Field of Science

Taxon of the Week: Leg or Breast?


This week's highlight taxon is one that is very familiar to me as a New Zealander, except it's not really. I've heard of these creatures since I was a little lad, and representations of them have been almost everywhere I've gone. I've never actually seen one. I don't know anyone who's every seen one. Probably no-one has seen one for hundreds of years, in fact. The Dinornithiformes are but a memory these days, long since converted into quarter-pack meals for Polynesian settlers. Ka ngaro i te ngaro a te moa - lost as the moa is lost.

The taxonomy of moa is complicated, but at present there are eleven species recognised as valid*. The order was unique to New Zealand - the "Australian" Dinornis queenslandiae De Vis, 1884, was based on a partial femur in the Queensland Museum, but this bone is now believed to have come from New Zealand and is assigned to Pachyornis elephantopus. In the past, Dinornithiformes has been divided into two families - the lightly built, more cursorial genus Dinornis in its own family and the other smaller and/or more heavily built genera in the Emeidae, but phylogenetic analysis has shown that Dinornis is nested within the Emeidae (Worthy & Holdaway, 2002).

*Eleven species were recognised in Worthy & Holdaway (2002), the most recent major review of Dinornithiformes. Bunce et al. (2003) reduced the number of species of Dinornis from three to two (see below), but Baker et al. (2005) increased the number of species of Megalapteryx from one to two.

The photo at the top of the page (from Wikipedia) shows the reconstructed Dinornis in the Auckland Museum. This specimen has been around for some time - it was built in 1913, though when the Natural History section of the museum was rebuilt it lost the tussock-land diorama it had previously inhabited (if I recall correctly) and moved into a glass case. The Auckland Museum moa stands about three metres tall, and modern interpretations would, unfortunately, label this a severely inaccurate reconstruction. It has been mounted in an unnaturally elevated stance, and should have been much more low-slung. A more realistic reconstruction was shown on a recent stamp issue, shown below (from New Zealand Birds):



Such a lowered reconstruction significantly lowers the height, but we're still looking at about two metres for the tallest Dinornis specimens. A moa of this size may have wighed over 150 kg (Worthy & Holdaway, 2002). Other species were smaller - the smallest was Megalapteryx didinus at probably about 40 kg. The super-heavy Pachyornis elephantopus was considerably shorter than large Dinornis, but may have weighed about the same amount. The image below comes from Nature, and shows three moa species against a 1.8 metre tall human. From left to right, the moas are a female Dinornis novaezealandiae, Megalapteryx didinus and Pachyornis elephantopus.



Despite their extinction prior to European settlement in New Zealand, a surprising amount of molecular data has been gleaned from ancient DNA studies of moa. Among other things, said molecular analyses have demonstrated that Dinornis displayed the highest degree of size dimorphism known from any bird (Bunce et al., 2003). Previously Dinornis had been divided into three species on the basis of size. Bunce et al. tested DNA from Dinornis remains for female-specific markers (birds differ from mammals in that it is the female that possesses different sex chromosomes [ZW], while the male has identical sex chromosomes [ZZ]). They found that all specimens that had been assigned to the smaller 'species' Dinornis struthoides were male, while all specimens of the larger 'species' D. novaezealandiae and D. giganteus were female. Molecular phylogenetic analysis also showed that these specimens were intermingled, with the major divide in the genus being not by size but by geography - the North Island and South Island populations (both containing representatives from all three 'species') were distinct, and were recognised as the separate (but morphologically indistinguishable) species Dinornis novaezealandiae (North Island) and D. robustus (South Island). While females showed a great deal of variation in size, the largest females would have been about 280% of the weight and 150% of the height of the largest males.

There is very little reliable information on the life habits of moa. By the time of European settlement, it had been long enough since the extinction of the moa that records of it in Maori oral tradition were seemingly few and far between, and those that were present had become significantly mythologised. Early researchers such as Owen and Haast interpreted moa as birds of open country, comparing them to modern ostriches and emus. However, the extensive New Zealand grasslands and fernlands these authors pointed to hadn't existed prior to human settlement, so moa were undoubtedly forest birds, foragers rather than grazers as also shown by preserved gizzard contents. Dinornis and Pachyornis seem to have had more fibrous diets with gizzards containing twigs and fibrous plants such as Phormium (the New Zealand flax) while Emeus and Euryapteryx with less robust bills had more selective diets of fruit and leaves. Interestingly, a well-preserved specimen of Euryapteryx geranoides showed a massive intrathoracic loop in the trachea, 1.2 metres long. In other birds such loops are associated with the ability to make loud, far-carrying calls, so moa (or at least Euryapteryx) would have been quite vocal birds in life.

REFERENCES

Baker, A. J., L. J. Huynen, O. Haddrath, C. D. Millar & D. M. Lambert. 2005. Reconstructing the tempo and mode of evolution in an extinct clade of birds with ancient DNA: The giant moas of New Zealand. Proceedings of the National Academy of Sciences of the USA 102(23): 8257-8262.

Bunce, M., T. H. Worthy, T. Ford, W. Hoppitt, E. Willerslev, A. Drummond & A. Cooper. 2003. Extreme reversed sexual size dimorphism in the extinct New Zealand moa Dinornis. Nature 425: 172-175.

