Fossil evidence unveils an early Cambrian origin for Bryozoa

AbstractBryozoans (also known as ectoprocts or moss animals) are aquatic, dominantly sessile, filter-feeding lophophorates that construct an organic or calcareous modular colonial (clonal) exoskeleton1–3. The presence of six major orders of bryozoans with advanced polymorphisms in lower Ordovician rocks strongly suggests a Cambrian origin for the largest and most diverse lophophorate phylum2,4–8. However, a lack of convincing bryozoan fossils from the Cambrian period has hampered resolution of the true origins and character assembly of the earliest members of the group. Here we interpret the millimetric, erect, bilaminate, secondarily phosphatized fossil Protomelission gatehousei9 from the early Cambrian of Australia and South China as a potential stem-group bryozoan. The monomorphic zooid capsules, modular construction, organic composition and simple linear budding growth geometry represent a mixture of organic Gymnolaemata and biomineralized Stenolaemata character traits, with phylogenetic analyses identifying P. gatehousei as a stem-group bryozoan. This aligns the origin of phylum Bryozoa with all other skeletonized phyla in Cambrian Age 3, pushing back its first occurrence by approximately 35 million years. It also reconciles the fossil record with molecular clock estimations of an early Cambrian origination and subsequent Ordovician radiation of Bryozoa following the acquisition of a carbonate skeleton10–13.

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Ancestral morphology of Ecdysozoa constrained by an early Cambrian stem group ecdysozoan

BMC Evolutionary Biology ◽

10.1186/s12862-020-01720-6 ◽

2020 ◽

Vol 20 (1) ◽

Author(s):

Richard J. Howard ◽

Gregory D. Edgecombe ◽

Xiaomei Shi ◽

Xianguang Hou ◽

Xiaoya Ma

Keyword(s):

Feeding Strategy ◽

Phylogenetic Analyses ◽

Morphological Diversity ◽

Early Cambrian ◽

Ancestral State ◽

Crown Group ◽

Stem Group ◽

Chengjiang Biota ◽

Fossil Records ◽

Experimental Decay

Abstract Background Ecdysozoa are the moulting protostomes, including arthropods, tardigrades, and nematodes. Both the molecular and fossil records indicate that Ecdysozoa is an ancient group originating in the terminal Proterozoic, and exceptional fossil biotas show their dominance and diversity at the beginning of the Phanerozoic. However, the nature of the ecdysozoan common ancestor has been difficult to ascertain due to the extreme morphological diversity of extant Ecdysozoa, and the lack of early diverging taxa in ancient fossil biotas. Results Here we re-describe Acosmia maotiania from the early Cambrian Chengjiang Biota of Yunnan Province, China and assign it to stem group Ecdysozoa. Acosmia features a two-part body, with an anterior proboscis bearing a terminal mouth and muscular pharynx, and a posterior annulated trunk with a through gut. Morphological phylogenetic analyses of the protostomes using parsimony, maximum likelihood and Bayesian inference, with coding informed by published experimental decay studies, each placed Acosmia as sister taxon to Cycloneuralia + Panarthropoda—i.e. stem group Ecdysozoa. Ancestral state probabilities were calculated for key ecdysozoan nodes, in order to test characters inferred from fossils to be ancestral for Ecdysozoa. Results support an ancestor of crown group ecdysozoans sharing an annulated vermiform body with a terminal mouth like Acosmia, but also possessing the pharyngeal armature and circumoral structures characteristic of Cambrian cycloneuralians and lobopodians. Conclusions Acosmia is the first taxon placed in the ecdysozoan stem group and provides a constraint to test hypotheses on the early evolution of Ecdysozoa. Our study suggests acquisition of pharyngeal armature, and therefore a change in feeding strategy (e.g. predation), may have characterised the origin and radiation of crown group ecdysozoans from Acosmia-like ancestors.

