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Carpoids are a puzzling group of fossil deuterostomes. Establishing their relationships with other deuterostomes has proven problematic; carpoids exhibit a unique character combination that is contentious to interpret. Carpoids are considered as (i) basal chordates/hemichordates, (ii) basal stem-echinoderms or (iii) derived echinoderms; the aim of this study is to assess the support for each of these models and, hence, derive a robust phylogenetic hypothesis for carpoid evolution. Enigmatic and controversial aspects of carpoid anatomy are investigated using tomographic techniques. Virtual reconstructions of the ctenocystoid Ctenocystis utahensis show two bilaterally symmetrical feeding grooves leading into an internal chamber, suggesting that ctenocystoids may have been pharyngeal basket feeders with a paired water vascular system. Reconstructions of a new genus of cinctan reveal a large depression in the interior of the ventral skeleton, which was possibly associated with an internal pharynx. The morphology of a specimen of the stylophoran Ceratocystis is elucidated, demonstrating that this individual belongs to a new species. An internal grill-like network of bars is visualized in models of the stylophoran Lagynocystis, suggesting the presence of gill bars and pharyngeal openings. The stylophoran Placocystites is shown to possess intricate internal skeletal structures, which may have supported unpreserved soft tissues.

2022

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.

Fossil carpoids possess a unique anatomy that is difficult to interpret; as a result, there are a number of competing phylogenetic hypotheses for carpoid taxa. Stratigraphic congruence indices provide a quantitative means of evaluating alternative cladograms where character coding is contentious; trees that show a statistically significant fit between stratigraphy and phylogeny are better supported by the fossil record. We here test the agreement between stratigraphic and cladistic data for 27 carpoid cladograms (24 have previously been published, three are novel). The results demonstrate that in analyses of subsets of carpoid taxa, the stratigraphic congruence of trees is not strongly affected by the interpretative model followed. However, when studying the relationships of carpoids with other deuterostomes, assuming that carpoids should be interpreted by reference to chordates/hemichordates (rather than echinoderms) leads to a poorer fit with the known stratigraphic ranges of taxa. Thus, the disputed calcichordate hypothesis (carpoids interpreted as stem and crown-group chordates and stem-group hemichordates) is much less congruent with stratigraphy than alternative models interpreting carpoids as stem or crown-group echinoderms.

Philosophical Transactions of The Royal Society B: Biological Sciences, 2008

Deuterostomes are a monophyletic group of animals that include the vertebrates, invertebrate chordates, ambulacrarians and xenoturbellids. Fossil representatives from most major deuterostome groups, including some phylum-level crown groups, are found in the Lower Cambrian, suggesting that evolutionary divergence occurred in the Late Precambrian, in agreement with some molecular clock estimates. Molecular phylogenies, larval morphology and the adult heart/kidney complex all support echinoderms and hemichordates as a sister grouping (Ambulacraria). Xenoturbellids are a relatively newly discovered phylum of worm-like deuterostomes that lacks a fossil record, but molecular evidence suggests that these animals are a sister group to the Ambulacraria. Within the chordates, cephalochordates share large stretches of chromosomal synteny with the vertebrates, have a complete Hox complex and are sister group to the vertebrates based on ribosomal and mitochondrial gene evidence. In contrast, tunicates have a highly derived adult body plan and are sister group to the vertebrates based on the analyses of concatenated genomic sequences. Cephalochordates and hemichordates share gill slits and an acellular cartilage, suggesting that the ancestral deuterostome also shared these features. Gene network data suggest that the deuterostome ancestor had an anterior-posterior body axis specified by Hox and Wnt genes, a dorsoventral axis specified by a BMP/chordin gradient, and was bilaterally symmetrical with left-right asymmetry determined by expression of nodal.

Proceedings of The National Academy of Sciences, 2000

The deuterostome phyla include Echinodermata, Hemichordata, and Chordata. Chordata is composed of three subphyla, Vertebrata, Cephalochordata (Branchiostoma), and Urochordata (Tunicata). Careful analysis of a new 18S rDNA data set indicates that deuterostomes are composed of two major clades: chordates and echinoderms ؉ hemichordates. This analysis strongly supports the monophyly of each of the four major deuterostome taxa: Vertebrata ؉ Cephalochordata, Urochordata, Hemichordata, and Echinodermata. Hemichordates include two distinct classes, the enteropneust worms and the colonial pterobranchs. Most previous hypotheses of deuterostome origins have assumed that the morphology of extant colonial pterobranchs resembles the ancestral deuterostome. We present a molecular phylogenetic analysis of hemichordates that challenges this long-held view. We used 18S rRNA to infer evolutionary relationships of the hemichordate classes Pterobranchia and Enteropneusta. Our data show that pterobranchs may be derived within enteropneust worms rather than being a sister clade to the enteropneusts. The nesting of the pterobranchs within the enteropneusts dramatically alters our view of the evolution of the chordate body plan and suggests that the ancestral deuterostome more closely resembled a mobile worm-like enteropneust than a sessile colonial pterobranch. ¶ To whom reprint requests should be addressed.

