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HOATZIN DIGITS



I haven't really been following the continuing debate over the 
homology of bird tetrapod digits, nor have I seen the 
recent paper on this subject. But I note a recent 
list suggestion that we should look at Hoatzin (_Opisthocomus_) 
digits - they might show some interesting ontogenetic things going 
on. While I know nothing about the embryology or ontogeny of Hoatzin 
digits, this caused a few figurative pennies to drop. Read on at your 
peril.

Most of you will, by now, be familiar with Robert Chandler's work on 
the North American phorusrhacoid _Titanis walleri_ (now known from 
Texas as well as Florida - Pleistocene palaeo-artists take note!). 
_Titanis_, and perhaps other phorusrhacoids too, actually had large 
clawed hands with a mobile pollex. Incidentally, earlier publications 
(e.g. Tonni on _Psilopterus_) had shown that some phorusrhacoids were 
equipped with sickle-claws on pedal digit II as well! Cretaceous 
theropods incarnate in the Neogene, oh yes.

At SVP '97, Chandler presented the preliminaries for a new 
phylogenetic hypothesis of phorusrhacoid affininities. As is expected 
from the overwhelming quantity of data, seriemas (Cariamidae) form a 
clade with the phorusrhacoids in Chandler's cladogram (and 
interesting that seriemas also bear sickle-claws... Farlow pers. 
comm.!).  The surprises were that the successive outgroups to this 
clade were touracos (Musophagidae) and Condors (Vulturidae 
[Cathartidae _auct._]). I could barely believe it. But then I 
consulted The Bible on Tertiary palaeornithology, Storrs Olson's 1985 
chapter from _Avian Biology_. The avian classification we all grew up 
with (the Gadow-Wetmore system) is deeply flawed, and Olson, more 
than anyone else, has done much to rectify it. 

In Olson's view, the phorusrhacoid-seriema clade (old name for this 
group is cariamans) are part of an endemic South American radiation 
of primitive land birds. Caracaras (Polyborinae, Falconidae) are 
raptorial members of this clade, and the most basal members are 
Hoatzins! Hoatzins (Opisthocomidae) have carpometacarpi strikingly 
like those of cariamans (bathornithids are part of this group too) - 
a shared derived characters is a robust, laterally bowed carpomet I -
 and Mourer-Chauvire has similarly proposed an affinity between them. 

OK, so the point of all this nonsense is.. Hoatzins (and it's 
pronounced 'what-zins' for those of you still saying 'hoat-zins') 
have unfused digits in their juvenile stages: could this be a 
primitive character for the hoatzin-cariaman clade, and could  
juvenile Hoatzin digits therefore be homologous with the 
newly-discovered digits of phorusrhacoids? I don't expect anyone to 
answer this - it calls for a lot of lab work, and lots more fossil 
discoveries. But it's a very interesting possibility.

On another tangent, if caracaras are part of this hoatzin-cariaman 
clade, and if touracos and condors are too, what of other birds of 
prey? Olson has suggested that accipitrids are most closely related 
to touracos, so they could be in here too. God help us if the 
proposed falconid-owl affinity is valid, and note that Sibley and 
Ahlquist's data is totally discordant.

"Jabba, you're a wonderful human being"

"I'm an alien abduction nightmare, won't you be my slave"

DARREN NAISH
darren.naish@port.ac.uk