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This document contains Chapters 22 to 23 CHAPTER 22 CHAETOGNATHS, ECHINODERMS, AND HEMICHORDATES CHAPTER OUTLINE 22.1. Phylum Chaetognatha A. Diversity (Figures 22.1, 22.2) 1. Chaetognatha’s evolutionary position is still much debated. 2. We depict Chaetognatha outside the protostome and deuterostome clades pending new studies. 3. They are pelagic marine predators commonly called arrow worms. 4. Arrow worms are highly specialized for their planktonic existence, although there are several benthic genera. 5. Horizontal fins bordering the trunk are used in flotation rather than in active swimming. B. Form and Function (Figure 22.2) 1. The body is unsegmented and includes a head, trunk, and postanal tail. 2. The head is a large vestibule with teeth flanked by chitinous spines for seizing prey. 3. Arrow worms are voracious feeders, living on planktonic forms, especially copepods. 4. A thin cuticle covers the body and the epidermis is single layered. 5. Arrow worms have a complete digestive system, a well-developed coelom, with a nervous system with a nerve ring. 6. The ciliary loop may detect water currents or may be chemosensory. 7. Vascular, respiratory, and excretory systems are entirely lacking. 8. Arrow worms are hermaphroditic with either cross- or self-fertilization. 9. Chaetognath embryogenesis suggests deuterostomes affinitities; however, the coelom is formed by a backward extension from the archenteron rather than by pinched-off coelomic sacs. 10. New studies indicate that cleavage planes in four-cell embryos (similar to crustaceans and nemotodes). 11. Some phylogenies based on nucleotide sequences place chaetognaths within Ecdysozoa, but there are no reports of molting of the thin cuticle. 22.2 Phylum Xenoturbellida A. Characteristics 1. Live in North Sea mud. 2. Two external furrows, one lateral, one running length of body, believed to be sensory. 3. No cephalization. 4. Mouth found at center of ventral surface; feed on bivalves and bivalve eggs. 5. Grouped within Ambulacraria due to similar ciliary structures and diffuse nervous system. 6. If Xenoturbellida is deuterstome, there is assumption that many ancestral characteristics have been lost within this group. 22.3 Clade Ambulacraria A. Ambulacraria is a superphylum that contains two deuterostome phyla: Echinodermata and Hemichordata. B. Members share a three-part (tripartite) coelom, similar larval forms, and an axial complex. C. Body form 1. It has a blind gut: coelom and excretory structures are absent. 2. The body is not cephalized, but does have a diffuse netlike nervous system. 3. Muscles are present. 4. There are no structured gonads, but sexual reproduction occurs. 22.4 Phylum Echinodermata (Figure 22.3) A. Characteristics 1. All members of the phylum have a calcareous skeleton. 2. The spiny endoskeleton consists of plates. 3. They have a unique water-vascular system. 4. They possess pedicellariae and dermal branchiae. 5. They have pentaradial symmetry in adults. B. Diversity 1. They are an ancient group extending back to the Cambrian period. 2. They likely descended from bilateral ancestors; their larvae are bilateral. 3. One theory is that they evolved radiality as an adaptation to sessile existence. 4. The body plan is derived from crinoid-like ancestors that became free-moving descendants later. 5. They lack ability to osmoregulate and this restricts them to marine environments. 6. No parasitic echinoderms are known; a few are commensals. 7. Asteroids or sea stars are mostly predators. (Figure 22.4) 8. Ophiuroids or brittle stars move by bending their jointed muscular arms and may be scavengers, browsers or commensal. (Figures 22.14, 22.17) 9. Holothurians or sea cucumbers are mostly suspension or deposit feeders. (Figure 22.24) 10. Echinoids or sea urchins are found on hard bottoms while sand dollars prefer sand substrate; they feed on detritus. (Figures 22.18-23) 11. Crinoids are sessile and flower-like as young and detach as adults; they are suspension feeders. (Figure 22.28) C. Ecology, Economics, and Research 1. Due to their spiny structure, echinoderms are not often preyed upon. 2. A few fish and otters are adapted to feed on sea urchins. 3. Humans sometimes eat the sea urchin gonads and the body wall of certain holothurians. 4. Sea stars feed on molluscs, crustaceans and other invertebrates; they may damage oyster beds. 5. The embryology of sea urchin eggs is very observable. 6. Artificial parthenogenesis was first described for sea urchin eggs; they develop without fertilization if treated with hypertonic seawater or subjected to other stimuli. 22.5. Class Asteroidea A. Diversity 1. Sea stars are common along shorelines and may aggregate on rocks. 2. Some sea stars live on muddy or sandy bottoms, or among coral reefs. 3. There are about 1500 living species. 