CHAPTER 23 Chordates 23-1
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23-3 The Chordates: Characteristics Structural Plan Name chordata comes from the notochord Rodlike, semirigid tissue enclosed in a sheath In most cases, extends the length of body lying between the gut and the nervous system Mainly serves to stiffen the body, providing skeletal scaffolding for the attachment of swimming muscles 5 Hallmark chordate characteristics Dorsal, tubular nerve cord Notochord Pharyngeal slits Endostyle Postanal tail
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The Chordates: Characteristics Chordates share features with some invertebrates: Bilateral symmetry Anterioposterior axis Coelom Tube-within-a-tube body plan Metamerism Cephalization 23-5
Traditional and Cladistic Classification of the Chordates Traditional and Cladistic Systems Diverge Traditional classification Convenient way to indicate the taxa included in each major group Cladistic systems Some traditional taxa (Agnatha and Reptilia) no longer used Reptiles are paraphyletic because they do not contain all of the descendants of recent common ancestor Reptiles, birds and mammals compose a monophyletic clade called Amniota 23-6
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Traditional and Cladistic Classification of the Chordates Reptiles can only be grouped as amniotes that are not birds or mammals No derived characters that group only reptiles to the exclusion of birds and mammals Likewise, agnathans (hagfishes and lampreys) are paraphyletic Most common recent ancestor is also an ancestor of all remaining vertebrates In contrast, the branches of a phylogenetic tree represent real lineages with geological information 23-8
Traditional and Cladistic Classification of the Chordates Traditional classification Protochordata (Acraniata) are separated from Vertebrata (Craniata) that have a skull Vertebrates may be divided into Agnatha (jawless) and Gnathostomata (having jaws) Vertebrates are also divided into Amniota, having an amnion, and Anamniota lacking an amnion Gnathostomata is subdivided into Pisces with fins and Tetrapoda, usually with two pair of limbs Many of these groupings are paraphyletic Alternative monophyletic taxa are suggested Some cladistic classifications exclude Myxini (hagfishes) from the group Vertebrata because they lack vertebrae, although retaining them in Craniata since they do have a cranium. 23-9
Notochord Five Chordate Hallmarks Always found at some embryonic stage First part of the endoskeleton to appear in the embryo Serves as an axis for muscle attachment Can bend without shortening and permits undulation In protochordates and jawless vertebrates, Persists throughout life 23-10
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Five Chordate Hallmarks In vertebrates Series of cartilaginous or bony vertebrae form from mesenchymal cells derived from blocks of mesodermal cells lateral to notochord In most vertebrates Notochord displaced by vertebrae Remnants may persist between or within vertebrae 23-12
Five Chordate Hallmarks Dorsal Tubular Nerve Cord In most invertebrate phyla Nerve cord is solid and ventral to alimentary canal In chordates Single, tubular cord is dorsal to alimentary canal Anterior end enlarges to form the brain Cord is produced in embryo by infolding of ectodermal cells on the dorsal side of body 23-13
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Five Chordate Hallmarks Pharyngeal Pouches and Slits Pharyngeal slits lead from pharyngeal cavity to the outside Form by the inpocketing of the ectoderm and the evagination of endoderm of pharynx In aquatic chordates 2 pockets break through to form pharyngeal slit In amniotes Pockets may not break through and only grooves are formed 23-15
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Five Chordate Hallmarks In tetrapods Pharyngeal pouches give rise to a variety of structures, including the Eustachian tube, middle ear cavity, tonsils and parathyroid glands Perforated pharynx functions as filterfeeding apparatus in protochordates Fishes added a capillary network with thin gas-permeable walls Led to evolution of gills 23-17
Five Chordate Hallmarks Endostyle or Thyroid Gland Recently, the endostyle was recognized as a shared chordate character Endostyle or its derivative, the thyroid gland, found in all chordates Some cells in endostyle secrete iodinated proteins homologous with the iodinatedhormone-secreting thyroid gland of adult lampreys and the remainder of vertebrates 23-18
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Five Chordate Hallmarks Postanal Tail Postanal tail, plus musculature, provided motility for larval tunicates and Amphioxus to swim. Efficiency increased in fishes but became smaller or vestigial in later lineages 23-20
