The ratites – the giant flightless birds of the Southern Hemisphere continents – are they a clade, with a separate common ancestor? And how do they tie into the modern changes in bird evolution?
I don’t know the answer, but at least I know what you’re asking, and I’ll be interested to see what prevailing opinion is.
Some of the latest research indicates that ratites are most likely a polyphyletic assemblage of flightless birds, and that each lineage lost flight independently. However, as with most such cases, that study is hardly likely to be the final word on the matter.
I think this question is complicated by the different meanings of “Ratite.” The question of whether ratites form a clade, and whether their common ancestor was flightless or volant, are different. It’s possible that all ratites evolved from a single common ancestor that could fly. If by “ratite” you mean just the extant flightless forms, then they’re probably not a clade if one excludes tinamous and extinct volant forms. But include those in the definition, and they’re a clade again, but the meaning of the word ratite would then potentiality change.
Exactly. The traditional definition is Ostriches + Emus + Rheas + Kiwis + Cassowaries. However, the “thorn”, so to speak, in the side of ratite phylogenies is where to place Tinamous. In a monophyletic version, tinamous lie outside the ratite clade. However, the paper I linked to finds that tinamous lie smack in the middle of Paleognath cladograms - they appear to be most closely related to (Cassowaries + Emus + Kiwis), the lot of those are all related to Rheas, and then that whole clade is sister to Struthio (ostriches). So, as long as tinamous are considered ratites, then it becomes a happy monophyletic clade. If tinamous are not considered to be ratites, however, then “ratite” becomes paraphyletic.
At this time, though, there is no real consensus as to where tinamous fall.
Thank you very much. (It doesn’t help that I mistakenly had regarded “Ratites” as synonymous with what are referred to as Palaeognathous birds, including the Tinamous as ‘ratites’ that had retained the ability to fly – this confused me at first.)
Is there any consensus on where the Paleognaths fit into the scheme of bird evolution, given the assortment of relatively recent (20 years or so) discoveries of Cretaceous transitional birds?
The consensus is that Palaeognaths are the most basal of Neornithes. The latest research seems to indicate that most lineages arose after the break-up of Gondwana, with the flightless members of the group appearing to have evolved very soon after the K-T extinction event (see, for example, this article).
Most of the Cretaceous birds coming out of China appear to be Neognathes.
Actually, there are only a handful of Cretaceous neognaths or potential neognaths known and I don’t believe many are from China. The vast majority of Cretaceous birds are enantiornithines, with a few more advanced and bunch of more primitive forms thrown in.
The details of the phylogeny are of course going to be controversial, but there’s an exhaustive list of Mesozoic birds here (scroll down past all the non-avialan theropods until you hit Archaeopteryx): http://home.comcast.net/~eoraptor/Phylogeny%20of%20Taxa.html
D’oh! For some reason, I meant to write Enantiornithines, but wrote Neognathes.
I blame the drugs.