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Phylogenetic position of Yanjiahella biscarpa within the Ambulacraria. Topology based on implicit enumeration with autocollapse search off and resampling via standard bootstrap methods. Bootstrap values shown under each node. Fifteen trees were found with a total length of 71 steps (Consistency Index = 0.648, Rentention Index = 0.821). Taxa illustrated by Nobumichi Tamura
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Deuterostomes are a morphologically disparate clade, encompassing the chordates (including vertebrates), the hemichordates (the vermiform enteropneusts and the colonial tube-dwelling pterobranchs) and the echinoderms (including starfish). Although deuterostomes are considered monophyletic, the inter-relationships between the three clades remain hig...
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... is covered with numerous irregular, ovoid to polygonal plates (Figs. 1a-c, 2a, c and Supplementary Figs. 2a, 3a, d, g). Plates are up to 2.5 mm in length and width with an unknown original composition. In some specimens, partial recovery between adjacent plates suggests that the theca might have been imbricately plated ( Fig. 2a and Supplementary Fig. 3a, c). However, plates seem to be grossly abutted in other specimens or areas, so that definite assumption over plating type would be overstated ( Fig. 1a and Supplementary Figs. 2a, 4b, d, h, 5a). Thecal plating is often preserved as slightly dislocated (a possible alternative explanation for apparent imbrication of plates) with ...
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... almost continuous transition between stalk and theca ( Fig. 2f, g and Supplementary Figs. 4a-d, 5a, e, i), so only the change in plating marks that boundary between these areas, with plates being absent in the stalk. Thecal plates have differentiated smooth (inner) surface and an outer (external) surface ornament of parallel ribs (Fig. 1c, 2a and Supplementary Figs. 3a, c). The proximal stalk is cylindrical (Figs. 1a, d, 2a, f, g and Supplementary Figs. 2b-c, h, 3a, e, 4f, i, 5e, i) and tapers distally (Fig. 1a, e and Supplementary Figs. 2c, h, 4g, 5a, e, i). The proximal stalk is transversely ridged (distance between two successive ridges about 100 µm); the ridges are usually very fine, but can be ...
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... appendages are long and slender and can extend up to 43 mm in length and may reach 0.6 mm in diameter. The appendages are not covered by plates, but sometimes transverse lines are observable, which could represent cases when two appendages are preserved juxtaposed to each other ( Fig. 2a and Supplementary Fig. 3c). The variable shape of the theca, the strong curvature of the distal stalk and the incomplete preservation of some specimens has hindered any reliable quantitative study. ...
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... with cephalochordates used as an outgroup taxon), each scored for 42 morphological characters (Supplementary Note 1 and Supplementary Table 1). Whether the analysis is performed under Traditional Search, New Technology Search, or Implicit Enumeration 32 and despite low bootstrap support, Y. biscarpa is constantly resolved as a basal echinoderm (Fig. 3 and Supplementary Figs. 6, 8, 9), reflecting our proposal that it represents a stem-group echinoderm. Only majority-rule consensus trees obtained under Traditional Search methods show Yanjiahella in an alternative position, as a basal hemichordate (50% majorityrule consensus tree, Supplementary Fig. 7a) or in a polytomy with ...
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... the majority of analyses, Y. biscarpa either occupies a basal position within the monophyletic total-group Echinodermata (Fig. 3 and Supplementary Figs. 6b, 8b) or is grouped with ctenocystoids as the sister group to the remaining echinoderms ( Supplementary Figs. 6a, 8a). In the latter cases though, the grouping of Y. biscarpa and ctenosystoids is never supported by any synapomorphies and is most likely a result of both taxa lacking characters present in more derived echinoderms. Indeed, the ...
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... and that the early history of the Echinodermata is firmly rooted in bilateral forms. This topology is largely consistent with previously published trees dealing with Cambrian echinoderms 33,34 (despite significant changes in the data matrices), indicating pentaradial symmetry evolved progressively from bilateral, asymmetrical, and triradial forms (Fig. 3). The presence of a posterior muscular appendage in Y. biscarpa confirms that the posterior appendage and its muscular component evolved numerous times among echinoderm lineages. Other stem-group echinoderms such as ctenocystoids lack a posterior appendage 34,35 , yet the posterior appendage of more derived taxa such as solute, ...
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... only in pterobranchs; however, recent fossil discoveries of enteropneusts from the Cambrian 23,25 have demonstrated that the construction of such external tubular structures are present in both clades. In our analysis, the ability to secrete an external tubular structure resulted in an unresolved phylogenetic signal within the hemichordate clade (Fig. 3). Although our phylogenetic analyses always recognized the enteropneusts and pterobranchs as belonging to two distinct, sister hemichordate clades (as expected), the exact relationships between the enteropneust taxa was less clear. The Cambrian enteropneusts were either considered as derived (with the crown group families as basal; ...
