Seymouria is an extinct genus of seymouriamorph from the Early Permian of North America and Europe. Although they were amphibians (in a biological sense), Seymouria were well-adapted to life on land, with many reptilian features—so many, in fact, that Seymouria was first thought to be a primitive reptile. It is primarily known from two species, Seymouria baylorensis and Seymouria sanjuanensis. The type species, S. baylorensis, is more robust and specialized, though its fossils have only been found in Texas. On the other hand, S. sanjuanensis is more abundant and widespread. This smaller species is known from multiple well-preserved fossils, including a block of six skeletons found in the Cutler Formation of New Mexico, and a pair of fully grown skeletons from the Tambach Formation of Germany, which were fossilized lying next to each other.
For the first half of the 20th century, Seymouria was considered one of the oldest and most "primitive" known reptiles. Paleontologists noted how the general body shape resembled that of early reptiles such as captorhinids, and that certain adaptations of the limbs, hip, and skull were also similar to that of early reptiles, rather than any species of modern or extinct amphibians known at the time. The strongly-built limbs and backbone also supported the idea that Seymouria was primarily terrestrial, spending very little time in the water. However, in the 1950s, fossilized tadpoles were discovered in Discosauriscus, which was a close relative of Seymouria in the group Seymouriamorpha. This shows that seymouriamorphs (including Seymouria) had a larval stage which lived in the water, therefore making Seymouria not a true reptile, but rather an amphibian (in the traditional, paraphyletic sense of the term). At that time, it was still thought to be closely related to reptiles, and many recent studies still support this hypothesis. If this hypothesis is correct, Seymouria is still an important transitional fossil documenting the acquisition of reptile-like skeletal features prior to the evolution of the amniotic egg, which characterizes amniotes (reptiles, mammals, and birds). However, under the alternative hypothesis that Seymouria is a stem-tetrapod, it has little relevance to the origin of amniotes.
History
Early history as a putative reptile Fossils of Seymouria were first found near the town of Seymour, in Baylor County, Texas (hence the name of the type species, Seymouria baylorensis, referring to both the town and county). The earliest fossils of the species to be collected were a cluster of individuals acquired by C.H. Sternberg in 1882. However, these fossils would not be properly prepared and identified as Seymouria until 1930.
Various paleontologists from around the world recovered their own Seymouria baylorensis fossils in the late 19th century and early 20th century. Seymouria was formally named and described in 1904 based on a pair of incomplete skulls, one of which was associated with a few pectoral and vertebral elements. These fossils were described by German paleontologist Ferdinand Broili, and are now stored in Munich. American paleontologist S.W. Williston later described a nearly complete skeleton in 1911, and noted that "Desmospondylus anomalus", a taxon he had recently named from fragmentary limbs and vertebrae, likely represented juvenile or even embryonic individuals of Seymouria.
Likewise, English paleontologist D.M.S. Watson noted in 1918 that Conodectes, a dubious genera named by Edward Drinker Cope back in 1896, was likely synonymous with Seymouria. Robert Broom (1922) argued that the genus should be referred to as Conodectes since that name was published first, but Alfred Romer (1928) objected, noting that the name Seymouria was too popular within the scientific community to be replaced. During this time, Seymouria was generally seen as a very early reptile, part of an evolutionary grade known as "cotylosaurs", which also included many other stout-bodied Permian reptiles or reptile-like tetrapods.
Proposed amphibian affinities Many paleontologists were uncertain about Seymouria's allegiance with the reptiles, noting many similarities with the embolomeres, which were unquestionably "labyrinthodont" amphibians. This combination of features from reptiles (i.e. other "cotylosaurs") and amphibians (i.e. embolomeres) was evidence that Seymouria was central to the evolutionary transition between the two groups. Regardless, not enough was known about its biology to conclude which group it was truly part of. Broom (1922) and Russian paleontologist Peter Sushkin (1925) supported a placement among the Amphibia, but most studies around this time tentatively considered it an extremely "primitive" reptile; these included a comprehensive redescription of material referred to the species, published by Theodore E. White in 1939.
However, indirect evidence that Seymouria was not biologically reptilian started to emerge by the 1940s. Around this time, several newly described genera were linked to Seymouria as part of the group Seymouriamorpha. Some seymouriamorphs, such as Kotlassia, had evidence of aquatic habits, and even Seymouria itself had occasionally been argued to possess lateral lines, sensory structures only usable underwater. Watson (1942) and Romer (1947) each reversed their stance on Seymouria's classification, placing it among the amphibians rather than the reptiles. Perhaps the most damning evidence came in 1952, when Czech paleontologist Zdeněk Špinar reported gills preserved in juvenile fossils of the seymouriamorph Discosauriscus. This unequivocally proved that seymouriamorphs had an aquatic larval stage, and thus were amphibians, biologically speaking. Nevertheless, the numerous similarities between Seymouria and reptiles supported the idea that seymouriamorphs were close to the ancestry of amniotes.
