secondary flightlessness (wordy)

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

A number of people seem to be distressed that Ji et al. make no mention of
the possibility of secondary flightlessness in the newly described Chinese
theropods.  (The do address, albeit briefly, the evidence for flightlessness
in general for these forms, particularly based on the shortness of their arms).


I do not wish to speak for Ji, Currie, Norell, and Ji in this matter.
However, I can say what suggests, to me, that these are probably not
secondarily flightless: namely, their phylogenetic position.

In the tree they provide, these taxa do not fall within a clade of taxa
which are unambiguously volant (i.e., "known fliers" to those with a
less-latinate vocabulary...).  Instead, _Caudipteryx_ is the sister taxon to
a clade of known fliers, and _Protarchaeopteryx_ forms a trichotomy with
Velociraptorinae and Avialae.

Now, a few of you out there (and we know who you are...) strongly support
particular scenarios where powered flight is, in your opinion, the only life
habit sufficient to explain certain features of maniraptoran, or
coelurosaurian, or dinosaurian anatomy.  Okay, scenario-driven hypotheses
(scenario first, phylogeny second) have a long tradition in evolutionary
biology.  Greg Paul has argued (in Predatory Dinosaurs of the World) that
most of what are now called "maniraptoriforms" are secondarily flightless;
George Olshevsky has argued here and elsewhere that even more inclusive
clades of dinosaurs might be secondarily flightless.

However, Ji et al., and I, are operating under a different mode of
evolutionary analysis.  In phylogenetic-driven hypotheses (phylogeny first,
scenario second), the position of a flightless form within a phylogeny
generated by some means (parsimony, maximum likelihood, etc.) will help
determine if its flightlessness is primary (i.e., it doesn't fly, nor did
any of its ancestors) or secondary (i.e., it doesn't fly, but its ancestors
did).

Under phylogenetic-driven hypotheses, all that would be required to convince
me that _Caudipteryx_, _Protarchaeopteryx_, _Mononykus_, _Velociraptor_,
_Allosaurus_, _Apatosaurus_, and/or _Triceratops_ were secondarily
flightless is to demonstrate that the most parsimonius phylogeny places
these taxa within a clade whose ancestral state is unambiguously volant.
This is the same standard which evolutionary biologists working on other
groups of organisms employ.

(For example, a flightless wasp species is deeply nested within a clade of
known flying insects, and so its flightlessness is regarded as secondary.
Ring-tailed lemurs, however, are not so nested within known fliers, and its
flightlessness is best regarded as primary).

However, as Alan Brush & others were talking about in the post-press
conference milling about time, there are gradations in what we mean by
"fliers", and it may be that one or both of the new Chinese forms had the
ability to get off the ground for short pulses of airborn transport.
Whether we call this "flight" is debatable.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661