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... between the enteropneust taxa was less clear. The Cambrian enteropneusts were either considered as derived (with the crown group families as basal; Supplementary Fig. 6a) or basal to the extant enteropneusts ( Supplementary Fig. 8a). This phylogenetic uncertainty is highlighted with bootstrap resampling as the group is always unresolved (Fig. 3 and Supplementary Figs. 6b and 8b). This ambiguity is most likely a result of insufficient morphological data to resolve meaningful relationships among hemichordate taxa, in particular the position of the Cambrian taxa, S. tenius and O. ...
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... Y. biscarpa exhibits a linear digestive tract that extends through the majority of its body, a configuration more reminiscent of enteropneusts. We therefore follow herein previous phylogenetic studies 23,25,29 in interpreting the tubicolous habit as an ancestral trait of the Enteropneusta that was ultimately lost in the crown group enteropneusts (Fig. ...
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... analysis. The phylogenetic analysis was conducted using TNT1.1 32 under implicit enumeration (Fig. 3), Traditional Search with 10,000 replicates ( Supplementary Figs. 6, 7), and New Technology Search, using Driven Search with Sectorial Search and Tree fusing options, and the analysis was set to find the minimum tree length 100 times ( Supplementary Figs. 8, 9). In all analyses, all characters were treated as unordered and with equal ...
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... is covered with numerous irregular, ovoid to polygonal plates (Figs. 1a-c, 2a, c and Supplementary Figs. 2a, 3a, d, g). Plates are up to 2.5 mm in length and width with an unknown original composition. In some specimens, partial recovery between adjacent plates suggests that the theca might have been imbricately plated ( Fig. 2a and Supplementary Fig. 3a, c). However, plates seem to be grossly abutted in other specimens or areas, so that definite assumption over plating type would be overstated ( Fig. 1a and Supplementary Figs. 2a, 4b, d, h, 5a). Thecal plating is often preserved as slightly dislocated (a possible alternative explanation for apparent imbrication of plates) with ...
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... almost continuous transition between stalk and theca ( Fig. 2f, g and Supplementary Figs. 4a-d, 5a, e, i), so only the change in plating marks that boundary between these areas, with plates being absent in the stalk. Thecal plates have differentiated smooth (inner) surface and an outer (external) surface ornament of parallel ribs (Fig. 1c, 2a and Supplementary Figs. 3a, c). The proximal stalk is cylindrical (Figs. 1a, d, 2a, f, g and Supplementary Figs. 2b-c, h, 3a, e, 4f, i, 5e, i) and tapers distally (Fig. 1a, e and Supplementary Figs. 2c, h, 4g, 5a, e, i). The proximal stalk is transversely ridged (distance between two successive ridges about 100 µm); the ridges are usually very fine, but can be ...
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... appendages are long and slender and can extend up to 43 mm in length and may reach 0.6 mm in diameter. The appendages are not covered by plates, but sometimes transverse lines are observable, which could represent cases when two appendages are preserved juxtaposed to each other ( Fig. 2a and Supplementary Fig. 3c). The variable shape of the theca, the strong curvature of the distal stalk and the incomplete preservation of some specimens has hindered any reliable quantitative study. ...
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... with cephalochordates used as an outgroup taxon), each scored for 42 morphological characters (Supplementary Note 1 and Supplementary Table 1). Whether the analysis is performed under Traditional Search, New Technology Search, or Implicit Enumeration 32 and despite low bootstrap support, Y. biscarpa is constantly resolved as a basal echinoderm (Fig. 3 and Supplementary Figs. 6, 8, 9), reflecting our proposal that it represents a stem-group echinoderm. Only majority-rule consensus trees obtained under Traditional Search methods show Yanjiahella in an alternative position, as a basal hemichordate (50% majorityrule consensus tree, Supplementary Fig. 7a) or in a polytomy with ...
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... the majority of analyses, Y. biscarpa either occupies a basal position within the monophyletic total-group Echinodermata (Fig. 3 and Supplementary Figs. 6b, 8b) or is grouped with ctenocystoids as the sister group to the remaining echinoderms ( Supplementary Figs. 6a, 8a). In the latter cases though, the grouping of Y. biscarpa and ctenosystoids is never supported by any synapomorphies and is most likely a result of both taxa lacking characters present in more derived echinoderms. Indeed, the ...
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... and that the early history of the Echinodermata is firmly rooted in bilateral forms. This topology is largely consistent with previously published trees dealing with Cambrian echinoderms 33,34 (despite significant changes in the data matrices), indicating pentaradial symmetry evolved progressively from bilateral, asymmetrical, and triradial forms (Fig. 3). The presence of a posterior muscular appendage in Y. biscarpa confirms that the posterior appendage and its muscular component evolved numerous times among echinoderm lineages. Other stem-group echinoderms such as ctenocystoids lack a posterior appendage 34,35 , yet the posterior appendage of more derived taxa such as solute, ...