4. They range from a centimeter across to about a meter across and may be brightly colored. 5. Asterias is common on the east coast of the U.S.; Pisaster is common on the west coast. B. Form and Function (Figures 22.4–22.12) 1. External Features (Figures 22.4-22.7) a. Sea stars have a central disc with tapering arms extending outward. b. The body is flattened and flexible, with a pigmented and ciliated epidermis. c. The mouth is on the underside or oral side. d. The ambulacrum runs from the mouth to the tip of each arm. e. Usually there are five arms but there may be more. f. The ambulacral groove is bordered by rows of tube feet. g. A large radial nerve is in the center of each ambulacral groove. h. Under the nerve is an extension of the coelom and the radial canal of the water-vascular system. i. In all other cases except crinoids, ossicles or other dermal tissue covers these structures. j. The aboral surface is spiny; at the base of the spines are groups of pincer-like pedicellariae. k. Pedicellariae keep the body surface free of debris. l. Papulae (dermal branchiae or skin gills) are soft projections lined with peritoneum and serve in respiration. m. On the aboral side is a circular madreporite that is a sieve leading to the water-vascular system. 2. Endoskeleton a. Under the epidermis is the mesodermal endoskeleton of small calcareous plates or ossicles. b. The plates are bound together by an unusual form of mutable collagen termed catch collagen, which is under neurological control. c. Ossicles are penetrated by a meshwork of spaces filled with fibers and dermal cells, the steroem. d. Muscles in the body wall move the rays and partially close the ambulacral grooves. 3. Coelom, Excretion, and Respiration a. The spacious body coelom filled with fluid is one coelomic compartment. b. The fluid contains amebocytes (coelomocytes). c. Ciliated peritoneal lining of the coelom circulates the fluid around the cavity and into papulae. d. Respiratory gases and nitrogenous waste ammonia diffuse across the papulae and tube feet. e. Some wastes are picked up by coelomocytes, which migrate to the exterior. 4. Water-Vascular System (Figures 22.5, 22.6, 22.8) a. This system is another coelomic compartment and is unique to echinoderms. b. It consists of a system of canals, tube feet and dermal ossicles. c. This system functions in locomotion and food-gathering as well as respiration and excretion. d. The system opens to the outside at the madreporite on the aboral side. (Figures 22.5 and 22.6) e. The madreporite leads to the stone canal, which joins the ring canal that encircles the mouth. f. Radial canals diverge from the ring canal and extend into each ray. g. 4-5 pairs of Tiedemann’s bodies attach to the ring canal and may produce coelomocytes. h. Polian vesicles may also be attached; they serve for fluid storage and regulation of internal pressure of the water vascular system. i. The inner end of each tube foot or podium is an ampulla that lies within the body coelom. j. The outer end of each tube foot bears a sucker. k. The water-vascular system operates hydraulically; valves in lateral canals prevent backflow. l. Muscles in the ampulla contract forcing fluid into and extending the podium. m. Small lateral canals, each with a one-way valve, connect the radial canal to the tube feet. n. Contraction of longitudinal muscles in the tube foot retracts it, forcing fluid back into ampulla. o. Small muscles in the end of the tube foot raise the middle of the end, creating suction. p. The sea star can move while being firmly adhered to the substrate. q. Tube feet are innervated by central nervous system; they move in one direction but not in unison. r. Cutting a radial nerve ends coordination in one arm; cutting a circumoral nerve ring stops all movement. 5. Feeding and Digestive System (Figures 22.8-22.9) a. The mouth on the oral side leads through a short esophagus to a large central stomach. b. The lower cardiac part of the stomach can be everted through the mouth during feeding. c. The upper stomach is smaller and is connected by ducts to a pair of pyloric ceca in each arm. d. The anus is inconspicuous and empties on the center at the top; some lack an intestine and anus. e. Sea stars consume a wide range of food; some eat sea urchins and regurgitate undigestible parts. f. Some feed on molluscs; they pull steadily until they can insert a stomach through the crack. g. Some sea stars feed on small particles that are carried up ambulacral grooves to the mouth. 6. Hemal System (Figure 22.9) a. This system of tissue strands encloses unlined sinuses; it is itself enclosed in perihemal channels. b. Hemal system appears to play a role in distributing nutrients. 