A long-headed Cambrian soft-bodied vertebrate from the American Great Basin region

The fossil record suggests that chordates might have been minor components of marine ecosystems during the first major diversification of animal life in the Cambrian. Vertebrates are represented by a handful of rare soft-bodied stem-lineage taxa known from Konservat-Lagerstätten, including Myllokunmingia and Yunnanozoon from the Stage 3 of South China, and Emmonsaspis and Metaspriggina from Stage 4-Drumian deposits of northeast USA and British Columbia. Here, we describe the first soft-bodied vertebrate from the American Great Basin, a region home to a dozen Cambrian Konservat-Lagerstätten. Found in the Drumian Marjum Formation of Utah, Nuucichthys rhynchocephalus gen. et sp. nov. is characterized by a finless torpedo-shaped body that includes a snout-like anterior head bearing anterolateral eyes, approximately 25 thick myomeres, a large branchial chamber with a keel and approximately seven putative dorsal bars and a spiniform caudal process. Using Bayesian inference, our analysis recovers Nuucichthys within the vertebrate stem, closer to the crown than Pikaia, Yunnanozoon and Myllokunmingia, where it forms a polytomy with its Laurentian relatives, Emmonsaspis and Metaspriggina, and a scion consisting of conodonts and crown-group vertebrates. Based on the eye orientation and absence of fins, we tentatively reconstruct Nuucichthys as a pelagic organism with limited swimming abilities (planktonektic).

Omnidirectional propulsion in a metachronal swimmer

Aquatic organisms often employ maneuverable and agile swimming behavior to escape from predators, find prey, or navigate through complex environments. Many of these organisms use metachronally coordinated appendages to execute complex maneuvers. However, though metachrony is used across body sizes ranging from microns to tens of centimeters, it is understudied compared to the swimming of fish, cetaceans, and other groups. In particular, metachronal coordination and control of multiple appendages for three-dimensional maneuvering is not fully understood. To explore the maneuvering capabilities of metachronal swimming, we combine 3D high-speed videography of freely swimming ctenophores (Bolinopsis vitrea) with reduced-order mathematical modeling. Experimental results show that ctenophores can quickly reorient, and perform tight turns while maintaining forward swimming speeds close to 70% of their observed maximum-performance comparable to or exceeding that of many vertebrates with more complex locomotor systems. We use a reduced-order model to investigate turning performance across a range of beat frequencies and appendage control strategies, and reveal that ctenophores are capable of near-omnidirectional turning. Based on both recorded and modeled swimming trajectories, we conclude that the ctenophore body plan enables a high degree of maneuverability and agility, and may be a useful starting point for future bioinspired aquatic vehicles.

A macroscopic free-swimming medusa from the middle Cambrian Burgess Shale

Cnidarians are regarded as one of the earliest-diverging animal phyla. One of the hallmarks of the cnidarian body plan is the evolution of a free-swimming medusa in some medusozoan classes, but the origin of this innovation remains poorly constrained by the fossil record and molecular data. Previously described macrofossils, putatively representing medusa stages of crown-group medusozoans from the Cambrian of Utah and South China, are here reinterpreted as ctenophore-grade organisms. Other putative Ediacaran to Cambrian medusozoan fossils consist mainly of microfossils and tubular forms. Here we describe Burgessomedusa phasmiformis gen. et sp. nov., the oldest unequivocal macroscopic free-swimming medusa in the fossil record. Our study is based on 182 exceptionally preserved body fossils from the middle Cambrian Burgess Shale (Raymond Quarry, British Columbia, Canada). Burgessomedusa possesses a cuboidal umbrella up to 20 cm high and over 90 short, finger-like tentacles. Phylogenetic analysis supports a medusozoan affinity, most likely as a stem group to Cubozoa or Acraspeda (a group including Staurozoa, Cubozoa and Scyphozoa). Burgessomedusa demonstrates an ancient origin for the free-swimming medusa life stage and supports a growing number of studies showing an early evolutionary diversification of Medusozoa, including of the crown group, during the late Precambrian-Cambrian transition.