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A superarmored lobopodian from the Cambrian of China and early disparity in the evolution of Onychophora

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1505596112 ◽

2015 ◽

Vol 112 (28) ◽

pp. 8678-8683 ◽

Cited By ~ 47

Author(s):

Jie Yang ◽

Javier Ortega-Hernández ◽

Sylvain Gerber ◽

Nicholas J. Butterfield ◽

Jin-bo Hou ◽

...

Keyword(s):

South China ◽

Cambrian Explosion ◽

Suspension Feeding ◽

Early Cambrian ◽

Early Paleozoic ◽

Crown Group ◽

Morphological Disparity ◽

Stem Group ◽

Velvet Worms ◽

Mode Of Life

We describe Collinsium ciliosum from the early Cambrian Xiaoshiba Lagerstätte in South China, an armored lobopodian with a remarkable degree of limb differentiation including a pair of antenna-like appendages, six pairs of elongate setiferous limbs for suspension feeding, and nine pairs of clawed annulated legs with an anchoring function. Collinsium belongs to a highly derived clade of lobopodians within stem group Onychophora, distinguished by a substantial dorsal armature of supernumerary and biomineralized spines (Family Luolishaniidae). As demonstrated here, luolishaniids display the highest degree of limb specialization among Paleozoic lobopodians, constitute more than one-third of the overall morphological disparity of stem group Onychophora, and are substantially more disparate than crown group representatives. Despite having higher disparity and appendage complexity than other lobopodians and extant velvet worms, the specialized mode of life embodied by luolishaniids became extinct during the Early Paleozoic. Collinsium and other superarmored lobopodians exploited a unique paleoecological niche during the Cambrian explosion.

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The dynamics of stem and crown groups

10.1101/633008 ◽

2019 ◽

Author(s):

Graham E. Budd ◽

Richard P. Mann

Keyword(s):

Molecular Clock ◽

Fossil Record ◽

Null Hypothesis ◽

Mass Extinctions ◽

Crown Group ◽

Stem Group ◽

Group Members ◽

Evolutionary Explanations ◽

Birth Death

ABSTRACTThe fossil record of the origins of major groups is of great interests to many biologists, especially when the fossil record apparently conflicts with timings based on molecular clock estimates. Here we model the diversity of “stem” (basal) and “crown” (modern) members of groups as seen in the fossil record, using a “birth-death model”. Under background conditions, the stem group members must diversify rapidly until the modern crown group emerges, at which point their diversity rapidly collapses, followed shortly by their extinction. Mass extinctions can disturb this pattern to create very diverse stem groups such as the dinosaurs and trilobites. Understanding these null-hypothesis patterns is essential for framing ecological and evolutionary explanations for how major groups originate and subsequently evolve.

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A new arthropod Pygmaclypeatus daziensis from the Early Cambrian Chengjiang Lagerstätte, South China

Journal of Paleontology ◽

10.1017/s0022336000033151 ◽

2000 ◽

Vol 74 (5) ◽

pp. 979-982 ◽

Cited By ~ 8

Author(s):

Xingliang Zhang ◽

Jian Han ◽

Degan Shu

Keyword(s):

