Hynobius nebulosus (Temminck & Schlegel, 1838)

Ishikawa, Kaoru, Taguchi, Yuki, Kobayashi, Ryomei, Anzai, Wataru, Hayashi, Toshinori & Tokita, Masayoshi, 2022, Cranial skeletogenesis of one of the largest amphibians, Andrias japonicus, provides insight into ontogenetic adaptations for feeding in salamanders, Zoological Journal of the Linnean Society 195 (1), pp. 299-314 : 304-305

Hynobius nebulosus ( Figs 3–5 )

Ossification has not started in phases I–VI. The sphenethmoid, otic capsule and exoccipital are still cartilaginous.The Meckel’s cartilages, first and second basibranchials, ceratohyal, hypohyal, first and second hypobranchials and first to fourth ceratobranchials are present as cartilaginous elements.

Phases VIII and IX are the earliest stages of ossification in this species. In phase VIII, the posterior part of the exoccipital begins to ossify. In phase IX, the lateral portion of the parietal starts to ossify.

In phase X, the anterior portions of the premaxilla and vomer start to ossify. The bones of the posterior skull (the parietal, squamosal, pterygoid, quadrate, prootic, opisthotic and columella) also begin to ossify. The pterygoid and parietal start to ossify from the posterior side. In the mandible, the dentary ossifies from the anteromedial side, and the medial part of the prearticular ossifies. Ossification of the angular, coronoid and articular are observed in some individuals. In the pharynx, ossification of the previously cartilaginous second hypobranchial has begun. The first and second basibranchials fuse to form a Y-shape. The fourth ceratobranchial disappears. The posterior part of the third ceratobranchial ossifies in some individuals.

In phase XI, the bones of the anterior skull (the maxilla, frontal, nasal, parasphenoid and orbitosphenoid) begin to ossify. The parietal begins to ossify from the anterolateral side and the parasphenoid from the medial side. Suture lines appear between the skull bones.

In phases XII and XIII, the fontanelle closes. In the pharynx, ossification of the anterior part of the second ceratobranchial begins in phase XII, and it is completely ossified in adults (phase XIII). The previously Y-shaped basibranchial becomes 8-shaped.

Figures

Figure 3. Cranial skeletogenesis (dorsal view) in Andrias japonicus, Hynobius nebulosus, Pleurodeles waltl and Ambystoma mexicanum. Pink elements are bone. Blue elements are cartilage. Scale bars are 1 mm, except those for phase XIII An. japonicus and Am. mexicanum, which are 10 mm. Abbreviations: cl, columella; exo, exoccipital; fr, frontal; ma, maxilla; na, nasal; oc, otic capsule; opi, opisthotic, osph, orbitosphenoid; pa, parietal; pfr, prefrontal; pma, premaxilla; po, prootic; psph, parasphenoid; pt, pterygoid; q, quadrate; seth, sphenothmoid; sq, squamosal; vo, vomer.

Figure 4. Select phases of cranial skeletogenesis (ventral view) in Andrias japonicus, Hynobius nebulosus, Pleurodeles waltl and Ambystoma mexicanum. Pink elements are bone. Blue elements are cartilage. Scale bars are 1 mm, except for the 10 mm scale bar for phase XIII An. japonicus. Abbreviations: cl, columella; exo, exoccipital; fr, frontal; ma, maxilla; na, nasal; oc, otic capsule; opi, opisthotic, osph, orbitosphenoid; pa, parietal; pfr, prefrontal; pma, premaxilla; po, prootic; psph, parasphenoid; pt, pterygoid; q, quadrate; seth, sphenothmoid; sq, squamosal; vo, vomer.

Figure 5. Osteogenesis of the lower jaw and hyobranchial skeletons (ventral view) at larval (phases I or II) and postmetamorphosis (phases XII or XIII) stages of Andrias japonicus, Hynobius nebulosus, Pleurodeles waltl and Ambystoma mexicanum. Pink elements are bone. Blue elements are cartilage. Scale bars: 1 mm.Abbreviations: ang, angular; ar, anterior radial; art, articular; bb, basibranchial; cb, ceratobranchial; ce, ceratohyal; den, dentary; hb, hypobranchial; hc, hypohyal; pr, posterior radial.