Elrathia is an extremely common genus of trilobites, most commonly found in the Cambrian strata of Utah. Along with Phacops rana, it's probably one of the first trilobites you think of when you hear the term. Elrathia is believed to have inhabited a highly specialized deep-sea environment known as the exaerobic zone. This is the thin zone on the border of hypoxic (low-oxygen) and anoxic (completely lacking oxygen) waters. So basically, less oxygen than hypoxis waters, but still enough O2 left for animals to survive. Elrathia is speculated to have fed on specialized sulfur-reducing bacteria, which grow abundantly in the exaerobic zone. It's also likely that Elrathia was an opportunist, congregating in huge hordes to feed on carcasses that fell to the sea floor.
Such an event has happened here. A large anomalocarid has died and fallen into the exaerobic zone, providing a feast for the Elrathia, along with a number of large, red nemertean worms. In the upper right, a mysterious sea dandelion-- a kind of siphonophore-- has been drawn by the sudden influx of food.