Developmental variation in pterygoid segmentation clarifies patterns of avian bony palate evolution

Key results

1. ShownAmong extant birds, the complete post-hatching segmentation sequence was observed only in Neoaves: a distinct hemipterygoid forms, separates from the pterygoid body and fuses to the caudal palatine.

   All 22 immature neoavian specimens showed evidence of a hemipterygoid (attached to the pterygoid, discrete, or fusing to the palatine), and it was merged into the palatine in all 14 adults. No transient discrete hemipterygoid or segmentation-consistent ontogenetic change in palatine and pterygoid shape was found in any surveyed palaeognath, anseriform or galliform. · NEOAVES; 4. DISCUSSION; Anseriformes; Mature

2. SuggestedNeoavian immatures can be ordered into five hypothetical stages, from an unsegmented rostral pterygoid projection to complete fusion with the palatine. A fully detached hemipterygoid (Stage 3) was rare, so many taxa may start fusion before detachment is complete.

   Stage 1 (no suture): immature Tauraco, Musophaga, Otidiphaps, Pteroglossus, Ara, Lorius. Stage 2 (sutured but still attached): e.g. Apus, Columba, Spheniscus, Morus, Nycticorax, Vultur. Stage 3: only one immature Gallirallus australis. Stage 4 (sutured to the palatine): Recurvirostra and Egretta, the latter with tentative signs of incomplete detachment that scan resolution could not settle. · STAGE 1 - UNSEGMENTED HEMIPTERYGOID (figs. 21…; STAGE 2 - PARTIALLY SEGMENTED HEMIPTERYGOID…; STAGE 4 – HEMIPTERYGOID PARTIALLY FUSED TO THE…; Neoaves

3. SuggestedIn most neoavians the adult pterygoid process of the palatine is a composite: the hemipterygoid supplies its dorsal part (sometimes a separate dorsal condyle) and the palatine its ventral part. The neoavian joint is therefore only partly homologous with the joints of other neognaths.

   In Stage 2 immatures the hemipterygoid sits in a shallow medial depression along the dorsal palatine crest, a depression that adults lack; the ventral condyle is formed by the palatine alone. The Stage 3 Gallirallus hemipterygoid matches the dorsal part of the adult pterygoid process, and an adult Opisthocomus retains an incompletely fused suture where the immature overlap lies. · STAGE 2 - PARTIALLY SEGMENTED HEMIPTERYGOID…; STAGE 3 - FULLY SEGMENTED HEMIPTERYGOID (figs…; Neoaves

4. SuggestedThe anatid rostral pterygoid projection, which persists into adulthood without segmenting, is proposed as homologous with the Stage 1 neoavian hemipterygoid, and the authors recommend calling it a 'hemipterygoid process'. Parts of the rostral projections of megapodes and Anhimidae may be equivalent.

   No distinct hemipterygoid appeared in any immature or adult anatid. The projection spans ~1/4 (Cygnus, Anser) to ~1/3 (Dendrocygna, Aythya, Spatula, Malacorhynchus, Anas) of immature pterygoid length and changes from blade-like to roughly cylindrical in adults; its shape and position match Stage 1 neoavians. Non-megapodiid galliforms had no rostral projection at any post-hatching stage. · Immature; Anseriformes; Potential homology of the anseriform rostral…; Galliformes

5. SuggestedDiscrete, unfused hemipterygoids reported in stem ornithurines (Ichthyornithes, Hesperornithes) lead the authors to argue that the hemipterygoid predates pterygoid segmentation. They favour segmentation as a Neoaves synapomorphy but say alternative scenarios remain viable.

   Ichthyornithine and hesperornithine hemipterygoids are described as very large relative to neoavian ones and unfused to pterygoid or palatine. Stem anseriforms Nettapterornis and Presbyornis, examined here, have short, robust rostral pterygoid projections like those of megapodes. The authors read these as hemipterygoid processes, implying hemipterygoid-pterygoid fusion is plesiomorphic for Galloanserae. · FOSSILS AND THE ORIGIN OF PTERYGOID SEGMENTATION; 5. CONCLUSIONS