Skulls of two specimens of the Lower Permian seymouriamorph tetrapod Seymouria, referred to Seymouria sanjuanensis Vaughn, 1966, with midline skull lengths of about 19.0 and 20.0 mm, respectively, and probably representing an early juvenile ontogenetic stage, are described from the Lower Permian Tambach Formation, lowermost formation of the Upper Rotliegend Group or Series, of the Bromacker quarry locality in the midregion of the Thuringian Forest near Gotha, central Germany. They represent the smallest known specimens of the genus and are compared with those of larger, previously described specimens of Seymouria that include from the same locality: a late juvenile and two fully adult specimens of S. sanjuanensis with skull lengths of 56.0 mm, and 87.9 and 94.6 mm, respectively, and fully adult specimens of S. sanjuanensis and Seymouria baylorensis Broili, 1904, with skull lengths of 86.0–93.0 and 99.0–124.0 mm, respectively, from the Lower Permian of North America. This partial but widely spaced ontogenetic series of the skull of S. sanjuanensis, which includes an early and late juvenile and fully mature adult stages, is the first to be documented for any Seymouriamorpha. The skulls of the two early juvenile specimens of S. sanjuanensis from the Bromacker quarry are compared with similarly sized supposed larval specimens of the very closely related seymouriamorph species Discosauriscus pulcherrimus and Discosauriscus austriacus (Makowsky 1876), from the Lower Permian of the Boskovice Furrow, Moravia, Czech Republic, in which the quadrate and articular were cartilaginous and the sensory grooves are present. Contrary to conditions in Discosauriscus, in the early juvenile specimens of S. sanjuanensis the quadrate and articular are ossified and the sensory grooves are absent. This indicates that Seymouria underwent metamorphosis at much earlier stage than Discosauriscus. In addition, the early juveniles of S. sanjuanensis can be distinguished from those of both species of Discosauriscus on the basis of several differences in the skull roof and palate. However, at this stage of growth three characters indicate that S. sanjuanensis is more similar to D. pulcherrimus than to D. austriacus: 1) a chevron-shaped postorbital; 2) rows of small denticles that radiate anteriorly from the midwidth of the posterior border of the palatal ramus of the pterygoid; and 3) presence of denticles arranged in rows or randomly scattered denticles on the anterior half of the parasphenoidal plate.
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