Ambystoma mexicanum
Creators: name: Ishikawa, Kaoru
name: Taguchi, Yuki
name: Kobayashi, Ryomei
name: Anzai, Wataru
name: Hayashi, Toshinori
name: Tokita, Masayoshi
Publisher: name: Zenodo
Year: 2022
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Amphibia
Caudata
Ambystomatidae
Ambystoma
Ambystoma mexicanum

description: Ambystoma mexicanum (Figs 3–5)   
 Atkins et al. (2019) described cranial skeletogenesis of Am. mexicanum and reported that the onset of ossification was at stage 46 of the staging table by Nye et al. (2003). The onset of ossification is at stage 56 in the present study. Given that the overall ossification sequence reported here is almost identical to that of Atkins et al. (2019), the difference in the timing of ossification onset could be attributable to differences in the rearing environment.   
In phases I–V, ossification has not started. In the skull, the sphenethmoid, otic capsule and exoccipital are cartilaginous. In the mandible and pharynx, the Meckel’s cartilages, first and second basibranchials, ceratohyal, hypohyal, first and second hypobranchials and first to fourth ceratobranchials are also cartilaginous.  
Phase VI is the first stage of ossification.The premaxilla, vomer, prootic, exoccipital, parasphenoid, dentary and coronoid begin to ossify. The former three bones are ossified from the anterior portion. The parasphenoid is ossified from the medial portion.  
 In phase VII, which corresponds to the last stage in the staging table of Am. mexicanum by Nye et al. (2003), the maxilla, parietal, palatine, squamosal and prearticular begin to ossify. The maxilla, palatine and prearticular ossify from the anterior side, whereas the parietal and squamosal ossify from the lateral side. Dentition is observed in the premaxilla, vomer, palatine, dentary and coronoid.   
 In juvenile Am. mexicanum corresponding to phases VIII–XI of the other three species examined, ossification of the pterygoid, opisthotic, columella, prearticular and angular begins. The pterygoid extends anteriorly and fuses with the palatine, forming a bony bar in the ventral side of the skull. Ossification progresses further in bones that began to ossify in the previous phase, but these bones do not yet contact each other. The architectural pattern of the hyobranchial skeleton is almost identical to the previous stage.   
In phase XII, the nasal, prefrontal, quadrate and orbitosphenoid are ossified, meaning that all bones that compose the adult skull are now ossified. However, even at this stage, some bones, such as the maxilla, nasal and exoccipital, do not contact neighbouring bones. Although the morphology of the hyobranchial skeleton does not change considerably from the previous stage, the second basibranchial begins to ossify.  
In phase XIII, all bones, including the nasal, are further ossified. However, even in the adult skull, gaps are present between the left and right premaxillae and between the premaxilla and the nasal.
description: Published as part of 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, pp. 299-314 in Zoological Journal of the Linnean Society 195 (1) on pages 305-306, DOI: 10.1093/zoolinnean/zlab038, http://zenodo.org/record/6530541