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Re: [dinosaur] Evolution of giant flightless birds (free pdf)



Ben Creisler <bcreisler@gmail.com> wrote:

> (Note regarding the recently proposed Vegaviidae.....
>
> "A fourth clade is represented by the Cretaceous Vegavis (Antarctica), which 
> was strongly excluded from Anseriformes")


Yes this study agrees with the Agnolin &c study that _Vegavis_ is not
a crown anseriform.  However, its sister group (Gastornithiformes
versus Anseriformes) varied according to the analysis.  To be honest,
I don't see the justification for erecting a whole new 'order'
(Vegaviiformes) for _Vegavis_ (and presumably other vegaviids).  The
extinct sylviornithids were found to be outside crown Galliformes, but
in this case Worthy &c extended the definition of Galliformes to
include Sylviornithidae.  Even if _Vegavis_ is hard to place, and
clearly outside the Anseriformes crown clade, this doesn't seem to be
a compelling reason for creating a new order.

The statement by Worthy &c that _Vegavis_ is "the only well-supported
neornithine from the Cretaceous" is interesting, because it reverses a
recent interpretation of the Nemegt avian _Teviornis_ (Late
Cretaceous, Mongolia) as a presbyornithid and a crown anseriform (De
Pietri et al. 2016 dx.doi.org/10.1098/rsos.150635).  The latter study
offered a fierce defense of the original referral of _Teviornis_ to
Presbyornithidae by Kurochkin et al. (2002), and pointedly rebutted
the assertion by Clarke & Norell (2004) that _Teviornis_ was neither a
presbyornithid or a crown anseriform.  (Hornerstown taxa like
_Anatalavis_ are presumably disregarded as Cretaceous neornithines -
either because they are dubiously Cretaceous, dubiously neornithine,
or both.)

Worthy &c infer that the most recent common ancestor of dromornithids
and gastornithids was volant.  This is not what their trees state -
but to be fair, this is based on the most parsimonious interpretation
of the distribution of characters, which would have the common
ancestor of this group being large and flightless, which "appears
biologically implausible, given their Paleocene age and wide
geographical distribution".  Makes sense.  According to this
hypothesis, there were two independent shifts to secondary
flightlessness in the dromornithid-gastornithid clade following the
dispersal of the separate volant ancestors of gastornithids and
dromornithids. If gastornithids are paraphyletic, there was an
additional loss of flight, with the different _Gastornis_ species
evolving from separate volant ancestors.  Thus, many of the features
shared by the _Gastornis_ species would be attributable to "convergent
evolution of gigantism and slow walking habit".  On that point, Worthy
&c flag the possibility (their words) that the two _Gastornis_ species
(_G. parisiensis_ and _G. giganteus_) might not be congeneric, since
there are substantial differences (>20 characters) between the two
species, and the Bayesian analysis recovered the two species as
paraphyletic.  Not mentioned is that if this finding is upheld it
would mean the resurrection of the genus _Diatryma_.

Overall, this study affirms something I've long been saying: That
flight in birds is a burden if they don't need it.  Flight places a
low ceiling on body mass, which is a disadvantage when processing
large amounts of poor-quality plant fodder.  Large flightless
herbivores dominate the base of Neornithes - both Palaeognathae and
Galloanserae.  One hypothesis is that terrestrial-based volant birds
(including those in continental ecosystems) became better adapted to
evade or defend against terrestrial predators - and over time spent
more time on the ground, and eventually lost the ability to fly.  Loss
of flight released the constraint on body size, allowing herbivorous
birds to become more massive. Huge body size had/has many advantages:
ability to feed on larger quantities of nutrient-poor plant material;
lower surface area / volume ratios for retaining body heat; and
improved ability to fend off or deter mammalian predators.