Worthy et al. 2013 reported:
“Until now, kiwi (Apteryx owenii, Apterygidae, Shaw 1813; Fig. 1) have had no pre-Quaternary fossil record to inform on the timing of their arrival in New Zealand or on their inter-ratite relationships.” They described two fossils (femur and quadrate) from the Early Miocene (Fig. 1; 19–16mya) which they named Proapteryx. “The new fossils indicate a markedly smaller and possibly volant bird, supporting a possible overwater dispersal origin to New Zealand of kiwi independent of moa. If the common ancestor of this early Miocene apterygid species and extant kiwi was similarly small and volant, then the phyletic dwarfing hypothesis to explain relatively small body size of kiwi compared with other ratites is incorrect.”
Apteryx owenii (Shaw 1813) The extant flightless kiwi has an elongate naris that extends to the tip of its beak. Maybe two teeth are there. Here it nests with Pseudocrypturus, but flightless traits push it toward Struthio, by convergence. in the pre-cladistic era, Calder (1978, 1984) considered the kiwi a phylogenetic dwarf derived from the larger moa, but that was invalidated by Worthy et al. 2013 and the large reptile tree.
Proapteryx micromeros (Worthy et. al. 2013) was a slender, tiny Miocene (18 mya) ancestor likely capable of flight.
Pseudocrypturus cercanaxius (Houde 1988; Early Eocene) was originally considered a northern hemisphere ancestor to ratites (like the ostrich, Struthio). Today these primitive flightless birds are chiefly restricted to the southern hemisphere. It could be that early birds did start in the South and had migrated to the North during the Paleocene (66-56 mya).
Since ratites are basal to extant birds, and Pseudocrypturus is basal to ratites (paleognaths), Pseudocrypturus is also quite similar to the ancestor of all extant birds despite its late appearance in the early Eocene. Perhaps something very much like it was one of the few survivors of the K-T extinction event.
It’s notable that Pseudocrypturus has long legs. Early ducks, like Presbyornis, and basal raptors, like Sagittarius, also had long legs. Evidence is building that this is the primitive condition for the clade of living birds arising from the K-T extinction event.
Worthy et al. nest Apteryx
within the order Casuariiformes, which includes cassowaries, emirs, and kiwi, but only in the absence of Pseudocrypturus.
The kiwi egg vs ventral pelvis issue
In most tetrapods, including humans, the egg/baby passes through the cloaca/vagina which passes through the two ischia. That was also likely the case with Archaeopteryx, because this is also the case with Gallus the chicken. In extant birds the ischia posterior tips no longer touch, but are widely separated. Going several steps further, in the kiwi the enormous egg is held in front of the pubis (Fig. 1), which is also in front of the ischia.
The following video of a kiwi laying an egg
shows the cloaca a substantial distance below the swirl that marks its tail. kiwi egg video click to play pretty much located at the tip of the long axis of the egg in figure 1 (maybe a little higher/closer to the tail).
In the LRT
Pseudocrypturus and Apteryx (Fig. 1) nest together and apart from the ratites. Pseudocrypturus is basal to all living birds. It probably first appeared in the Early Cretaceous. It was found in the Paleocene.
Calder WA 1978. The kiwi. Scientific American 239(1):132–142.
Calder WA 1984. Size, function and life history. 448 pp. Cambridge (Harvard U Press).
Houde PW 1986. Ostrich ancestors found in the northern hemisphere suggest new hypothesis of ratite origins. Nature 324:563–565.
Houde PW 1988. Paleognathus birds from the early Tertiary of the northern hemisphere. Publications of the Nuttall Ornithological Club 22. 147 pp.
Shaw 1813. Naturalist’s Miscellany 19:
Worthy, TH. et al. (5 coauthors) 2013. Miocene fossils show that kiwi (Apteryx, Apterygidae) are probably not phyletic dwarves. Paleornithological Research 2013, Proceedings of the 8th International Meeting of the Society of Avian Paleontology and Evolution. Retrieved 16 September 2017.