Cambrian comb jellies from Utah illuminate the early evolution of nervous and sensory systems in ctenophores

Ctenophores are a group of predatory macroinvertebrates whose controversial phylogenetic position has prompted several competing hypotheses regarding the evolution of animal organ systems. Although ctenophores date back at least to the Cambrian, they have a poor fossil record due to their gelatinous bodies. Here, we describe two ctenophore species from the Cambrian of Utah, which illuminate the early evolution of nervous and sensory features in the phylum. Thalassostaphylos elegans has 16 comb rows, an oral skirt, and an apical organ with polar fields. Ctenorhabdotus campanelliformis has 24 comb rows, an oral skirt, an apical organ enclosed by a capsule and neurological tissues preserved as carbonaceous films. These are concentrated around the apical organ and ciliated furrows, which connect to a circumoral nerve ring via longitudinal axons. C. campanelliformis deviates from the neuroanatomy of living ctenophores and demonstrates a substantial complexity in the nervous system of Cambrian ctenophores.

•

Two species of rare fossil ctenophores are described from the Cambrian of Utah

•

Fossil ctenophores preserve remains of nervous tissue and sensory structures

•

Neurological structures include an oral nerve ring and giant longitudinal axons

•

Cambrian ctenophores had a more complex neuroanatomy than living species

A late-surviving stem-ctenophore from the Late Devonian of Miguasha (Canada)

Like other soft-bodied organisms, ctenophores (comb jellies) produce fossils only under exceptional taphonomic conditions. Here, we present the first record of a Late Devonian ctenophore from the Escuminac Formation from Miguasha in eastern Canada. Based on the 18-fold symmetry of this disc-shaped fossil, we assign it to the total-group Ctenophora. Our phylogenetic analyses suggest that the new taxon Daihuoides jakobvintheri gen. et sp. nov. falls near Cambrian stem ctenophores such as 'dinomischids' and 'scleroctenophorans'. Accordingly, Daihuoides is a Lazarus-taxon, which post-dates its older relatives by over 140 million years, and overlaps temporally with modern ctenophores, whose oldest representatives are known from the Early Devonian. Our analyses also indicate that the fossil record of ctenophores does not provide strong evidence for or against the phylogenomic hypothesis that ctenophores are sister to all other metazoans.

Cambrian Sessile, Suspension Feeding Stem-Group Ctenophores and Evolution of the Comb Jelly Body Plan

The origin of ctenophores (comb jellies) is obscured by their controversial phylogenetic position, with recent phylogenomic analyses resolving either sponges or ctenophores as the sister group of all other animals. Fossil taxa can provide morphological evidence that may elucidate the origins of derived characters and shared ancestries among divergent taxa, providing a means to "break" long branches in phylogenetic trees. Here we describe new fossil material from the early Cambrian Chengjiang Biota, Yunnan Province, China, including the putative cnidarian Xianguangia, the new taxon Daihua sanqiong gen et sp. nov., and Dinomischus venustus, informally referred to as "dinomischids" here. "Dinomischids" possess a basal calyx encircled by 18 tentacles that surround the mouth. The tentacles carry pinnules, each with a row of stiff filamentous structures interpreted as very large compound cilia of a size otherwise only known in ctenophores. Together with the Cambrian tulip animal Siphusauctum and the armored Cambrian scleroctenophores, they exhibit anatomies that trace ctenophores to a sessile, polypoid stem lineage. This body plan resembles the polypoid, tentaculate morphology of cnidarians, including a blind gastric cavity partitioned by mesenteries. We propose that comb rows are derived from tentacles with paired sets of pinnules that each bear a row of compound cilia. The scleroctenophores exhibit paired comb rows, also observed in Siphusauctum, in addition to an organic skeleton, shared as well by Dinomischus, Daihua, and Xianguangia. We formulate a hypothesis in which ctenophores evolved from sessile, polypoid suspension feeders, sharing similarities with cnidarians that suggest either a close relationship between these two phyla, a striking pattern of early convergent evolution, or an ancestral condition for either metazoans or eumetazoans.

