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I believe that Piciformes is very likely NOT holo(mono)phyletic (i.e. paraphyletic at best, but perhaps polphyletic). Haven't seen evidence that Apodiformes has the same problem, but who knows.
As for TRUE reversals, the only clear candidates would be: Losses of genes. As long as the gene (or genes) for a character are still present, there is always the potential of "turned-off" genes getting turned back on. But once the reversal of "gene loss" occurs, the probability of ever getting the same gene back are astronomically remote (unless you are one of those genetically "promiscuous" prokaryotes, and even then it's extremely improbable).
Other than gene loss, "reversals" are a relative term and subject to controversies over probabilities. This is just one of many homoplastic (homoplasic, homoplasious) complexities that challenge evolutionary systematists. And it's especially true in paleontology, given the near total absence of genetic and other molecular data from fossils (at least given our current technology).
------ Ken
Darren Naish wrote:

Philidor11 wrote...

> Can anyone suggest an example of a true reversal?

Mandibular dentition in one genus of marsupial tree frog. More debatably,
tabular bones in chrysochlorids (golden moles).

Incidentally, I read Chris Brochu's paper on archosaur systematics in the recent
_Journal of Paleontology_ last night. Excellent review, but, in the bit about
bird relationships, Chris says that no-one doubts the monophyly of Piciformes
(woodpeckers, barbets, toucans, honeyguides, jacamars) or Apodiformes
(swifts, swiftlets and hummingbirds). Though monophyly of these groups is
currently the consensus view, this is a little surprising: piciform monophyly has
actually been quite controversial and formed the subject of various
argumentative papers by Olson, Cracraft and Raikow. Apodiform monophyly
has also been contested and some authors have separated hummingbirds (as the
Trochiliformes) from the other apodiforms. Anyway, back to work.

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