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Re: Island-dwelling dinosaurs (was Re: Gargantuavis neck vertebra)



On Sun, June 17, 2012 3:15 pm, David Marjanovic wrote:
> Am 15.06.2012 um 07:01 schrieb Tim Williams:
>
>>  David Marjanovic <david.marjanovic@gmx.at> wrote:
>>
>> >> The _Gargantuavis_ type material and the new neck vertebra come
>> >> from the Ibero-Armorican island. Cretaceous birds of Europe may
>> >> have been especially prone to secondary flightlessness.
>> >
>> > Despite the contemporary presence of dromaeosaurids and
>> > abelisaurids of usual sizes? (Buffetaut pointed this out as a
>> > mystery in his talk.)
>>
>>  No mystery at all.
>
> I wasn't talking about the size but about the flightlessness. Why would
> a bird become flightless in the presence of terrestrial predators? The
> ratites, gastornithids, dromornithids and others now seem to have become
> flightless in the Paleocene, which was almost devoid of large
> terrestrial predators.

Do we really know this? Especially for the ratites: how good is our timing
of the origin of flightlessness in this bunch?

Furthermore, crocodylomorph and squamate terrestrial predators were
present on both Laurasia and Gondwana at this time.

Additionally, phorusrhacids seem to have developed flightlessness in the
presence of marsupial terrestrial carnivores.

It may be that we are conflating the origins of flightlessness in island
communities with flightlessness in continental ones: perhaps they are
produced by different sets of selective pressures.

An analogy: the origin of gigantism in island populations may stem from
causes very different from the origins of gigantism in (for example)
sauropods and proboscideans.

Thomas R. Holtz, Jr.
Email: tholtz@umd.edu   Phone: 301-405-4084
Office: Centreville 1216
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
http://www.geol.umd.edu/~tholtz/
Fax: 301-314-9661

Faculty Director, Science & Global Change Program, College Park Scholars
http://www.geol.umd.edu/sgc
Fax: 301-314-9843

Mailing Address:        Thomas R. Holtz, Jr.
                        Department of Geology
                        Building 237, Room 1117
                        University of Maryland
                        College Park, MD 20742 USA