Ctenophore relationships and their placement as the sister group to all other animals

Ctenophora, compromising approximately 200 described species, is an important lineage for understanding metazoan evolution and is of great ecological and economic importance. Ctenophore diversity includes species with unique colloblasts used for prey capture, smooth and striated muscles, benthic and pelagic lifestyles, and locomotion with ciliated paddles or muscular propulsion. However, ancestral states of traits are debated and relationships among many lineages are unresolved. Here, using 27 newly sequenced ctenophore transcriptomes, publicly available data, and methods to control systematic error we establish the placement of Ctenophora as the sister group to all other animals and refine phylogenetic relationships within ctenophores. Molecular clock analyses suggest modern ctenophore diversity originated approximately 350MYA ± 88 MY, conflicting with previous hypotheses of approximately 65 MYA. We recover Euplokamis dunlapae, a species with striated muscles, as the sister lineage to other sampled ctenophores. Ancestral state reconstruction shows the most recent common ancestor of extant ctenophores was pelagic, possessed tentacles, was bioluminescent, and did not have separate sexes. Our results imply at least two transitions from a pelagic to a benthic lifestyle within Ctenophora, suggesting such transitions were more common in animal diversification than appreciated.

Three Cambrian fossils assembled into an extinct body plan of cnidarian affinity

The early Cambrian problematica Xianguangia sinica, Chengjiangopenna wangii, and Galeaplumosus abilus from the Chengjiang biota (Yunnan, China) have caused much controversy in the past and their phylogenetic placements remain unresolved. Here we show, based on exceptionally preserved material (85 new specimens plus type material), that specimens previously assigned to these three species are in fact parts of the same organism and propose that C. wangii and G. abilus are junior synonyms of X. sinica Our reconstruction of the complete animal reveals an extinct body plan that combines the characteristics of the three described species and is distinct from all known fossil and living taxa. This animal resembled a cnidarian polyp in overall morphology and having a gastric cavity partitioned by septum-like structures. However, it possessed an additional body cavity within its holdfast, an anchoring pit on the basal disk, and feather-like tentacles with densely ciliated pinnules arranged in an alternating pattern, indicating that it was a suspension feeder rather than a predatory actiniarian. Phylogenetic analyses using Bayesian inference and maximum parsimony suggest that X. sinica is a stem-group cnidarian. This relationship implies that the last common ancestor of X. sinica and crown cnidarians was probably a benthic, polypoid animal with a partitioned gastric cavity and a single mouth/anus opening. This extinct body plan suggests that feeding strategies of stem cnidarians may have been drastically different from that of their crown relatives, which are almost exclusively predators, and reveals that the morphological disparity of total-group Cnidaria is greater than previously assumed.

Gastric pouches and the mucociliary sole: setting the stage for nervous system evolution

Prerequisite for tracing nervous system evolution is understanding of the body plan, feeding behaviour and locomotion of the first animals in which neurons evolved. Here, a comprehensive scenario is presented for the diversification of cell types in early metazoans, which enhanced feeding efficiency and led to the emergence of larger animals that were able to move. Starting from cup-shaped, gastraea-like animals with outer and inner choanoflagellate-like cells, two major innovations are discussed that set the stage for nervous system evolution. First, the invention of a mucociliary sole entailed a switch from intra- to extracellular digestion and increased the concentration of nutrients flowing into the gastric cavity. In these animals, an initial nerve net may have evolved via division of labour from mechanosensory-contractile cells in the lateral body wall, enabling coordinated movement of the growing body that involved both mucociliary creeping and changes of body shape. Second, the inner surface of the animals folded into metameric series of gastric pouches, which optimized nutrient resorption and allowed larger body sizes. The concomitant acquisition of bilateral symmetry may have allowed more directed locomotion and, with more demanding coordinative tasks, triggered the evolution of specialized nervous subsystems. Animals of this organizational state would have resembled Ediacarian fossils such as Dickinsonia and may have been close to the cnidarian–bilaterian ancestor. In the bilaterian lineage, the mucociliary sole was used mostly for creeping, or frequently lost. One possible remnant is the enigmatic Reissner's fibre in the ventral neural tube of cephalochordates and vertebrates.