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History Ancestry and Evolution Earliest protochordates were soft-bodied and would not have left many fossils Most work has been conducted on developmental stages where early features are conserved A theory that chordates evolved within the protostome lineage was discarded due to embryonic evidence Deuterostomes are a natural grouping that have a common origin in Precambrian seas 23-22
Ancestry and Evolution Anatomical, developmental, and molecular evidence indicate that chordates arose about 570 million years ago from a lineage related to echinoderms and hemichordates Molecular data suggest that a clade containing both echinoderms and hemichordates is the sister group of chordates 23-23
Subphylum Urochordata: Tunicata Diversity Approximately 3000 species of tunicates identified Occur in all seas and at all depth Most are sessile as adults although a few are free-living Tunic is a tough, nonliving test that surrounds them and contains cellulose In most species, only the larval form bears all the chordate hallmarks Adults lose many of these characters 23-24
Subphylum Urochordata: Tunicata During adult metamorphosis Notochord and tail disappear Dorsal nerve cord is reduced Urochordata is divided into 3 classes Ascidiacea Appendicularia Thaliacea 23-25
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Subphylum Urochordata: Tunicata Form and Function of Ascidians Called sea squirts because they discharge a jet of water when disturbed Most attached to rocks or pilings as adults Among most abundant intertidal animals Colonial and solitary ascidians have their own test Compound forms share a common test In some compound ascidians Each has own incurrent siphon but share the excurrent siphon 23-28
Subphylum Urochordata: Tunicata Mantle lines the tunic Incurrent or oral siphon marks anterior side Excurrent or atrial siphon marks dorsal side Water entering the incurrent siphon passes through a ciliated perforated pharynx with an elaborate basketwork Water flows though slits into atrial cavity and out through excurrent siphon Feeding depends on the formation of a mucous net that is secreted by the endostyle 23-29
Subphylum Urochordata: Tunicata Cilia on gill bars of pharynx pull mucus into a sheet Particles trapped in sheet are worked into a rope and carried back to the esophagus and stomach Heart drives blood first in one direction, then in reverse Organisms also concentrate very rare elements, such as vanadium, in dramatically high concentrations Nervous system has one nerve ganglion and a plexus of nerves on dorsal side of pharynx 23-30
Subphylum Urochordata: Tunicata Subneural gland samples incoming water and may have an endocrine function Hermaphroditic with a single ovary and a single testes Fertilization is external Adult sea squirts Retain 1 of the 5 chordate features: pharyngeal slits Tadpole larvae Have all 5 chordate characteristics Larva does not feed, but swims awhile before attaching and developing into a sessile adult 23-31
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Subphylum Urochordata: Tunicata Form and Function of Thalacians Salps are pelagic with a lemon-shaped, transparent body Pump water through body by muscular contraction rather than ciliary action Alternate sexual and asexual generations Increase in number rapidly with abundant food supply 23-33
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Subphylum Urochordata: Tunicata Form and Function of Appendicularian Resemble the larval stages of other tunicates Each builds a delicate hollow sphere of mucus interlaced with passages for water entry Phytoplankton and bacteria trapped on a feeding filter inside sphere are drawn into mouth through a tube When filters become clogged with wastes, they are left behind and a new sphere is built Paedomorphic Sexually mature individuals that retain the larval body form of ancestors 23-35
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Diversity Lancelets 23-37 Subphylum Cephalochordata Slender, laterally flattened, translucent animals about 5 7 cm long Live in sandy bottoms of coastal waters around the world Originally bore the generic name Amphioxus, but by priority are now in the genus Branchiostoma Still referred to by general name, amphioxous About 25 species of amphioxus are described 5 occur in North American coastal waters
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Subphylum Cephalochordata Form and Function Amphioxus has the five distinctive characteristics of chordates in simple form Water enters the mouth driven by cilia in the buccal cavity and pharynx Water passes through pharyngeal slits where food is trapped in mucus secreted by the endostyle Food is moved through gut via cilia concentrated in areas called the ileocolic ring 23-39
