Tail length varies in living Malagasy prosimians from long (e.g., Lemur catta) to short (e.g., Indri indri). The dimensions of the sacral canal are correlated with tail length in some primates (Ankel 1965, 1972). The "sacral index" is the ratio of the size of the sacral hiatus to the size of the cranial opening of the sacral canal; it is large when the tail is long and small when the tail is short/absent. Here we compare the sacral index in living lemurs, lorises and macaques to relative tail length (tail length as a percentage of body length). We then examine the sacral index in a sample of giant extinct lemurs in order to assess relative tail length in categorical terms (absent or vestigial, medium, or long).

Across our entire extant sample, the rank order correlation (rho) between sacral index and relative tail length is 0.9 (p<0.05) at the family level and 0.83 (p<0.001) at the species level. Rho is also positive and significant for prosimians alone (p<0.001), Malagasy prosimians only (p<0.05), indriids only (p<0.01 ), and macaques only (p<0.05). Among the large-bodied subfossil lemurs, the sacral index suggests that

Pachylemur, Archaeolemur and Hadropithecus possessed relatively long tails. Caudal vertebrae are known for these quadrupedal taxa and serve to corroborate our inferences. We reconstruct the tail of Palaeopropithecus, a highly suspensory sloth lemur, as vestigial. We believe that these results are promising and will extend our

analysis to other extinct species, including Babakotia and Megaladapis, for which sacra are also known.