Additional species and fossils
In 1966, Peter Paul Vaughn described an assortment of Seymouria skulls from the Organ Rock Shale of Utah. These remains represented a new species, Seymouria sanjuanensis. Fossils of this species are now understood to be more abundant and widespread than those of Seymouria baylorensis. Several more species were later named by Paul E. Olson, although their validity has been more questionable than that of S. sanjuanensis. For example, Seymouria agilis (Olson, 1980), known from a nearly complete skeleton from the Chickasha Formation of Oklahoma, was reassigned by Michel Laurin and Robert R. Reisz to the parareptile Macroleter in 2001. Seymouria grandis, described a year earlier from a braincase found in Texas, has not been re-referred to any other tetrapod, but it remains poorly known. Langston (1963) reported a femur indistinguishable from that of S. baylorensis in Permian sediments at Prince Edward Island on the Eastern coast of Canada. Seymouria-like skeletal remains are also known from the Richards Spur Quarry in Oklahoma, as first described by Sullivan & Reisz (1999).
A block of sediment containing six S. sanjuanensis skeletons was found in the Cutler Formation of New Mexico, as described by Berman, Reisz, & Eberth (1987). In 1993, Berman & Martens reported the first Seymouria remains outside of North America, when they described S. sanjuanensis fossils from the Tambach Formation of Germany. The Tambach Formation has produced S. sanjuanensis fossils of a similar quality to those of the Cutler Formation. For example, in 2000 Berman and his colleagues described the "Tambach Lovers", two complete and fully articulated skeletons of S. sanjuanensis fossilized lying next to each other (though it cannot be determined whether they were a couple killed during courtship). The Tambach Formation has also produced the developmentally youngest known fossils of Seymouria, assisting comparisons to Discosauriscus, which is known primarily from juveniles.
Description
Seymouria individuals were robustly-built animals, with a large head, short neck, stocky limbs, and broad feet. Even the largest specimens were fairly small, only about 2 ft (60 cm) long. The skull was boxy and roughly triangular when seen from above, but it was lower and longer than that of most other seymouriamorphs. The vertebrae had broad, swollen neural arches (the portion above the spinal cord). As a whole the body shape was similar to that of contemporary reptiles and reptile-like tetrapods such as captorhinids, diadectomorphs, and parareptiles. Collectively these types of animals have been referred to as "cotylosaurs" in the past, although they do not form a clade (a natural, relations-based grouping).
Skull
The skull was composed of many smaller plate-like bones. The configuration of skull bones present in Seymouria was very similar to that of far more ancient tetrapods and tetrapod relatives. For example, it retains an intertemporal bone, which is the plesiomorphic ("primitive") condition present in animals like Ventastega and embolomeres. The skull bones were heavily textured, as was typical for ancient amphibians and captorhinid reptiles. In addition, the rear part of the skull had a large incision stretching along its side. This incision is termed an otic notch, and a similar incision in the same general area is common to most Paleozoic amphibians ("labyrinthodonts", as they are sometimes called), but unknown in amniotes. The lower edge of the otic notch was formed by the squamosal bone, while the upper edge was formed by downturned flanges of the supratemporal and tabular bones (known as otic flanges). The tabular also has a second downturned flange visible from the rear of the skull; this flange (known as an occipital flange) connected to the braincase and partially obscured the space between the braincase and the side of the skull. The development of the otic and occipital flanges is greater in Seymouria (particularly S. baylorensis) than in any other seymouriamorph.
The sensory apparatus of the skull also deserves mention for an array of unique features. The orbits (eye sockets) were about midway down the length of the skull, although they were a bit closer to the snout in juveniles. They were more rhomboidal than the circular orbits of other seymouriamorphs, with an acute front edge. Several authors have noted that a few specimens of Seymouria possessed indistinct grooves present in bones surrounding the orbits and in front of the otic notch. These grooves were likely remnants of a lateral line system, a web of pressure-sensing organs useful for aquatic animals, including the presumed larval stage of Seymouria. Many specimens do not retain any remnant of their lateral lines, not even juveniles. Near the middle of the parietal bones was a small hole known as a pineal foramen, which held a sensory organ known as a parietal eye. The pineal foramen is smaller in Seymouria than in other seymouriamorphs.