Dromaius novaehollandiae (Latham, 1790)

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 : 185-191

DROMAIUS NOVAEHOLLANDIAE

Stage 31

No ossification has occurred. The trabecula communis extends under the eyes, but not far rostral to them resulting in a very short prenasal process. The process is slightly flexed ventrally. Meckel’s cartilage is in articulation with the quadrate cartilage, but is short as the beak is only weakly developed. The quadrate cartilage itself is triradiate. The infrapolar process is small, and is located dorsal to the quadrate cartilage. The pars canaliculi of the auditory capsule are round in shape and fully chondrified. The ceratohyal and hypohyal are fused to form a curved element directly posterior to the quadrate cartilage.

Stage 32

There is no ossification present in the skull. The prenasal process exceeds Meckel’s cartilage in length, and remains triangular in lateral view. It remains slightly flexed ventrally. The parietotectal cartilage is

RM, Redpath Museum.

Asterisk denotes elements that ossify variably with respect to stage.

growing posteriorly over the nasal capsule, and is dorsoventrally flattened. The postorbital cartilage has formed behind the orbit. The pars canaliculi of the auditory capsule are dorsally elongated, becoming more ovoid than in the previous stage. Anteroventrally, they are in close contact with the quadrate cartilage. The columella is situated posterior to the quadrate cartilage. Meckel’s cartilage is considerably more elongate than in the previous stage, and has developed a retroarticular process. The basibranchial portion of the hyoid apparatus is approximately equal to the retroarticular cartilage in posterior extent.

Stage 33

The skull remains unossified. The prenasal process has straightened and forms a distinct angle with the posterior nasal septum, although outgrowth of the beak has been limited. Meckel’s cartilage remains considerably shorter than the prenasal process. The infrapolar process has increased in prominence. The foramen for the ophthalmic artery is oval, with its long axis parallel to the long axis of the skull.

Stage 34

The prenasal process is slightly more elongate than in the previous stage, but the greatest amount of growth has been in the lower jaw, which is now almost equal to the prenasal process in anterior extent. The two rami of Meckel’s cartilage share a broad contact. The area between the parietotectal cartilage and the prenasal process is not well chondrified. The external narial opening cuts a trough in the underlying trabecula, but does not form a discrete perforation in the nasal septum because of weak chondrification. The angular is ossifying.

Later in this stage ( Fig. 1A), the squamosal ossifies around the quadrate articulation. The palatine, pterygoid, jugal, and quadratojugal are also ossifying. In the lower jaw, the dentary and supra-angular ossify. The ossification of the jugal is variable, as it is absent in some stage 35 individuals (RM 8021, RM 8053).

Stage 35

The jugal process of the premaxilla is present. The angle between the prenasal process and the nasal capsule has decreased relative to stage 34. The base of the interorbital cartilage curves ventrally to a point just rostral to the external nares before it flattens out to form the prenasal process. The external narial opening is slit-like, with its long axis parallel to the prenasal process. The perforation in the underlying cartilage corresponding to the narial opening is complete.

Later in this stage, the parasphenoid rostrum, palatal and frontal processes of the premaxilla, the vomer, and lacrimal are ossifying. The jugal and frontal processes of the premaxilla ossify from separate centres, as do the dorsal and ventral portions of the orbital process of the lacrimal. The dorsal ossification centre of the orbital process of the lacrimal Dromaius novae- D. novaehollandiae Rhea Struthio Eudromia

Element hollandiae (RM) (YPM) americana camelus elegans

Skull

Basioccipital 14–21 8 8 14–15 5 Exoccipital 14 7 8 16 7 Supraoccipital 18 7 6 14 7 Parasphenoid rostrum 6 3 3 5 4 Parasphenoid ala 12 7 6 16 7 Parasphenoid lamina 14 4 4 16 4 Basisphenoid 10–12 4–7 6 8–10 4–6 Laterosphenoid 13–18 10 8 16 7

Prootic 25 19 8 17 7 Opisthotic 22 12 8 18 7

Epiotic 21 13 8 18 8 Squamosal 3 2–4 2 4 3

Parietal 12 6 6 7 4

Frontal 11 6 6 8 4 Lacrimal 6 4 2 6 4 Mesethmoid 18 8 8 14 7 Trabeculae 22–24 23

Nasal 8 3 4 6 3 Premaxilla 4 2 2 5 2

Maxilla 7 3 2 4 3 Palatine 3 3 2 5 3 Pterygoid 3 3 2 3–5 3–4 Vomer 5 3 2 6 3

Jugal 3–5 3 2 5 3 Quadratojugal 2 1–3 2 4 2 Quadrate 11 6 5 11 4 Ectethmoid

Dentary 2 1 4 3 3 Supra-angular 2 1 2 4 3

Angular 1 3 2 4 3

Splenial 7 4 4 5 3 Prearticular 9 5 4 10 4–6 Articular 23 13–18 9 19

Mandibular

Entoglossal

Basihyal

Urohyal

Ceratobranchial 13 6 5 9–11 4 Epibranchial

Postcranial axial skeleton

Cervical centra 15 7 5 13 7 Thoracic centra 15 7 4 11 4–6 Synsacral centra 16 7 7 11 4–6 Caudal centra 19–24 11–13 16–18

Pygostyle 27–29 21 20

Cervical neural arch 17 9 8 14 7 Thoracic neural arch 20 9–11 9 14 8 Synsacral transverse processes 27 15–16 10 19

Caudal transverse processes 17 20–22

Synsacral arch 19–21 11 21

Cervical ribs 17 7 7 14 8

Dorsal ribs 3–7 4 5 7 4

Sternal ribs 15 7 9 22–23

Uncinate processes 11

RM, Redpath Museum; YPM, Peabody Museum.

