Life restoration of an advanced stegosaur- I mean an ankylosaur (Ankylosaurus magniventris) by Emily Willoughby, CC-BY-SA.
When digging into the history of North American fossil interpretation for the eventual next edition of my Beasts of Antiquity series, one thing that I found a bit weird was the constant reference to ankylosaurids and nodosaurids as types of stegosaurs. To a modern reader, this seems off. After all, the group of armored dinosaurs, Thyreophora ("shield bearers"), is divided into two major groups: Ankylosauria and Stegosauria, each with a few well supported subgroups. It makes sense that these close relatives might once have been classified together, and stegosaurs were discovered first, lending them priority of name. But what changed? Neither ankylosaurs nor stegosaurs are particularly large groups (especially the stegosaurs), and it seems odd that 20th century taxonomists would want to raise a group as small as the modern idea of Stegosauria to the level of "suborder". Why were ankylosaurs eventually spun off, leaving the more primitive stegosaurs behind? I decided to do a little digging to find out.