The anus empties into the excurrent siphon, which expels wastes and water. Tunicates are found in shallow ocean waters around the world. A cranium is a bony, cartilaginous, or fibrous structure surrounding the brain, jaw, and facial bones Figure.
Vertebrates are named for the vertebral column, composed of vertebrae —a series of separate, irregularly shaped bones joined together to form a backbone Figure.
Initially, the vertebrae form in segments around the embryonic notochord, but eventually replace it in adults. In most derived vertebrates, the notochord becomes the nucleus pulposus of the intervertebral discs that cushion and support adjacent vertebrae. Traditional phylogenies place the cephalochordates as a sister clade to the chordates, a view that has been supported by most current molecular analyses.
This hypothesis is further supported by the discovery of a fossil in China from the genus Haikouella. This organism seems to be an intermediate form between cephalochordates and vertebrates.
The Haikouella fossils are about million years old and appear similar to modern lancelets. These organisms had a brain and eyes, as do vertebrates, but lack the skull found in craniates. Vertebrates are the largest group of chordates, with more than 62, living species, which are grouped based on anatomical and physiological traits. More than one classification and naming scheme is used for these animals. Virtually all modern cladists classify birds within Reptilia, which correctly reflects their evolutionary heritage.
Thus, we now have the nonavian reptiles and the avian reptiles in our reptilian classification. We consider them separately only for convenience. Further, we will consider hagfishes and lampreys together as jawless fishes, the Agnatha , although emerging classification schemes separate them into chordate jawless fishes the hagfishes and vertebrate jawless fishes the lampreys.
Tetrapods include amphibians, reptiles, birds, and mammals, and technically could also refer to the extinct fishlike groups that gave rise to the tetrapods. Tetrapods can be further divided into two groups: amphibians and amniotes. Amniotes are animals whose eggs contain four extraembryonic membranes yolk sac, amnion, chorion, and allantois that provide nutrition and a water-retaining environment for their embryos.
Amniotes are adapted for terrestrial living, and include mammals, reptiles, and birds. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms. Most tunicates live on the ocean floor and are suspension feeders. Which of the two invertebrate chordate clades is more closely related to the vertebrates continues to be debated. Vertebrata is named for the vertebral column, which is a feature of almost all members of this clade.
The name Craniata organisms with a cranium is considered to be synonymous with Vertebrata. Figure Which of the following statements about common features of chordates is true? Which of the following is not contained in phylum Chordata? Hagfish, lampreys, sharks, and tuna are all chordates that can also be classified into which group? The characteristic features of the phylum Chordata are a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. What is the structural advantage of the notochord in the human embryo?
In his book Before the backbone , Henry Gee recounts a great number of theories that, over the last century and a half, have invoked almost every other major living animal group as the ancestors of vertebrates Gee Mercifully, there is now much less equivocation over the relationships of vertebrates to their living relatives, none of which are thought of as being ancestral.
Rather, vertebrates and their nearest kin—the invertebrate chordates, the hemichordates and the echinoderms—are more correctly perceived as living representatives of distinct genealogical lineages that separated one from another deep in geological time. It is the aim of many paleontologists, comparative anatomists, embryologists, and molecular biologists to uncover the genealogical relationships of these animals—their family tree—and to test this tree with evidence that bears on the question of how these distinct organismal designs.
Explanations of the emerging evolutionary pattern range from traditional Darwinian gradualistic evolution to those that invoke explosive diversifications seized upon by creationists and intelligent designers as evidence for irreducible complexity Meyer a , b but which are actually consistent with natural causal mechanisms McLennan Whatever the pattern and processes, a holistic understanding of the origin and diversification of vertebrates can only be obtained by a holistic approach, integrating all relevant strands of evidence into a framework of evolutionary relationships established on the basis of the only universal characteristics shared by all organisms: molecular sequence data.