Proceedings of the National Academy of Sciences of the United States of America, 2000

The deuterostome phyla include Echinodermata, Hemichordata, and Chordata. Chordata is composed of three subphyla, Vertebrata, Cephalochordata (Branchiostoma), and Urochordata (Tunicata). Careful analysis of a new 18S rDNA data set indicates that deuterostomes are composed of two major clades: chordates and echinoderms + hemichordates. This analysis strongly supports the monophyly of each of the four major deuterostome taxa: Vertebrata + Cephalochordata, Urochordata, Hemichordata, and Echinodermata. Hemichordates include two distinct classes, the enteropneust worms and the colonial pterobranchs. Most previous hypotheses of deuterostome origins have assumed that the morphology of extant colonial pterobranchs resembles the ancestral deuterostome. We present a molecular phylogenetic analysis of hemichordates that challenges this long-held view. We used 18S rRNA to infer evolutionary relationships of the hemichordate classes Pterobranchia and Enteropneusta. Our data show that pterobranch...

Deuterostomia is one of the major monophyletic groups in the animal kingdom. Interest in deuterostome evolution stems from the diversity of life cycles and anatomies present in this group and moreover from the fact that vertebrates are part of Deuterostomia. Phylogeny is a prerequisite for the understanding of evolution. In the present review I critically assess our current understanding of deuterostome phylogeny based on molecular and morphological data. Conflict in phylogenetic signal in molecular phylogenies is revealed utilizing data display networks and split analyses. Phylogenetic argumentation supports the hypothesis that the fossil Pikaia gracilens is a member of the stem lineage of Craniata . A first experimental data matrix based on “evo-devo ” characters is presented and it is demonstrated that such a cladistic approach holds great potential for phylogenetic analysis of molecular developmental studies. The phylogenetic analysis of morphological characters is in conflict w...

Proceedings of The Royal Society B: Biological Sciences, 2002

We report the discovery of a new agnathan specimen from the Lower Cambrian Chengjiang Lagersta ¨tte of China and thereby provide new evidence on the myomeres (V-shaped), the branchial apparatus (gill filaments and arches), the dorsal fin and the gonads (24-26) of the earliest vertebrates. The new specimen and the co-occurring Myllokunmingia fengjiaoa and Haikouichthys ercaicunensis represent a single species, which is a primitive member of the crown group craniates (vertebrates) and post-dates the origin of the myxinoids (hagfish). The origin of the vertebrate clade is at least as old as Early Cambrian.

The evolutionary origin of vertebrates has been debated ad nauseam by anatomists, paleontologists, embryologists, and physiologists, but it is only now that molecular phylogenetics is providing a more rigorous framework for the placement of vertebrates among their invertebrate relatives that we can begin to arrive at concrete conclusions concerning the nature of ancient ancestors and the sequence in which characteristic anatomical features were acquired. Vertebrates tunicates and cephalochordates together comprise the chordate phylum, which along with echinoderms and hemichordates constitute the deuterostomes. The origin of vertebrates and of jawed vertebrates is characterized by a doubling of the vertebrate genome, leading to hypotheses that this genomic event drove organismal macroevolution. However, this perspective of evolutionary history, based on living organisms alone, is an artifact. Phylogenetic trees that integrate fossil vertebrates among their living relatives demonstrate the gradual and piecemeal assembly of the gnathostome body plan. Unfortunately, it has not been possible to demonstrate gradual assembly of the vertebrate body plan. This is not because vertebrates are irreducibly complex but because many of the characters that distinguish vertebrates from invertebrates are embryological and cellular and, therefore, inherently unfossilizable.

