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

Abstract

Malformed 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 trilobite Estaingia bilobata from the Emu Bay Shale Konservat-Lagerstätte (Cambrian Series 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 on E. bilobata. Considering the position of malformed and normal specimens of E. bilobata in 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 malformed E. bilobata specimens demonstrates how rarely injuries, developmental malfunctions, and pathological infestations occurred within the species.

The Emu Bay Shale: A unique early Cambrian Lagerstätte from a tectonically active basin

“…A similar scenario has been suggested for arthropod molt remains from the Burgess Shale ( 83 ). The arthropod W. barbarahardyae may have visited the benthos to feed on E. bilobata —as shown by cololites ( 84 , 85 )—and swimming individuals of the vetulicolian N. aldridgei appear to have been occasionally swept up by sediment gravity flows that buried them in silt and fine sand ( 14 , 86 ). This suggests that both W. barbarahardyae and N. aldridgei may have spent time near the seafloor, either on the delta front or on the prodelta during times when oxygen levels were favorable.…”

Five new malformed trilobites from Cambrian and Ordovician deposits from the Natural History Museum

“…Documentation of injured specimens housed in other collections will expand this preliminary sample and permit the left–right behavioural asymmetry hypothesis to be re-addressed ( Babcock & Robison, 1989 ; Babcock, 1993 ). Recent examination of injury patterns in Cambrian trilobites have demonstrated little evidence for injury asymmetry ( Pates & Bicknell, 2019 ; Bicknell et al, 2022a ; Bicknell et al, 2023a ). However, with 80% of Ogygo.…”

“…klotzi unilateral injuries being right sided, this injury distribution may indeed reflect a population-level pattern ( Table 2 ). Illustrating this condition with a statistical dataset of one species (following Pates et al, 2017 ; Bicknell, Paterson & Hopkins, 2019 ; Bicknell et al, 2022a ; Bicknell et al, 2023a ; Pates & Bicknell, 2019 ) will uncover interesting injury patterns and represents a clear direction for exploring this topic further.…”

“…Injuries commonly show evidence for cicatrisation and/or segment repair and regeneration—records of a successful moulting after an injury. Occasionally, abnormal structures, such as fusion of exoskeletal sections or lack of segment expression, are associated with injuries ( Owen, 1985 ; Bicknell et al, 2022a ; Bicknell et al, 2023a ). This reflects abnormal recovery from the injury.…”

Trilobites in rock enrol: a comment on ‘Developmental and functional controls on enrolment in an ancient, extinct arthropod’ by Esteve and Hughes (2023)

“…Interestingly, teratological expression of segment articulation, where trunk segments remained partially fused to each other and were thus unable to flex during enrolment, was found only in adult morphotypes of A. koninckii , and was strikingly common at approximately 10% of specimens. This is of particular interest, because such thoracic articulation teratologies are generally rare in trilobites [ 13 ], probably due to their inherent interference with movement and protective behaviours like enrolment, leading to the premature death of affected individuals. It seems that the different enrolment style in A. koninckii individuals with more trunk segments may have been resistant to this articulation teratology, or indeed the (large) adults themselves were less likely to be predated, reducing the need for perfect enrolment and causing this teratology to be less strongly selected against.…”