Similarly sized skulls of juvenile specimens of the seymouriid and discosauriscid seymouriamorph tetrapods Seymouria and Discosauriscus are compared for the first time. Comparisons are based on a single juvenile skull of Seymouria sanjuanensis from the Lower Permian Bromacker locality, Germany, measuring 56 mm in length and several skulls of Discosauriscus austriacus from the Lower Permian Boskovice Furrow, Czech Republic, reaching a maximum length of 62 mm. Establishment of their stage of development as juvenile is based on the small size of S. sanjuanensis compared to adults of the same species and previous comparisons of various-sized specimens of D. austriacus to complete ontogenetic series of several temnospondyls. In addition, a similar degree of cranial ossification, although slightly more advanced in the S. sanjuanensis skull, is also cited as evidence of very similar ontogenetic stages of development. However, D. austriacus is remarkable in retaining remnants of the lateral line canals, suggesting that a longer premetamorphic period of its life history was aquatic. Despite representing similar, juvenile stages of development, the S. sanjuanensis and D. austriacus specimens can be distinguished clearly from one another on structural and proportional cranial features, thus dispelling suggestions that Seymouria may represent the adult stage of development of Discosauriscus, which is known only by larval and juvenile stages of development.
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