Ptilopoda (lit. "feather feet") are a clade of Hadrorhachia.
First emerging around 4067ma, Ptilopods are filter-feeding Acanthopods, feeding on the vast swarms of plankton near the ocean surface. To this end, their four feeding arms are lined with long, tightly spaced bristles used to sieve plankton from the water. These bristles, combined with their subcarangiform swimming, makes them slower than other clades of Hadorhachids. While this doesn't impede their ability to hunt, it does make them more easily preyed upon by Temnopods, who serve as their primary predators.
Like pelagic filter feeders seen on Earth, Ptilopods are the largest clade of Acanthopods, measuring up to nine meters long. This is close to the upper limit for Acanthopods, due to their gill-driven respiration: growing to larger sizes would require the gill surfaces to become impractically large.
For roughly 140Ma years, Ptilopods monopolized the role of large filter feeders. However, following the disruption of the oceanic ecosystem, Ptilopods declined in number, triggering a massive disruption in the pelagic ecosystem. The Thalattotheres, more specifically Isopterygians, largely overtook the Ptilopods' role as filter feeders. However, the Ptilopods have persisted into the present day, as their gill-based respiration allows them to hunt and survive in waters too deep for the Thalattotheres.