Establishing a phylogenetic framework of relationships among organisms is an essential prerequisite to uncovering the patterns and mechanisms of evolution. Traditionally, the evidence to support evolutionary trees has been derived from analysis of skeletal, muscular, and nervous systems, development and embryology, and cell characteristics. Invariably, these different aspects of organismal biology were studied in isolation and resulted in conflicting ideas of animal relationships that have been difficult to resolve Jenner and Schram However, at the deepest levels in the genealogy of animals, such as the splits between phyla and groups of phyla, embryological and cell characters have held sway, not least because there are no skeletal characters shared between phyla.
Most animals are bilaterally symmetrical—or show evidence of a bilateral ancestry in their embryology—and fall into one of two major groups: the protostomes first mouths or the deuterostomes second mouths , the names of which betray the embryological characters on which they were defined Footnote 1. Vertebrates belong to the chordate phylum and along with hemichordates acorn worms and pterobranchs and echinoderms sea lilies, star fish, sea urchins, sea cucumbers have long been considered deuterostomes.
Whether there are additional deuterostomes has been the subject of long-running debate. Classically, chaetognaths arrow worms and lophophorates—bryozoans moss animals and brachiopods lamp shells —have been considered deuterostomes, based on similarities of the larvae from which the more familiar adults develop, but this is no longer the prevailing view.
Such instability stems from the fact that, while features shared by phyla may well reflect their kinship, the absence of features is more difficult to interpret Donoghue and Purnell ; Jenner It may indicate that in one clade the characters in question were never present, providing evidence against a close evolutionary relationship with clades that possess them, or it could be that the characters were originally inherited by both clades from their common ancestor but were subsequently lost in one.
Indeed, even what appear to be shared similarities between phyla can be unreliable because they may reflect evolutionary convergence and independent acquisition rather than common ancestry. Because of these problems and because the universe of available data had largely been exhausted, attempts to decipher the relationships between animal phyla could not reach a consensus and it is hard to see how this situation would have changed were it not for the availability of genetic sequence data.
Analysis of this rapidly expanding molecular dataset has provided a robust and essentially independent test of the theories of evolutionary relationships previously derived from anatomical and developmental data. Perhaps the most surprising result of the molecular phylogenetic revolution has been that the majority of the classical groupings of animals, and their evolutionary relationships, have withstood testing, even in the face of ever-greater molecular datasets, representing ever-more lineages e.
The composition of some major clades and the details of their relationships have, however, been the subject of dramatic change. For example, the lophophorates and chaetognaths have been excised from the deuterostomes and are now recognized to be protostomes although their precise relations within the protostomes remain an open question.
The residual deuterostomes have also been rearranged: hemichordates and chordates were considered more closely related to one another than to echinoderms, but there is now very strong molecular support for the Ambulacraria grouping of hemichordates and echinoderms, to the exclusion of chordates Fig. Xenoturbella , a worm-like animal of hitherto enigmatic affinity, is recognized as a fourth very minor phylum of just two species that is more closely related to ambulacrarians than chordates Bourlat et al.
The interrelationships of the deuterostome phyla within the context of Metazoa. Primarily based upon Dunn et al. With this increasingly reliable phylogenetic framework in place, it is now possible to turn to the question of how the major groups of deuterostomes emerged through evolutionary history. This is not a trivial challenge: the deuterostomes may be a small grouping of just four phyla, but the body plans that characterize the phyla are as anatomically disparate as any.
Tradition would have us start our discussion with the last common ancestor of all deuterostomes and move up the evolutionary tree considering each subsequent branch in turn. However, as well as being overly deterministic, perpetuating the erroneous teleological impression that evolution progressed towards vertebrates this would not reflect certainties concerning knowledge of relationships and the sequence of character evolution, which are far better resolved within phyla than between phyla.
So we will take a different route here, starting with our own familiar branch, the vertebrates, and working backwards to more distant and unfamiliar relatives, attempting to describe events in evolutionary history that are well constrained, through to those that are not so well constrained.
