:-( AFAIK American Museum Novitates are completely unavailable here...
Must be very "deep" if it took them 7 years!
Pneumatization in an embryo (rather, fetus) is not only peculiar, it is _bizarroid_*: I have a copy of chapter 5 -- John McLelland: Anatomy of the lungs and air sacs, p. 221 -- 275 -- of a thick book on avian anatomy here. I can't identify the book, because I didn't copy it myself. (Chapter 4 is by J. H. Brackenbury, judging from the last 5 references, it is about how songbirds sing.) Anyway, on p. 272, it says: "It is generally accepted that pneumatization of the postcranial skeleton always takes place after hatching. The timing of development has recently been investigated by Hogg (1984b) in the domestic fowl. This study showed that great variations exist in the onset of development between different regions and between individual birds within a region. Furthermore, significant differences were also observed in the rate of development of pneumatization of the humerus between different breeds and between pullets and cockerels of the same breed. The earliest onset of pneumatization recorded by Hogg in the different regions in pullets of Golden Comet chickens was as follows: cervical vertebrae, 35 days; thoracic vertebrae, 63 days; synsacral vertebrae, 77 days; vertebral ribs, 91 days; sternum, 140 days; humerus, 35 days (pneumatization was not found in the os coxae [apparently pelvis] and coracoid bone in the Golden Comet birds between the onset of the study at hatching and the termination of the study at 182 days). In White Leghorn domestic fowl pneumatization was first noted in the humerus at 28 days and in the coracoid bone at 63 days."
*That's not invented by me, it is a French _expression_ (with 2 dots on the i); BTW, "bizarre" itself comes from the most appropriate language, Basque.
Is the carpometacarpus a synapomorphy of some clade, or has it evolved several times?
Especially considering that the describers of *Potamornis*, including HP GSP, want to resurrect Odontognathae, based on e. g. jaw features...