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.

Watson noted in 1918 that Conodectes, a dubious genera named by Edward Drinker Cope back in 1896, was likely synonymous with Seymouria.

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.

Broom (1922)[16] and Russian paleontologist Peter Sushkin (1925)[17] 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.

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.

Perhaps the most damning evidence came in 1952, when Czech paleontologist Zdeněk Špinar reported gills preserved in juvenile fossils of the seymouriamorph Discosauriscus.

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.

Langston (1963) reported a femur indistinguishable from that of S. baylorensis in Permian sediments at Prince Edward Island on the Eastern coast of Canada.

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).

[8] The Tambach Formation has also produced the developmentally youngest known fossils of Seymouria, assisting comparisons to Discosauriscus, which is known primarily from juveniles.

[15] 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.

[9] The inner ear of Seymouria baylorensis retains a cochlear recess located behind (rather than below) the vestibule, and its anterior semicircular canal was likely encompassed by a cartilaginous (rather than bony) supraoccipital.

The palate is generally solid bone, with only vestigial interpteryoid vacuities (a pair of holes adjacent to the midline) separated by a long and thin cultriform process (the front blade of the base of the braincase).

[15] The mandible also retained at least one large hole along its inner edge known as a meckelian fenestra, although this feature was only confirmed during a 2005 re-investigation of one of the Cutler Formation specimens.

The system of grooves and nerve openings on the side of the braincase were unusually similar to those of the fish Megalichthys, and the cartilaginous base is another plesiomorphic feature.

The neural arches, which lie above the pleurocentra, are swollen into broad structures with table-like zygapophyses (joint plates) about three times as wide as the pleurocentrum itself.

The ulna is similar, but longer due to the possession of a pronounced olecranon process, as is common in terrestrial tetrapods but rare in amphibious or aquatic ones.

Likewise, some differences relating to the proportions of the rear of the skull may be considered to be an artifact of the fact that most S. sanjuanensis specimens were not fully grown prior to the discovery of the "Tambach lovers", which were adult members of the species.

He argued that the robust limbs and wide-set body supported the idea that it was a strong, terrestrial animal with a sprawling gait.

However, he also noted that Permian trackways generally support the idea that terrestrial tetrapods from this time period were not belly-draggers, but instead were strong enough to keep their bodies off of the ground.

As with other paleontologists around the time, Romer assumed that Seymouria had a reptilian (or amniote) mode of reproduction, with eggs laid on dry land and protected from the elements by an amnion membrane.

[15] White also drew attention to the unusual swollen vertebrae, which would have facilitated lateral (side-to-side) movement but prohibit any torsion (twisting) of the backbone.

He supposed that Seymouria was also a good swimmer, since he (erroneously) estimated that the animal had a deep and powerful tail similar to that of modern crocodilians.

However, he also noted that it would have been vulnerable to semiaquatic or aquatic predators, and that Seymouria fossils were more common in terrestrial deposits as a result of its habitat preferences.

Berman et al. (2000) supported this hypothesis, as the Tambach Formation preserved Seymouria fossils while also completely lacking aquatic animals.

Based on this, he also supported the idea that Seymouria females gave birth to large-yolked eggs on land, as with turtles and crocodilians.

[15] Vaughn (1966) later found a correlation between chevron acquisition and certain skull proportions in Seymouria sanjuanensis, and proposed that they too were examples of sexual dimorphism.

Although it was possible that genital size was variable among males to the extent of impacting the skeleton, the more likely explanation was that the differences White had observed were caused by individual skeletal variation, evolutionary divergence, or some other factor unrelated to sexual dimorphism.

Seymouria are inferred to have undergone metamorphosis very early in life, likely due to environmental stresses from fluctuating wet and dry seasons.