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... only in pterobranchs; however, recent fossil discoveries of enteropneusts from the Cambrian 23,25 have demonstrated that the construction of such external tubular structures are present in both clades. In our analysis, the ability to secrete an external tubular structure resulted in an unresolved phylogenetic signal within the hemichordate clade (Fig. 3). Although our phylogenetic analyses always recognized the enteropneusts and pterobranchs as belonging to two distinct, sister hemichordate clades (as expected), the exact relationships between the enteropneust taxa was less clear. The Cambrian enteropneusts were either considered as derived (with the crown group families as basal; ...
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... between the enteropneust taxa was less clear. The Cambrian enteropneusts were either considered as derived (with the crown group families as basal; Supplementary Fig. 6a) or basal to the extant enteropneusts ( Supplementary Fig. 8a). This phylogenetic uncertainty is highlighted with bootstrap resampling as the group is always unresolved (Fig. 3 and Supplementary Figs. 6b and 8b). This ambiguity is most likely a result of insufficient morphological data to resolve meaningful relationships among hemichordate taxa, in particular the position of the Cambrian taxa, S. tenius and O. ...
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... Y. biscarpa exhibits a linear digestive tract that extends through the majority of its body, a configuration more reminiscent of enteropneusts. We therefore follow herein previous phylogenetic studies 23,25,29 in interpreting the tubicolous habit as an ancestral trait of the Enteropneusta that was ultimately lost in the crown group enteropneusts (Fig. ...
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... analysis. The phylogenetic analysis was conducted using TNT1.1 32 under implicit enumeration (Fig. 3), Traditional Search with 10,000 replicates ( Supplementary Figs. 6, 7), and New Technology Search, using Driven Search with Sectorial Search and Tree fusing options, and the analysis was set to find the minimum tree length 100 times ( Supplementary Figs. 8, 9). In all analyses, all characters were treated as unordered and with equal ...
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Echinoderms exhibit remarkable powers of autotomy. For instance, crinoids can shed arm and stalk portions when attacked by predators. In some species, it has been reported that the autotomized arms display vigorous movements, which are thought to divert the attention of predators. This phenomenon, however, has not been well explored. Here we presen...
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... Comparative anatomy, developmental biology, and molecular phylogenetics have provided valuable insights into patterns of character evolution, 2-5 but important aspects of the evolution of the group remain unclear due to the widely divergent body plans exhibited by extant deuterostomes. Putative stem deuterostomes, 6-8 stem chordates, 9,10 stem echinoderms, [11][12][13] and stem hemichordates [14][15][16] have been reported from the Cambrian, potentially bridging these morphological gaps, but the phylogenetic positions of these taxa are contentious. One such fossil is the enigmatic metazoan Rotadiscus grandis. ...
... 29 Until now, the apparent absence of any derived characters shared with extant phyla had led to their exclusion from phylogenetic analyses concerned with deuterostome evolution. 8,13,16 Our novel interpretations of the anatomy of Rotadiscus, based on exceptionally preserved fossil specimens, allow us to rigorously constrain the phylogenetic position of eldonioids for the first time, thereby elucidating their evolutionary significance. Historically, uncertainty regarding the plesiomorphic character states of deuterostomes has hindered efforts to reconstruct ancestral body plans 2-5 ; our analyses suggest this is because major characters, including a post-anal region, gill bars, and a U-shaped gut, evolved through convergence rather than shared ancestry. ...
Deuterostomes are characterized by some of the most widely divergent body plans in the animal kingdom. These striking morphological differences have hindered efforts to predict ancestral characters, with the origin and earliest evolution of the group remaining ambiguous. Several iconic Cambrian fossils have been suggested to be early deuterostomes and hence could help elucidate ancestral character states. However, their phylogenetic relationships are controversial. Here, we describe new, exceptionally preserved specimens of the discoidal metazoan Rotadiscus grandis from the early Cambrian Chengjiang biota of China. These reveal a previously unknown double spiral structure, which we interpret as a chordate-like covering to a coelomopore, located adjacent to a horseshoe-shaped tentacle complex. The tentacles differ in key aspects from those seen in lophophorates and are instead more similar to the tentacular systems of extant pterobranchs and echinoderms. Thus, Rotadiscus exhibits a chimeric combination of ambulacrarian and chordate characters. Phylogenetic analyses recover Rotadiscus and closely related fossil taxa as stem ambulacrarians, filling a significant morphological gap in the deuterostome tree of life. These results allow us to reconstruct the ancestral body plans of major clades of deuterostomes, revealing that key traits of extant forms, such as a post-anal region, gill bars, and a U-shaped gut, evolved through convergence.