Named after one of their most conspicuous anatomical characters, jaws, gnathostomes are much more than just a pretty smile, and the list of features that distinguishes them from their living jawless relatives contains what might be generally, but incorrectly, considered as distinctive attributes of the vertebrate body plan. This includes teeth and a mineralized internal skeleton forming a braincase, jaws and, in fish, gill supports , a backbone, trunk, and appendages.
The different tissues of the mineralized skeleton—bone, cartilage, dentine, and enamel—are also unique, among living animals, to gnathostomes. Features of the brain and inner ear also distinguish gnathostomes, together with a good many other characters that are too detailed to recount here for more details, see Donoghue et al.
In fact, the number of anatomical characters that distinguish living jawed from jawless vertebrates is even greater than the complementary suite of features that distinguish vertebrates from invertebrates see below.
The interrelationships of the principal groups of living chordates and vertebrates. Primarily based upon Delsuc et al. A vast number of characters distinguish vertebrates from their nearest invertebrate relatives among chordates. Some of the more obvious features include a distinct anatomical head with a distinct brain, paired sensory organs, together with a number of specialized cell types that are responsible for the development and function of the nervous system and skeleton.
The majority of these vertebrate-specific characters can be accounted for by a couple of embryological innovations that have long been thought exclusive to vertebrates, but for which evidence of evolutionary rudiments has grudgingly been found in the invertebrate chordates Donoghue et al. These innovations are neurogenic placodes and neural crest cells Gans and Northcutt During development, neurogenic placodes give rise to the sensory organs of the central and peripheral nervous systems, including the eyes, nasal organs, inner ear, and the lateral line system of fishes.
Neural crest cells are a specialized population of migratory cells which behave like stem cells in that they have the potential to differentiate into a broad spectrum of specialized cell types, such as specialized neurons and glia of the nervous system, pigment cells, and cells of the dermis Le Douarin and Kalcheim It is also from neural crest cells that the gnathostome's dermal and pharyngeal bone, cartilage, and dentine develop.
Because most of these anatomical characters are exclusive to vertebrates, it has been thought that neurogenic placodes and neural crest cells were the key innovations that underpinned a rapid evolutionary emergence of vertebrates.
This is now known to be an oversimplification: certain sense organs in tunicates, for example, develop from placodes in a manner comparable to their vertebrate counterparts, and their development is regulated by a common suite of molecular factors Mazet et al.
Similarly, some tunicates also possess a migratory cell population that emerges during the early development of the nervous system, and these cells differentiate into pigment cells—a characteristic derivative of neural crest—utilizing an equivalent repertoire of genes Jeffery ; Jeffery et al. Nevertheless, the diversity and functionality of sensory organs and neural crest cell derivatives in invertebrate chordates are far less than in their vertebrate counterparts, and without doubt the evolution of the potentiality of neurogenic placodes and neural crest was key to the emergence of vertebrates Donoghue et al.
Understanding what distinguished vertebrates from other animals is only part of the picture. We also need to know the details of how they fit into the Tree of Life. Here, once again, molecular phylogenetics has weighed heavily upon classical interpretations.
Generally, new data have confirmed the monophyly of vertebrates and chordates but see Lartillot and Philippe , but in other areas of the deuterostome tree decades of dogma have been overturned. In particular, debate over the relationships among the principal lineages of living vertebrates has become polarized into two conflicting hypotheses, each derived from different kinds of data. Molecular analyses indicate that the hagfishes and lampreys comprise a clade to the exclusion of jawed vertebrates Bourlat et al.
Whichever hypothesis turns out to be correct, it is unlikely to have major consequences for understanding the broader picture of vertebrate and gnathostome origins. Perhaps the biggest surprise of recent molecular revisions to the chordate tree is the recognition that the peculiar tunicates sea squirts and not cephalochordates amphioxus are the closest invertebrate relatives of the vertebrates Delsuc et al.
While this conclusion, at first quite shocking, is now generally accepted, it has very few implications for what we understand of the nature of common ancestor of all chordates. Under either scenario, this ancestor must have been an amphioxus-like organism because segmented muscles and paired pharyngeal gill slits are characters general to deuterostomes, and a notochord is a character general to chordates i.
