Number of Ossification Centers in the Anuran Cranial Bones Depends upon the Rate of Development: Experimental Evidence

Abstract

Cranial development was compared in Rana temporaria (Anura; Ranidae) and Bombina variegata (Anura; Discoglossidae) specimens reared at normal, high, and low level of thyroid hormone (TH). Sequence and timing of cranial ossification as well as rate of a particular bone’s differentiation and growth depend upon TH level. Also, bones which normally develop from a single ossification center (frontoparietal, parasphenoid, vomer, squamosal, angulosplenial, septomaxilla, pterygoid, and premaxilla) were revealed to form from two or more ossification centers in specimens reared at high and low TH level. This indicates them to develop from several early precalcified anlagen which normally fuse prior to the beginning of calcification thus appearing as a single ossification center. Changes in the rate of a particular bone’s differentiation and growth caused by manipulation with a TH level may cause precocious beginning of their calcification or their delayed fusion. As a result, they begin to calcify prior to their fusion thus appearing as several separate ossification centers. Number of ossification centers depends upon the rate of cranial and overall somatic development. Interspecies and individual differences in the rate of cranial and overall ontogeny are responsible for the inter- and intraspecific variability in the number of ossification centers recorded in anurans.