THOMAS, K.R.; University of Chicago: Evolutionary conservation and modularity in the cranial ossification sequences of ostariophysan fishes

This study documents the ossification sequence of skull bones in two ostariophysan species and attempts to address two questions: 1) How conserved are cranial ossification sequences among ostariophysan fishes? and 2) Is there evidence for modularity in the evolution of cranial ossification sequences? Data on skull ossification were obtained from cleared and stained developmental series of Chanos chanos (Gonorynchiformes: Chanidae) and Hemigrammus erythrozonus (Characiformes: Characidae). Median ossification sequences were constructed for both species and compared to literature data available for two other species. Results indicate that cranial ossification sequences are generally conserved among ostariophysans, showing relatively high, significant rank correlations (Spearman r). Additionally, 70% of sequence unit characters constructed from the ossification sequences are invariant among those ostariophysans considered. To test for evidence of modularity, a priori hypothesized modules were designed by grouping bones according to developmental, functional or structural criteria. Bones in hypothesized modules were tested for coordinated changes using randomization methods. Results indicate that certain sets of cranial bones, such as bones of the hyoid bar or bones of the gill arches, change their relative timing of ossification in a significantly coordinated manner, showing evidence for modularity in the evolution of ossification sequences.