March 18, 2014 | 115
As blasphemous and offensive as it seems to say it, birds are pretty samey. Generally speaking, they’re small flying things with long forelimbs, proportionally large heads with big, globular braincases, and grasping feet where an enlarged first toe (the hallux) opposes the remaining three. A shape like this was – so both the fossil record and inferences made from cladograms show us – ancestral for modern birds, so any bird that deviates from it is weird indeed. Cue ratites. Gigantic, long-legged, flightless birds with proportionally small heads, short, ridiculously short, or absent wings, they are the closest that any bird group comes to recapturing the body form (and presumably lifestyle) of non-bird dinosaurs.
Conventionally, the term ‘ratite’ is used for the kiwi-emu-rhea-ostrich clade (even though it always feels a bit weird to regard kiwi as ratites). The 11 or so recently extinct moa of New Zealand and the also recently extinct elephant birds or aepyornithids of Madagascar are also clearly members of this group, and then there are a handful of fossil groups as well. But there’s another modern group we have to consider here: the tinamous of South, Central and southern North America. All 40 or so species are small compared to ratites, capable of flight, and superficially galliform-like. They lack the anatomical specialisations that make ratites so remarkable, like an unkeeled, raft-like sternum, reduced, atrophied or absent forelimbs, proportionally long legs and neck, loose, ‘decomposed’ plumage, and so on.
However, despite the fact that they were often regarded as neognaths close to galliforms in the past, tinamous clearly share morphological and molecular characters with ratites. Together, both are united within Palaeognathae, the bird clade typically imagined to be anatomically (and perhaps behaviourally and ecologically) archaic compared to all other modern birds. Because I’m British, I call these birds palaeognaths (rather than ‘paleognaths’), and this makes sense given that the group is termed Palaeognathae, not ‘Paleognathae’.
Palaeognathae means ‘ancient jaws’, this being a reference to the idea that palaeognaths have an archaic palatal structure compared to the other modern birds, the neognaths. Anyone well-read on bird anatomy or the history of bird classification will be extremely familiar with the idea that modern birds can be divided into those with a ‘palaeognathous palate’ versus those with a ‘neognathous palate’, but it’s always been a source of frustration to me that this distinction is never well explained. I’m going to add to that frustration right now by not explaining it here, either, but I promise that I will do so in the near future.
Ratite distribution, or the ‘Cretaceous Kiwi Hypothesis’
One of the most curious things about ratites is that they’re widespread, with a distribution strongly tied to the Gondwanan continents (fossils from Eurasia notwithstanding). Modern ratites are all flightless, so their common ancestor was, so it’s been assumed, presumably flightless too (though… wait for the next article for much more on this issue). Seeing as Gondwana broke up during the Cretaceous, does the far-flung presence of big, flightless birds mean that they owe their distribution to overland dispersal and vicariance rather than overwater dispersal? Exactly this has been argued by several authors, most notably Joel Cracraft (1974, 2001), and it’s reminiscent of Thomas Huxley’s notion of 1867 that living ratites are but “waifs and strays” – the remnants of a once near-global radiation of species. [World map below by Hoshie.]
If ratites only moved around the world using terrestrial connections, their lineages must go back a long way. As in, well into the Late Cretaceous, at least. After all, you need moa and kiwi ancestors to move onto Zealandia before it breaks away from the eastern margin of Gondwana during the Late Cretaceous, elephant birds have to get onto Madagascar before it becomes an island during the Late Cretaceous and they have to get onto Indo-Madagascar before it became isolated during the Early Cretaceous, you need rheas to get into South America before it becomes an island in the Cenozoic, and so on and on. In other words, there must have been ‘Cretaceous kiwi’ and such, if this model is correct (though, strictly speaking, these birds would have been stem-members of the kiwi and other lineages, and not necessarily modern-looking species with all the unusual features we associate with their lineages).
There are no fossil ratites anywhere near this old (the oldest, in fact, are from the Paleocene). As it happens, there are good reasons for thinking that palaeognaths were present in the Cretaceous (Lee et al. 2014), but even studies of this sort ‘only’ posit palaeognath origins at about 83 million years ago, plus they concern the origin of the lineage that led to both modern palaeognaths and the Paleogene lithornithids (a group of long-jawed, archaic flying palaeognaths), not to the origin of modern-type ratites. [Adjacent image by Ghedoghedo.]
In short, neither the fossil record nor the pattern of palaeognath or ratite phylogeny supports the idea that these birds were diversifying, or even in existence, at the times required for a vicariant explanation for their distribution. The conclusion from this? They’ve gotten to where they are more recently, (mostly) via overwater dispersal. So, did this “overwater dispersal” require rafting of the sort always predicted for terrestrial animals, or did they fly? We’ll come back to this issue in the next article…
Ratites as ‘overgrown chicks’
Are the unusual features of ratites primitive characters that they’ve retained from earlier kinds of birds, or novelties that have arisen more recently, partly through paedomorphism or neoteny (the retention of juvenile features into adulthood)? The neoteny idea for ratite evolution was promoted by Gavin De Beer (1956) who argued that the ‘fluffy’ plumage, unfused cranial sutures and palaeognathous palates of ratites might all be due to neoteny. By removing the thyroid glands of developing starling, Dawson et al. (1994) were able to make experimental subjects express ratite-like features in their adult form. Dawson (1996) even suggested that ratite intelligence and their propensity to swallow inappropriate objects might be due to neoteny (after all, babies tend to be dumb and inquisitive). Molecular support for the neoteny hypothesis was published by Härlid & Arnason (1998) who used mtDNA data to show that ratites are deeply nested within neognaths. This result is almost certainly due to poor sampling, however, and other molecular studies show that it’s very unlikely to be correct.