South China ◽

Fossil Record ◽

South Australia ◽

Early Cambrian ◽

Burgess Shale ◽

Closely Related Species ◽

Stratigraphic Position ◽

Chengjiang Lagerstätte ◽

Early Metazoan ◽

North Greenland

The early Cambrian Chengjiang Lagerstatte, generally regarded as late Atdabanian (Qian and Bengtson, 1989; Bengtson et al., 1990), has become celebrated for perhaps the earliest biota of soft-bodied organisms known from the fossil record and has proven to be critical to our understanding of early metazoan evolution. The Sirius Passet fauna from Peary Land, North Greenland, another important repository of soft-bodied and poorly sclerotized fossils, was also claimed as Early Cambrian (Conway Morris et al., 1987; Budd, 1995). The exact stratigraphic position of the Sirius Passet fauna (Buen Formation) is still uncertain, although the possibility of late Atdabanian age was proposed (Vidal and Peel, 1993). Recent work dates it in the “Nevadella” Biozone (Budd and Peel, 1998). It therefore appears to be simultaneous with or perhaps slightly younger than Chengjiang Lagerstatte, Eoredlichia Biozone (Zhuravlev, 1995). The Emu Bay Shale of Kangaroo Island, South Australia, has long been famous as a source of magnificent specimens of the trilobites Redlichia takooensis and Hsunaspis bilobata. It is additionally important as the only site in Australia so far to yield a Burgess-Shale-type biota (Glaessner, 1979; Nedin, 1992). The Emu Bay Shale was considered late Early Cambrian in age (Daily, 1956; Öpik, 1975). But Zhang et al.(1980) reassessed its age based on data from the Chinese Early Cambrian. The occurrence of Redlichia takooensis and closely related species of Hsunaspis indicates an equivalence to the Tsanglangpuian in the Chinese sequence, and the contemporary South Australia fauna correlate with the Botomian of Siberia (Bengtson et al., 1990). Thus the Emu Bay Shale is younger than the upper Atdabanian Chengjiang Lagerstatte, Chiungchussuian.

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An early Cambrian ecdysozoan with a terminal mouth but no anus

10.1101/2020.09.04.283960 ◽

2020 ◽

Author(s):

Yunhuan Liu ◽

Huaqiao Zhang ◽

Shuhai Xiao ◽

Tiequan Shao ◽

Baichuan Duan

Keyword(s):

Molecular Clock ◽

Phylogenetic Analyses ◽

Early Cambrian ◽

Ecological Diversity ◽

Animal Group ◽

Total Group ◽

New Material

ABSTRACTThe ecdysozoans are the most diverse animal group on Earth1, 2. Molecular clock studies indicate that the ecdysozoans may have diverged and diversified in the Ediacaran Period3, 4, but unambiguous ecdysozoan fossils first appear in the earliest Cambrian and are limited to cycloneuralians5–7. Here we report new material of the early Cambrian microscopic animal Saccorhytus coronarius, which was previously interpreted as a deuterostome8. Saccorhytus coronarius is reconstructed as a millimetric and ellipsoidal meiobenthic animal with a spinose armor and an anterior mouth but no anus. Purported pharyngeal gills in support of the deuterostome hypothesis8 are shown to be taphonomic artifacts. Phylogenetic analyses indicate that Saccorhytus coronarius belongs to the total-group Ecdysozoa, highlighting the morphological and ecological diversity of early Cambrian ecdysozoans.

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The dynamics of stem and crown groups

Science Advances ◽

10.1126/sciadv.aaz1626 ◽

2020 ◽

Vol 6 (8) ◽

pp. eaaz1626 ◽

Cited By ~ 6

Author(s):

Graham E. Budd ◽

Richard P. Mann

Keyword(s):

Molecular Clock ◽

Fossil Record ◽

Large Scale ◽

Mass Extinctions ◽

Crown Group ◽

Stem Group ◽

Considerable Controversy ◽

Long Stem ◽

Evolutionary Explanations ◽

Birth Death

The fossil record of the origins of major groups such as animals and birds has generated considerable controversy, especially when it conflicts with timings based on molecular clock estimates. Here, we model the diversity of “stem” (basal) and “crown” (modern) members of groups using a “birth-death model,” the results of which qualitatively match many large-scale patterns seen in the fossil record. Typically, the stem group diversifies rapidly until the crown group emerges, at which point its diversity collapses, followed shortly by its extinction. Mass extinctions can disturb this pattern and create long stem groups such as the dinosaurs. Crown groups are unlikely to emerge either cryptically or just before mass extinctions, in contradiction to popular hypotheses such as the “phylogenetic fuse”. The patterns revealed provide an essential context for framing ecological and evolutionary explanations for how major groups originate, and strengthen our confidence in the reliability of the fossil record.

