Ecdysis in a stem-group euarthropod from the early Cambrian of China

Abstract

Moulting is a fundamental component of the ecdysozoan life cycle, but the fossil record of this strategy is susceptible to preservation biases, making evidence of ecdysis in soft-bodied organisms extremely rare. Here, we report an exceptional specimen of the fuxianhuiid Alacaris mirabilis preserved in the act of moulting from the Cambrian (Stage 3) Xiaoshiba Lagerstätte, South China. The specimen displays a flattened and wrinkled head shield, inverted overlap of the trunk tergites over the head shield, and duplication of exoskeletal elements including the posterior body margins and telson. We interpret this fossil as a discarded exoskeleton overlying the carcass of an emerging individual. The moulting behaviour of A. mirabilis evokes that of decapods, in which the carapace is separated posteriorly and rotated forward from the body, forming a wide gape for the emerging individual. A. mirabilis illuminates the moult strategy of stem-group Euarthropoda, offers the stratigraphically and phylogenetically earliest direct evidence of ecdysis within total-group Euarthropoda, and represents one of the oldest examples of this growth strategy in the evolution of Ecdysozoa.

Computed tomography sheds new light on the affinities of the enigmatic euarthropod Jianshania furcatus from the early Cambrian Chengjiang biota

“…Xiaocaris nov. specifically resembles Alacaris from the Cambrian (Stage 3) Xiaoshiba biota in overall appearance and in that the procurved posterior margin of the head shield is more pronounced in these taxa compared to all other fuxianhuiids (Fig. 4a, b) [51,60], but differs in having a lower tergite count. Furthermore, all chengjiangocaridids are distinguished by a head shield with an approximately 1:1 length/width ratio, five anteriormost small tergites, and their endopods have an almost conical outline without developed ventral endites.…”

“…8). Xiaocaris nov. shares some similarities with members of Chengjiangocarididae [5,21,51,60], particularly the subtrapezoidal outline of the trunk tergites with straight tergopleural margins, the presence of up to four appendage pairs per tergite, and a more gradual posterior tapering of the body when compared with Fuxianhuia or Guangweicaris. Xiaocaris nov. specifically resembles Alacaris from the Cambrian (Stage 3) Xiaoshiba biota in overall appearance and in that the procurved posterior margin of the head shield is more pronounced in these taxa compared to all other fuxianhuiids (Fig.…”

“…For example, a single enigmatic euarthropod from the Wuliuan Kaili Formation has been regarded as a putative fuxianhuiid with preserved gut contents [67], but the affinity of this fossil remains highly dubious owing to its poor preservation. Alacaris mirabilis from the Xiaoshiba biota is the only fuxianhuiid with a differentiated and enlarged protopodite with prominent gnathobasic spines, but only restricted to three pairs of post-oral appendages the anterior end of the body, whereas the endopods through most of the trunk have an almost conical construction with rounded ventral edges [51,60]. In addition to Alacaris, specimens of Chengjiangocaris from the Xiaoshiba biota demonstrate the presence of minute medially directed spine-like projections on the bases on the post-oral appendages that form a food groove [51], but otherwise the endopods lack well-developed endites suggesting limited food processing capabilities.…”

Origin of ecdysis: fossil evidence from 535-million-year-old scalidophoran worms

“…It is generally difficult to distinguish moults (exuviae) from carapaces unless the animal is preserved in the act of shedding its old exoskeleton [34,41]. Suggested criteria for the identification of fossil moults are mainly applicable to arthropods having a relatively hard carapace such as trilobites [10], and include the presence of suture lines as well as repetitive associations and configurations of specific exoskeletal elements [10].…”

“…Ecdysis is relatively well documented in early Palaeozoic panarthropods such as trilobites [10,29 -31], and has also been recognized in more basal groups such as anomalocaridids [32] and lobopodians [33]. A recent study describes the apparent moulting process of a fuxianhuiid euarthropod from the early Cambrian Xiaoshiba Lagerstätte of China [34].…”