Strikingly, the burst of embryological and anatomical innovation at the origin of vertebrates and gnathostomes seems to correspond to radical events in the evolution of their genomes.
At some stage, along the lineage leading to living vertebrates after its separation from tunicates, the vertebrate genome was duplicated, at once doubling the repertoire of coding and noncoding DNA. Similarly, at some point after the living jawless vertebrates branched off, a second duplication took place prior to the appearance of the last common ancestor of living gnathostomes.
In each case of duplication, one gene set was available to maintain its existing functions, but a matching set was free to evolve without compromising the original function of the genes. Thus, new gene functions could emerge to regulate the development of organisms and their organs, leading rapidly to innovation see McLennan for discussion of duplication and cooption in evolution generally.
There is abundant evidence that this occurred: 1 all living vertebrates can be shown to possess, or to have possessed, two copies of the genes possessed by invertebrate chordates; 2 all living gnathostomes can be shown to possess, or to have possessed, at least four copies Holland et al.
Of course, not all duplicate genes have been retained, but there is good evidence for the selective retention of genes that are essential to the anatomical and developmental innovations of vertebrates and gnathostomes. For instance, of the duplicate genes retained, there is a particular pattern of retention of genes that encode protein products associated with the extracellular matrices that are essential for skeletal development Huxley-Jones et al.
There has been much debate about the tempo and mode of organismal evolution associated with genome duplication. Some have argued for a direct causal linkage such that genome duplication drives a geologically instantaneous burst of evolution Sidow ; Wagner et al. The principal evidence marshaled in support of the evolutionary burst hypothesis is the observation that living vertebrates and gnathostomes are distinguished from living invertebrate chordates and jawless vertebrates, respectively, by very large inventories of anatomical and developmental characters—surely only genome duplication can explain the emergence of so many characters in concert?
So far, we have discussed only living gnathostomes, vertebrates, and chordates, but this provides an extremely incomplete perspective on vertebrate organismal evolution because current vertebrate diversity is only a small subset of the vertebrates that have ever lived. Many major branches from the vertebrate Tree of Life have been extinguished, and we are familiar with very many of them and the information that they provide on the pattern, timing, and tempo of evolution.
For instance, dinosaurs record the gradual assembly of the body plan of modern birds Padian and Chiappe and there is a comparably rich record of synapsid reptiles that record the gradual assembly of the body plan of modern mammals Kemp ; Angielczyk No one, therefore, would attempt to explain the emergence of birds or mammals directly from reptilian ancestors in a geologically instantaneous event.
The same is true of gnathostome origins. Interleaved in the Tree of Life, between living jawless and jawed vertebrates, is a parade of long-dead and in some cases bizarre-looking fish that records clearly how gnathostome characters were acquired over a period of at least 80 million years Fig. Another planktonic group of Urocordates includes the larvaceans.
These animals live inside intricate mucous "houses" and retain their larval tail throughout their lives. This tail drives a gentle current of water through the house, propelling the organism through the water. The photograph below shows the organism, Oikopleura vanhoeffeni , inside its house, creating a current by movements of its tail. Acknowledgements to Dr. Alexander Bochdansky, Queens University, Canada, for the above pictures of thaliacean and larvacean Urochordates.
How will we recognize the non-native tunicate species, Ciona savignyi when it appears? What do we know about this species? For more information, go to C. Some kinds of tunicates live alone, and are called solitary tunicates. Others, including the two forms shown here, have the ability to bud off additional individuals from the first to arrive, and these grow into colonies. These are openings that connect the pharynx or throat to the outside of the neck. In some primitive species, the slits are used to filter food out of the water.
In other species, like fish, the pharyngeal slits have gills. In other species, like mammals, the pharyngeal slits are only present during the embryonic stage. Chordates also have a dorsal nerve cord that runs down the length of the organism.
The dorsal nerve cord has pairs of nerves that connect to the organism's muscles.
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