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Re: Ah ha! That's where therizinosaurs came from
Your note stimulated my curiosity about moas. I did some research on them and
they are fascinating, I have ordered a book by Alan Tennyson and will be doing
more reading on them.
But I did want to report what I'd learned so far regarding the biological
similarities and differences between moas and dromaeosaurs. My point is just to
compare and contrast the two families, and assess their relative levels of
diversity, in light of the hypothesis that both may be radiations of
secondarily flightless maniraptorans.
Moas seem to have achieved a diversity of probably only 9 species in 6 genera,
with possibly two more known from Miocene fossils. (Tennyson et al. 2010).
Their body masses ranged from 20kg to around 240kg. Moas achieved this
diversity in a minimum of 22 to 25 million years after NZ was inundated, though
many suspect there were refugia that extended the moa lineage back much further.
Dromaeosaurs have about 32 to 35 described species in maybe 30 genera. Their
body masses range from 200 grams in Microraptor zhaoianus (Turner et al., 2007)
to 500kg in Utahraptor. They range from teeth known from the Middle Jurassic to
end Cretaceous, so maybe 100 million years total.
There are many similarities and differences we could tally. Most, but not all,
ratites and other large flightless birds seem to have tended to become
graviportal browsers. The dromaeosaurs may have ranged from sleek hunters to
tiny gleaners. The large flightless birds seem to have been geographically
restricted, while dromaeosaurs were cosmopolitan. Feduccia also reports that
flightless birds tend to develop longer life spans. That could be tested in
dromaeosaurs against basal avialans and Archaeopteryx to see if it was
On Aug 9, 2011, at 5:45 PM, Ronald Orenstein wrote:
> Leave out "achieved worldwide distribution" and the moas come pretty close.
> Ronald Orenstein
> 1825 Shady Creek Court
> Mississauga, ON L5L 3W2
> From: Jason Brougham <firstname.lastname@example.org>
> To: email@example.com
> Cc: DML <firstname.lastname@example.org>
> Sent: Tuesday, August 9, 2011 4:57:17 PM
> Subject: Re: Ah ha! That's where therizinosaurs came from
> Yes, let's keep the point in mind: that no family of ratites or other
> flightless crown group birds that I am aware of has become flightless,
> achieved the morphological distinction of a new family, and then radiated
> widely, diversified, achieved a range of body sizes spanning two orders of
> magnitude, and achieved global distribution, in the way that dromaeosaurs
> did, if we assume that BCF is true.
> On Aug 9, 2011, at 4:26 PM, David Marjanovic wrote:
> >> >> Secondly, failure of any recent group of flightless birds to
> >> >> spread over the world is in part due to the recent positioning of
> >> >> the continents (ie there are oceans in the way). Presumably
> >> >> flightless birds have only dispersed over a land connection (I
> >> >> can't think offhand of an example of a flightless bird spreading
> >> >> by, say, rafting).
> >> >
> >> > Gondwana and Lauarsia were separated, but they shared several
> >> > groups of maniraptorans in common.
> > That separation only came about in the Late Jurassic and was rather
> > intermittent. In contrast, I don't think there ever was a unitary Laurasia
> > that wasn't cut in two by various seas through Europe.
> >> >> Third, the fauna of mammalian predators and nest-robbers has been
> >> >> quite different since the K-T from what it was earlier, and the
> >> >> barriers to radiation of flightless birds on continents may not
> >> >> therefore be identical to what they might have been in the
> >> >> Mesozoic.
> >> >
> >> > Different, certainly, but there is no reason to think the
> >> > differences were consequential. Mammals were feeding on dinosaurs
> >> > in the Mesozoic, they could glide, they could swim, no reason to
> >> > think they couldn't rob nests.
> > Never mind nest robbers. It now seems that the flightless paleognaths
> > became flightless in the early Paleocene on the continents as if they were
> > islands -- due to the lack of big predators after the K-Pg mass extinction.
> >> > But, in the case of rails, the morphological changes
> >> > associated with flightlessness were so recent and superficial that
> >> > the resulting species did not move into new families, unlike with
> >> > the ratites. In maniraptorans we had, I guess, Troodontidae and
> >> > Dromaeosauridae that Paul might say descended from Archaepterygids,
> >> > though he may group them all in the same family.
> > Isn't that just a matter of time? Let those flightless rails sit on their
> > islands for 30 million years (...longer than most of the islands in
> > question will exist, but never mind...) and then tell me what ranks you'd
> > assign to them.
> Jason Brougham
> Senior Principal Preparator
> American Museum of Natural History
> (212) 496 3544
Senior Principal Preparator
American Museum of Natural History
(212) 496 3544