... Although the earliest quantitative phylogenetic analyses recovered a range of conflicting relationships (Paul & Smith, 1984;Smith, 1984;Sumrall, 1997), rigorous phylogenetic treatments of Cambrian echinoderms have converged upon similar topologies. For example, the results of Topper et al. (2019) for echinoderm lineages are largely congruent with those of and Zamora & Rahman (2014) despite significant differences in the underlying character matrices. Notably, these analyses recover inconsistencies between phylogenetic positions and the first appearances in the fossil record of major groups. ...
... Hunter & Ortega-Hern andez, 2021). Stylophorans were not included in the most recent phylogenetic investigations that spanned multiple Cambrian stem-group echinoderms Topper et al., 2019;Zamora et al., 2020), or in earlier studies of echinoderm phylogeny that placed 'carpoids' at the base of the echinoderm tree (Paul & Smith, 1984;Smith, 1984). The analysis of Sumrall (1997) recovered stylophorans in a clade with other 'carpoids' nested within blastozoans, although this blastozoan-carpoid relationship has not received subsequent support. ...
... A number of enigmatic taxa have been assigned to Echinodermata over the years but remain problematica due to poor preservation, lack of diagnostic traits, and/or other morphological inconsistencies that preclude confident placement in the phylum. These include Arkarua (Gehling, 1987; and Tribrachidium (Glaessner & Wade, 1966) from the Ediacaran of southern Australia, and Yanjiahella (Topper et al., 2019;Zamora et al., 2020) from the Early Cambrian (Fortunian) of China. These taxa are discussed at greater length below in Section V. Other taxa originally identified as echinoderms have since been established as members of other phyla, such as the middle Cambrian Echmatocrinus (Sprinkle, 1973) from the Burgess Shale of Canada, which is now recognized as a cnidarian (Conway Morris, 1993;Ausich & Babcock, 1998. ...
Deuterostomes are the major division of animal life which includes sea stars, acorn worms, and humans, among a wide variety of ecologically and morphologically disparate taxa. However, their early evolution is poorly understood, due in part to their disparity, which makes identifying commonalities difficult, as well as their relatively poor early fossil record. Here, we review the available morphological, palaeontological, developmental, and molecular data to establish a framework for exploring the origins of this important and enigmatic group. Recent fossil discoveries strongly support a vermiform ancestor to the group Hemichordata, and a fusiform active swimmer as ancestor to Chordata. The diverse and anatomically bewildering variety of forms among the early echinoderms show evidence of both bilateral and radial symmetry. We consider four characteristics most critical for understanding the form and function of the last common ancestor to Deuterostomia: Hox gene expression patterns, larval morphology, the capacity for biomineralization, and the morphology of the pharyngeal region. We posit a deuterostome last common ancestor with a similar antero-posterior gene regulatory system to that found in modern acorn worms and cephalochordates, a simple planktonic larval form, which was later elaborated in the ambulacrarian lineage, the ability to secrete calcium minerals in a limited fashion, and a pharyngeal respiratory region composed of simple pores. This animal was likely to be motile in adult form, as opposed to the sessile origins that have been historically suggested. Recent debates regarding deuterostome monophyly as well as the wide array of deuterostome-affiliated problematica further suggest the possibility that those features were not only present in the last common ancestor of Deuterostomia, but potentially in the ur-bilaterian. The morphology and development of the early deuterostomes, therefore, underpin some of the most significant questions in the study of metazoan evolution.
... The early history of deuterostomes, the group composed of the chordates, echinoderms and hemichordates 1 , is still controversial, not least because of a paucity of stem representatives of these clades [2][3][4][5] . The early Cambrian microscopic animal Saccorhytus coronarius was interpreted as an early deuterostome on the basis of purported pharyngeal openings, providing evidence for a meiofaunal ancestry 6 and an explanation for the temporal mismatch between palaeontological and molecular clock timescales of animal evolution [6][7][8] . ...
... 1-3 and Extended Data Figs. [2][3][4][5][6][7][8] and Shizhonggou section 15 (Extended Data Fig. 9) in Shaanxi Province, South China (see Extended Data Fig. 1 for localities). We characterized S. coronarius using both scanning electron microscopy (SEM) and synchrotron radiation X-ray tomographic microscopy 16 (SRXTM), interpreted its morphology through a taphonomic lens, reconstructed its three-dimensional morphology and, on the basis of an expanded morphological database of metazoans, carried out a comprehensive phylogenetic analysis to establish its affinity 6,12,13 . ...
