PaleoBios, Volume 16, Number 4, Pages 1-8, December 8,1995 Comparative Cranial Anatomy of Seymouria sanjuanensis (Tetrapoda: Batrachosauria) from the Lower Permian of Utah and New Mexico Michel Laurin Department of Zoology, Erindale College, University of Toronto, Mississauga, ON, L5L 1C6, Canada Current address: Museum of Paleontology, University of California, Berkeley, CA 94720, USA ABSTRACT The Lower Permian seymouriamorph (Tetrapoda: Batrachosauria) Seymouria sanjuanensis was erected on the basis of several specimens from Utah, and specimens from New Mexico were subsequently described. Further preparation of two specimens of 5. sanjuanensis reveals additional differences between S. sanjuanensis and the better known S. baylorensis. These include the exclusion of the supratemporal from the tabular horn, a shorter and narrower anterior process of the jugal, a vertical jugal-squamosal suture, a palatine wedged in the pterygoid, and a larger lateral exposure of the articular in 5. sanjuanensis. As in S. baylorensis, there is no mandibular fenestra. Seymouria is more similar to the "discosauriscid" Ariekanerpeton than previously recognized. However, most similarities between S. sanjuanensis and Ariekanerpeton are probably size-related, ontogenetic, or primi- tive. Presence of posteroventral and anteroventral notches in the orbit of S. sanjuanensis and 5. baylorensis indicates that these two species are more closely related to each other than to Ariekanerpeton. INTRODUCTION Seymouria has long been considered to repre- sent an early reptile or a link between amphibians and reptiles (Broili, 1904; White, 1939). As such, it has figured extensively in discussions of the origin and early evolution of amniotes (Gauthier et al., 1988; Laurin and Reisz, 1995). More recently (Laurin, 1994), Seymouria has been argued to be a close relative of the crown-group of tetrapods (a crown-group is a clade that includes the last com- mon ancestor of all the living taxa, and all its de- scendants). Therefore, any improvements of our knowledge of this early tetrapod may shed new light on the origin of modern tetrapods. This study is a preliminary step in a re-evaluation of the origin of amniotes and amphibians. Seymouria is best known from the mature, fairly large specimens of S. baylorensis collected in Leonardian (late Early Permian) localities of North- Central Texas. These were the basis of our knowl- edge of the genus from the beginning of this cen- tury, until the discovery by Vaughn (1966) of speci- mens belonging to a new, slightly smaller species, S. sanjuanensis. This second species extended the stratigraphic and geographic range of Seymouria deep into the Cutler Formation (early to middle Wolfcampian, earliest Early Permian) and into Utah (Vaughn, 1966), New Mexico (Berman et al., 1987), and Germany (Berman and Martens, 1993). This latter discovery is the first record of Seymouria out- side North America. Until this time, Lower Per- mian seymouriamorphs were divided into the North American genus Seymouria, represented by mid-sized to large postmetamorphic specimens, and Eurasian "discosauriscids", represented by lar- vae and small to mid-sized post-metamorphic indi- viduals. One of the newly discovered German specimens of S. sanjuanensis is very small and falls within the size range of "discosauriscids" (Berman and Martens, 1993). These discoveries, along with a recent study of the "discosauriscid" Ariekanerpeton (Laurin, 1994), suggest that the differences between Seymouria and "discosauriscids" may have been over-emphasized and that seymouriamorph sys- tematics needs to be revised. Because S. sanjuanensis is the species of Seymouria represented by the small- est and most immature specimens, comparisons be- tween S. sanjuanensis and Ariekanerpeton are easier than comparisons between S. baylorensis and Ariekanerpeton. Furthermore, new anatomical data on Ariekanerpeton (Laurin, 1994) allow more detailed comparisons with Seymouria than were previously possible. Therefore, S. sanjuanensis will be com- pared to S. baylorensis and to the Russian seymouriamorph Ariekanerpeton. I performed additional preparation on two specimens initially described by Berman et al. (1987). This material provides new data on S. sanjuanensis and forms the basis of comparisons