Malformed individuals of the trilobiteEstaingia bilobatafrom the Cambrian Emu Bay Shale and their palaeobiological implications

Abstract

AbstractMalformed trilobite specimens present important insight into understanding how this extinct arthropod group recovered from developmental or moulting malfunctions, pathologies, and injuries. Previously documented examples of malformed trilobite specimens are often considered in isolation, with few studies reporting on multiple malformations in the same species. Here we report malformed specimens of the ellipsocephaloid trilobiteEstaingia bilobatafrom the Emu Bay ShaleKonservat-Lagerstätte(CambrianSeries 2, Stage 4) on Kangaroo Island, South Australia. Ten malformed specimens exhibiting injuries, pathologies, and a range of teratologies are documented. Furthermore, five examples of mangled exoskeletons are presented, indicative of predation onE. bilobata. Considering the position of malformed and normal specimens ofE. bilobatain bivariate space, we demonstrate that the majority of malformed specimens cluster among the larger individuals. Such specimens may exemplify larger forms successfully escaping predation attempts, but could equally represent individuals exhibiting old injuries that were made during earlier (smaller) growth stages that have healed through subsequent moulting events. The available evidence from the Emu Bay Shale suggests that this small, extremely abundant trilobite likely played an important role in the structure of the local ecosystem, occupying a low trophic level and being preyed upon by multiple durophagous arthropods. Furthermore, the scarcity of malformedE. bilobataspecimens demonstrates how rarely injuries, developmental malfunctions, and pathological infestations occurred within the species.