The early history of deuterostomes, the group composed of the chordates, echinoderms and hemichordates1, is still controversial, not least because of a paucity of stem representatives of these clades2–5. The early Cambrian microscopic animal Saccorhytus coronarius was interpreted as an early deuterostome on the basis of purported pharyngeal openings, providing evidence for a meiofaunal ancestry6 and an explanation for the temporal mismatch between palaeontological and molecular clock timescales of animal evolution6–8. Here we report new material of S. coronarius, which is reconstructed as a millimetric and ellipsoidal meiobenthic animal with spinose armour and a terminal mouth but no anus. Purported pharyngeal openings in support of the deuterostome hypothesis6 are shown to be taphonomic artefacts. Phylogenetic analyses indicate that S. coronarius belongs to total-group Ecdysozoa, expanding the morphological disparity and ecological diversity of early Cambrian ecdysozoans. Analyses of newly discovered specimens of the early Cambrian microscopic animal Saccorhytus coronarius support its taxonomic classification in total-group Ecdysozoa.
... The potential earliest representative of echinoderms, Yanjiahella without clear attribution to a recognized group of Echinodermata and some controversial characters (Zamora et al., 2020) was redescribed from the Terreneuvian (541-534 Ma). For Yanjiahella a straight gut was possibly assessed, but with simultaneous evidence for the presence of a terminal-attaching, long, relatively narrow stalk (notably, without a gut inside) and a general semi-sedentary/sedentary appearance (Topper et al., 2019;Zamora et al., 2020). ...
Various evaluations of the last common bilaterian ancestor ( lcba ) currently suggest that it resembled either a microscopic, non-segmented motile adult; or, on the contrary, a complex segmented adult motile urbilaterian. These fundamental inconsistencies remain largely unexplained. A majority of multidisciplinary data regarding sedentary adult ancestral bilaterian organization is overlooked. The sedentary-pelagic model is supported now by a number of novel developmental, paleontological and molecular phylogenetic data: (1) data in support of sedentary sponges, in the adult stage, as sister to all other Metazoa; (2) a similarity of molecular developmental pathways in both adults and larvae across sedentary sponges, cnidarians, and bilaterians; (3) a cnidarian-bilaterian relationship, including a unique sharing of a bona fide Hox-gene cluster, of which the evolutionary appearance does not connect directly to a bilaterian motile organization; (4) the presence of sedentary and tube-dwelling representatives of the main bilaterian clades in the early Cambrian; (5) an absence of definite taxonomic attribution of Ediacaran taxa reconstructed as motile to any true bilaterian phyla; (6) a similarity of tube morphology (and the clear presence of a protoconch-like apical structure of the Ediacaran sedentary Cloudinidae) among shells of the early Cambrian, and later true bilaterians, such as semi-sedentary hyoliths and motile molluscs; (7) recent data that provide growing evidence for a complex urbilaterian, despite a continuous molecular phylogenetic controversy. The present review compares the main existing models and reconciles the sedentary model of an urbilaterian and the model of a larva-like lcba with a unified sedentary(adult)-pelagic(larva) model of the lcba .
... To establish the relationships between Yorkicystis and other early echinoderms, parsimony and Bayesian phylogenetic analyses were performed. We chose to focus exclusively on pentaradial forms because all recent phylogenetic analyses [16][17][18][19][20][21] have recovered these as derived in echinoderm phylogeny. We selected all the major pentaradial morphotypes present in the Cambrian and early Ordovician for which fossil material is sufficiently well known, including a spiral form (Helicocystis), early edrioasteroids (Kailidiscus and Stromatocystites), isorophid edrioasteroids (Isorophus and Argodiscus), plesiomorphic blastozoans (Lepidocystis, Gogia and Vyscystis), an early glyptocystitid (Ridersia) and the earliest crinoids (Titanocrinus and Apektocrinus). ...
... While a small number of Ediacaran and lower Cambrian fossils have previously been considered as uncalcified echinoderms (e.g. [18,[36][37][38]), their echinoderm affinities are highly dubious and debated [20,22,39,40]. However, comparable patterns of incomplete calcification have been documented in other echinoderm groups based on younger fossil and living forms. ...
... Early echinoderms displayed great plasticity in terms of their body plan construction, with bilateral, asymmetrical, triradial and pentaradial forms described from the Cambrian [22,52]. Recent phylogenetic analyses [18,20,17,19] place bilateral and asymmetrical forms as stem-group echinoderms, indicating that some of the synapomorphies of crown-group echinoderms, such as pentaradial symmetry and ambulacra with flooring and cover plates, are not plesiomorphic for the phylum [16,22]. Our phylogenetic analyses recover Yorkicystis as a derived pentaradial form, most closely related to Cambrian edrioasteroids such as Kailidiscus, with the absence of skeleton in the extraxial part of the body owing to secondary loss. ...