A vanished history of skeletonization in Cambrian comb jellies

Ctenophores are traditionally regarded as "lower" metazoans, sharing with cnidarians a diploblastic grade of organization. Unlike cnidarians, where skeletonization (biomineralization and sclerotization) evolved repeatedly among ecologically important taxa (for example, scleractinians and octocorals), living ctenophores are characteristically soft-bodied animals. We report six sclerotized and armored ctenophores from the early Cambrian period. They have diagnostic ctenophore features (for example, an octamerous symmetry, oral-aboral axis, aboral sense organ, and octaradially arranged ctene rows). Unlike most modern counterparts, however, they lack tentacles, have a sclerotized framework, and have eight pairs of ctene rows. They are resolved as a monophyletic group (Scleroctenophora new class) within the ctenophores. This clade reveals a cryptic history and sheds new light on the early evolution of this basal animal phylum. Skeletonization also occurs in some other Cambrian animal groups whose extant members are exclusively soft-bodied, suggesting the ecological importance of skeletonization in the Cambrian explosion.

Convergent evolution of neural systems in ctenophores

Neurons are defined as polarized secretory cells specializing in directional propagation of electrical signals leading to the release of extracellular messengers - features that enable them to transmit information, primarily chemical in nature, beyond their immediate neighbors without affecting all intervening cells en route. Multiple origins of neurons and synapses from different classes of ancestral secretory cells might have occurred more than once during ~600 million years of animal evolution with independent events of nervous system centralization from a common bilaterian/cnidarian ancestor without the bona fide central nervous system. Ctenophores, or comb jellies, represent an example of extensive parallel evolution in neural systems. First, recent genome analyses place ctenophores as a sister group to other animals. Second, ctenophores have a smaller complement of pan-animal genes controlling canonical neurogenic, synaptic, muscle and immune systems, and developmental pathways than most other metazoans. However, comb jellies are carnivorous marine animals with a complex neuromuscular organization and sophisticated patterns of behavior. To sustain these functions, they have evolved a number of unique molecular innovations supporting the hypothesis of massive homoplasies in the organization of integrative and locomotory systems. Third, many bilaterian/cnidarian neuron-specific genes and 'classical' neurotransmitter pathways are either absent or, if present, not expressed in ctenophore neurons (e.g. the bilaterian/cnidarian neurotransmitter, γ-amino butyric acid or GABA, is localized in muscles and presumed bilaterian neuron-specific RNA-binding protein Elav is found in non-neuronal cells). Finally, metabolomic and pharmacological data failed to detect either the presence or any physiological action of serotonin, dopamine, noradrenaline, adrenaline, octopamine, acetylcholine or histamine - consistent with the hypothesis that ctenophore neural systems evolved independently from those in other animals. Glutamate and a diverse range of secretory peptides are first candidates for ctenophore neurotransmitters. Nevertheless, it is expected that other classes of signal and neurogenic molecules would be discovered in ctenophores as the next step to decipher one of the most distinct types of neural organization in the animal kingdom.

Eoandromeda and the origin of Ctenophora

The Ediacaran fossil Eoandromeda octobrachiata had a high conical body with eight arms in helicospiral arrangement along the flanks. The arms carried transverse bands proposed to be homologous to ctenophore ctenes (comb plates). Eoandromeda is interpreted as an early stem-group ctenophore, characterized by the synapomorphies ctenes, comb rows, and octoradial symmetry but lacking crown-group synapomorphies such as tentacles, statoliths, polar fields, and biradial symmetry. It probably had a pelagic mode of life. The early appearance in the fossil record of octoradial ctenophores is most consistent with the Planulozoa hypothesis (Ctenophora is the sister group of Cnidaria + Bilateria) of metazoan phylogeny.

