the lateral orbit. Without a stiffened lateral orbit, deformation would displace soft tissues, when contraction of the anterior temporalis muscle takes place, thus impeding eye movement. Occurrence A complete postorbital bar has evolved at least eleven times as a convergent adaptation in nine mammalian orders. Postorbital bars are characteristic to the following clades: Order Primates Order Scandentia Order Perissodactyla Subfamily Equinae Order Artiodactyla Suborder Neoselenodontia Postorbital bars have furthermore developed individually in the following taxa: Clade Metatheria Order †Sparassodonta †Thylacosmilus Order Diprotodontia †Thylacoleo Clade Eutheria Order Carnivora Barbourofelis Order †Litopterna †Thoatherium †Diadiaphorus Order Sirenia Trichechus senegalensis Order Artiodactyla Balaenoptera acutorostrata The presence of a postorbital bar in the extinct Oviraptorosauria species Avimimus portentosus was one of several defining characteristics that suggested to paleontologists that the species was more morphologically different from avian species than previously thought, affecting interpretation of the rate of evolution from dinosaurs to birds. Postorbital process Postorbital bars are likely derived from well-developed postorbital processes, an intermediate condition where a small gap retains between the process and the zygomatic arch. Well-developed postorbital processes have evolved separately within the orders of the Dermoptera and Hyracoidae and the Chiropteran families of Emballonuridae and Pteropodidae and to varying degrees within many carnivorian taxa. Complete postorbital bars and well-developed postorbital processes, retaining gaps of mere centimetres, spanned by the postorbital ligament, occur as polymorphisms within a number of pteropodid and hyracoid taxa. References Primate anatomy Skull Mammal anatomy