Skeletal remains of an adult (estimated age at death ≈ 27-30 years) Prehispanic male with fusion in the sacroiliac joints showed a heterotopic ossification consisting of a flat, 10 × 5 cm bone formation covering the posterior aspect of the right sacroiliac joint. The bones were recovered from a burial cave containing remains of 4 individuals, located on the side of a ravine, in Buenavista del Norte (NW corner of Tenerife, Canary Islands), at ≈ 450 m altitude. This individual was probably affected by ankylosing spondylitis (AS) that led to bony fusion of the pelvic bones with the sacrum, but several unusual features prompted us to compare the pelvis with those belonging to two modern individuals affected by AS (20