NON-PAEDOMORPHIC RATITES

[darren.naish@port.ac.uk]

The big thing Matt Troutman and George had going on paedomorphosis 
and the evolution of secondary flightlessness in non-avian theropods 
has pretty much died down, so this message could be coming in at the 
worst, or the best, time. Or.. forgive me, for I know not what I do.

Matt argued that flightlessness in birds ***ONLY*** arises via 
paedomorphosis (viz., the retention of juvenile traits of ancestors 
into the adults of descendants). The evidence for this is the wide 
occurrence in all Recent flightless birds of juvenile characters 
(e.g. down-like plumage, thick-walled leg bones). Some of you may 
have noted that, during the discussion, I pointed out that avian 
flightlessness may not _necessarily_ have always evolved via 
paedomorphosis. In fact, the whole reason it has become assumed that 
it _has_ (that is, avian flightlessness requires paedomorphosis) is 
political, rather than scientific - IMO. 

The two most vocal, most widely published, and most widely read of 
all palaeornithologists, Storrs Olson and Alan Feduccia, are both 
vehement advocates of the paedomorphic theory for the evolution of 
flightlessness. For example, Olson (1985) briefly reviewed the 
evidence for paedomorphosis vs. other mechanisms in the evolution of 
ratites, and came down firmly on the side of paedomorphosis - yet he 
did not make it clear why a paedomorphic explanation for certain 
characters (e.g. the divided ramphotheca of palaeognaths) was the 
favoured one. Feduccia, in his new book (in which he suggests that 
hesperornithiformes may have been viviparous and that hupehsuchians 
and a hand-cranked analysis of grebe and loon relationships prove 
that cladistics is totally unreliable - - in which case one wonders 
why cladograms are used, uncritised, later on in the book), cites 
several works supporting paedomorphosis as the mechanism explaining 
avian flightlessness - e.g. the work on thyroidised starlings. 

I am not criticising these workers, I am merely making the point 
that, because they have done so much to popularise the idea that 
flightlessness evolved via paedomorphosis, it has virtually become 
dogma in semi-popular palaeornithology.

However, the whole paedomorphic thing is a theory, not a fact, and 
there is another theory that may fit the facts better. 

Paedomorphosis was (if memory serves) one of several developmental 
terms coined by de Beer in the '30s (yes I am that old*). It has been 
successfully applied as a concept to the neotenic larvae of certain 
caudate amphibians, and is increasingly recognised in marine reptiles 
where it was apparently advantageous to have late-ossifying or 
non-ossifying limb elements. Indeed, from all the recent work that 
Mike Caldwell has published on paedomorphic trends in mosasaurs, 
ichthyosaurs and sauropterygians, it seems evident that 
paedomorphosis leads to increasingly poorly ossified structures, not 
vice versa (read on).

De Beer also coined the term hypermorphosis, this time meaning the 
evolutionary development of features in descendants extended beyond 
the adult stage of ancestors, or 'overstepping'. The very fact that 
flightless birds, particularly ratites, are so big compared to other 
birds implies that they are hypermorphic - they apparently have grown 
to sizes extending well beyond those of their ancestors. Other 
features, some previously explained as paedomorphic - the 
well-developed aftershaft of ratite feathers, uniformly distributed 
feathers (arguably overstepping of the very extensive apteria seen in 
juvenile carinates), ossification of the procoracoid, thick-walled 
long bones - can be equally, or more, parsimoniously interpreted 
as hypermorphic. Diverse designs in ratite pelves imply 
hypermorphosis, because paedomorphosis is supposed to have a 
homogenising effect and embryonic bird pelves are all similar.

It is also important to note that paedomorphosis and hypermorphosis 
are not mutually exclusive, as both processes may have occurred 
within the same group of animals, but acted on different parts of 
the body. 

Thus, paedomorphosis is not the only mechanism that can be invoked to 
explain the evolution of avian flightlessness, and it is not totally 
clear that it is THE reason explaining cases of avian 
flightlessness, as Elzanowski has extensively argued. That Mesozoic 
non-avian theropods do not exhibit features that have been 
interpreted as paedomorphic or hypermorphic  does not rule out the 
possibility of them evolving flightlessness via other means. Worse, 
they do have characters that could be interpreted as hypermorphic and 
therefore of possible flightlessness. If, that is, that's how you 
wanted to interpret them...

"_Sphere_: corals, sea snakes, sunlight (and a film that just didn't 
make any sense) in 1000 ft of Pacific seawater"

*Just kidding.

DARREN NAISH
darren.naish@port.ac.uk