Summary The effects of thyroid hormone (TH) level changes on skeletal development were studied in cyprinids Labeobarbus intermedius (Barbinae) and Danio rerio (Danioninae). TH was revealed to influence development of a skull, axial skeleton, paired fins and girdles, and squamation. Usually, high TH‐level promotes the premature onset and acceleration of ontogenetic processes, whereas TH‐deficiency delays and retards them. However, skeletal elements differ in TH‐responsiveness. Alterations of TH level induce developmental changes in some structures but do not cause any changes in others. As a result, the normal sequence of ontogenetic events and the temporal interrelations between ontogenetic processes undergo changes resulting in changes in adult morphology.

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Summary Large African barbs (genus Labeobarbus) of Lake Tana (Ethiopia) form a species flock, which includes 15 species differing in biology, ecology, and shape features, including skull proportions. Early craniogenesis was studied in three species of Lake Tana barbs: L. intermedius, a proposed ancestor of Tana species flock, L. brevicephalus, a small‐sized and early‐maturating barb, and L. megastoma, a large‐sized, piscivorous species. The general sequence of cranial cartilage and bone appearance is the same for the barb species studied. However, they differ greatly in temporal pattern of craniogenesis. As compared to L. intermedius, bony skull development is accelerated relative to chondrocranial development in L. brevicephalus and retarded in L. megastoma. These differences in temporal pattern of craniogenesis are proposed to account for the different cranial proportions displayed by these barbs.