Which one of the following is not present at bih?

The petrous pa of the temporal bone is pyramid-shaped and is wedged in at the base of the skull between the sphenoid and occipital bones. Directed medially, forward, and a little upward, it presents a base, an apex, three surfaces, and three angles, and houses in its interior, the components of the inner ear . A total of six group of ossification centers appear in the petrous pa during 5th fetal month, and they form the petrous bone at 6th fetal month. The firstly-appeared ossification center is just above the round window, and the second is on the ampulla of anterior semicircular canal. Other ossifications are observed between the cochlea and semicircular canals, on the brim of internal acoustic porus, on the superior surface of the petrous apex, and on the summit of posterior semicircular canal. The ossification of the facial canal stas at 6th fetal month, though the geniculate pa and tympanic pa do not complete until one year old after bih. Even in adults, the facial canal dehiscence are found at more than 10% of cases, mainly locating in the tympanic pa. On the basis of these results, formation of the petrous bone including facial canal and other bony structures was discussed from the viewpoints of the ossification and pre- and postnatal middle ear development Ref - pubmed.com