Subphylum Cephalochordata Food particles separated from mucus are passed into hepatic cecum where they are phagocytized Filtered water leaves body by an atriopore Closed circulatory system is complex but lacks a heart Blood is pumped by peristaltic contractions in ventral aorta, passes upward through branchial arteries in pharyngeal bars to paired dorsal aortas Blood moves by microcirculation through tissues and returns to ventral aorta 23-40
Subphylum Cephalochordata Blood lacks erythrocytes and hemoglobin and mainly transports nutrients Hollow nerve cord lies above the notochord Pairs of spinal nerve roots emerge at each trunk segment Sense organs are simple, including an unpaired ocellus that functions as a photoreceptor Anterior nerve cord is not enlarged, yet is homologous to vertebrate brain 23-41
Subphylum Cephalochordata Reproduction Sexes are separate Gametes are set free in the atrium and pass out through atriopore Fertilization is external Cleavage is holoblastic and a gastrula forms by invagination Larvae soon hatch and gradually assume the shape of adults 23-42
Basic Plan Amphioxus possesses features that suggest the vertebrate plan Cecum is a diverticulum resembling the vertebrate pancreas in secreting digestive enzymes Trunk muscles resemble vertebrate patterns Possess basic circulatory plan of more advanced chordates Many zoologists consider amphioxus a living descendant of ancestors that gave rise to both cephalochordates and vertebrates Would make them the living sister group of the vertebrates 23-43 Subphylum Cephalochordata
Subphylum Vertebrate (Craniata) Adaptations That Guided Vertebrate Evolution Earliest vertebrates Were substantially larger than protochordates Considerably more active than protochordates Characterized by increased speed and mobility resulting from modifications of skeletal structures and muscles Higher activity level and size of vertebrates Requires structures specialized in the location, capture, and digestion of food and adaptations designed to support a high metabolic rate 23-44
Subphylum Vertebrate (Craniata) Musculoskeletal Modifications Most vertebrates possess both an exoskeleton and endoskeleton of cartilage or bone Endoskeleton permits almost unlimited body size with much greater economy of building materials Endoskeleton forms excellent jointed scaffolding for attachment of segmented muscles 23-45
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Subphylum Vertebrate (Craniata) Segmented body muscles (myomeres) changed from V-shaped muscles of cephalochordates to W-shaped muscles of vertebrates Also unique to vertebrates are presence of fin rays of dermal origin in fins, aiding in swimming Endoskeleton probably composed initially of cartilage and later gave way to bone 23-47
Subphylum Vertebrate (Craniata) Endoskeleton of living hagfishes, lampreys, sharks and their kin, and even some bony fishes, such as sturgeons, mostly composed of cartilage Structural strength of bone is superior to cartilage Makes it ideal for muscle attachment in areas of high mechanical stress Perhaps bone evolved, in part, as a means of minereal regulation Phosphorus and calcium are used for many physiological processes In particularly high demand in organisms with high metabolic rates 23-48
Subphylum Vertebrate (Craniata) Some of the most primitive fishes, including Ostracoderms and placoderms were partly covered in a bony, dermal armor Modified in later fishes as scales Many of the bones encasing brain of advanced vertebrates develop from cells that originate from the dermis Most vertebrates are protected with keratinized structures derived from the epidermis Reptilian scales, hair, feathers, claws, and horns 23-49
Subphylum Vertebrate (Craniata) Physiology Modifications of digestive, respiratory, circulatory, and excretory systems to meet increased metabolic demand Perforated pharynx evolved as a filterfeeding device in early chordates Water with suspended food particles was drawn through the pharynx by ciliary action and trapped by mucus secreted by the endostyle 23-50
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Subphylum Vertebrate (Craniata) In larger, predatory vertebrates, pharynx was modified into a muscular apparatus that pumped water through pharynx With the origin of highly vascularized gills, function of pharynx shifted to primarily gas exchange To manage increased ingestion of food Gut shifted from movement of food by ciliary action to muscular action Accessory digestive glands, the liver and pancreas, produced secretions that aided digestion 23-52
Subphylum Vertebrate (Craniata) Transport of nutrients gases, and other substances was enhanced by Ventral 3-chambered heart Sinus venosus Atrium Ventricle Erythrocytes containing hemoglobin Vertebrates possess paired, glomerular kidneys to remove metabolic waste products and regulated body fluid composition 23-53
