A SOLUTION TO THE INVERSE “T. REX PROBLEM” IN PTERYGOTID EURYPTERIDS

Pterygotid eurypterids (Pterygotidae) are the largest marine arthropods of all time, with estimated maximum body lengths of ~2.5 m. Their characteristic feature is greatly expanded chelicerae, terminating in a strongly toothed chela, capable of existing considerable force in large individuals. Early pterygotid reconstructions showed the chelicerae with multiple podomeres. It eventually became established that despite their great size, pterygotids retained the three-segmented chelicerae, with only one ramus proximal to the chela. This ramus is longer than the chela. This has created a key problem in understanding feeding in pterygotids: how did chelicerae bring food to the ventral side of the prosoma and gnathobases? Detailed range of motion studies demonstrate that even at maximum flexion, the distal chela sections are still markedly anterior to the front of the prosoma. This separation can be considered an arthropod equivalent to the conundrum of the short forelimbs of T. rex; in this case, the limbs are too long. The apparent problem is compounded by the reliance on the pterygotid reconstruction in Clarke and Ruedemann (1912) that presents chelicerae projecting antero-laterally and nearly equally sized prosomal appendages II-V arranged laterally about the carapace, well posterior to the chelicerae. A review of available pterygotid specimens, particularly Acutiramus, demonstrate that this reconstruction is incorrect. Other than the swimming appendages, all prosomal appendages project anteriorly, and insert proximal to the midline. Furthermore, following Selden (1986), appendage II is reduced compared to III-V. We suggest these appendages functioned together as a food basket. The flexed chelicerae brought food items into reach of appendages III-V, which would have conveyed them medially to appendage II, and then to the gnathobases. As actively swimming animals, pterygotid prosomal appendages acted together to gather and process prey from a large volume of water in front of them, as opposed to utility in walking. To this end, we present a new model for pterygotids that focuses on a revised view of their prosomal appendages.