Thecodontosaurus was named for its teeth. Thecodont teeth are situated so that the base is completely enclosed in a bony socket, like our teeth and the teeth of other mammals, crocodilians, and dinosaurs. To help visualize this refer to the image below.
Over 245 fragmentary specimens of Thecodontosaurus are known, all attributed to this single species; not for a lack of trying though as 14 other species have been named only to be reassigned to other genera or re-folded into the single valid species. A second species was speculated in 2000 by Benton et al., 2000 when observing more robust specimens of Thecodontosaurus. In the same paper the authors stated that the more robust morphology was equally as likely a result of sexual dimorphism as it was a potential additional species. Regardless of the findings of the paper as regards sexual dimorphism or secondary species, two morphologies are acknowledged simply as gracile and robust.