An early Cambrian polyp reveals an anemone-like ancestor for medusozoan cnidarians

Extant cnidarians are a disparate phylum of non-bilaterians and their diploblastic body plan represents a key step in animal evolution. Anthozoans (anemones, corals) are benthic polyps, while adult medusozoans (jellyfishes) are dominantly pelagic medusae. A sessile polyp is present in both groups and is widely conceived as the ancestral form of their last common ancestor. However, the nature and anatomy of this ancestral polyp, particularly of medusozoans, are controversial, owing to the divergent body plans of both groups in the extant lineages and the rarity of medusozoan soft tissues in the fossil record. Here we redescribe the enigmatic Conicula striata Luo et Hu from the early Cambrian Chengjiang biota, south China, which has previously been interpreted as a polyp, lophophorate or deuterostome. We show that C. striata possessed features of both anthozoans and medusozoans. Its stalked polyp and fully encasing conical, annulated organic skeleton (periderm) are features of medusozoans. However, the gut is partitioned by ~28 mesenteries, and has a tubular pharynx, resembling anthozoans. Our phylogenetic analysis recovers C. striata as a stem medusozoan, indicating that the enormously diverse medusozoans were derived from an anemone-like ancestor, with the pharynx lost and number of mesenteries reduced prior to the origin of crown group Medusozoa.

Discussion on ‘Tabelliscolex (Cricocosmiidae: Palaeoscolecidomorpha) from the early Cambrian Chengjiang Biota and the evolution of seriation in Ecdysozoa’ by Shi et al. 2021 (JGS, jgs2021-060)

Exceptionally preserved fossils are key to reconstructing the origin of the modern animal body plans in the Cambrian radiation. The Panarthropod phyla Euarthropoda, Onychophora and Tardigrada have roots in a ‘lobopodian’ grade typified by broadly cylindrical organisms with sclerotized dorsal plates and paired ventral projections. A similar anatomical configuration has been taken to link certain palaeoscolecid worms with the earliest ecdysozoans. Shi et al. (2021) contend that these similarities evolved convergently, and that palaeoscolecids are priapulan relatives with little bearing on the panarthropod evolution.Here we show that this conclusion holds only under a particular treatment of inapplicable character states with known shortcomings. When inapplicable tokens are handled more rigorously, palaeoscolecids are most parsimoniously reconstructed as stem-group panarthropods with homologous dorsal plates and ventral projections – highlighting the degree to which the treatment of inapplicable data can influence fundamental evolutionary conclusions. As the position of palaeoscolecids depends so strongly on the underlying methodology, and is highly uncertain under a Bayesian approach, we consider it premature to exclude the possibility that panarthropods evolved from a grade of palaeoscolecids with dorsal plates and ventral projections.Supplementary material:https://doi.org/10.6084/m9.figshare.16419522

Differentiated appendages in Isoxys illuminate origin of arthropodization

The Cambrian fossil record has produced remarkable insights into the origin of euarthropods, particularly the evolution of their versatile body plan of segments bearing specialized, jointed appendages for different functions including feeding and locomotion [01, 02]. Early euarthropod evolution involved a major transition from lobopodian-like taxa [03, 04, 05] to organisms featuring a fully sclerotized trunk (arthrodization) and limbs (arthropodization) [02, 06, 07, 08]. However, the precise origin of arthropodization remains controversial because some of the earliest branching euarthropods possess a broad dorsal carapace that obscures critical details of the trunk and appendage organization [09, 10, 11, 12, 13, 14, 15]. Here, we demonstrate the presence of fully arthropodized ventral appendages in the upper stem-group euarthropod Isoxys curvirostratus from the early Cambrian Chengjiang biota in South China. Micro-computed tomography reveals the detailed three-dimensional structure of the biramous appendages in I. curvirostratus for the first time. In addition to the raptorial frontal appendages I. curvirostratus also possesses two batches of morphologically distinct biramous limbs, with the first batch consisting of four pairs of short cephalic appendages bearing prominent endites with a feeding function, followed by a second batch of elongate trunk appendages for locomotion. Each biramous limb bears an endopod with more than 12 well-defined podomeres, and an exopod consisting of a slender shaft carrying approximately a dozen paddle-shaped lamellae. Our findings clarify the enigmatic appendicular organization of Isoxys, one of the most ubiquitous euarthropods in Cambrian Burgess Shale-type deposits worldwide [01, 10, 11, 12, 14, 15, 16, 17, 18]. Critically, our new material shows that the trunk of I. curvirostratus was not arthrodized. The phylogenetic position of isoxyiids as possibly the earliest branching members of Deuteropoda [01, 02, 07, 15, 19], suggests that arthropodized biramous appendages evolved before the pattern of full trunk arthrodization that characterizes most extant and extinct members of this successful animal phylum.

