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A.T. White on the Origins of the Ostrich and Emu

The internal relationships of ratites are still more or less anyone's guess.

It's possible to find even recent papers by good scientists which come up with completely hair-brained results, such as ostriches being the sister of kiwis. Bertelli & Porzecanski (2004). There's a general sense that the ostriches split off early, just after tinamous, and that the emus diverged late. However, nothing is really fixed. E.g. van Tuinen et al. (1998); Cooper et al. (2001); Edwards et al. (2002).

That's the bad news. The good news is that the vicariance theory is looking better and better for other reasons. The vicariance theory states that ostriches split from other ratites because they were physically isolated from them in Africa, when it broke off from Gondwana at about 90 Ma. This requires ostriches to have split off early since paleontology puts them in Africa, and Africa broke free from Gondwana quite early.

The problem has been that molecular clock models based on mitochondrial DNA work say that the ostrich split was too recent to permit either path. Hedges' group, who wanted to hang on to both molecular clocks and vicariance, have to invoke rafting as a way to get ostriches to Africa. van Tuinen et al. (1998). It now turns out that, even if one generally buys into molecular clocks, ostriches don't seem to wear the same clock as other birds. Cooper et al. (2001). More refined calculations, show a basal ostrich divergence time of 75Ma, which matches the split between Africa and Gondwana much better. Id.

Vicariance is further bolstered by recent findings that Africa was really quite isolated, even for flying birds, for a substantial period of time. Ericson et al. (2003); Beresford et al. (2005). Thus, even if ratites were still flying birds in the Late Cretaceous, they were stuck on the African ship once it left the all the Gondwanan connections behind.

Finally, the whole radiation of modern birds keeps getting pushed back, particularly with the recent definitive finding of a duck/goose type bird with the unlikely name of Vegavis iaai from the Cretaceous of Antarctica. Clarke et al (2005). As those authors point out, the existence of an (almost) duck in the Late Maastrichtian (last age of the Cretaceous), means that a lot of other modern birds lines must have diverged by 67 Ma. Since everyone concedes that the ratites evolved before any other modern bird, that likely puts them at least as early as 75 Ma, probably substantially earlier, and gives the ostrich ancestor more time to catch his African boat.

OK. If ostriches and emus split off from each other 75+ Ma, why do they look so much the same? The answer seems to be that, as you said, they are both specialized for running. In fact, they are more specialized for running than almost any other animal.

As the locomotion expert, John Hutchinson, recently stated, "... these two birds have relatively the largest total hindlimb extensor muscles yet recorded for any tetrapods." Hutchinson (2004)

So, potentially, they are the champion runners of all time (not fastest, but a combination of speed, power, and endurance). This is particularly true of the emu, which devotes almost 1/3 of its entire body mass, bones and all, to leg extensor muscles. Both birds have light compact bodies.  That kind of compact body shortens the horizontal moment arm created by the body, allowing quick changes in direction, while the low body mass not only takes less energy to move, but also reduces the vertical moment arm created by the long legs, making it easier to maintain balance (listen up, SUV owners!).

 Both have short, almost horizontal femurs and very long shins and ankles. Hutchinson (2004). These maximize acceleration and mechanical advantage while running. Both have long necks and good vision which allow them to take advantage of that speed in open country.

Like specialization for flying, extreme specialization for running, or anything else, tends to make animals look the same , even if they started with quite different body plans. If, like ostrich and emu, they diverged from a single ancestor who was (most likely) already specialized for running, they will remain very similar, no matter how long they have been isolated.

Cites:

Beresford P, FK Barker, PG Ryan & TM Crowe (2005), African endemics span the tree of songbirds (Passeri): Molecular systematics of several evolutionary 'enigmas'. Proc. R. Soc. B 272: 849-858

Bertelli, S & AL Porzecanski (2004), Tinamou (Tinamidae) systematics: a preliminary combined analysis of morphology and molecules. Ornitol. Neotrop. 15 Supp., 7 pp.

JA Clarke, CP Tambussi, JI Noriega, GM Erickson & RA Ketcham (2005), Definitive fossil evidence for the extant avian radiation in the Cretaceous. Nature 433: 305-308.

Cooper, A, C Lalueza-Fox, S Anderson, A Rambaut, J Austin & R Ward (2001), Complete mitochondrial genome sequences of two extinct moas clarify ratite evolution. Nature 409: 704-707.

Edwards, SV, B Fertil, A Giron & PJ Deschavanne (2002), A genomic schism in birds revealed by phylogenetic analysis of DNA strings. Syst. Biol. 51:599-613.

Ericson, PGP, M Irestedt & US Johansson (2003), Evolution, biogeography, and patterns of diversification in passerine birds. J. Avian Biol. 34: 3-15.

JG Groth & GF Barrowclough (1998), Basal divergences in birds and the phylogenetic utility of the nuclear RAG-1 gene. Mol. Phylo. & Evol. 12:115-123.

Hutchinson, JR (2004), Biomechanical modeling and sensitivity analysis of bipedal running ability. I. Extant taxa. J. Morph. 262: 421-440.

van Tuinen, M, CG Sibley, & SB Hedges (1998), Phylogeny and biogeography of ratite birds inferred from DNA sequences of the mitochondrial ribosomal genes. Mol. Biol. Evol. 15: 370-376.

AT White