Sironemelia is caused due to effect in:

Gastrulation itself may be disrupted by genetic abnormalities and toxic insults. In caudal dysgenesis (sirenomelia), insufficient mesoderm is formed in the caudalmost region of the embryo. Because this mesoderm contributes to formation of the lower limbs, urogenital system (intermediate mesoderm), and lumbosacral veebrae, abnormalities in these structures ensue. Affected individuals exhibit a variable range of defects, including hypoplasia and fusion of the lower limbs, veebral abnormalities, renal agenesis, imperforate anus, and anomalies of the genital organs Ref: Langman's embryology 11th edition Chapter 5.