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Re size of Mesozoic birds, Nick Longrich says...

> The exceptions- Patagopteryx and Hesperornis- 
> are both flightless. 

Buffetaut et al. described a synsacrum of what would appear to be a
large (ostrich-sized) bird, found in Upper Cret French rocks. They
compared the material with that of flamingoes, but emphasised that
this didn't necessarily denote relationship. If the material is of a
bird, which they were sure was the case (the same degree of
intervertebral fusion hasn't yet been recorded in non- avians, if I
remember correctly), it shows that big ones were around at this time -
but then, we don't know if it was volant or not. If so, we have the
very interesting possibility of a very big - say, teratorn sized -
flying bird in the late Cretaceous! This specimen asides, the biggest
volant Mesozoic bird I'm aware of is an 'eagle-sized' enantiornithine
that Phil Currie is working on.

Nessov has described some pretty big bird taxa from the Upper Cret of
Asia, but, needless to say, these are based on little bits, such as
jaw tips. Some of these he has referred to extant orders, including
(the biggest one I can think of) a Cretaceous frigate bird. In
discussing these forms, he notes that azhdarchids always had pointy,
stork-type bills, while these large marine birds had hooked beaks:
were sympatric big birds and pterosaurs living side-by-side, and not

To return to the French synsacrum, if this bird, if it was a bird, was
flightless, we again have a remarkable possibility - an ostrich-sized
bird living alongside non-avian dinosaurs! If this is a reality, it
resurrects the image that prevailed (see Swinton's 'Fossil Birds')
before it was realised that 'Caenagnathiformes' were related to
_Oviraptor_ - Sternberg originally described these theropods (from
lower jaws) as large birds.

Incidentally, Buffetaut et al. suggested that at least some of the
numerous French Upper Cret. fossil eggs might be birdy ones, in view
of this discovery.

> The lack of large birds seems to be the result of competition from
> pterosaurs, no?

Maybe, maybe not. Vagaries of the fossil record as usual.

If megalancosaurids are truly ornithodiran, they provide powerful
support for BCF. Can the affinities of these critters be resolved?