Echinoderms are characterized by a distinctive high-magnesium calcite
endoskeleton as adults, but elements of this have been drastically reduced
in some groups. Herein, we describe a new pentaradial echinoderm, Yorkicystis
haefneri n. gen. n. sp., which provides, to our knowledge, the oldest
evidence of secondary non-mineralization of the echinoderm skeleton.
This material was collected from the Cambrian Kinzers Formation in York
(Pennsylvania, USA) and is dated as ca 510 Ma. Detailed morphological
observations demonstrate that the ambulacra (i.e. axial region) are composed
of flooring and cover plates, but the rest of the body (i.e. extraxial region) is
preserved as a dark film and lacks any evidence of skeletal plating. Moreover,
X-ray fluorescence analysis reveals that the axial region is elevated in
iron. Based on our morphological and chemical data and on taphonomic
comparisons with other fossils from the Kinzers Formation, we infer that
the axial region was originally calcified, while the extraxial region was
non-mineralized. Phylogenetic analyses recover Yorkicystis as an edrioasteroid,
indicating that this partial absence of skeleton resulted from a secondary
reduction. We hypothesize that skeletal reduction resulted from lack of
expression of the skeletogenic gene regulatory network in the extraxial
body wall during development. Secondary reduction of the skeleton in
Yorkicystis might have allowed for greater flexibility of the body wall.
... 18). And until the recent work by Steiner et al. (2020) in Hubei, South China, E-C biostratigraphic data from this area were most commonly collected from outcrop Topper et al., 2019), while chemostratigraphic data were obtained from drill core (Ishikawa et al., 2008). ...
The diversification of animals during the Cambrian Period is one of the most significant evolutionary events in Earth’s history. However, the sequence of events leading to the origin of ‘modern’ ecosystems and the exact temporal relationship between Ediacaran and Cambrian faunas are uncertain, as identification of the Ediacaran–Cambrian boundary and global correlation through this interval remains problematic. Here we review the controversies surrounding global correlation of the base of the Cambrian and present new high-resolution biostratigraphic, lithostratigraphic and δ¹³C chemostratigraphic data for terminal Ediacaran to basal Cambrian strata in the Zavkhan Basin of Mongolia. This predominantly carbonate sequence, through the Zuun-Arts and Bayangol formations in southwestern Mongolia, captures a distinct, negative δ¹³C excursion close to the top of the Zuun-Arts Formation recognized as the BAsal Cambrian carbon isotope Excursion (BACE). In this location, the nadir of the BACE closely coincides with first occurrence of the characteristic early Cambrian protoconodont Protohertzina anabarica. Despite recent suggestions that there is an evolutionary continuum of biomineralizing animals across the Ediacaran–Cambrian transition, we suggest that this continuum is restricted to tubular forms and that skeletal taxa such as Protohertzina depict ‘true’ Cambrian representatives and are restricted entirely to the Cambrian. Employing the first appearance of the trace fossil Treptichnus pedum to define the base of the Cambrian suffers significant drawbacks, particularly in carbonate settings where it is not commonly preserved. As T. pedum is the only proxy available to correlate the Cambrian Global boundary Stratotype Section and Point (GSSP) defined at Fortune Head, Newfoundland, we suggest that the GSSP be redefined elsewhere, in a new stratigraphic section that contains secondary markers that permit global correlation. We propose the nadir of the BACE as the favored candidate to define the base of the Cambrian. However, it is essential that the BACE be complemented with secondary markers. In many global sections the nadir of the BACE and the first occurrence of the genus Protohertzina are closely juxtaposed, as are the BACE and T. pedum. Hence these taxa provide essential biostratigraphic control on the BACE and increase potential for effective global correlation. We also recommend that an Auxiliary boundary Stratotype Section and Point (ASSP) be simultaneously established in order to incorporate additional markers that will aid global correlation of the Ediacaran–Cambrian boundary. The BAY4/5 section through the upper Zuun-Arts and Bayangol formations yields key shelly fossils and δ¹³C values and is therefore an ideal candidate for consideration as the GSSP for the Ediacaran–Cambrian boundary.
... These five clades correspond to the Crinoidea (sea lilies and feather stars), the Asteroidea (sea stars), the Ophiuroidea (brittle stars), the Holothuroidea (sea cucumbers) and the Echinoidea (sea urchins). Their representatives are all char acterized by a typical pentaradial symmetry that is thought to have secondarily evolved from a bilateral ancestral form Topper et al. 2019). ...
