Thoracolumbar vertebral number:: the first skeletal synapomorphy for afrotherian mammals

There is overwhelming molecular support for the monophyly of a supra-ordinal clade of living African placental mammals, the Afrotheria, but there is not a single unequivocal morphological synapomorphy for this group. We conducted a survey of thoraco-lumbar vertebral numbers across mammals, based on the examination of specimens representing 86 living and 12 fossil species and a thorough review of the anatomical literature. A total of 19 thoraco-lumbar vertebrae is plesiomorphic for mammals, eutherians and metatherians. Metatherians show no variation in this plesiomorphic condition, suggesting the presence of a developmental constraint, in view of the contrasting variability of the equally old eutherian clade. Several deviations from the plesiomorphic condition have evolved independently in the course of placental phylogeny. Optimization of the information on a published phylogenetic framework based on molecular sequence data reveals that an increase in the number of thoracolumbar vertebrae is the first unambiguous skeletal synapomorphy of Afrotheria.