Hadrorhachia (lit. "stout spine") is a clade of Acanthopodia, common to pelagic environments and analogous to cartilaginous fish on Earth.
Hadrorhachia first emerged around 4060ma, when it diverged from the Orthorhachia branch.
The first Hadrorhachids used subcarangiform swimming, having a truncate caudal fin, and measured around 2 meters in length. While some species retain this form of locomotion, other species have evolved to use the other two forms of body-driven swimming. Native to pelagic environments, all observed Hadrorhachids employ some form of counter-shading.
Hadrorhachids dominated the pelagic ecosystem for over 150 million years, until the global climate shift resulted in the disruption of the oceanic ecosystem. While the Hadrorhachids survived, their top-level roles in the food chain where mostly overtaken by the Thalattotheres. However, the Hadrorhachids remain dominant in deeper waters due to their gill-based respiration.
- Ptilopoda: Filter feeders, comprise the largest Acanthopods ever seen
- Teleopterida: Pelagic predators