Pterygotid eurypterid palaeoecology: praedichnia and palaeocommunities

Pterygotid eurypterids (Chelicerata) were all regarded as active apex nektonic predators, particularly in the Silurian. The chelicerae and lateral eyes of Acutiramus cummingsi were then interpreted as being inconsistent with a predatory lifestyle. Jaekelopterus, Pterygotus and Erettopterus were interpreted as having more acute vision than Acutiramus, but their chelicerae implied they had different ecologies, with not all taxa as top predators. The chelicerae of Acutiramus are here interpreted as being much more robust than was previously assumed, and their eyes as adapted to hunting at depth, at night or in murky water, so neither precludes a predatory lifestyle. Previous models of the mode of life of pterygotid eurypterids are assessed, based on a review of their trace fossils, functional morphology and a new analysis of their biotic associations. All pterygotid taxa are here interpreted as slow swimming vagrant and ambush predators, using their chelicerae to rapidly capture prey, and their more robust coxal gnathobases and metastoma to process it. Slimonia, and Acutiramus with more gracile cutting chelicerae, tend to associate with, and likely specialised on, lightly-armoured phyllocarid crustaceans. Erettopterus and Pterygotus, with more robust chelicerae, tend to associate with more heavily armoured thelodonts and osteostracans, respectively. Jaekelopterus tend to associate with osteicthyans, placoderms and pteraspids. Praedichnia (predation traces) on the pteraspids Lechriaspis and Larnovaspis, eurypterids Eurypterus and Acutiramus and trilobite Spinisscutellum, and their coprolites, provide further evidence for such predatory interactions. The cheliceral morphology, visual acuity, associations and fossil record all support the hypothesis that Acutiramus is actually more basal to Jaekelopterus and Pterygotus, the largest ever arthropods, with Jaekelopterus now estimated at 2.59 m long