A new stalked filter-feeder from the middle Cambrian Burgess Shale, British Columbia, Canada

Burgess Shale-type deposits provide invaluable insights into the early evolution of body plans and the ecological structure of Cambrian communities, but a number of species, continue to defy phylogenetic interpretations. Here we extend this list to include a new soft-bodied animal, Siphusauctum gregarium n. gen. and n. sp., from the Tulip Beds (Campsite Cliff Shale Member, Burgess Shale Formation) of Mount Stephen (Yoho National Park, British Columbia). With 1,133 specimens collected, S. gregarium is clearly the most abundant animal from this locality.This stalked animal (reaching at least 20 cm in length), has a large ovoid calyx connected to a narrow bilayered stem and a small flattened or bulb-like holdfast. The calyx is enclosed by a flexible sheath with six small openings at the base, and a central terminal anus near the top encircled by indistinct openings. A prominent organ, represented by six radially symmetrical segments with comb-like elements, surrounds an internal body cavity with a large stomach, conical median gut and straight intestine. Siphusauctum gregarium was probably an active filter-feeder, with water passing through the calyx openings, capturing food particles with its comb-like elements. It often occurs in large assemblages on single bedding planes suggesting a gregarious lifestyle, with the animal living in high tier clusters. These were probably buried en masse more or less in-situ by rapid mud flow events.Siphusauctum gregarium resembles Dinomischus, another Cambrian enigmatic stalked animal. Principal points of comparison include a long stem with a calyx containing a visceral mass and bract-like elements, and a similar lifestyle albeit occupying different tiering levels. The presence in both animals of a digestive tract with a potential stomach and anus suggest a grade of organization within bilaterians, but relationships with extant phyla are not straightforward. Thus, the broader affinities of S. gregarium remain largely unconstrained.

Arthropod visual predators in the early pelagic ecosystem: evidence from the Burgess Shale and Chengjiang biotas

Exceptional fossil specimens with preserved soft parts from the Maotianshan Shale (ca 520 Myr ago) and the Burgess Shale (505 Myr ago) biotas indicate that the worldwide distributed bivalved arthropod Isoxys was probably a non-benthic visual predator. New lines of evidence come from the functional morphology of its powerful prehensile frontal appendages that, combined with large spherical eyes, are thought to have played a key role in the recognition and capture of swimming or epibenthic prey. The swimming and steering of this arthropod was achieved by the beating of multiple setose exopods and a flap-like telson. The appendage morphology of Isoxys indicates possible phylogenetical relationships with the megacheirans, a widespread group of assumed predator arthropods characterized by a pre-oral 'great appendage'. Evidence from functional morphology and taphonomy suggests that Isoxys was able to migrate through the water column and was possibly exploiting hyperbenthic niches for food. Although certainly not unique, the case of Isoxys supports the idea that off-bottom animal interactions such as predation, associated with complex feeding strategies and behaviours (e.g. vertical migration and hunting) were established by the Early Cambrian. It also suggests that a prototype of a pelagic food chain had already started to build-up at least in the lower levels of the water column.

The earliest fossil record of the animals and its significance

The fossil record of the earliest animals has been enlivened in recent years by a series of spectacular discoveries, including embryos, from the Ediacaran to the Cambrian, but many issues, not least of dating and interpretation, remain controversial. In particular, aspects of taphonomy of the earliest fossils require careful consideration before pronouncements about their affinities. Nevertheless, a reasonable case can now be made for the extension of the fossil record of at least basal animals (sponges and perhaps cnidarians) to a period of time significantly before the beginning of the Cambrian. The Cambrian explosion itself still seems to represent the arrival of the bilaterians, and many new fossils in recent years have added significant data on the origin of the three major bilaterian clades. Why animals appear so late in the fossil record is still unclear, but the recent trend to embrace rising oxygen levels as being the proximate cause remains unproven and may even involve a degree of circularity.

