A small order of weak-flying, partridge-like birds and giant, flightless ratite birds found in the southern continents. Their relationship to other birds is unknown. The struthioniforms are characterized by a palaeognathous palate, a break in the postnasal strut, close approximation of the zygomatic process to the quadrate, and the structure of the rhamphotheca (the horny sheath covering a bird's beak). They are frequently placed in a separate superorder, the Palaeognathae. However, that distinction places too much emphasis on their separation from other birds. For many years, most scientists believed that the palaeognathous birds were not related to each other, and separated them into six or seven orders; however, careful analysis of their cranial anatomy has demonstrated the monophyly of the group. Even though most investigators continue to place the tinamous in a separate order, they are very close to the rheas in some anatomical features; hence, all of the palaeognathous birds will be included here in a single order. Contrary to common opinion, no evidence supports the concept that the palaeognathous birds are primitive among living birds.