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Re: origin of Palaeognathous Pallet
From: John Bois <email@example.com>
This was from an abstract from SVP (I think) a year or so ago. If the
ostrich and emu descended (as it were) from different volant ratites,
doesn't this challenge the no-neoteny idea for ratite origins. Or, is
there something about primitive ratites that predisposed them to dominate
the flightless niche?
HETEROCHRONY SUGGESTS MULTIPLE FLIGHT LOSS EVENTS IN THE
MARSHALL, Cynthia L., Museum of the Rockies, Montana State
University,>Bozeman, MT, USA>Studied wing development/loss in ratite
embryos. Finds very different>sequence in emus vs. ostriches.>Proposes
that emus and clan were small birds that became big, and that ostriches
were large fliers.
Good point, although not so much as the study by Gussekloo, this study does
weigh against a neotanous origin of the palaeognathous pallet. The neotany
in ratites theorist (Feduccia 1999) suggests that an extensive suite of
embryonic features, including those that unite the ratites, could be
retained by an under-developed chick, who better competes without the
expensive burden of flight-related features. Other workers have suggested
that this ontogenous arrested-evolution could have been caused by radiation,
vitamin deficiencies, or lower levels of thyroid hormones.
Cynthia observed wings in the emu and ostrich embryos growing at a slower
rate for the emu and a longer rate for the ostrich than the ancestral
condition, the ancestral condition being that exhibited by a tinamou and a
chicken. If ratites were united solely by neotany, you would not expect to
see different wing growth rates though out their ontogeny. Additionally, you
would not think that an ostrich would loose flight apparatus through
neotany, enough to gain a palaeognathous pallet, only to later grow longer
wings for an extended time compared to the ancestral condition.
Although I don?t see how Lydia?s study proves that the ostrich evolved from
a bird that had reached maximum flight size, as she suggests, her study does
weigh against the ostrich and emu evolving from the exact same volant bird.
Perhaps there was a southern radiation of ratites, evolved directly from the
proto-tinamou; and an additional northern radiation which evolved from a
bigger and better flyer. However, the rhea actually has some pretty fair
sized wings, and DNA studies reveal that he is more closely related to the
ostrich than the emu.
I?ve been wondering why tinamous never become flightless. While South
America is not the Splendid Isolation that it once was, you would think that
there would be at least one recent example of a 2F tinamou somewhere.
Perhaps the sternum of the tinamou ossifies too early in ontogeny for
neotany to take place. This would not have been the case in the
proto-tinamou, the common ancestor of both the tinamou and the ratites.
(See, I am pro-neotany to an extent).
P.S. On a related note, I noticed that Houde 1987 examined Hesperornis for
similarities in the pallet. He noted that it did contain some of the
elements of the ratite paleognathous pallet, but that as constructed in the
model, its jaw was much too derived to be directly related. Addtionally he
found the bone structure to be different, the structure of the ratites being
unique, and that of the tinamous half way there. Hesperonis could have
derived its skull thorough neotany, and then evolved the derived jaw later.
Too bad we cannot watch the embryo grow.
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