Cranial skeletogenesis of one of the largest amphibians, Andrias japonicus, provides insight into ontogenetic adaptations for feeding in salamanders Author Ishikawa, Kaoru Author Taguchi, Yuki Author Kobayashi, Ryomei Author Anzai, Wataru Author Hayashi, Toshinori Author Tokita, Masayoshi text Zoological Journal of the Linnean Society 2022 2022-05-01 195 1 299 314 https://academic.oup.com/zoolinnean/article/195/1/299/6317662 journal article 55609 10.1093/zoolinnean/zlab038 eee5a5c5-ecbf-4c54-8aae-3b7d3342301f 0024-4082 6530541 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. Table 2. Descriptions of landmarks digitized onto skull specimens 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.

Numbers	Definition
1	Most anterior points of the premaxilla
2–3	Most anterior points in the joint between the
premaxilla and maxilla
4–5	Most lateral points in the joint between the
prefrontal and maxilla
6–7	Most posterior points of the maxilla
8–9	Most anterior points on the suture between
the parietal and pterygoid
10–11	Most anterior points of the pterygoid
12	Centre of the joint between the frontal and
parietal
13–14	Most posterolateral points of the pterygoid
15–16	Most anterolateral points of the jaw joint
17–18	Most posteromedial points of squamosal
19–20	Most posterolateral points of the jaw joint
21	Most posteromedial point of the parietal
22–23	Most posterior points of the exoccipital

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. Figure 1. The skull of an adult Japanese giant salamander, Andrias japonicus . Left, dorsal view of the skull, with cartilages stained with Alcian Blue and bones with Alizarin Red. Scale bar: 10 mm. Right, landmarks on the skull. Abbreviations: exo, exoccipital; fr, frontal; ma, maxilla; na, nasal; osph, orbitosphenoid; pa, parietal; pfr, prefrontal; pma, premaxilla; pt, pterygoid; q, quadrate; sq, squamosal. 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. Table 3. Ossification phases of cranial skeletogenesis of the four salamander species examined in this study

Phase	Andrias japonicus	Hynobius nebulosus	Pleurodeles waltl	Ambystoma
mexicanum
I	–	–	–	–
II	Columella, dentary,	–	–	–
parasphenoid,
prearticular,
premaxilla,
squamosal, vomer
III	Angular, coronoid	–	–	–
IV	Parietal	–	–	–
V	Maxilla	–	–	–
VI	Exoccipital, frontal,	–	Dentary, exoccipital,	Coronoid,
dennasal, pterygoid	opisthotic, palatine,	tary, exoccipital,
parasphenoid, parietal,	paraphenoid,
prootic, prearticular,	premaxilla, prootic,
premaxilla, pterygoid,	vomer
squamosal, vomer
VII	Exoccipital,	–	Angular, columella, cor-	Maxilla, palatine,
hypobranchial II,	onoid, frontal, maxilla,	parasphenoid,
parprefrontal, quadrate	prefrontal, quadrate	ietal, squamosal
VIII	Orbitosphenoid	Exoccipital	Basibranchial II, nasal,	Angular, columella,
orbitosphenoid	frontal, opisthotic,
IX	–	Parietal	–	prearticular,
pteryX	–	Angular, articular, colu-	–	goid
mella, coronoid, dentary,
opisthotic, hypobranchial II,
prearticular, premaxilla,
pterygoid, prootic, quadrate
XI	–	Ceratobranchial II, frontal,	–
maxilla, nasal,
orbitosphenoid, parasphenoid
XII	Articular,	–	Articular, ceratobran-	Articular,
ceratobranchial II	chial II, ceratohyal,	basibranchial II,
hypobranchial II	nasal,
orbitosphenoid, prefrontal
XIII	–	–	–	–

Bones ossified in each phase are listed in alphabetical order. 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.