Echinoderms, a sister group of chordates, is a group of exclusively marine animals. The Echinodermata is an ancient phylum dating to at least 450 million years old and including more than 10,000 extant species present throughout the world’s oceans. The modern echinoderms belong to five classes: Echinoidea (e.g., sea urchins and sand dollars), Asteroidea (e.g., starfishes), Ophiuroidea (e.g., brittle stars), Holothuroidea (e.g., sea cucumbers) and Crinoidea (e.g., sea lilies and feather stars). Sea urchin represents a well-established marine model in biological sciences. This chapter provides a general description of echinoderms and focuses on the significant advances in cell and developmental biology that the study of sea urchins has made possible. During the last decade, many genomic data concerning echinoderms and sea urchins in particular have become available. These new molecular tools have facilitated gene regulation analysis during development and have boosted the possibilities offered by sea urchins as experimental models.
... The model was largely based on first fossil appearances of metazoan lineages and thus may vary with emerging discoveries ). More recently, a possible trackway of a bilaterian with paired appendages was reported from the late Ediacaran Shibantan biota (Chen et al. 2018) and a number of putative Rigby (1986), Wood and Zhuravlev (2012), and Murdock (2020); Demospongiae from Bengtson et al. (1990), Müller et al. (2008), Wood and Zhuravlev (2012), and Murdock (2020); Calcarea from Kruse et al. (1995), Mehl (1995, 1996), Wood and Zhuravlev (2012), Murdock (2020); Archaeocyatha from Rozanov and Zhuravlev (1992), Wood and Zhuravlev (2012); Anthozoa from Tynan (1983), Sorauf and Savarese (1995); Protoconodonta from Khomentovsky and Karlova (2005), McIlroy and Szaniawski (2010); Halwaxiida from Conway Morris and Peel (1990), Vinther and Nielsen (2005); Hyolitha from Vinn (2006), Kouchinsky (2001), Skovsted and Peel (2011); Cambroclavida from Conway Morris et al. (1997), Steiner et al. (2007); Paracarinachitida from Conway Morris and Chen (1991), Wood and Zhuravlev (2012); Tianzhushanellidae from Kouchinsky et al. (2012); Helcionelloida from Khomentovsky and Karlova (2005), and Wood and Zhuravlev (2012); Polyplacophora from Carter and Hall (1990), Treves et al. (2003), Hoare and Pojeta (2006); Scaphopoda from Peel (2006), Murdock (2020); Stenothecoida from Rozanov and Zhuravlev (1992), Ushatinskaya and Zhuravlev (1994), Wood and Zhuravlev (2012); Rostroconchina from Kouchinsky (2000), Khomentovsky and Karlova (2005), Murdock (2020); Bivalvia from Runnegar and Bentley (1983), Runnegar (1985); Gastropoda from Parkhaev (2017), Murdock (2020); Cephalopoda from Chen and Teichert (1983), Landing and Kröger (2009); Tommotiida from Khomentovsky and Karlova (2005), Skovsted et al. (2008); Mobergellidae from Rozanov and Zhuravlev (1992), Skovsted (2003), Topper and Skovsted (2017); Linguliforma from Skovsted and Holmer (2003), Khomentovsky and Karlova (2005), Murdock (2020); Rhynchonelliformea from Ushatinskaya and Zhuravlev (1994), Ushatinskaya and Malakhovskaya (2006); Bryozoa from Taylor and Weedon (2000) Bate and East (1972); Echinodermata from Rozanov and Zhuravlev (1992), Guensburg and Sprinkle (2001); Calcichordata from Dominguez et al. (2002), Lefebvre (2007); Conodonta from Bengtson (1983), Miller (1984); Ascidiacea from Lowenstam and Abbott (1975), Chen et al. (2003); Vertebrata from Rücklin et al. (2012) deuterostomes were recovered from the Fortunian Stage, e.g., meiofaunal deuterostomes (Han et al. 2017; reconsidered as a primitive scalidophoran or the nearest common ancestor of the Bilateria in Shu and Han 2020), a stem group echinoderm (Topper et al. 2019(Topper et al. , 2020; but see Zamora et al. 2020), and enteropneust hemichordates (Maletz 2019). These discoveries are critical to constrain the duration of the Cambrian Explosion and hence remain to be tested and verified. ...
