Pseudotrilobites

Pseudotrilobites (tribe Iugotagmatini) are a group of Caenarthrid aquatic isopods found in the Interthalassic and Medithalassic oceans. They are relatively basal within their family and never managed to take advantage of decaying biofilm food sources like their cousins the mudeaters. Instead, they remained small and unspecialized through the Ice Age, having no major role in any ecosystem until relatively recently. However, as the seabeds were picked clean and sandy sediments once again took over, they found themselves flourishing by the end of the Early Arthrocene. With strong mouthparts capable of processing solid food, they expanded into niches that other Caenarthrids couldn't evolve to occupy. Today, three genera exist, each with its own unique variation on the basic pseudotrilobite anatomy. This includes a wide, skirted body, which gives the impression of a three-lobed structure and is the basis for their common name. Its function is to protect the woodlouse, preventing it from being flipped over by carnivores to expose its soft underside. Pseudotrilobites also have relatively good eyesight for Caenarthrids, most of which use touch and scent to navigate their environment. Those living in shallow waters are active during the day and use their large eyes to scan for both predators and prey, while deep-ocean forms use bioluminescence to light the sea floor ahead of them.

The largest pseudotrilobites, belonging to the type genus Iugotagma, use this "headlight" strategy to find scraps of marine snow, sessile filter-feeders, and the corpses of larger epipelagic animals. The Fallfinder (I. vagrans) specializes on the latter, cruising just above the sand for weeks at a time until it stumbles upon a sunken fish, marine tetrapod, or even an especially large taitube worm that it can feast on. Its long journey is rewarded by the fact that it's often the first to arrive at these fallen feasts, allowing it to selectively feed on the most nutritious parts before other scavengers arrive. It may fight to defend its meal for a while, but typically retreats when it encounters one of the many sea creatures more powerful than itself. Fallfinders can go months without eating and thus can afford to leave the less-tasty scraps to someone else.

On the other hand, Surferbugs (Asperaquarthrus) lead a far more energy-intensive lifestyle, living on rough, rocky coastlines where powerful waves force them to cling tightly to any shelter they can find. Rockslime cliffs are a common habitat, but surferbugs can also be found just offshore around shoals of stone that find themselves fully submerged at high tide. In either case, the crustaceans forage when the water rises around them. They have a particular taste for animals dashed against the rocks and land creatures swept away in the current, neither of which put up much of a fight. On occasion, they may also make passes at swimming prey like Copabiniid copepods, which share their typical hunting grounds.

The last and most speciose pseudotrilobite clade is the genus Radignathus. These have strong, downward-facing mandibles built to destroy hard objects. With effort, they can crack arthropod shells, pierce the glassy exoskeletons of Vitrovolvox algae, and even chew through soft rock to access burrowing animals within. They are also some of the only isopods that can put up a fight against predatory seawis, as the birds generally give up their attacks when they feel their beaks being chewed on. Radignathus pseudotrilobites can leave the sea for short periods, though like all aquatic isopods they cannot breathe air due to their soft gills, which tend to collapse in on themselves without water to support them. Still, with the ability to survive without breathing for nearly half an hour, members of this genus can get a lot of scavenging done before returning for a fresh supply of oxygen. They scour the beach at low tide, sometimes dragging large food items back below the surface. Small species and young individuals may live in the substrate, retreating into the sand instead of risking death in open water. Others may choose to take refuge in tide pools, entering a brief dormant state to avoid suffocation as conditions quickly become anoxic.