New Head, Brain, and Sensory Systems Shift from filter feeding to active predation Required new sensory, motor, and integrative controls for location and capture of larger prey Anterior end of nerve cord enlarged as a tripartite brain Forebrain, midbrain, and hindbrain Brain was protected by cartilaginous or bony cranium Paired special sense organs for vision, equilibrium, and sound evolved Other receptors that evolved Mechanoreceptors, chemoreceptors, electroreceptors, and olfactory receptors 23-54 Subphylum Vertebrate (Craniata)
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Subphylum Vertebrate (Craniata) Neural Crest, Ectodermal Placodes, and Hox Genes Development of vertebrate head and special sense organs Largely result of two embryonic innovations present only in vertebrates Neural crest and ectodermal placodes 23-56
Subphylum Vertebrate (Craniata) Neural crest Derived from a population of ectodermal cells lying along length of the embryonic neural tube Contributes to formation of many different structures Most of the cranium, pharyngeal skeleton, tooth dentine, some cranial nerves, ganglia, Schwann cells, and some endocrine glands May also regulate development of adjacent tissue, such as tooth enamel and pharyngeal muscles (branchiomeres) 23-57
Subphylum Vertebrate (Craniata) Ectodermal placodes Plate-like ectodermal thickenings appearing on either side of neural tube Give rise to olfactory epithelium, lens of eye, inner ear epithelium, some ganglia, some cranial nerves, lateral-line mechanoreceptors, and electroreceptors Placodes also induce formation of taste buds Vertebrate head with sensory structures located adjacent to mouth (later equipped with preycapturing jaws) stemmed from the creation of new cell types 23-58
Subphylum Vertebrate (Craniata) Hox Genes Recent studies of the distribution of homeoboxcontaining genes that control the body plan of chordate embryos suggest that the Hox genes were duplicated at about the time of the origin of vertebrates One copy of Hox genes is found in Amphioxus and other invertebrates Living gnathostomes have 4 copies Perhaps additional copies of genes that control body plan provided genetic material free to evolve a more complex kind of animal 23-59
Subphylum Vertebrate (Craniata) The Search for the Vertebrate Ancestral Stock Jawless ostracoderms from the early Paleozoic vertebrate fossil record Share organ system development with living vertebrates Indicates organ systems must have originated in early vertebrate and invertebrate lineage 23-60
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Subphylum Vertebrate (Craniata) Haikouella lanceolata Small fishlike creature known from over 300 fossil specimens provides a wealth of information on evolution of vertebrates Possessed a notochord, pharynx, and dorsal nerve cord Had pharyngeal muscles, paired eyes, and an enlarged brain Not a vertebrate because it lacks distinctive vertebrate characteristics such as a cranium, an ear, and telencephalon (anterior lobe of the brain) 23-62
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Subphylum Vertebrate (Craniata) It is transitional in morphology between cephalochordates and vertebrates Some hypothesize Haikouella is the sister taxon of vertebrates 23-64
Evolutionary History Garstang s Hypothesis of Chordate Larval Evolution Chordates pursued 2 paths in early evolution 1 st path led to the sedentary urochordates The 2 nd to active, mobile cephalochordates and vertebrates In 1928, Walter Garstang of England, suggested the chordate ancestral lineage retained into adulthood the larval form of sessile tunicate-like animals Paedomorphosis: Evolutionary retention of larval traits in an adult body Occurs in some amphibians 23-65
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Evolutionary History Garstang s hypothesis has been challenged recently Molecular data along with information from fossils suggests Ancestor of deuterostomes was free-swimming The sessile ascidians represent a derived body form Free-swimming appendicularians are most similar in body form to ancestral chordates 23-67
23-68 Evolutionary History Position of Amphioxus Long been considered the closest living relative to earliest vertebrates Now not considered a direct ancestor although it may closely resemble ancestor Lacks a brain and specialized sensory equipment of vertebrates No gills in the pharynx and no mouth for pumping water Recent studies of homeobox containing genes Suggest that ancestor of both amphioxus and vertebrates was cephalized
Evolutionary History Many zoologists still consider cephalochordates closest living relative of vertebrates However, Amphioxus is unlike the most recent common ancestor of vertebrates Lacks the tripartite brain, chambered heart, special sensory organs, muscular gut and pharynx, and neural crest tissue inferred to have been present in ancestor Larger fins of some extinct cephalochordates suggest they were more free-swimming than modern Amphioxus 23-69
