by the process of folding of the cartilaginous nasal capsule during fetal development, outpouchings of the main nasal chamber become enlarged (primary pneumatization) (Witmer, 1999; Macrini, 2014). The resulting paranasal recesses may or may not expand farther into the bodies of facial and basicranial bones (secondary pneumatization). Histori- cally, the result of these processes, paranasal recesses and sinuses, respectively, have not always been distin- guished (see further discussion in Rossie, 2006). Yet, in either case the nasal fossa becomes subdivided into a central nasal chamber and more peripheral paranasal chambers (Smith et al., 2014; Curtis and Van Valken- burgh, 2014)[PMID:25312359]
In Aves: The nasal capsule is dorsoventrally divided into two parts: the upper part, the ectethmoid, serves olfaction and is composed of the lamina cribosa, the crista galli apophysis and the conchae. The lower part, the mesethmoid, is a thick cartilage bar extending from the corpus sphenoidalis to the rostral extremity of the nose (Fig. 1A-B). In the avian embryo, the mesethmoid constitutes the cartilage primordium of the upper beak.
In most mammals, the nasal capsule remains unossified, except in mammals where the ethmoid portion ossifies to form the turbinates
In avians, the mesethmoid supports upper beak formation, whereas the ectethmoid comprises elements of the olfactory system, including the lamina cribosa, the crista galli apophysis and the conchae.