An abnormality of the fossa cranii posterior (the posterior fossa), which is made up primarily of the occipital bone and which surrounds to the foramen magnum. The phenotype is Abnormal posterior cranial fossa morphology (Human Phenotype Ontology identifier HP_0000932). Also known as: Abnormality of the posterior cranial fossa, Abnormality of the posterior fossa, Posterior fossa anomaly.