Exceptionally preserved jellyfishes from the Middle Cambrian

Cnidarians represent an early diverging animal group and thus insight into their origin and diversification is key to understanding metazoan evolution. Further, cnidarian jellyfish comprise an important component of modern marine planktonic ecosystems. Here we report on exceptionally preserved cnidarian jellyfish fossils from the Middle Cambrian (∼505 million years old) Marjum Formation of Utah. These are the first described Cambrian jellyfish fossils to display exquisite preservation of soft part anatomy including detailed features of structures interpreted as trailing tentacles and subumbrellar and exumbrellar surfaces. If the interpretation of these preserved characters is correct, their presence is diagnostic of modern jellyfish taxa. These new discoveries may provide insight into the scope of cnidarian diversity shortly after the Cambrian radiation, and would reinforce the notion that important taxonomic components of the modern planktonic realm were in place by the Cambrian period.

Darwin's dilemma: the realities of the Cambrian 'explosion'

The Cambrian 'explosion' is widely regarded as one of the fulcrum points in the history of life, yet its origins and causes remain deeply controversial. New data from the fossil record, especially of Burgess Shale-type Lagerstätten, indicate, however, that the assembly of bodyplans is not only largely a Cambrian phenomenon, but can already be documented in fair detail. This speaks against a much more ancient origin of the metazoans, and current work is doing much to reconcile the apparent discrepancies between the fossil record, including the Ediacaran assemblages of latest Neoproterozoic age and molecular 'clocks'. Hypotheses to explain the Cambrian 'explosion' continue to be generated, but the recurrent confusion of cause and effect suggests that the wrong sort of question is being asked. Here I propose that despite its step-like function this evolutionary event is the inevitable consequence of Earth and biospheric change.

Possible ctenophoran affinities of the Precambrian "sea-pen" Rangea

The Namibian Kuibis Quartzite fossils of Rangea are preserved three-dimensionally owing to incomplete collapse of the soft tissues under the load of instantaneously deposited sand. The process of fossilization did not reproduce the original external morphology of the organism but rather the inner surface of collapsed organs, presumably a system of sacs connected by a medial canal. The body of Rangea had tetraradial symmetry, a body plan shared also by the White Sea Russian fossil Bomakellia and possibly some other Precambrian frond-like fossils. They all had a complex internal anatomy, smooth surface of the body, and radial membranes, making their alleged colonial nature unlikely. Despite a different style of preservation, the Middle Cambrian Burgess Shale frond-like Thaumaptilon shows several anatomical similarities to Rangea. The body plan of the Burgess Shale ctenophore Fasciculus, with its numerous, pinnately arranged comb organs, is in many respects transitional between Thaumaptilon and the Early Cambrian ctenophore Maotianoascus from the Chengjiang fauna of South China. It is proposed that the irregularly distributed dark spots on the fusiform units of the petaloid of Thaumaptilon represent a kind of macrocilia and that the units are homologous with the ctenophoran comb organs. These superficial structures were underlain by the complex serial organs, well represented in the fossils of Rangea. The Precambrian "sea-pens" were thus probably sedentary ancestors of the ctenophores.

A molecular phylogenetic framework for the phylum Ctenophora using 18S rRNA genes

This paper presents the first molecular phylogenetic analysis of the phylum Ctenophora, by use of 18S ribosomal RNA sequences from most of the major taxa. The ctenophores form a distinct monophyletic group that, based on this gene phylogeny, is most closely related to the cnidarians. Our results suggest that the ancestral ctenophore was tentaculate and cydippid-like and that the presently recognized order Cydippida forms a polyphyletic group. The other ctenophore orders that we studied (Lobata, Beroida, and Platyctenida) are secondarily derived from cydippid-like ancestors, a conclusion that is also supported by developmental and morphological data. The very short evolutionary distances between characterized ctenophore 18S rRNA gene sequences suggests that extant ctenophores are derived from a recent common ancestor. This has important consequences for future studies and for an understanding of the evolution of the metazoans.