"A new cinctan (Protocinctus mansillaensis gen. et sp. nov.), from the early Middle Cambrian of the Iberian Chains (north-east Spain), is described with the aid of X-ray microtomography and three-dimensional computer models. Investigation in this manner was possible because of the unusual condition of the fossils, which are preserved as recrystallized calcite. Protocinctus gen nov. possesses an elongate body with a single left anterior feeding groove and an open posterior marginal frame (in ventral view): this combination of characters is unique amongst cinctans. Through the study of original specimens and ‘virtual fossils’ it was possible to reconstruct the palaeobiology of Protocinctus gen. nov.: cinctans are interpreted as pharyngeal basket feeders with a U-shaped gut, using their posterior appendage to aid stability on the sediment surface. Cinctans are critical to understanding the evolutionary history of the echinoderm stem group, as they illustrate the transition from a paired water vascular system (basal) to one constructed from just the left hydrocoel (derived). The former condition is also observed in another group of stem-group echinoderms, the ctenocystoids, and is hypothesized for the latest common ancestor of the echinoderms and hemichordates."

The International Journal of Developmental Biology, 2009

Pallid anchovy fillet, friendly filtering, peacefully laying and little lancelet are some of the nicknames and adjectives the cephalochordate amphioxus has received throughout the last two centuries. Traditionally regarded as the living representative of the last ancestor of vertebrates, amphioxus has recently been promoted to the privileged position of being the most ancient chordate. The preliminary analysis of its prototypical genome is nearly completed, and its hidden secrets towards the understanding of the primitive chordate and deuterostome genomes will soon see the light. Amphioxus embryonic development and body plan have remained in evolutionary stasis since the cephalochordate lineage split from the chordate ancestor about 500 million years ago. In contrast, amphioxus research is far from being at a standstill; in Europe, thanks to the international cooperation and the Banyuls Oceanographic Station, amphioxus embryos are obtained on demand during the spawning season. We summarise here our progress towards the dream of the experimental manipulation of the amphioxus embryo, to enter the era of Experimental Evo-Devo.

Phytomorphology, 1996

The vascular skeleton of the archaic winteraceous carpels reveals some features that question the contemporary carpel theory based mainly on the resemblance of a carpel with a conduplicately folded leaf or leaf in general. The occurrence of the double vascular supply to the ovules from both dorsal and ventral carpellary strands and the double nature of these systems, seen clearly in Drimys lallceolma, questions the monomery of the carpel and re-defines it as a dimerous structure in which the carpel has an independent dorsal and ventral vascular system. With due regard to the occurrence of the archaic coincidence of ovule placentation and the stigmatic crest, it allows us to trace the origin of true carpels back to the dimerous Lepfosfrobus type of cupules, redefined in this respect, as amphicid capria. The occurrence of the abaxially oriented ovule placentation in some relic winteraceous genera such as Bubbia and BellioLunl, together with the abaxially oriented microsynangia in Degelleria, BellioLum, Lacforis and others are indicative of the derivation of. the most adaxially oriented ovule placentation and microsynangia position from the ancestral abaxial condition. Thus, the proto-angiospermous forerunners should have abaxially oriented synangiate structures turning directly to the adaxial condition by the overtopping of ovule placentae and rnicrosynangia. This was possible without an additional hypothetical stage of purely marginal placentation as evidenced by the vascular architecture of relic winteraceous gynoecia.

Journal of Vertebrate Paleontology, 2001

Carpodaptes'' jepseni is a morphologic intermediate between Carpodaptes and Carpolestes, with the number and position of cusps on p4 more consistent with placement in Carpodaptes but relative size of p4 more like Carpolestes. The type and only previously known specimen of ''C.'' jepseni, a partial dentary with p4-m2, is from Divide Quarry (Tiffanian Land-Mammal Age) in the Fort Union Formation of the Bighorn Basin, Wyoming. New specimens of ''C.'' jepseni from Divide Quarry include a nearly complete dentary with p4-m3 and alveoli for all anterior teeth, and the first known upper dentitions with P1-M2 and an alveolus for C1. Specimens of Carpodaptes cygneus are also described from Divide Quarry, demonstrating the occurrence of two distinct carpolestid species at the same locality.

