D Bullock & J Rackham mention vertebral fusion in "Epiphysial fusion & tooth
eruption of feral goats from Moffatdale, Dumfries & Galloway, Scotland", in
Wilson, Grigson & Payne (eds) 1982 Ageing & sexing animal bones from
archaeological sites, BAR 109, 73-80, specifically p. 74. they don't give
data specifically for axis but they do mention a sequence of fusion.