Abnormal xiphosurids, with possible application to Cambrian trilobites

Xiphosurida comprise an archetypal arthropod group of considerable interest to both biological and palaeontological researchers. This appeal is generated by a combination of unique anatomical features, utility as modern analogues for extinct arthropod groups, and an impressive fossil record. Although xiphosurids have been extensively studied, there are few published examples of abnormal specimens. Abnormalities in xiphosurids have mostly been attributed to injuries (either self-inflicted, from mating, or predation) or teratologies (developmental and genetic malfunctions). Here we summarise all previously recorded extant xiphosurid abnormalities and describe new examples of injuries and teratologies to Limulus polyphemus and Tachypleus tridentatus. Furthermore, we present the first evidence of injured fossil xiphosurids: Euproops danae and Mesolimulus walchi. We identify two main groups of telson teratologies and document new 'U' shaped cephalothoracic injuries to the anterior cephalothoracic margins of L. polyphemus and T. tridentatus. We show 'V' and 'W' shaped injuries to E. danae and M. walchi cephalothoracic sections. A further specimen of E. danae is described, which likely represents plastic deformation of a recently moulted exoskeleton, rather than an abnormality sensu stricto. We compare injuries on extant xiphosurids to extinct Cambrian trilobite injuries to suggest that rare cephalic injuries to trilobites were incurred during soft-shelled exoskeletal stages. Reviewing xiphosurid injuries through time is a pivotal step towards understanding how Recent and extinct arthropods responded to injuries.