Bryozoans from the early Cambrian Chengjiang Biota

Bryozoa are colonial, aquatic, generally sessile coelomate metazoans, with a retractable lophophore and U-shaped digestive tract. They are widely distributed and highly diverse in extant marine and freshwater environments. Current confident fossils of the earliest bryozoan only occur in the Early Ordovician, although molecular estimation date their origin in or prior to the early Cambrian, rendering the origin and early evolution of bryozoans are intensely debated. Here, we describe two cyclostomes, Panpipora yunnanensis gen. et. sp. nov. and Corynotrypa sp., having been recovered from the Cambrian Chengjiang Biota (~ 518 m.y.) in Southwest China. The bryozoans represent the earliest fossils of Bryozoa, predating the record of the phylum by more than 30 million years. Our finding reveals that the Bryozoa, as all other skeletonised metazoans, first evolved during the Cambrian Explosion, and it is congruent with estimation of molecular clock that the cyclostomes is the earliest biomineralised clade. Corynotrypa might be derived from unmineralized ctenostomes by stolonal mineralization and change of growth direction of polypides from vertical to creeping. The unmineralised frontal exterior wall might be a share character existed in all ancestral clades. Encrusting specimens that inhabited in muddy environments have been confirmed here, further suggesting that encrusting bryozoans underwent a profound ecological migration from soft- to hard-bottom facies from Cambrian to Ordovician.

Before trilobite legs: Pygmaclypeatus daziensis reconsidered and the ancestral appendicular organization of Cambrian artiopods

The Cambrian Stage 3 Chengjiang biota in South China is one of the most influential Konservat Lagerstatten worldwide thanks to the fossilization of diverse nonbiomineralizing organisms through pyritization. Despite their contributions to understanding the evolution of early animals, several Chengjiang species remain poorly known due to their scarcity and/or incomplete preservation. Here, we use micro-computed tomography to reveal in detail the ventral appendage organization of the enigmatic non-trilobite artiopod Pygmaclypeatus daziensis, one of the rarest euarthropods in Chengjiang, and explore its functional ecology and broader evolutionary significance. P. daziensis possesses a set of uniramous antennae and 14 pairs of post-antennal biramous appendages, the latter of which show an unexpectedly high degree of heteronomy based on the localized differentiation of the protopodite, endopod and exopod along the antero-posterior body axis. The small body size (less than 2 cm), presence of delicate spinose endites, and well-developed exopods with multiple paddle-shaped lamellae on the appendages of P. daziensis indicate a nekto-benthic mode of life, and a scavenging/detritus feeding strategy. P. daziensis shows that appendage heteronomy is phylogenetically widespread within Artiopoda, the megadiverse clade that includes trilobites and their relatives with nonbiomineralizing exoskeletons and suggests that a single exopod lobe with paddle like lamellae is ancestral for this clade.

Tabelliscolex (Cricocosmiidae: Palaeoscolecidomorpha) from the early Cambrian Chengjiang Biota, and the evolution of seriation in Ecdysozoa

Cricocosmiidae is a clade of palaeoscolecid-like worms from the Chengjiang Biota, China (Cambrian Stage 3). In contrast to palaeoscolecids sensu stricto, which exhibit tessellating micro-plate trunk ornamentation, cricocosmiids possess larger, serially repeated sets of trunk sclerites bearing resemblance to lobopodian trunk sclerites (e.g., Microdictyon spp.). Cricocosmiidae were therefore proposed as stem-group Panarthropoda in some studies but are recovered as stem-group Priapulida in most phylogenetic analyses. The affinity of cricoscosmiids within Ecdysozoa is therefore of much interest, as is testing the homology of these seriated structures. We report four new specimens of the rare cricocosmiid Tabelliscolex hexagonus, yielding new details of the ventral trunk projections, sclerites and proboscis. New data confirm T. hexagonus possessed paired ventral trunk projections in a consistent seriated pattern, which is also reported from new material of Cricocosmia jinningensis (Cricocosmiidae) and Mafangscolex yunnanensis (Palaeoscolecida sensu stricto). Even when the seriated sclerites and ventral projections of cricocosmiids are coded as homologous with the seriated trunk sclerites and paired appendages, respectively, of lobopodian panarthropods, our tree searches indicate they are convergent. Cricocosmiidae is nested within a monophyletic “Palaeoscolecida sensu lato” clade (Palaeoscolecidomorpha nov.) in stem-group Priapulida. Our study indicates that morphological seriation has independent origins in Scalidophora and Panarthropoda.Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosionSupplementary material:https://doi.org/10.6084/m9.figshare.c.5551565

