Sauer (e.g., Sauer 1972) recognized a form of fossil eggshell displaying pore morphology that he identified as struthious (Type S) in structure, specifically in that it resembled eggshells of Struthio camelus camelus. Other fossil eggshells displaying a similar pattern had been referred to as Psammornis (see in Sauer 1969) or Struthiolithus (see in Mikhailov 1997), and Sauer (1972:9) felt that "actually there is no need to separate these specimens from the genus Struthio." Sauer distinguished this struthious Type S eggshell from the aepyornithid-type (Type A) by the fact that the pore structure in Type S eggshell was dominated solely by small circular pores, with a total absence of groove-type pores. Later, in describing the eggshells from the Mio-Pliocene of Ourzazate, Morocco, Sauer and Sauer (1978) presented their discovery of fossil eggshell intermediate in pore morphology between the previously recognized Type A and Type S eggshells. This interpretation came about with the discovery at Ourzazate of eggshell fragments that exhibited areas occupied solely by singular, small round pores, along with areas occupied by the typical subparallel groove pores, and with a gradational zone in between. In fact, among the specimens from the Baynunah Formation, several pieces can be seen which capture a transition zone between an area populated purely by small circular pores to one expressing primarily the subparallel bent and forked groove pores. The eggshell fragments shown in Figure 3 have been chosen to represent these variations in pore morphology. Pore morphology is known to vary across different regions of an egg (personal observations on eggs of Rhea and Aepyornis), making it possible that Sauer's Type A and Type S morphologies, and those similar variations represented among the Baynunah aepyornithid-type specimens, simply reflect fragments from different parts of the egg (see also Sauer and Sauer 1978).
Grellet-Tinner (2000) described and coded 22 eggshell characters, most of which are based on the radial microstructure of the eggshell. His cladistic analysis indicated that aepyornithid-type eggshell (his Struthiolithus) shares autapomorphies with Aepyornis, and that both of these are sister taxa to Struthio. We note that the sister-relationship of Aepyornithid-type eggshell with Aepyornis, as based on characters of the cross-sectional microstructure, is in congruence with the gross morphology of the eggshells. Grellet-Tinner (2000) refers Aepyornthid-type eggshell to the oogenus Struthiolithus. It is not clear why this is done as he does not justify a similarity between all Aepyornithid-type eggshells and all those of Struthiolithus (e.g., Brandt 1874; Lowe 1931; Sauer 1968; Mikhailov 1997). Although we agree with Grellet-Tinner (2000) that many samples of Aepyornthid-type and Struthiolithus eggshell are indistinguishable, we do not at this point feel it is justified to refer all Aepyornithid-type eggshell to Struthiolithus. Furthermore, it would not be advantageous to refer Aepyornithid-type eggshell to Aepyornis nor to ascribe a new ootaxon name to aepyornithid-type eggshell type without a comprehensive study and revision of its representative samples in relation to those of all the other ratites.
Skeletal remains from Namibia identified as Struthio coppensi (Mourer-Chauviré et al. 1996a) extend the age of the genus Struthio to the Early Miocene. Mourer-Chauviré et al. (1996a) claim that the didactyl tarsometatarsus of Struthio coppensi is associated with eggshells of aepyornithid-type from Namibia. If aepyornithoid-type eggshells represent a non-monophyletic cluster diagnosed on the basis of primitive pore characters, then it remains possible that the Struthionidae developed their characteristic eggshell morphology from an aepyornithid-type ancestor.
Eggshells from Plio-Pleistocene deposits are generally attributed to Struthio asiaticus, skeletal remains of which were described by Lydekker (1884). These eggshells display a pore morphology that is reportedly indistinguishable from that of Struthio camelus molybdophanes (Bidwell 1910; Sauer 1968; Sahni et al. 1990). As noted by Sahni et al. (1990), the attribution of the eggshell material to Struthio asiaticus is a product of practicality rather than a statement of phylogenetic affinity. Other fossil ratite eggshell have been included in the following taxa or ootaxa: Struthio pannonicus (includes skeletal remains, Kretzoi 1954); Struthiolithus chersonensis (Brandt 1873, 1874); Struthio asiaticus (includes skeletal remains, Milne-Edwards 1871); Struthio wimani (founded on skeletal remains, Lowe 1931); Struthio anderssoni (Lowe 1931); Struthio mongolicus (Lowe 1931). See Sauer (1968) for a review of these forms. The validity and systematic relationships of these and other ratite ootaxa are in need of study.
Fossil skeletal remains of ratites have been discovered and described (Lowe 1931, 1933; Kretzoi 1954; Kurochkin and Lungu 1970; Mourer-Chauviré, et al. 1996a, 1996b) although it still remains to coherently synthesize all the available evidence and revise the different conflicting evolutionary hypotheses established on the basis of these remains. Further comparative work, particularly cladistic work including both eggshell and skeletal material, should illuminate the evolutionary history of the Ratitae.