Exites in Cambrian arthropods and homology of arthropod limb branches

Unveiling the ventral morphology of a rare early Cambrian great appendage arthropod from the Chengjiang biota of China

BACKGROUND

The early Cambrian arthropod clade Megacheira, also referred to as great appendage arthropods, comprised a group of diminutive and elongated predators during the early Palaeozoic era, around 518 million years ago. In addition to those identified in the mid-Cambrian Burgess Shale biota, numerous species are documented in the renowned 518-million-year-old Chengjiang biota of South China. Notably, one species, Tanglangia longicaudata, has remained inadequately understood due to limited available material and technological constraints. In this study, we, for the first time, examined eight fossil specimens (six individuals) utilizing state-of-the-art μCT and computer-based 3D rendering techniques to unveil the hitherto hidden ventral and appendicular morphology of this species.

RESULTS

We have identified a set of slender endopodites gradually narrowing distally, along with a leaf-shaped exopodite adorned with fringed setae along its margins, and a small putative exite attached to the basipodite. Our techniques have further revealed the presence of four pairs of biramous appendages in the head, aligning with the recently reported six-segmented head in other early euarthropods. Additionally, we have discerned two peduncle elements for the great appendage. These findings underscore that, despite the morphological diversity observed in early euarthropods, there exists similarity in appendicular morphology across various groups. In addition, we critically examine the existing literature on this taxon, disentangling previous mislabelings, mentions, descriptions, and, most importantly, illustrations.

CONCLUSIONS

The μCT-based investigation of fossil material of Tanglangia longicaudata, a distinctive early Cambrian euarthropod from the renowned Chengjiang biota, enhances our comprehensive understanding of the evolutionary morphology of the Megacheira. Its overall morphological features, including large cup-shaped eyes, raptorial great appendages, and a remarkably elongated telson, suggest its potential ecological role as a crepuscular predator and adept swimmer in turbid waters.

The early Cambrian Kylinxia zhangi and evolution of the arthropod head

The early Cambrian Kylinxia zhangi occupies a pivotal position in arthropod evolution, branching from the euarthropod stem lineage between radiodonts (Anomalocaris and relatives) and "great-appendage" arthropods.1,2 Its combination of appendage and exoskeletal features is viewed as uniquely bridging the morphologies of so-called "lower" and "upper" stem-group euarthropods.3,4 Microtomographic study of new specimens of Kylinxia refines and corrects previous interpretation of head structures in this species. Phylogenetic analyses incorporating new data reinforce the placement of Kylinxia in the euarthropod stem group but support new hypotheses of head evolution. The head of Kylinxia is composed of six segments, as in extant mandibulates, e.g., insects.5 In Kylinxia, these are an anterior sclerite associated with an unpaired median eye and paired lateral eyes (thus three rather than five eyes as was previously described1), deutocerebral frontal-most appendages, and four pairs of biramous appendages (rather than two pairs of uniramous appendages). Phylogenetic trees suggest that a six-segmented head in the euarthropod crown group was already acquired by a common ancestor with Kylinxia. The segmental alignment and homology of spinose frontal-most appendages between radiodonts and upper stem-group euarthropods6,7,8,9,10 is bolstered by morphological similarities and inferred phylogenetic continuity between Kylinxia and other stem-group euarthropods.

Interpreting fossilized nervous tissues

Paleoneuranatomy is an emerging subfield of paleontological research with great potential for the study of evolution. However, the interpretation of fossilized nervous tissues is a difficult task and presently lacks a rigorous methodology. We critically review here cases of neural tissue preservation reported in Cambrian arthropods, following a set of fundamental paleontological criteria for their recognition. These criteria are based on a variety of taphonomic parameters and account for morphoanatomical complexity. Application of these criteria shows that firm evidence for fossilized nervous tissues is less abundant and detailed than previously reported, and we synthesize here evidence that has stronger support. We argue that the vascular system, and in particular its lacunae, may be central to the understanding of many of the fossilized peri-intestinal features known across Cambrian arthropods. In conclusion, our results suggest the need for caution in the interpretation of evidence for fossilized neural tissue, which will increase the accuracy of evolutionary scenarios. Also see the video abstract here: https://youtu.be/2_JlQepRTb0.

Ventral Morphology of the Non-Trilobite Artiopod Retifacies abnormalis Hou, Chen & Lu, 1989, from the Early Cambrian Chengjiang Biota, China

Simple Summary

This study reveals the detailed ventral morphology of the enigmatic Cambrian non-biomineralized euarthropod Retifacies abnormalis from the early Cambrian of South China through the use of micro-computer tomography and three-dimensional modelling. The ventral morphology of R. abnormalis includes five pairs of cephalic appendages in three forms (one uniramous antenna pair, three uniramous appendage pairs, one biramous appendage pair), biramous trunk appendages with multilamellar exites, a variable number of pygidial appendage pairs (five or six) and a multi-articulated tailspine with two pairs of short lobe-like accessories. The new anatomical data inform the ecology and evolution of R. abnormalis in the broader evolutionary context of trilobite-like euarthropods known from sites of exceptional preservation. R. abnormalis possessed a higher degree of appendage differentiation along the body than initially thought, in parallel to similar discoveries of other trilobitomorphs such as Naraoia spinosa, Pygmaclypeatus daziensis, and Sinoburius lunaris. This discovery provides additional support to the hypothesis that early trilobitomorphs were ancestrally characterized by heteronomous ventral appendages with various degrees of functional specialization for feeding and respiration.

