Dual role of enrolment for moulting and protection in a Cambrian trilobite from Poland

The spinose ellipsocephalid Strenuella polonica Czarnocki, 1926 (Cambrian Series 2, Holy Cross Mountains, Poland) exhibits various enrolment postures, ranging from outstretched to fully enrolled. Previous studies explored the possible protective role of these postures, especially full enrolment, while other postures, such as cranidium flexed, have been associated with moulting. We suggest that the dorsal axial thoracic spines may have aided moulting by providing grip against the sediment. A sample of 85 specimens of S. polonica was analysed to investigate whether the proportions of moults and carcasses differed with enrolment posture, and whether the dorsal axial thoracic spines assisted with moulting. Moults and carcasses were found in all postures, but more carcasses have an enrolled thorax, spreading the dorsal axial thoracic spines, and more moults have an outstretched thorax. Pairwise testing for differences in moult to carcass proportions across all postures was not significant, but when sorted by whether dorsal axial thoracic spines were spread out, the results were significant. Therefore, while thoracic enrolment (spreading dorsal axial spines) likely evolved as a protective response to environmental factors and predation, it may have been used for moulting, as moults occurred in all postures, including enrolled thoraces. S. polonica’s spinosity is also indicative of adaptation to predation, likely paralleling an increase in predation intensity during theCambrian. Infaunal moulting may have contributed to the three-dimensional preserva-tion of enrolled moults, and possibly protected against predation during moulting. If so,this suggests a stratigraphically early case of infaunal moulting.