Variation and timing of the cranial ossification sequence of the oriental fire-bellied toad, Bombina orientalis (Amphibia, Discoglossidae).

Abstract

The sequence of appearance of the 17 different skull bones in the oriental fire-bellied toad, Bombina orientalis, is described. Data are based primarily on samples of ten or 11 laboratory-reared specimens of each of 11 Gosner developmental stages (36-46) representing middle through late metamorphosis. Ossification commences as early as stage 37 (hind limb with all five toes distinct), but the full complement of adult bones is not attained until stage 46 (metamorphosis complete). Number of bones present at intermediate stages is poorly correlated with external morphology. As many as four Gosner developmental stages elapse before a given bone is present in all specimens following the stage at which it may first appear. The modal ossification sequence is frontoparietal, exoccipital, parasphenoid, septomaxilla, premaxilla, vomer, nasal, maxilla, angulosplenial, dentary, squamosal, quadratojugal, pterygoid, prootic, interfrontal, sphenethmoid, and mentomeckelian. Most specimens are consistent with this sequence, despite the poor correlation between cranial ossification and external development as assayed by Gosner stage. The timing of cranial ossification in Bombina orientalis differs in many respects from that described for two other, distantly related anurans, the leopard frog (Rana pipiens) and the western toad (Bufo boreas). These include the total number and sequence of appearance of bones, and the timing of ossification relative to the development of external morphology. Interspecific variation may reflect differences in the timing of the tissue interactions known to underlie skeletal differentiation and evolution.