Enrolment and trunk segmentation of a Cambrian eodiscoid trilobite

Abstract

Completely enrolled, phosphatized, 3‐D specimens of the eodiscinid trilobite Tsunyidiscus yanjiazhaiensis from Cambrian Stage 3 of South China exhibit much morphological detail and show variation in delicate coaptative structures associated with their trunk segmentation. The relationship between enrolment mechanism and trunk segmentation during ontogeny confirms a unique developmental pattern among the early Cambrian eodiscoid trilobites, revealing how these animals controlled the rate of segment increase and release during post‐embryonic development in their life cycles.

First confident evidence of moulting in eodiscid trilobites from the Cambrian Stage 3 of South China

“…2009; Dai et al . 2019). The opening of facial suture and rostral suture would have allowed the emergence forward of the post-ecdysial trilobite.…”

“…Eodiscids could enrol and several examples have been reported (e.g. Jell, 1975;Zhang & Clarkson, 1993;Cederström et al 2009;Dai et al 2019). The opening of facial suture and rostral suture would have allowed the emergence forward of the post-ecdysial trilobite.…”

Developmental traits and life strategy of redlichiid trilobites

“…Unlike micropygous redlichiids, all these taxa have an isopygous or subisopygous body patterning, with a relatively large pygidium containing more segments throughout ontogeny. A more complex condition in the segment exchange between the thorax and pygidium makes the trunk segmentation of these groups seemingly ‘irregular’ (see Dai et al ., 2021 a , b ), and muddied our understanding of trilobite segmentation in the context of their lifestyle and functional morphology (see Dai et al ., 2019 ). It is likely that different body patterning associated with the shape and size of the head and trunk could play a key role in trilobite development and provide insights into how various trilobites controlled their body morphology (including size, shape and segment number in both the thorax and caudal plate) through different rates of segment production and liberation.…”

“…This condition might also be present in some derived species such as A. koninckii in which variation in pygidial segment number (3 and 4) during ontogeny is also known in different meraspid degrees (see Hughes & Chapman, 1995 ). Multiple instars in each meraspid degree, which might be caused by intraspecific variation or individual differences in the rate of somitogenesis and tagmosis, suggest that they could build their body structures with some flexibility through different developmental trajectories (Dai et al ., 2019 ). This pattern has not yet been observed in any redlichiid taxa.…”

“…This pattern is very similar to that of many eodiscoids for which ontogenetic sequences have been documented. For example, in the collections of Tsunyidiscus (Dai & Zhang, 2011 ; Dai et al ., 2016 ), Sinodiscus (Dai & Zhang, 2013 a ) and Pagetia (Dai et al ., 2019 ), there are usually hundreds to thousands of articulated and disarticulated holaspid specimens, but only a few dozen meraspides. This distribution is in direct contrast to that of other polymerid trilobites.…”