Phorusrhacoids group together on the basis of cranial and pelvic features. Increase in size is convergent among to lineages, with one primitive, seriama-like lineage still capable of flight, and behaviorally in this context may have been similar. Only tinamous today among ratites fly, but comparison to the fossil record shows that primitively, ratites lost flight perhaps several times. That arm and sternal features vary among ratites makes it more difficult to assess any particular trend in flight loss about them, but maybe _among_ them.