[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: origin of Palaeognathous Pallet



From: John Bois <jbois@umd5.umd.edu>

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 RATITES 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).

Thanks,

Evan Robinson

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.

_________________________________________________________________
Get ready for school! Find articles, homework help and more in the Back to School Guide! http://special.msn.com/network/04backtoschool.armx