Struthio camelus (Parker, 1866)

Maxwell, Erin E., 2009, Comparative ossification and development of the skull in palaeognathous birds (Aves: Palaeognathae), Zoological Journal of the Linnean Society 156 (1), pp. 184-200 : 191-193

STRUTHIO CAMELUS

Day 12: This embryo is poorly prepared; however, some general observations are possible. The beak is broad, and the prenasal process does not extend far rostral to the eyes. The distal tip of the prenasal process is directed ventrally. The lower jaw extends only as far as the anterior margin of the orbit. The pars canaliculi of the auditory capsule are ovoid, with the long axes orientated dorsoventrally. They are not in contact posteriorly.

Day 15 ( Fig. 1F): There is no ossification in the skull. The prenasal process is straight and narrow. The roof of the nasal capsule is chondrified. It widens posteriorly, and contacts the orbits laterally. There is an angle formed between the nasal capsule and the prenasal process. The contact between these two structures occurs near the proximal end of the prenasal

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process. The cartilages of both the upper and lower jaw are elongate; the upper jaw extends further anteriorly than the lower jaw. The lower jaw is y-shaped, with the two rami contacting each other medially along a broad contact. The retroarticular process of Meckel’s cartilage extends posterior to the quadrate articulation. The pars canaliculi retain their ovoid morphology, and do not contact each other posteriorly. There is a dorsally directed chondrification originating from the anteroventral margin of the pars canaliculi, ventral to the external auditory meatus. This represents the metotic cartilage. The definitive occipital arch is ventral and medial to the pars canaliculi, and is distinct from this element. The quadrate cartilage and stapes are present.

Day 16: The beak is slightly longer than in the younger embryo. The prenasal process is slightly hooked at the tip, and is also wider. The pterygoid is ossifying. The dentary is also ossifying from the ventral surface of the mandibular symphysis. The timing of onset of ossification is variable, with some day 17 embryos lacking ossified skull elements (YPM 112440).

Day 17: The maxilla is weakly ossified near the base of the ascending process. The squamosal is ossifying around the quadrate articulation. The quadratojugal, supra-angular, and angular are also ossifying.

Day 19: The parasphenoid rostrum, parietal, lacrimal, nasal, premaxilla, palatine, vomer, jugal, and splenial are ossifying. The lacrimal, parietal, nasal, and vomer are variable in their timing of ossification, being absent in some day 21 embryos (YPM 112444).

Day 21: The hyoid apparatus is more elongate, extending posterior to the external auditory meatus. There is a process of the metotic cartilage overlying the quadrate articulation. The squamosal is anterior to this. The prenasal process is not completely enveloped by the premaxilla. Its anterior tip has a spatulate morphology. The frontal is ossifying, as is the ceratobranchial. The ceratobranchials are variable in the timing of ossification, remaining cartilaginous in some day 22 embryos (YPM 112446, YPM 112447).

Day 22: The basisphenoid and prearticular are ossified. The premaxilla completely covers the prenasal process. The basisphenoid is variable in its timing of ossification, being absent from some day 23 to day 25 embryos (YPM 112448, YPM 112450).

Day 23 ( Fig. 1G): The quadrate is ossifying ( Fig. 2A ); this is variable as it is cartilaginous in some day 24 embryos (YPM 112449). The maxilla lacks a distinct ascending process, although it is triangular at its midpoint. The nasal lacks a descending process.

Day 26: The basioccipital is ossifying from paired linear ossification centres along the cranial midline. The parasphenoid alae and lamina are now ossified.

Day 28: The two ossification centres of the basioccipital have fused into a single oblong element ( Fig. 2B ). The supraoccipital is ossifying, as are the laterosphenoid, exoccipital, and prootic. Both the lamina dorsalis and interorbital septum of the mesethmoid are ossified. The prearticular extends along the medial surface of the lower jaw, and posteriorly along the anterior portion of the medial process. The nasal has an ossified descending process. The ossification of the prootic and the lamina dorsalis of the mesethmoid are variable in timing of occurrence, and remain cartilaginous in some day 30–32 embryos (YPM 112454, YPM 112455, YPM 112458).

Day 30: There is a large patch of reduced ossification in the centre of the squamosal, corresponding to changes in bone architecture. This is similar to what was seen in stage 40+ (day 22) R. americana , and stage 39 M. gallopavo ( Maxwell, 2008a).

Day 31: The stapes is ossified. This is followed by the ossification of the opisthotic and epiotic. The epiotic ossifies from the anterolateral margin of the supraoccipital. The calvarium is very heavily ossified, with all dermal roofing elements contacting each other ( Fig. 1H).

Day 36: The nasal trabeculae are ossifying, but this event is very variable in timing and they remain cartilaginous in some day 37–38 individuals (YPM 112462, YPM 112464, YPM 112465).

Day 38: The articular is ossifying from the dorsal surface of the medial process.

Figures

Figure 2. Palatal view of selected palaeognath embryos. A, B, Struthio camelus (modified from Parker, 1866). C, Rhea americana, stage 37 (day 17 of incubation, RM 7219). D, Rhea americana (modified from Müller, 1963). E, Dromaius novaehollandiae, stage 37 (day 28 of incubation, RM 8026). F, Eudromia elegans, day 14 of incubation (YPM 112524). Scale bars = 5 mm. Abbreviations: bo, basioccipital; exo, exoccipital; mx, maxilla; pal, palatine; pmx, premaxilla; psl, parasphenoid lamina; psr, parasphenoid rostrum; pt, pterygoid; q, quadrate; v, vomer.