Current Biology, 2014

2007

, and Smithsonian Marine Station s0010 INTRODUCTION s0020 A. HISTORY OF HYPOTHESES OF CHORDATE ORIGINS p0010 Vertebrates share several distinct morphological characters with three invertebrate groups: lancelets, tunicates, and hemichordates (Figure 6.1). Tunicates, lancelets, and vertebrates have traditionally been considered to be a monophyletic group-the chordates-that shares five morphologic features: a notochord, a dorsal neural tube, an endostyle, a muscular postanal tail, and pharyngeal gill slits. Hemichordates share some of these chordate features; the pharyngeal gill slits (Figure 6.2), an endostyle, and a postanal tail. Previously, hemichordates were thought to contain a notochord homolog (the stomochord) and a dorsal neural tube in the neck region, but recent evidence from developmental genetics has questioned these homologies. Developmental genetics and genomics have allowed for the reexamination of the question of chordate origins by comparing developmental gene expression in embryos of different phyla. This powerful approach has allowed new insights into the molecular mechanisms underlying morphologic changes. Genomics has allowed for investigations into the phylogenetic relationships of the chordates and their invertebrate relatives and for the comparison of the shared genetic pathways in related embryos. We review current research on this topic and show that our view of the chordate ancestor has changed during the past 10 years. For years, the chordate ancestor has been considered to be a filter-feeding, tunicate-like animal with a tiny chordate tadpole larva. However, recent evidence from my laboratory and others has shown that the chordate ancestor was more likely a benthic worm with a mouth and pharyngeal gill slits supported by cartilaginous gill bars (

Proceedings of the Royal Society B, 2022

Deuterostomes comprise three phyla with radically different body plans. Phylogenetic bracketing of the living deuterostome clades suggests the latest common ancestor of echinoderms, hemichordates and chordates was a bilaterally symmetrical worm with pharyngeal openings, with these characters lost in echinoderms. Early fossil echinoderms with pharyngeal openings have been described, but their interpretation is highly controversial. Here, we critically evaluate the evidence for pharyngeal structures (gill bars) in the extinct stylophoran echinoderms Lagynocystis pyramidalis and Jaekelocarpus oklahomensis using virtual models based on high-resolution X-ray tomography scans of three-dimensionally preserved fossil specimens. Multivariate analyses of the size, spacing and arrangement of the internal bars in these fossils indicate they are substantially more similar to gill bars in modern enteropneust hemichordates and cephalochordates than to other internal bar-like structures in fossil blastozoan echinoderms. The close similarity between the internal bars of the stylophorans L. pyramidalis and J. oklahomensis and the gill bars of extant chordates and hemichordates is strong evidence for their homology. Differences between these internal bars and bar-like elements of the respiratory systems in blastozoans suggest these structures might have arisen through parallel evolution across deuterostomes, perhaps underpinned by a common developmental genetic mechanism.

Nature, 2005

Stylophora are a peculiar extinct group of asymmetrical deuterostomes whose biological affinity has been fiercely debated . Disarticulated skeletal elements of a ceratocystid stylophoran recovered from the earliest Middle Cambrian of Morocco are not only the oldest stylophorans in the fossil record, but their exceptional preservation provides crucial data on the microstructure of its skeleton. Stylophoran plates are constructed of a threedimensional mesh, termed 'stereom', identical to that of living echinoderms in which stereom microstructure provides a reliable guide to the nature of the investing soft tissues . Using modern echinoderm anatomy to interpret stereom microstructure of stylophoran elements, here we show that the large proximal lumen of their appendage was filled with muscle and that ligamentary tissues bound distal elements firmly together. We find no evidence for a mouth in the proximal lumen and no evidence that the covering plates of the appendage were articulated. Thus, although skeletal structure suggests that stylophorans are echinoderms, their appendage was not a feeding arm but a muscular locomotory organ.

A new cinctan (Protocinctus mansillaensis gen. et sp. nov.), from the early Middle Cambrian of the Iberian Chains (north-east Spain), is described with the aid of X-ray microtomography and three-dimensional computer models. Investigation in this manner was possible because of the unusual condition of the fossils, which are preserved as recrystallized calcite. Protocinctus gen nov. possesses an elongate body with a single left anterior feeding groove and an open posterior marginal frame (in ventral view): this combination of characters is unique amongst cinctans. Through the study of original specimens and ‘virtual fossils’ it was possible to reconstruct the palaeobiology of Protocinctus gen. nov.: cinctans are interpreted as pharyngeal basket feeders with a U-shaped gut, using their posterior appendage to aid stability on the sediment surface. Cinctans are critical to understanding the evolutionary history of the echinoderm stem group, as they illustrate the transition from a paired water vascular system (basal) to one constructed from just the left hydrocoel (derived). The former condition is also observed in another group of stem-group echinoderms, the ctenocystoids, and is hypothesized for the latest common ancestor of the echinoderms and hemichordates.