If an element is unnumbered, it was unossified in all specimens examined. If two numbers are given, these represent the range of ranks over which a variable element can ossify.

forms slightly before the ventral centre. The orbital process forms in contact with a cartilaginous lateral extension of the ectethmoid. The vomer is ossifying from paired centres.

The lacrimal, parasphenoid rostrum, and frontal process of the premaxilla ossify variably, as they are occasionally unossified in early stage 36 individuals (RM 8022).

Stage 36

The beak has outgrown to the point where the angle between the cartilaginous prenasal process and the nasal septum has been eliminated. The splenial is ossifying along the medial margin of the lower jaw. The jugal and palatal processes of the maxilla are ossifying. The jugal and frontal processes of the premaxilla retain separate ossification centres. This is followed by the ossification of the nasal from a single centre lying along the roof of the nasal capsule. The prearticular ossifies along the posterior medial margin of the lower jaw. The squamosal is long and thin, forming a spur paralleling the otic process of the quadrate cartilage and also forming an arch around the external auditory meatus. The dentary is ossifying from multiple centres along the anterior portion of the lower jaw. The hyoid apparatus remains short and stout.

Late in this stage, the basisphenoid ossifies. The two ossification centres of the lacrimal remain unfused, as do the two ossification centres of the premaxilla.

Stage 37 ( Fig. 2E )

The frontal is ossifying along the dorsal margin of the orbit. The body of the quadrate is ossified. The ossification centres of the lacrimal have fused, as have the premaxillary ossification centres. Later in this stage, the parietal and the parasphenoid alae ossify. The parasphenoid alae form a plate of bone ventral to the postorbital cartilage; this cartilage remains unossified.

Stage 38 ( Fig. 1B)

The premaxilla has completely surrounded the cartilaginous prenasal process. The frontal has expanded its ossified area ventrally into the orbit. The ceratobranchials are ossifying. The laterosphenoid is also ossifying from its ventrolateral corner. This is followed by the ossification of the exoccipitals and parasphenoid lamina. The parasphenoid lamina ossifies from right and left paired ossification centres located posterior to the parasphenoid rostrum and basisphenoid. Otoliths are calcified.

The laterosphenoid is variable in its timing of ossification, as it is cartilaginous in some stage 40+ individuals (RM 8047).

Stage 39

The lacrimal has a large foramen on the anterior surface of the descending process. The supraoccipital is ossifying from a single centre; this appears to be variable as two centres on either side of the cranial midline were observed in some individuals. The laterosphenoid is ossified along its entire ventral edge. The mesethmoid is ossifying from two centres – one on the anteroventral edge of the interorbital septum, the second located in the lamina dorsalis representing the ossification of the parietotectal cartilage. The lamina dorsalis has extensive dorsal exposure in D. novaehollandiae , first as cartilage and later as bone. It is bordered rostrally by the frontal process of the premaxilla (which does not contact the frontal), laterally by the nasals and frontals and in later stages, posteriorly by the frontals.

The supraoccipital and mesethmoid are variable in their timing of ossification, remaining unossified in some stage 40+ individuals (RM 8047).

Stage 40+

Day 36: The basioccipital is ossifying as a single linear ossification centre on the cranial midline. The ossified area of the laterosphenoid has expanded to include the entire lateral edge. The epiotic is beginning to ossify from the medial margin of the supraoccipital. It does not yet have lateral exposure. The ossified portion of the parietal has expanded to reach the posterior wall of the orbit. There is an extra medial ossification centre of the parasphenoid lamina, located between the two principal wings of that element.

Day 38: The basioccipital is elongate and diamondshaped, broadening posteriorly. The laterosphenoid is entirely ossified. The opisthotic ossifies separately from the exoccipital, and is exposed along the lateral wall of the braincase. The ossified portion of the lamina dorsalis of the mesethmoid contacts the frontals posteriorly, but is not overlapped by any elements. The interorbital septum is ossified over the anterior third of the element. The articular is ossified, beginning from the centre of the jaw joint. There also appears to be some bony ossicles in the jaw joint that are separate from the articular, and may represent sesamoid elements termed ossicula articularia ( Jollie, 1957).

Day 43 ( Fig. 1C): The opisthotic contacts the squamosal, forming the posterior border of the external auditory meatus.

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Day 45: The prootic ossifies medial to the squamosal. It is not visible in lateral view.

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.