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

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.

A simplified summary of the "island rule" is that big taxa get smaller
(dwarfism) and small taxa get bigger (gigantism).  Certain studies
(e.g., see papers by Shai Meiri &c) have determined that, as a
*general* rule, the former is true for large mammals, and the latter
is true for birds (where gigantism is often associated with
flightlessness); but these studies found no statistical support for
other aspects of the island rule for either mammals or birds in
general.  Nevertheless, there were certain exceptions, such as murid
rodents (e.g., _Papagomys_), which tend to get larger on islands; but
such examples of insular gigantism were atypical of small mammals in

Certain large dinosaurs do appear to conform to the island rule, such
as the titanosaur _Magyarosaurus_ and the hadrosaur _Telmatosaurus_
from the Hateg island, as discussed by Benton et al. (2010).  However,
Benton &c also note that Hateg dromaeosaurid and troodontid theropods
appear to be no smaller than their counterparts from North America and
Asia.  This was affirmed by the discovery of _Balaur_ by Csiki et al.
(2011), who further noted that "predators on these [European] islands
were not necessarily small, geographically endemic, or primitive."

So while we might expect a secondarily flightless, island-dwelling and
presumably herbivorous bird like _Gargantuavis_ to get bigger, we
might not expect to see any profound shift in body size by co-existing
Ibero-Armorican predatory dromaeosaurids and abelisaurids.