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Monophyly of ratite flightlessness? (was Re: More on Argentavis)



----- Original Message ----- From: "Tim Williams" <twilliams_alpha@hotmail.com>
Sent: Thursday, July 05, 2007 4:16 AM


Evelyn Sobielski wrote:

Ratites are a peculiar case, their flightlessness is
probably very highly convergent (based on place and
time of fossil record of paleognaths).

There is some disagreement on this. However, Cooper et al. (2001)'s mitochondrial phylogenetic study support "a Late Cretaceous vicariant speciation of ratite taxa, followed by the subsequent dispersal of the kiwi to New Zealand." This implies that dispersals were land-based, with the ancestors of the ostrich and elephant-bird each crossing the Kerguelen Plateau to Indo-Madagascar and (for the ostrich lineage) on to Eurasia; and kiwi ancestors crossing over to NZ from Australia via the Norfolk Ridge or Lord Howe Rise some time later (65-72 Mya). This means that flightlessness *could* be primitive for ratites, although I agree that it is not direct proof that this was the case.

The topology found by Cooper et al. does not seem to test the hypothesis of ratite monophyly, because all non-ratites are apparently outgroups. However, molecular analyses have always found monophyletic ratites, so I think ratite monophyly is a safe assumption.


For context, here's the tree by Cooper et al. (fig. 2), with bootstrap values:

,--chicken
|--tinamous
`--+--rhea
  `--65--100--*Emeus* (moa)
     |  `--*Dinornis* (moa)
     `--62--ostrich
        `--67--? *Mullerornis* (elephantbird)
           |--kiwi
           `--100--cassowary
              `--emu

Concerning divergence dates, however, I let Cooper et al. speak for themselves (p. 705sq.):

"There are two potential calibration points: first, Mid to Late Oligocene fossils which show that the emu and cassowary lineages had clearly separated sometime before 25 Myr ago (ref. 15), probably around 30--35 Myr (W. Boles, personal communication); second, the geological split between New Zealand and Australia/Antarctica around 82--85 Myr (refs 16, 17), which has been related to the basal separation of the moa from the other ratite taxa^14,18,19. Unfortunately, like many terrestrial avian groups the rest of the ratite fossil record is relatively poor, with only the rhea and ostrich known from the early Tertiary^4,9,20. These fossils can provide only a minimum age for these long unbranched lineages (Fig. 2).
Minimal divergence dates for the ratite lineages were estimated using the geological calibration point (Table 2). These molecular estimates predict an emu/cassowary split at 33--39 Myr, encouragingly consistent with the palaeontological data. The concordance between the molecular estimates and the two calibration points give strong support for a Late Cretaceous origin for ratites and, by implication, for other modern avian orders. These divergence dates are compatible with vicariance events during the break-up of Gondwana, apart from the kiwi, for which a subsequent dispersal event is required."


With six years of hindsight, I think I can comfortably say that these two paragraphs from the Bronze Age of molecular dating sound remarkably naïve. To wit:

- Cooper et al. used _*A SINGLE CALIBRATION POINT*_, for crying out loud. Nothing good can come out of that; the more calibration points, and the more regularly spaced on the tree (young and old, inside and outside the ingroup), the better (Brochu 2004, Marjanovic & Laurin 2007 :-° ).
- That calibration point rests on the _assumption_ that flight was lost once or at most twice within ratites (that would be once in rheas and once in the ancestor of all other ratites). That, obviously, should have been among their conclusions, not among their basic assumptions.
- Ironically, that assumption requires that the kiwi reached NZ from Australia by some other means than flying long after the moa walked there. Cooper et al. repeatedly point this out. So why didn't they postulate the kiwi-(emu + cassowary) divergence at 82 to 85 Ma and avoid the oceangoing kiwi problem? I don't think they've tried what would happen -- and I think what would happen is that the origin of ratites would be dated to, say, the Permian.
- Using that speculative and single calibration point produces an estimate for the "emu/cassowary split at 33--39 Myr". This is "encouragingly consistent with the" guess that 5 to 10 million years may have been necessary to derive the oldest known emu, the reportedly 25 Ma old *Emuarius gidju*, from the MRCA of emus and cassowaries. Whoever "W. Boles" is, guesses about the speed of morphological evolution are just that: guesses. They can't be used as evidence.
- "The concordance between the molecular estimates and the two calibration points" is an unambiguous failure of peer-review. There is only one calibration point, as made explicit two sentences before. This calibration point was used to calculate the molecular divergence dates, and those dates are vaguely consistent with a guess based on another "potential calibration point" which was not used to derive the molecular dates. In other words, it's not circular, but Cooper et al. write as if it were. It's just vague. -- All we can say is that the estimate for the emu-cassowary divergence is consistent with the age of the oldest known representative of that clade in not being younger than that age, but that had to be expected (Brochu 2004, me & Laurin 2007) from a Santonian to Campanian calibration point close to the base of the tree.


Conclusions: Cooper et al. (2001) have not found any evidence for Cretaceous ratites (let alone "other modern avian orders") or for 2 or fewer losses of flight in ratites. Take the topology they have found seriously, but ignore their attempts at molecular dating.