20 species were analyzed.
• Species were categorized by tail type: tailless, nonprehensile short-tailed, nonprehensile long-tailed, and prehensile-tailed.
• The developed model was applied to reconstruct tail types for four extinct primate species.

Main Results:

• Tailless primates exhibited significantly thinner sacral cortical bone than tail-bearing primates.
• Nonprehensile short-tailed primates had thinner cortical bone than nonprehensile long-tailed primates.
• Cortical bone thickness did not differentiate between prehensile-tailed and nonprehensile long-tailed primates, with phylogeny being a strong influence.

Conclusions:

• Sacral cortical bone thickness, within a phylogenetic context, can successfully allocate extinct primate species to tail type categories.
• Reconstructions for extinct species included Epipliopithecus vindobonensis (tailless), Archaeolemur edwardsi (long-tailed), and Megaladapis grandidieri and Palaeopropithecus kelyus (short-tailed or tailless).