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A mitogenomic phylogeny of chitons (Mollusca: Polyplacophora)

BMC Evolutionary Biology ◽

10.1186/s12862-019-1573-2 ◽

2020 ◽

Vol 20 (1) ◽

Cited By ~ 2

Author(s):

Iker Irisarri ◽

Juan E. Uribe ◽

Douglas J. Eernisse ◽

Rafael Zardoya

Keyword(s):

Molecular Clock ◽

Gene Order ◽

Fossil Record ◽

Phylogenetic Analyses ◽

Integrative Taxonomy ◽

Sensitivity Analyses ◽

Molecular Clocks ◽

Additional Species ◽

Phylogeny And Evolution ◽

Conserved Gene

Abstract Background Polyplacophora, or chitons, have long fascinated malacologists for their distinct and rather conserved morphology and lifestyle compared to other mollusk classes. However, key aspects of their phylogeny and evolution remain unclear due to the few morphological, molecular, or combined phylogenetic analyses, particularly those addressing the relationships among the major chiton lineages. Results Here, we present a mitogenomic phylogeny of chitons based on 13 newly sequenced mitochondrial genomes along with eight available ones and RNAseq-derived mitochondrial sequences from four additional species. Reconstructed phylogenies largely agreed with the latest advances in chiton systematics and integrative taxonomy but we identified some conflicts that call for taxonomic revisions. Despite an overall conserved gene order in chiton mitogenomes, we described three new rearrangements that might have taxonomic utility and reconstructed the most likely scenario of gene order change in this group. Our phylogeny was time-calibrated using various fossils and relaxed molecular clocks, and the robustness of these analyses was assessed with several sensitivity analyses. The inferred ages largely agreed with previous molecular clock estimates and the fossil record, but we also noted that the ambiguities inherent to the chiton fossil record might confound molecular clock analyses. Conclusions In light of the reconstructed time-calibrated framework, we discuss the evolution of key morphological features and call for a continued effort towards clarifying the phylogeny and evolution of chitons.

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Occurrence of the eudemersal radiodont Cambroraster in the early Cambrian Chengjiang Lagerstätte and the diversity of hurdiid ecomorphotypes

Geological Magazine ◽

10.1017/s0016756820000187 ◽

2020 ◽

Vol 157 (7) ◽

pp. 1200-1206 ◽

Cited By ~ 3

Author(s):

Yu Liu ◽

Rudy Lerosey-Aubril ◽

Denis Audo ◽

Dayou Zhai ◽

Huijuan Mai ◽

...

Keyword(s):

New Species ◽

South China ◽

Early Cambrian ◽

Burgess Shale ◽

Stem Group ◽

Lower Palaeozoic ◽

Trophic Webs ◽

Chengjiang Lagerstätte ◽

A New Species

AbstractRadiodonts are a diverse clade of Lower Palaeozoic stem-group euarthropods that played a key role in the emergence of complex marine trophic webs. The latest addition to the group, Cambroraster falcatus, was recently described from the Wuliuan Burgess Shale, and is characterized by a unique horseshoe-shaped central carapace element. Here we report the discovery of Cambroraster sp. nov. A, a new species from the Cambrian Stage 3 Chengjiang Lagerstätte of South China. The new occurrence of Cambroraster demonstrates that some of the earliest known radiodonts had already evolved a highly derived carapace morphology adapted to an essentially eudemersal life as sediment foragers.

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

Journal of the Geological Society ◽

10.1144/jgs2021-060 ◽

2021 ◽

pp. jgs2021-060

Author(s):

Xiaomei Shi ◽

Richard J. Howard ◽

Gregory D. Edgecombe ◽

Xianguang Hou ◽

Xiaoya Ma

Keyword(s):

Phylogenetic Analyses ◽

Sensu Stricto ◽

Cambrian Explosion ◽

Early Cambrian ◽

Content Type ◽

Link Type ◽

Stem Group ◽

New Material ◽

Supplementary Material ◽

Chengjiang Biota

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

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