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Abstract
Biomineralised trilobite exoskeletons provide a 250 million year record of abnormalities in one of the most diverse arthropod groups in history. One type of abnormality—repaired injuries—have allowed palaeobiologists to document records of Paleozoic predation, accidental damage, and complications in moulting experienced by the group. Although Cambrian trilobite injuries are fairly well documented, the illustration of new injured specimens will produce a more complete understanding of Cambrian prey items. To align with this perspective, nine new abnormal specimens displaying healed injuries from the Smithsonian National Museum of Natural History collection are documented. The injury pattern conforms to the suggestion of lateralised prey defence or predator preference, but it is highlighted that the root cause for such patterns is obscured by the lumping of data across different palaeoecological and environmental conditions. Further studies of Cambrian trilobites with injuries represent a key direction for uncovering evidence for the Cambrian escalation event.
Highlights
The Cambrian Explosion—a rapid and stunning increase in animal diversity and disparity during the earliest Paleozoic, over 500 million years ago—likely resulted from a combination of biological, ecological, and environmental factors (Smith & Harper, 2013; Zhang et al, 2014; Bicknell & Paterson, 2018)
Mummaspis oblisooculatus Fritz, 1992, USNM PAL 443790, Mural Formation (Cambrian Series 2, Stage 4, age taken from Ortega Hernández, Esteve & Butterfield, 2013), Alberta, Canada
Comparing the nature of the abnormalities documented here with examples in other publications suggests that the specimens described in this contribution display repaired injuries, rather than examples of developmental or genetic aberrations (Owen, 1985; Conway Morris & Jenkins, 1985; Babcock & Robison, 1989; Babcock, 1993; Babcock, 2003; Figure 6 Glossopleura gigantea Resser, 1939, UNSM PAL 729419, Spence Shale Member, Langston Formation (Miaolingian Series, Wuliuan). (A) Complete specimen. (B) Close up of abnormality in box in (A) illustrating single segment injuries (SSIs) on 4th thoracic segment
Summary
The Cambrian Explosion—a rapid and stunning increase in animal diversity and disparity during the earliest Paleozoic, over 500 million years ago—likely resulted from a combination of biological, ecological, and environmental factors (Smith & Harper, 2013; Zhang et al, 2014; Bicknell & Paterson, 2018). The rise of predation has been considered a key evolutionary innovation that helped drive and shape morphological and diversity trajectories of different Cambrian groups (Vermeij, 1989; Conway Morris, 1998; Bengtson, 2002; Babcock, 2003; Wood & Zhuravlev, 2012; Bicknell & Paterson, 2018; Pates & Bicknell, 2019), including the rise of biomineralisation in the exoskeletons and shells of prey animals (Vermeij, 1989; Vermeij, 2013; Conway Morris & Jenkins, 1985; Babcock, 1993; Babcock, 2003; Conway Morris & Bengtson, 1994; Bicknell & Paterson, 2018).
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