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Re: Looking for references on the osteology of the Hoatzin



Dan Chure <danchure@easilink.com> wrote:

>  I am looking for a description of the osteology of the Hoatzin more recent
> than Heilmann and references therein. My searches haven't turn anything up.
> Given how it keeps popping up in discussions about Archaeopteryx and bird
> origins I foolishly thought I would have no trouble finding such a
> publication.


Apparently young turacos (Musophagidae) also use wing-claws to clamber
around in trees.  This has been mentioned fairly often, especially in
studies that have proposed a unique link between the hoatzin and
turacos - such as the deliriously titled "Phylogenetic relationships
of the enigmatic hoatzin (_Opisthocomus hoazin_) resolved using
mitochondrial and nuclear gene sequences" (Hughes & Baker, 1999; MBE
16: 1300-1307.)  But for some reason it's the young hoatzins that get
all the love.


Young hoatzins do tend to keep their wing-claws for a long time though
- up to 70-100 days (often longer).  This is apparently because it can
take young hoatzins this long before they are able to fly.  This is a
consequence of the feeble flight abilities of this species, which is
in turn due to the expanded foregut for fermentation.  The sternum and
the attached flight musculature are accordingly reduced because they
are crowded out by the immense crop.  It also means that in order to
support themselves in the trees, hoatzins use not just "conventional"
perching (i..e, with the feet) but sternal perching as well, during
which the front of the body rests on a branch by way of a special
sternal callus (a horny patch of skin over the rear tip of the
sternum).


Adult hoatzins are like scrawny, smelly, two-legged cows with wings -
and excellent perching abilities.  So although I'm calling bulldust on
the notion that juvie hoatzins are excellent analogs for tree-climbing
basal paravians (such as _Archaeopteryx_), the adults may be better
analogs for the first flying birds.  I don't mean to suggest that the
first flying birds were fermenting folivores like the hoatzin.  But
the recent _Xiaotingia_ study suggests that the line leading to birds
was at least herbivorous; and _Jeholornis_ (based on the number of
seeds in the type specimen) does appear to have had a big crop.  Also,
as with early birds, the flight abilities of the hoatzin are weak, due
to the truncated sternum and undersized musculature - which limits
hoatzins to glides or short bursts of flight between trees.






Cheers

Tim