Abstract

The artiopodans represent a diverse group of euarthropods with a typically flattened dorsal exoskeleton that covers numerous pairs of biramous ventral appendages, and which are ubiquitous faunal components of the 518-million-year-old Chengjiang Lagerstätte in South China. Despite their abundance, several Chengjiang artiopodans remain poorly known, such as the large euarthropoda Retifacies abnormalis, Hou, Chen & Lu, 1989, which is distinguished by the presence of mesh-like ornamentation on its dorsal exoskeleton. Although only a few ventral details were described in a single study in 25 years, it has been frequently featured in phylogenetic analyses that explore the relationships between Cambrian euarthropods. Here, we employ micro-CT and fluorescent microphotography to investigate the exceptionally preserved ventral morphology of R. abnormalis and explore its phylogenetic implications through maximum parsimony and Bayesian inference. Detailed morphology revealed here better supports R. abnormalis as a sister group to the diminutive artiopod Pygmaclypeatus daziensis, also known from Chengjiang, and strengthens the close relationship of these taxa that have been suggested by previous studies as early-branching representatives of Trilobitomorpha. Cephalic appendages suggest this animal might be a scavenger, possibly feeding on soft-bodied organisms. Different pairs of pygidial appendages suggest an anamorphic post-embryonic ontogeny, which adds to the understanding of the developmental mode of Cambrian artiopods, and further supports the statement that post-hatching segment addition occurred in the ancestor of Euarthropoda.

The Daphnia carapace and other novel structures evolved via the cryptic persistence of serial homologs

Understanding how novel structures arise is a central question in evolution. Novel structures are often defined as structures that are not derived from (homologous to) any structure in the ancestor.¹ The carapace of the crustacean Daphnia magna is a bivalved "cape" of exoskeleton. Shiga et al.² proposed that the carapace of crustaceans like Daphnia and many other plate-like outgrowths in arthropods are novel structures that arose through the repeated co-option of genes like vestigial that also pattern insect wings.^2-4 To determine whether the Daphnia carapace is a novel structure, we compare previous functional work² with the expression of genes known to pattern the proximal leg region (pannier, araucan, and vestigial)^5,6 between Daphnia, Parhyale, and Tribolium. Our results suggest that the Daphnia carapace did not arise by co-option but instead derived from an exite (lateral leg lobe) that emerges from an ancestral proximal leg segment that was incorporated into the Daphnia body wall. The Daphnia carapace, therefore, appears to be homologous to the Parhyale tergal plate and the insect wing.⁵ Remarkably, the vestigial-positive tissue that gives rise to the Daphnia carapace appears to be present in Parhyale⁷ and Tribolium as a small, inconspicuous protrusion. Thus, rather than a novel structure resulting from gene co-option, the Daphnia carapace appears to have arisen from a shared, ancestral tissue (morphogenetic field) that persists in a cryptic state in other arthropod lineages. Cryptic persistence of unrecognized serial homologs may thus be a general solution for the origin of novel structures.

The evolution of biramous appendages revealed by a carapace-bearing Cambrian arthropod

Biramous appendages are a common feature among modern marine arthropods that evolved deep in arthropod phylogeny. The branched appendage of Cambrian arthropods has long been considered as the ancient biramous limb, sparking numerous investigations on its origin and evolution. Here, we report a new arthropod, Erratus sperare gen. et sp. nov., from the Lower Cambrian (Stage 3, 520 Ma) Chengjiang biota of Yunnan, China, with unique trunk appendages formed of lateral anomalocaridid-type flaps and ventral subconical endopods. These appendages represent an intermediate stage of biramous limb evolution, i.e. from 'two pairs of flap appendages' in radiodonts to 'flap + endopod' in Erratus, to 'exopod + endopod' in the rest of carapace-bearing arthropods that populate the basal region of the upper-stem lineage arthropods (deuteropods). The new species occupies a phylogenetic position at the first node closer to deuteropods than to radiodonts, and therefore pinpoints the earliest occurrence of the endopod within Deuteropoda. The primitive endopod is weakly sclerotized, and has unspecialized segments without endites or claw. The findings might support previous claims that the outer branch of the biramous limb of fossil marine arthropods, such as trilobites, is not a true exopod, but is instead a modified exite. This article is part of the theme issue 'The impact of Chinese palaeontology on evolutionary research'.