The palaeognaths (infraclass Palaeognathae within subclass Aves) represent the sister clade to the Neognathae within the birds (Class Aves). They comprise the ostriches (Order Struthioniformes) and the emus, kiwis, rheas and tinamous (clade Notopalaeognathae), as illustrated below:

The earliest known fossil representative of the stem-Palaeognathae is Lithornis celetius, found in the Fort Union Formation of Middle Paleocene (Selandian) age at Bangtail Quarry in Park County, Montana, USA (Houde, 1988; Worthy et al, 2017). A restoration of a member of the genus Lithornis is shown below:

The oldest known crown-group palaeognath is Diogenornis fragilis, a member of the Notopalaeognathae clade found in the Late Paleocene of the Itaboraí Basin, Brazil (Alvarenga, 1983; Yonezawa et al, 2017). No public-images are available for this species.

Image credits - Palaeognaths
Header (Common ostriches)     By Bernard DUPONT from FRANCE (Common Ostriches (Struthio camelus)) [CC BY-SA 2.0  (https://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons
Lithornis sp.     By Sheatherius [CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons