Splitting up the Palaeognathae

Distinct from earlier DNA and morphological studies
members of the Euornithes (extant birds and their closest kin) are undergoing tree topology shifts when they enter the large reptile tree (LRT, 1076 taxa). We’ve seen this before with other reptile and mammal clades.

Today we’ll be talking about
the base of the Euornithes, the ratites and tinamous (= Palaeognathae).

Ratites have no keel on their sternum.
But that keel fails to appear on several flightless birds in several clades. Wikipedia reports, “Flightlessness is a trait that evolved independently multiple times in different ratite lineages. The systematics involved [in the ratites] have been in flux.”

Wikipedia reports,
“There are three extinct groups [of Palaeognathae], the Lithornithiformes (Lithornis + Pseudocrypturus.), the Dinornithiformes (moas) and the Aepyornithiformes (elephant birds), that are undisputed members of Palaeognathae.” 

Disputing those traditional assignments,
in the LRT (Fig. 1):

  1. the moa, DInornis, nests with parrots
  2. the elephant bird, Aepyornis, nests with the ostrich, Struthio.

When I added
the kiwi
Apteryx Fig. 2) and elephant bird (Aepyornis (Fig. 3) to the LRT a monophyletic clade(?) Ratites + tinamous (= Palaeognathae; pink taxa in Fig. 1) was not recovered. The remaining ratites are not a clade, but a grade of basal birds with tinamous, like Rhynchotus, nesting basal to and the proximal outgroup to the clade Neognathae.

And yes, the Solnhofen bird
Jurapteryx recurva (= Eichstätt specimen of Archaeopteryx) is the basalmost member (= last common ancestor) of the Euornithes. That means, someday we’ll be finding palaeognathid ostrich, cassowary and tinamou ancestors in the Early and Late Cretaceous. That has not happened yet (to my knowledge).

Currently filling this Cretaceous gap
are the toothed birds Yanornis, Apsaravis, Ichthyornis and HesperornisThey now nest within the Euornithes (the clade of extant birds). Evidently teeth redeveloped in this clade as they did in Pelagornis, the giant albatross-like bird with bony teeth. Earlier we looked at the reappearance of digit ‘0’ in screamers, so old genes can and do reassert themselves in birds.

Without this clade of toothed Cretaceous birds
there would have been, a long Cretaceous gap in the fossil record of Euornithes. I’m sure this gap will be filled someday with toothless birds. When it is filled phylogenetic bracketing indicates they’re going to look like dippers, like Cinclus, and screamers, like Chauna. As mentioned earlier, this gap is currently not filled, nor even hinted at (Fig. 1).

Figure 2. Jurapteryx, Pseudocrypturus, Apteryx and Proapteryx to scale.

Figure 2. Jurapteryx, Pseudocrypturus, Apteryx and Proapteryx to scale. Now we know why the gastralia disappeared in this clade!

When you put both Pseudocrypturus and Apteryx together
to scale (Fig. 2) the several reasons (traits) why they nest together become more obvious. This is contra recent DNA studies that nest elephant birds with kiwis (Mitchell et al. 2014). That study represents one more incidence of the loss of validity with DNA over large phylogenetic distances along with the typical problem of taxon exclusion that the LRT attempts to minimize.

Archaeopteryx (Jurapteryx) recurva 
(JM2257; the Eichstätt specimen; Howgate 1985) is one of the smaller Solnhofen birds. Here it nests as the last common ancestor of all extant birds. A gap spanning the entire Cretaceous separates this taxon from extant taxa and their kin. As in other bird lines, the loss of tail length, the fusion of the pygostyle and the fusion of manus elements are convergent.

Pseudocrypturus cercanaxius 
(Houde 1988; Early Eocene) was originally considered a northern hemisphere ancestor to ratites (like the ostrich, Struthio). That is true, but Pseudocrypturus is also close to the ancestry of all extant birds. Today 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) or earlier. 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.

Apteryx 
(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 linking it toward Struthio are 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 LRT.

Note that
Proapteryx (Worthy et al. 2013; Miocene), known from a partial femur and coracoid, falls within the size range of Jurapteryx (Late Jurassic). Proapteryx likely was volant.

References
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.
Howgate ME 1985. Problems of the osteology of Archaeopteryx: is the Eichstätt specimen a distinct genus?. In Hecht, Ostrom, Viohl, and Wellnhofer (eds.), The Beginnings of Birds: Proceedings of the International Archaeopteryx Conference, Eichstätt 1984. Freunde des Jura-Museums Eichstätt, Eichstätt 105-112.
Mitchell KJ (seven coauthors) 2014. Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution. Science. 344 (6186): 898–900.
Shaw 1813. Naturalist’s Miscellany 19:
Worthy TH et al. 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.

wiki/Jurapteryx
wiki/Pseudocrypturus
wiki/Apteryx, Kiwi
wiki/Proapteryx
wiki/Dipper
wiki/Proapteryx

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