Evolutionary History Ammocoete Larvae of Lampreys as a Model of the Primitive Vertebrate Body Plan Lampreys have a larval stage, ammocoete, that closely resembles the amphioxus Ammocoete larvae were originally considered petromyzontidan adults Ammocoete larval mouth resembles the amphioxus but draws water in by muscular pumping Endostyle, mucus, body muscles, notochord and circulatory system closely resemble amphioxus 23-70
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Evolutionary History Ammocoetes equipped with 2-chambered heart 3-part brain Median nostril Auditory vesicles Thyroid and pituitary gland Extensive pharyngeal filaments (serve in respiration) True liver, gallbladder, and pancreatic tissue Ammocoete larva Has the most primitive condition of this set of vertebrate structures 23-72
Evolutionary History The Earliest Vertebrates Ostracoderms Until recently, earliest known vertebrate fossils were armored jawless fishes called ostracoderms Found in the late Cambrian deposits in United States, Bolivia and Australia Small, heavily armored, jawless, and lacked paired fins During the last 10 years, several 530-million-yearold fossils were discovered in the Chengjiang deposits belonging to one or two fishlike vertebrates Myllokunmingia and Haikouichthys 23-73
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Evolutionary History These fossils push back the origin of vertebrates to at least the early Cambrian Fossils showed many vertebrate characters including a heart, paired eyes, otic capsules, and rudimentary vertebrae Earliest Ostracoderms Equipped with bone in dermis Lacked paired fins that later fishes used for stability Not considered to be a natural evolutionary assemblage, but a convenience for describing several groups of heavily armored extinct jawless fishes, such as the heterostracans 23-75
Evolutionary History Heterostracans Early group of ostracoderms Represent an awkward design that probably filtered particles from the ocean bottom Sucked in water by muscular pumping Some believe they may have been able to feed on soft-bodied animals Devonian saw a major radiation of heterostracans producing numerous peculiar-looking forms, never evolved jaws or paired fins Extinct near end of Devonian 23-76
Evolutionary History Osteostracans Coexisted with heterostracans Developed paired pectoral fins that stabilized movement Jawless, toothless mouth Sensory lateral line, paired eyes, and inner ears with semicircular canals Head was well armored, but lacked axial skeleton or vertebrae A typical osteostracan was Cephalaspis Small marine animal covered with a heavy, dermal armor of cellular bone, including a single-piece head shield Likely had a sophisticated nervous system and sense organs, similar to those of modern lampreys 23-77
Evolutionary History Anapsids More streamlined than other Ostracoderms Impressive radiation in Silurian and Devonian periods Streamlined and more closely resembled modern lamprey All Ostracoderms became extinct by end of Devonian period For decades strange microscopic tooth-like fossils called conodonts have been used to date paleozoic marine sediments without having any idea what kind of creature originally possessed these elements 23-78
Evolutionary History Complete conodont animals have been discovered (early 1980s) Phosphatized toothlike elements, W-shaped myomeres, cranium, notochord, and paired eye and otic capsules, conodonts clearly indicate they belong to vertebrate clade Exact position is unclear 23-79
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Evolutionary History Early Jawed Vertebrates Gnathostomes All living and extinct jawed vertebrates Living agnathans (jawless vertebrates), the lampreys and hagfishes, often called cyclostomes Gnathostomes constitute a monophyletic group Presence of jaws is a derived character state shared by all jawed fishes and tetrapods Agnathans, defined by the absence of jaws, is paraphyletic 23-81
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Evolutionary History Evidence indicates jaws arose by modification of 1 st two cartilaginous gill arches Both gill arches and jaws form from upper and lower bars that bend forward and are hinged Both are derived from neural crest cells rather than from mesodermal tissue as are most bones Jaw musculature is homologous to the musculature that originally supported gills 23-83
Evolutionary History Mandibular arch may have first become enlarged to assist gill ventilation Perhaps to meet increasing metabolic demands of early vertebrates Placoderms appeared in the early Devonian and were heavily armored Acanthodians are included in a clade that underwent a great radiation into bony fishes that dominate waters today 23-84
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