What it is
A sturdy land animal of the early Permian, named for the town of Seymour in Baylor County, Texas, where it was found. Its bones show it spent most of its life on land.
Filed as a reptile
In 1911 Samuel Williston ranked it lowest among known reptiles. In 1918 D. M. S. Watson titled a paper "On Seymouria, the most primitive known reptile". Even its champions saw amphibian features in its skull, but in 1939 Theodore White still called it the most primitive reptile known at all well.
Refiled
By 1947 the question was how the animal grew up. Alfred Romer wrote that a reptile, by definition, lays an egg that skips the water-living larva, and he moved Seymouria to the amphibian side. Five years later, gills were reported on young fossils of its cousin Discosauriscus. No larva of Seymouria itself has been found.