The anterolateral portion (squama) arises from the mesenchyme at 8 weeks of embryogenesis while the petromastoid portion develops later from a cartilaginous center at 6 months of fetal development.

[1] In certain people, it can contain an emissary vein, referred to as the petrosquamosal sinus.

Being aware of this anatomic variant with preoperative CT scanning can be important to prevent bleeding in certain types of otolaryngological surgeries.

Some authors have theorized that a persistent venous sinus reflects an arrest in embryologic development.

[1] This article incorporates text in the public domain from page 142 of the 20th edition of Gray's Anatomy (1918)