Which description applies to the prominent transverse lines located on the anterior surface of the sacrum?

These lines (there is four of them) are residuals of the fusion of the five sacrum vertebrae that are fused. These "vertebrae" or segments present a characteristics, being the first of them very similar to a lumbar vertebra, large, and the other vertebrae are progressively smaller and flattened. They also present a curved shape being anteriorly concave.