A predatory bivalved euarthropod from the Cambrian (Stage 3) Xiaoshiba Lagerstätte, South China

Abstract

Bivalved euarthropods represent a conspicuous component of exceptionally-preserved fossil biotas throughout the Lower Palaeozoic. However, most of these taxa are known from isolated valves, and thus there is a limited understanding of their morphological organization and palaeoecology in the context of early animal-dominated communities. The bivalved euarthropod Clypecaris serrata sp. nov., recovered from the Cambrian (Stage 3) Hongjingshao Formation in Kunming, southern China, is characterized by having a robust first pair of raptorial appendages that bear well-developed ventral-facing spines, paired dorsal spines on the trunk, and posteriorly oriented serrations on the anteroventral margins of both valves. The raptorial limbs of C. serrata were adapted for grasping prey employing a descending stroke for transporting it close the mouth, whereas the backwards-facing marginal serrations of the bivalved carapace may have helped to secure the food items during feeding. The new taxon offers novel insights on the morphology of the enigmatic genus Clypecaris, and indicates that the possession of paired dorsal spines is a diagnostic trait of the Family Clypecarididae within upper stem-group Euarthropoda. C. serrata evinces functional adaptations for an active predatory lifestyle within the context of Cambrian bivalved euarthropods, and contributes towards the better understanding of feeding diversity in early ecosystems.

Three-Dimensionally Preserved Appendages in an Early Cambrian Stem-Group Pancrustacean

“…The sediment filled tubular gut in E. multinodosa ( Figure S3E) closely resembles that of the morphologically similar bivalved euarthropods Clypecaris pteroidea [9] and C. serrata [34] and suggests that all these taxa may have had a relatively simple detritus-feeding autecology. However, the presence of threedimensional gut tracts filled with sediment in various euarthropods from the Chengjiang Lagerst€ atte has been regarded instead as a result of early permineralization of initially organically preserved and chemically reactive gut glands that later become replaced by clay minerals [37].…”

“…Our findings emphasize the utility of novel imaging techniques to extract phylogenetically informative morphological data from exceptional fossils that are affected by decay and preservation mode [32,33]. Although the presence of a first pair of antenniform appendages is well documented in several euarthropods, such as hymenocarines [10,11,13,14], clypecaridids [9,34], and waptiids [8,16,27], the traditional view is that most of these forms possessed largely homonomous post-antennular appendages with minimal functional specialization [5,12,13,15,20]. Alongside recent redescriptions of Burgess Shale taxa [14,16,35], E. multinodosa directly challenges this assumption, and demonstrates a level of appendicular complexity that approximates that of crown-group pancrustaceans by integrating morphologically-and implicitly functionally-differentiated tritocerebral antennae with mandibles and maxillules (Figures 1D and 2A-2D).…”

“…The complex appendicular organization of E. multinodosa impacts the phylogenetic affinities of morphologically similar Cambrian bivalved euarthropods known from Burgess Shaletype deposits, such as clypecaridids [9,34], and waptiids [8,16,27,35]. Until recently [16], both groups had been systematically excluded from cladistic analyses of Lower Palaeozoic euarthropods [5,[12][13][14][15].…”

“…Our new data on E. multinodosa helps to clarify some of these issues, as it demonstrates for the first time the presence of tritocerebral antennae in a macroscopic bivalved euarthropod from a Cambrian Burgess Shale-type deposit. E. multinodosa and other clypecaridids [9,34] share various similarities with waptiids [16,27,35] and hymenocarines [10,11,13,14,26,43], including the presence of paired dorsal valves, an anterior sclerite, differentiated cephalic appendages, biramous limbs with filamentous exopods, appendage-less abdomen composed of cylindrical segments, and paired caudal rami. In this context, the observed morphology of E. multinodosa suggests that other clypecaridids, waptiids, and hymenocarines may also possess tritocerebral antennae and gnathal appendages, as well as trunk epipodites.…”

Reviewing the bases for a nomenclatural uniformization of the highest taxonomic levels in arthropods

“…In certain recent works (e.g. Yang et al 2015Yang et al , 2016, 'stem-group Arthropoda' and 'stem-group Euarthropoda' have also been used as a taxonomic rank. This usage is inappropriate, not so much because it designates a paraphyletic grouping (many Linnaean ranks persist in spite of fluctuations in the configuration of cladograms), but because (1) it is not a character-based systematic entity, falsifiable through a diagnosis, and (2) it contradicts the very essence of the stem-group concept, which is to qualify an existing taxonomic rank.…”