The largest specimen of Apateon and the life history pathway of neoteny in the Paleozoic temnospondyl family Branchiosauridae

摘要

Two distinct developmental trajectories, metamorphosis and neoteny (the retention oflarval somatic features in adult animals), have been reported for the small gill-bearingbranchiosaurids of the Late Carboniferous and Early Permian of central Europe. Basedon a very large specimen of the species Apateon caducus (Ammon, 1889), anatomicalfeatures characteristic for the neotenic phenotype of branchiosaurids are described.Large neotenes lack changes that occur during a short phase of transformation intoterrestrial adults (metamorphosis), such as ossification of the braincase and palatoqua-drate and intercentra, further ossification of the girdles and formation of muscle attach-ment scars and processes on the limb bones. They also lack a distinct sculpturing ofthe dermal skull roofing elements with deep polygonal ridges and grooves. Instead,larval somatic features are retained including ossified branchial denticles indicative ofopen gill slits and accentuated larval-type sculpturing of the dermal skull roof. Largesize, high degree of ossification as compared to the larvae, and the presence of unci-nate processes on the ribs clearly demonstrate an adult ontogenetic stage. Neotenesremained in the aquatic environment throughout their life and were most likely notcapable of effective terrestrial locomotion. The frequency distribution of the two phe-notypes in modern salamander populations and the environmental cues that influencethe development of them provide a comparative framework for the discussion of theevolution of the two life history pathways in branchiosaurids.