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Seymouriamorphs are a small butwidespread group of early terrestrial choanates. They are known from theLower to Upper Permian and have been found in Europe, Asia, and NorthAmerica (Fig. 1). Six genera and eight species are known. Many additional specieshave been erected, but most of them are not valid (Klembara and Janiga,1993).
Figure 1. Geographic distribution of seymouriamorph localities. These localities are located in Eurasia and North America, but in the Permian, the continents were in different positions and Eurasia was represented by several smaller plates.
Small aquatic larvae bearing external gills are known in someseymouriamorphs (Ivakhnenko, 1981; Kuznetsov and Ivakhnenko, 1981;Klembara, 1995). The adults, when known, appear to have been terrestrial.The skull length of the known specimens ranges from about 6 mm in thesmallest larvea (Ivakhnenko, 1981) to 15 cm in presumably matureindividuals. The monophyly of this group has long been doubted because ofsize and ontogenetic differences between various taxa. Seymouria andKotlassia are only known from relatively large and mature specimens (Fig. 2) whereasall other known seymouriamorphs are represented by larval and small tomid-sized postmetamorphic specimens and were previously calleddiscosauriscids (this is now known to be a paraphyletic group). However,recent discoveries of fairly large specimens of Discosauriscus (Klembaraand Meszáros, 1992) and Ariekanerpeton (Laurin, 1996b) and relatively small specimens of Seymouria (Berman and Martens, 1993) have bridged this gap.
Figure 2. Seymouriamorph skulls in dorsal (A-C) and left lateral (D-F) views.Seymouria baylorensis (A, D). Seymouria sanjuanensis (B, E).Ariekanerpeton sigalovi (E, F). The specimens on which the reconstructionsof S. sanjuanensis and A. sigalovi are based are probably not mature andthese taxa may have reached approximately the same size as S. baylorensis.Modified from Laurin (1995).