Worthy, T. H., & R. N. Holdaway. 2001. The Lost World of the Moa: Prehistoric life of New Zealand. Indiana University Press: Bloomington (Indiana).

15 comments:

  1. Are they closely related to the kiwi? I seem to recall a Gould essay that suggested this.

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  2. Ha! You know not what you ask...

    Most earlier authors supported a moa-kiwi connection, and morphological analyses have tended to support this, usually with the New Zealand ratite clade fairly low down on the tree. Molecular data, on the other hand, separates the two. Moa are still low on the tree (possibly even outside the clade of living ratites) while kiwis are sister to the Casuariiformes (emus and cassowaries).

    I'm inclined to support the molecular data, more because of the kiwis than the moa. Kiwis are highly apomorphic compared to other living ratites, being the only clade of small insectivores/vermivores instead of large herbivores, and I suspect that their oddball nature may be giving them a spuriously low position in the tree. I have to admit that I don't necessarily have a lot of hard support for my position, though.

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  3. Thanks! That is what I was after.

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  4. Also the fact that moa uniquely amongst birds didn't have wings at all, while kiwi do.

    I've always wondered what moa did with their head while they slept. Perhaps they died out of never being able to get a good night's sleep :)

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  5. Also the fact that moa uniquely amongst birds didn't have wings at all, while kiwi do.

    Not uniquely, as it happens. Another New Zealand bird decided to ditch those useless little sticking-out bits - the adzebill Aptornis. Aptornis was a large bird with a ridiculously over-powered skull and dubious habits - it was probably a generalist feeder, possibly using its beak to break open fallen logs to get at animals, insects, etc. within.

    The affinities of Aptornis are arguably even more obscure than those of the moa. It is a member of the polyphyletic "Gruiformes". Morphological data connects it with Rhynochetos, the endangered kagu of New Caledonia, but suffers from the extreme wierdness of Aptornis. Molecular data connects it with the Rallidae (rails), but suffers from the fact that only the merest fragment of ancient DNA has been recovered. If the Metaves-Coronaves division of modern birds is correct, then Rhynochetos is Metaves while Rallidae is Coronaves, so this is a pretty significant conflict between results. In this case, I'd favour the morphology, because the amount of molecular data is still just pitiful.

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  6. Whoops, it seems I must correct myself. Aptornis had stupidly small wing bones, maybe even smaller proportionally than a kiwis, but it did still have them. They probably wouldn't have been visible under the feathers, but then neither are a kiwi's.

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  7. Hasn't there also been a cladistic analysis that got Aptornis as sister of the Galloanserae?

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  8. I don't know if it was a full cladistic analysis or not, but Weber & Hesse (1995) did suggest a relationship to Galloanserae based on features of the lower jaw. Worthy & Holdaway (2002) suggested that these similarities might be functional convergences, and noted one feature that was supposed to be unique to Galloanserae and Aptornis to be very similar to a character of the rail Porphyrio. I haven't personally read the Weber & Hesse study to comment on it, I must admit.

    So if you also add the fact that Richard Owen originally described Aptornis as a species of Dinornis, the adzebill has at some time or another been assigned to every one of the major clades of modern birds - Palaeognathae, Galloanserae, Metaves, Coronaves. Can any other bird claim such loose associations?

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  9. When we drove past an emu farm recently, I asked my girlfriend if she would let me clone a moa and ride it around Davis. She seemed dubious...

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  10. Chris;

    Curious about the observations about no wings. Given I am an engineer not a biologist please bear with me? How do we know it is a bird? In other words, did it loose wings or arms? I understand it hasn't a breastbone either.

    Clifford M Dubery

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  11. Clifford: Despite the lack of wings, moa are still undoubtedly birds - there are still a large number of other characters shared with other birds, and more specifically with other ratites. Take a look at the skull and the legs, for instance. The absence of wings in moa is a derived feature, so moa would have indeed descended from an ancestor with wings (or at least forelimbs). Moa and other ratites do have a breastbone - what they lack compared to other birds is the carina, the deep keel on the front of the breastbone that anchors the enlarged wing muscles. Other flightless birds show various stages of reduction of the carina, but I don't know whether any non-ratite birds have entirely lost it as in ratites.

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  12. Oh and Neil: if what I've seen on the telly of ostrich-racing is any indication, I'd rather doubt the suitability of moa as a mount. You'd be far better off using your cloned moa for the world's biggest Chicken McNugget.

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  13. Christopher; thank you for clearing that up in my mind for what it is worth:), Certainly a curious animal, and the ecological placement of the Kiwi (I watched an ABC dodo on the subject), in the scavenger area, like a rodent. Living in Australia, we can see the adaptation to niches in the environment where more common Northern Hemisphere rodents occupy.

    Cheers and keep up the good work.

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  14. Thanks for the answer! Another bird that has also been shoved around a lot would be Gastornis (Diatryma). It has been regarded as a ratite, an anseriform (probably correctly), a giant heron, a relative of albatrosses, a relative of parrots...
    The hoatzin was also placed nearly everywhere, from "most primitive living bird" to passeriform.

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