Intraspecific variation in the Cambrian: new observations on the morphology of the Chengjiang euarthropod Sinoburius lunaris

Background The Chengjiang biota from southwest China (518-million-years old, early Cambrian) has yielded nearly 300 species, of which more than 80 species represent early chelicerates, crustaceans and relatives. The application of µCT-techniques combined with 3D software (e.g., Drishti), has been shown to be a powerful tool in revealing and analyzing 3D features of the Chengjiang euarthropods. In order to address several open questions that remained from previous studies on the morphology of the xandarellid euarthropod Sinoburius lunaris, we reinvestigated the µCT data with Amira to obtain a different approach of visualization and to generate new volume-rendered models. Furthermore, we used Blender to design 3D models showing aspects of intraspecific variation. Results New findings are: (1) antennulae consist of additional proximal articles that have not been detected before; (2) compared to other appendages, the second post-antennular appendage has a unique shape, and its endopod is comprised of only five articles (instead of seven); (3) the pygidium bears four pairs of appendages which are observed in all specimens. On the other hand, differences between specimens also have been detected. These include the presence/absence of diplotergites resulting in different numbers of post-antennular appendages and tergites and different distances between the tip of the hypostome and the anterior margin of the head shield. Conclusions Those new observations reveal intraspecific variation among Chengjiang euarthropods not observed before and encourage considerations about possible sexual dimorphic pairs or ontogenetic stages. Sinoburius lunaris is a variable species with respect to its morphological characters, cautioning that taxon-specific variabilities need to be considered when exploring new species.

New vauxiid sponges from the Chengjiang Biota and their evolutionary significance

Aspiculate demosponges are rarely described in geological history on account of the absence of spicules that are stable and resistant to degradation. One exception is the exquisite preservation of sponges without any mineralised skeletons discovered in Lagerstätten (e.g. the Burgess Shale). The Chengjiang Biota, an early example of a Burgess Shale-type Biota in South China (Cambrian Series 2, Stage 3), is one of the only examples of convincing aspiculate sponges until now. Here, we describe Vauxia pregracilenta sp. nov. and V. paraleioia sp. nov., as well as two poorly preserved vauxiid specimens (Vauxia sp.) in open nomenclature, from the Chengjiang Biota. V. pregracilenta has a fan-like holdfast and branches in various sizes, as well as a typical two-layered net-like skeleton, without spicules. The endosomal layer is hexagonal, while the dermal layer is sub-rectangular. V. paraleioia is characterised by a two-layered subconical skeleton, with the dermal layer ornamented with vertical surface grooves. The openings of the dermal and endosomal layers of V. paraleioia are both hexagonal but of different sizes. These newly discovered Vauxia species indicate that the aspiculate sponges were diversified in the early Cambrian period. Partial silicification of the fibres of aspiculate Vauxia are confirmed from the Chengjiang Biota.Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosion

Fuxianhuiids are mandibulates and share affinities with total-group Myriapoda

In spite of their unrivalled ecological success, the origins of terrestrial mandibulates have long remained virtually unknown. In recent years, claims have been made based on phylogenetic results that fuxianhuiids, iconic fossils of the Chengjiang biota and purported basal euarthropods, were in fact mandibulates, allied with the problematic euthycarcinoids as the closest sister-group to Mandibulata or even stem myriapods. A re-examination of euthycarcinoid mouthparts has since brought additional support to this view. Here, we reinvestigated the anterior anatomy of the Cambrian euarthropod Fuxianhuia and its relatives. We demonstrate that the fuxianhuiid head is that of a mandibulate, sharing similarities with hymenocarines, including a limbless intercalary segment. The hypostome is a sub-triangular sclerite partially fused but anatomically independent from a large, bilobed labral plate, as observed in many extant mandibulate taxa as well as euthycarcinoids. The so-called ‘specialized post-antenn(ular) appendages' are therefore the mandibles, with a tripartite palp and large dented gnathal lobe partially covered by the hypostome-labrum complex. Under a ‘deep split' phylogenetic scenario, Fuxianhuia and its relatives are here resolved as sister-group to crown Mandibulata.Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosionSupplementary material: Additional remarks on terminology, morphological interpretations and a list of modified/new characters used for the phylogenetic analysis are available at https://doi.org/10.6084/m9.figshare.c.5305042