Andrias japonicus (Temminck, 1836)

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 : 302-304

Andrias japonicus ( Figs 2–5 )

In phase I, ossification has not yet begun, and the cranial skeleton is composed of only cartilaginous elements. Sphenethmoid and otic capsules are visible, and the mandible consists exclusively of Meckel’s cartilages. In the pharynx, the basibranchial and the first to third ceratobranchials are observed as cartilaginous structures.

Ossification of the skull begins in phase II. The premaxilla, squamosal, dentary and columella begin to ossify. In some individuals, ossification of the vomer, parasphenoid and prearticular is also detected. An ossification centre is detected in the medial portion of the premaxilla, the anterior portion of the vomer and the posterolateral portion of the parasphenoid. The lateral part of the frontal and parietal bones begins to ossify along the anterior–posterior axis. In the pharynx, the hypohyal, ceratohyal, first and second hypobranchials and fourth ceratobranchial appear as cartilaginous structures.

Ossification of the angular, coronoid and parietal begin in phases III and IV. The coronoid starts to ossify from the medial side and the parietal from the lateral side.

In phase V, ossification of the maxilla, exoccipital and angular begins. The maxilla starts to ossify from the medial side, the exoccipital from the posterior side and the angular from the anterior side. Ossification of the premaxilla and dentary is advanced, and several teeth have developed on these bones.

In phase VI, ossification begins at the medial side of the nasal, the lateral side of the frontal and the anterior portion of the pterygoid. Ossification of the second ceratobranchial is observed in some individuals.

In phase VII, the prefrontal and the second ceratobranchial are ossified in all individuals. Ossification of the frontal and parietal progresses from the lateral portion to the medial portion during phases VI and VII. A fontanelle and suture lines are observed in the dorsomedial portion of the skull.

In phase VIII, the frontal and parietal are divided by a suture line. The orbitosphenoid begins to ossify.

In phase IX, the third and fourth ceratobranchial disappear.

The second ceratobranchial ossifies in phase XII (the subadult stage). All bone elements present in the skull of the adult (phase XIII) are ossified.

Figures

Figure 2. Cranial skeletogenesis in the Japanese giant salamander, Andrias japonicus. Cranial bones and cartilages are shown for eight of 12 developmental phases. Scale bars: 1 mm. Abbreviations: cl, columella; exo, exoccipital; fr, frontal; ma, maxilla; na, nasal; oc, otic capsule; osph, orbitosphenoid; pa, parietal; pfr, prefrontal; pma, premaxilla; psph, parasphenoid; pt, pterygoid; q, quadrate; seth, sphenothmoid; sq, squamosal; vo, vomer.

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.