While the neoteny hypothesis has long been popular, and while it’s promoted in what (unfortunately) remains the only comprehensive volume on bird evolution (Feduccia 1996), it’s important to be very sceptical about it. Features like a modified skull and palate and loose, shaggy plumage might result from neoteny, but they could just as easily be the result of natural selection that doesn’t involve neoteny: a shaggy plumage and specific cranial configuration, for example, could be advantageous for several reasons and you don’t need to invoke reliance on a specific physiological process to account for it. In some animals, open and unfused cranial sutures appear to be retained because they’re advantageous as goes the dissipation of stresses or because they allow flexion at certain points (Rafferty et al. 2003), for example, while the idea that the ratite palate is neotenous is almost certainly completely erroneous (Bock 1963, Elzanowski 1989).
In any case, cranial sutures aren’t unfused in all ratites, plus ratites aren’t neotenic in many aspects of anatomy: their skulls are proportionally tiny, not proportionally large, their leg bones are thick-boned and seemingly ‘over-designed’, not juvenile-like in form or proportion, and their pelvic girdles are both highly variable (contrary to the theoretical predictions of neoteny) and extremely well ossified, often with accessory structures not seen in other birds. Elzanowski (1989) argued that de Beer’s hypothesis of neoteny for ratites – enthusiastically promoted by Alan Feduccia (1996) and colleagues – was politically motivated as well as weakly supported. Since 1930, de Beer was leading a new ‘anti-recapitulationism school’ and hence was seemingly opposed to the idea that the ‘primitive’ characters seen in ratites might really be retentions from an earlier stage in evolution. Rather, he wanted them to be evolutionary novelties that had arisen from neognath-like ancestors.
In short, while it’s true that some ratite features do recall those that arise in neognaths forced to become neotenous adults (Dawson et al. 1994), the idea that neoteny is an important driving force behind ratite evolution is problematic and probably false. Firstly, their supposed neotenic features are either primitive characters retained from earlier birds, or novelties that have arisen for good reason, just as they have done – convergently – in some flightless neognaths. Secondly, ratites clearly aren’t neotenic in several organ systems, nor overall, but more plausibly look ‘hypermorphic’ more than they do ‘paedomorphic’. Thirdly, the idea that ratites are neotenic was originally proposed to help support an agenda (de Beer’s anti-recapitulationism of the 1930s, 40s and 50s), and has been supported since by workers promoting another agenda – this time, the idea that ratites might have evolved independently from diverse neognath ancestors.
There’s lot more I want to say on ratites and other palaeognaths, and in the next article we’ll start by looking at the ‘ratite polyphyly’ hypothesis.
For previous Tet Zoo articles on ratites and neornithine bird evolution in general, see…
Refs – -
Bock, W. J. 1963. The cranial evidence for ratite affinities. Proceedings of the XIII International Ornithological Congress, 39-54.
Cracraft, J. 1974. Phylogeny and evolution of the ratite birds. Ibis 116, 494-521.
- . 2001. Avian evolution, Gondwana biogeography and the Cretaceous-Tertiary mass extinction event. Proceedings of the Royal Society of London B 268, 459-469.
Dawson, A. 1996. Neoteny and the thyroid in ratites. Reviews of Reproduction 1, 78-81.
- ., McNaughton, F. J., Goldsmith, A. R. & Dgen, A. A. 1994. Ratite-like neoteny induced by neonatal thyroidectomy of European starlings, Sturnus vulgaris. Journal of Zoology 232, 633-639.
De Beer, G. 1956. The evolution of ratites. Bulletin of the British Museum of Natural History (Zoology) 4, 59-70.
Elzanowski, A. 1989. Ontogeny and evolution of the ratites. In Ouellet, H. (ed) Acta XIX Congresseus Internationalis Ornithologici, volume 2. University of Ottawa Press (Ottawa), pp. 2037-2046.
Feduccia, A. 1996. The Origin and Evolution of Birds. Yale University Press, New Haven & London.
Härlid, A. & Arnason, U. 1999. Analyses of mitochondrial DNA nest ratite birds within the Neognathae: supporting a neotenous origin of ratite morphological characters. Proceedings of the Royal Society of London B 266, 305-309.
Lee, M. S. Y., Cau, A., Naish, D. & Dyke, G. J. 2014. Morphological clocks in paleontology, and a Mid-Cretaceous origin of crown Aves. Systematic Biology doi:10.1093/sysbio/syt110
Naish, D. 2012. Birds. In Brett-Surman, M. K., Holtz, T. R. & Farlow, J. O. (eds) The Complete Dinosaur (Second Edition). Indiana University Press (Bloomington & Indianapolis), pp. 379-423.
Rafferty, K. L., Herring, S. W. & Marshall, C. D. 2003. Biomechanics of the rostrum and the role of facial sutures. Journal of Morphology 257, 33-44.