Discussion

Although a comprehensive analysis of the pattern of egg evolution among extant amniotes is compromised by limited data, a robust phylogenetic approach will be possible ultimately as information on egg morphology and physiology of modern species increases. However, answers to fundamental questions concerning the origin of the egg and the polarity of character transformation will be more elusive because of the absence of living representatives of a suitable outgroup. The closest outgroup to modern Amniota, Lissamphibia, is a phylogenetically distant taxon with specialized reproduction and development.

Recent phylogenetic analyses identify Synapsida as the sister group to all other Amniota, which are represented by Reptilia among extant species (Gauthier et al., 1988b; Lombard and Sumida, 1992). Based on this phylogenetic hypothesis, the egg of extant oviparous Reptilia is distinct from that of extant oviparous Synapsida, represented by Monotremata (Fig. 6). The structural characteristics of the eggs of Monotremata, (1) meroblastic cleavage, (2) proteinaceous shell membrane, (3) choriovitelline membrane as the definitive yolk sac, (4) amniochorion formed by folding, and (5) expanded allantoic vesicle confined to the embryonic pole of the egg, are likely plesiomorphic for Amniota. Monotremes share oviparity, and oviductal secretions of glycoprotein and keratinous protein with modern Amphibia (Hughes, 1977). Meroblastic cleavage, a derived trait, is shared with Reptilia. Two of the characters shared between Reptilia and Monotremata, an amniochorion and allantoic vesicle, are synapomorphies for Amniota.

The distribution of characters among extant taxa (Fig. 6) indicates that the ancestral condition for Reptilia was an oviparous, lecithotrophic egg. The egg cleavage pattern was meroblastic. Oviductal contributions were a proteinaceous and mineralized shell membrane and a large mass of albumen. The eggshell was pliable. The sequence of yolk sac development included a bilaminar omphalopleure, a choriovitelline membrane and a splanchnopleure. The amniochorion was formed from folds and an expanded allantoic vesicle surrounded the egg contents in terminal stages of incubation. The reptilian egg contains four derived characters not present in monotremes, a mineralized eggshell membrane, a large mass of albumen, the presence of a yolk sac that is entirely splanchnopleuric and an allantoic vesicle that surrounds the egg contents.

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