The Cambrian Explosion by nature is a three-phased explosion of animal body plans alongside episodic biomineralization, pulsed change of generic diversity, body size variation, and progressive increase of ecosystem complexity. The Cambrian was a time of crown groups nested by numbers of stem groups with a high-rank taxonomy of Linnaean system (classes and above). Some stem groups temporarily succeeded while others were ephemeral and underrepresented by few taxa. The high number of stem groups in the early history of animals is a major reason for morphological gaps across phyla that we see today. Most phylum-level clades achieved their maximal disparity (or morphological breadth) during the time interval close to their first appearance in the fossil record during the early Cambrian, whereas others, principally arthropods and chordates, exhibit a progressive exploration of morphospace in subsequent Phanerozoic. The overall envelope of metazoan morphospace occupation was already broad in the early Cambrian though it did not reach maximal disparity nor has diminished significantly as a consequence of extinction since the Cambrian. Intrinsic and extrinsic causes were extensively discussed but they are merely prerequisites for the Cambrian Explosion. Without the molecular evolution, there could be no Cambrian Explosion. However, the developmental system is alone insufficient to explain Cambrian Explosion. Time-equivalent environmental changes were often considered as extrinsic causes, but the time coincidence is also insufficient to establish causality. Like any other evolutionary event, it is the ecology that make the Cambrian Explosion possible though ecological processes failed to cause a burst of new body plans in the subsequent evolutionary radiations. The Cambrian Explosion is a polythetic event in natural history and manifested in many aspects. No simple, single cause can explain the entire phenomenon.
... Many sections here have been suggested as standard stratigraphic sections in China (Chen et al., 2006;Wang et al., 2009), and the depositional succession along the Three Gorges area is regarded as an auxiliary stratotype section of the traditional lower Cambrian in South China (Wang et al., 1987;Zhang and Hua, 2005;Zhu et al., 2007;. The depositional succession through the Ediacaran-Cambrian Series 2 interval yields abundant shale-hosted fossils that have contributed significantly to the study of early animal evolution (Guo et al., 2014;Fu et al., 2019;Topper et al., 2019). The depositional sequence in the study area includes, in ascending order, the Ediacaran Dengying Formation, the lower Cambrian Yanjiahe Formation, Shuijingtuo Formation, Shipai Formation, Tianheban Formation, and Shilongdong Formation ( Fig. 1.3). ...
Diverse and abundant fossil taxa have been described in the lower Cambrian Shipai Formation in the Three Gorges area of Hubei Province, South China, but the taxonomy and diversity of the co-occurring brachiopod fauna are still far from clear. Here we describe the brachiopod fauna recovered from the Shipai Formation in the Three Gorges area of South China, including representatives of the subphylum Linguliformea: linguloids ( Lingulellotreta ergalievi , Eoobolus malongensis , and Neobolidae gen. indet. sp. indet.), and an acrotretoid ( Linnarssonia sapushanensis ); and representatives from the subphylum Rhynchonelliformea: the calcareous-shelled Kutorginates ( Kutorgina sinensis , Kutorgina sp., and Nisusia liantuoensis ). This brachiopod assemblage and the first occurrence of Linnarssonia sapushanensis shell beds permit correlation of the Shipai Formation in the Three Gorges area of Hubei Province with the Stage 4 Wulongqing Formation in the Wuding area of eastern Yunnan. This correlation is further strengthened by the first appearance datum (FAD) of the rhynchonelliform brachiopod Nisusia in the upper silty mudstone of both the Shipai and Wulongqing formations. The new well-preserved material, derived from siliciclastic rocks, also gives critical new insights into the fine shell structure of L . sapushanensis . Microstructural studies on micromorphic acrotretoids (like Linnarssonia ) have previously been restricted to fossils that were acid-etched from limestones. This is the first study to carry out detailed comparative ultrastructural studies on acrotretoid shells preserved in siliciclastic rocks. This work reveals a hollow tube and solid column microstructure in the acrotretoid shells from the Shipai Formation, which is likely to be equivalent of traditional column and central canal observed in shells dissolved from limestones.
... Another lost body plan evolved among early Cambrian echinoderms as a group of asymmetric spiral-bodied echinoderms, the oldest echinoderms with ambulacra represented by genera Helicoplacus and Helicocystis (see [11,23,136,157,158]). Helicoplacoids were spindle-shaped animals with a mouth sited laterally, an apical anus, and three ambulacral areas armored by rectangular platelets, which were arranged in spiral rows [11,136]. ...
In this review, we consider transformations of axial symmetry in metazoan evolution and development, the genetic basis, and phenotypic expressions of different axial body plans. In addition to the main symmetry types in metazoan body plans, such as rotation (radial symmetry), reflection (mirror and glide reflection symmetry), and translation (metamerism), many biological objects show scale (fractal) symmetry as well as some symmetry-type combinations. Some genetic mechanisms of axial pattern establishment, creating a coordinate system of a metazoan body plan, bilaterian segmentation, and left–right symmetry/asymmetry, are analysed. Data on the crucial contribution of coupled functions of the Wnt, BMP, Notch, and Hedgehog signaling pathways (all pathways are designated according to the abbreviated or full names of genes or their protein products; for details, see below) and the axial Hox-code in the formation and maintenance of metazoan body plans are necessary for an understanding of the evolutionary diversification and phenotypic expression of various types of axial symmetry. The lost body plans of some extinct Ediacaran and early Cambrian metazoans are also considered in comparison with axial body plans and posterior growth in living animals.