Colonial origin for Emetazoa: major morphological transitions and the origin of bilaterian complexity

A new hypothesis for the evolution of Bilateria is presented. It is based on a reinterpretation of the morphological characters shared by protostomes and deuterostomes, which, when taken together with developmental processes shared by the two lineages, lead to the inescapable conclusion that the last common ancestor of Bilateria was complex. It possessed a head, a segmented trunk, and a tail. The segmented trunk was further divided into two sections. A dorsal brain innervated one or more sensory cells, which included photoreceptors. "Appendages" or outgrowths were present. The bilaterian ancestor also possessed serially repeated "segments" that were expressed ontogenetically as blocks of mesoderm or somites with adjoining fields of ectoderm or neuroectoderm. It displayed serially repeated gonads (gonocoels), each with a gonoduct and gonopore to the exterior, and serially repeated "coeloms" with connections to both the gut and the exterior (gill slits and pores). Podocytes, some of which were serially repeated in the trunk, formed sites of ultrafiltration. In addition, the bilaterian ancestor had unsegmented coeloms and a contractile blood vessel or "heart" formed by coelomic myoepithelial cells. These cells and their underlying basement membrane confine the hemocoelic fluid, or blood, in the connective tissue compartment. A possible scenario to account for this particular suite of characters is one in which a colony of organisms with a cnidarian grade of organization became individuated into a new entity with a bilaterian grade of organization. The transformation postulated encompassed three major transitions in the evolution of animals. These transitions included the origins of Metazoa, Eumetazoa, and Bilateria and involved the successive development of poriferan, cnidarian, and bilaterian grades of organization. Two models are presented for the sponge-to-cnidarian transition. In both models the loss of a flow-through pattern of water circulation in poriferans and the establishment of a single opening and epithelia sensu stricto in cnidarians are considered crucial events. In the model offered for the cnidarian-to-bilaterian transition, the last common ancestor of Eumetazoa is considered to have had a colonial, cnidarian-grade of organization. The ancestral cnidarian body plan would have been similar to that exhibited by pennatulacean anthozoans. It is postulated that a colonial organization could have provided a preadaptive framework for the evolution of the complex and modularized body plan of the triploblastic ancestor of Bilateria. Thus, one can explore the possibility that problematica such as ctenophores, the Ediacaran biota, archaeocyaths, and Yunnanozoon reflect the fact that complexity originated early and involved the evolution of a macroscopic compartmented ancestor. Bilaterian complexity can be understood in terms of Beklemishev "cycles" of duplication and colony individuation. Two such cycles appear to have transpired in the early evolution of Metazoa. The first gave rise to a multicellular organism with a sponge grade of organization and the second to the modularized ancestor of Bilateria. The latter episode may have been favored by the ecological conditions in the late Proterozoic. Whatever its cause, the individuation of a cnidarian-grade colony furnishes a possible explanation for the rapid diversification of bilaterians in the late Vendian and Cambrian. The creation of a complex yet versatile prototype, which could be rapidly modified by selection into a profusion of body plans, is postulated to have affected the timing, mode, and extent of the "Cambrian explosion." During the radiations, selective loss or simplification may have been as creative a force as innovation. Finally, colony individuation may have been a unique historical event that imprinted the development of bilaterians as the zootype and phylotypic stage. (ABSTRACT TRUNCATED)

Internal and external relationships of the Cnidaria: implications of primary and predicted secondary structure of the 5'-end of the 23S-like rDNA

Since both internal (class-level) and external relationships of the Cnidaria remain unclear on the basis of analyses of 18S and (partial) 16S rDNA sequence data, we examined the informativeness of the 5'-end of the 23S-like rDNA. Here we describe analyses of both primary and predicted secondary structure data for this region from the ctenophore Bolinopsis sp., the placozoan Trichoplax adhaerens, the sponge Hymeniacidon heliophila, and representatives of all four cnidarian classes. Primary sequence analyses clearly resolved the Cnidaria from other lower Metazoa, supported sister group relationships between the Scyphozoa and Cubozoa and between the Ctenophora and the Placozoa, and confirmed the basal status of the Anthozoa within the Cnidaria. Additionally, in the ctenophore, placozoan and sponge, non-canonical base pairing is required to maintain the secondary structure of the B12 region, whereas amongst the Cnidaria this is not the case. Although the phylogenetic significance of this molecular character is unclear, our analyses do not support the close relationship between Cnidaria and Placozoa suggested by previous studies.