Developmental dynamics of two closely related species Bombina bombina and B. variegata (Amphibia: Anura) in the context of their fossil evidence

Abstract

The available fossil record of Bombina suggests that the genus appeared at the beginning of the Miocene. An isolated occurrence of early Miocene Bombina in Siberia, together with the relatively common occurrence of Bombina in the Miocene of Europe, suggest a possible continuous distribution to East Asia, as is the case with some other Miocene amphibian taxa. The determinable fossil record of B. bombina from the Pliocene precedes that of B. variegata from the Early Pleistocene by about 1 Mya. This also implies that B. variegata differentiated from B. bombina before the Middle Pleistocene continental glaciations. The younger geochronological age of B. variegata compared with that of B. bombina, is reflected in the development of its skeleton. Although both pass through morphologically the same stages of skeletal development, B. variegata is slightly retarded. The developmental retardation of the skeleton of B. variegata culminates in the adult stage, which may last several years after reaching sexual maturity. During this period, the different rates of ossification and growth in both species result in hypo-ossification and underdevelopment and thus in the emergence of differential characteristics. These are less distinguishable in juveniles, but more pronounced in fully grown adults.