7. Nervous System a. The oral system of a nerve ring and radial nerves coordinate the tube feet. b. A deep hyponeural system aboral to the oral system forms a ring around the anus and extends into the roof of each ray. c. The epidermal nerve plexus coordinates responses of the dermal branchiae to tactile stimulation. d. Tactile organs are scattered over the surface and an ocellus is at the tip of each arm. e. They react to touch, temperature, chemicals and light intensity; they are mainly active at night. 8. Reproductive System, Regeneration, and Autonomy (Figures 22.6A, 22.10) a. Most have separate sexes; a pair of gonads is in each interradial space. b. Fertilization is external; in early summer, eggs and sperm are shed into the water. c. Echinoderms also regenerate lost parts; they can cast off injured arms and regenerate new ones. d. An arm can regenerate a new sea star if at least one-fifth of the central disc is present. 9. Development (Figures 22.11, 22.12) a. Development is quite different among different sea star lineages. b. In most cases, embryonating eggs are dispersed in the water and hatch to free-swimming larvae. c. Embryogenesis shows a typical primitive deuterostome pattern. d. Coelomic compartments, called somatocoels, arise from legs of the posterior blastocoel. e. The left hydrocoel becomes the water-vascular system; the left axocoel becomes the stone canal and perihemal channels. f. The free-swimming larva has cilia arranged in bands and is called a bipinnaria. g. Ciliated tracts become larval arms. h. When the larva grows three adhesive arms and a sucker at the anterior, it is called a brachiolaria. i. A brachiolaria then attaches to the substrate and undergoes metamorphosis into a radial juvenile. j. As its arms and tube feet appear, the animal detaches from its stalk and becomes a young sea star. 10. Sea Daisies (Figure 22.13) a. These small, disc-shaped animals were discovered in deep water off New Zealand. b. Described in 1986, only two species are known. c. Phylogenetic analysis of rDNA places them within Asteroidea. d. They are pentaradial but have no arms. e. Tube feet are located around the periphery of the disc rather than in ambulacral areas. f. The water-vascular system has an outer ring and a hydropore homologous to the madreporite that connects the inner ring canal to the aboral surface. g. One species has a shallow, sac-like stomach. h. The other species has no digestive tract; a velum covers the oral surface and absorbs nutrients. 22.6. Class Ophiuroidea (Figures 22.14–22.17) A. Form and Function (Figures 22.14-- 22.16) 1. This group is largest in number (> 2000) of species and probably in abundance. 2. The arms of the brittle stars are slender and distinct from the central disc. 3. They lack pedicellariae or papulae and the ambulacral groove is closed and coated with ossicles. 4. Tube feet lack suckers. 5. The madreporite is on the oral surface. 6. The tube feet lack ampullae; protrusion is generated by proximal muscles. 7. Each jointed arm has a column of articulated ossicles called vertebrae. 8. Arms are moved in pairs for locomotion. 9. Five movable plates act as jaws and surround the mouth; there is no anus. 10. Skin is leathery and surface cilia are mostly lacking. 11. Visceral organs are all in the central disc; the arms are too slender to accomodate them. 12. The stomach is saclike; there is no intestine. 13. The water-vascular, nervous and hemal systems resemble those of sea stars. 14. Five invaginations called bursae open to the oral surface by genital slits at the bases of the arms. 15. Gonads on the wall of each bursa discharge ripe sex cells into the water for external fertilization. 16. Sexes are usually separate but a few are hermaphroditic. 17. The larva is an ophiopluteus. 18. The larva has ciliated bands that extend onto delicate and beautiful larval arms. 19. In contrast to sea stars, they lack any attached phases during metamorphosis. 20. Regeneration and autotomy are more pronounced than in sea stars; they are very fragile. C. Behavior and Ecology 1. Brittle stars are secretive and live on hard or sandy bottoms where little light penetrates, often under rocks or in kelp holdfasts. 2. They browse on food or suspension feed. 3. Basket stars perch on corals and extend their branched arms to capture plankton. 22.7. Class Echinoidea (Figures 22.18–22.23) A. Diversity 1. There are roughly 950 species of living echinoids. 2. Sea urchins lack arms but their tests show the five-part symmetry. 3. The up-folding brings the ambulacral areas up to the area of the anus. 4. Most sea urchins have a hemispherical shape with radial symmetry and long spines. 5. Sand dollars and heart urchins (irregular echinoids) have become bilateral with short spines. 6. Regular urchins move by tube feet; irregular urchins move by their spines. 7. Echinoids occur from intertidal regions to deep ocean. B. Form and Function 1. The echinoid test has ten double rows of plates with movable, stiff spines. 2. The tube feet extend along the five ambulacral rows. 3. The spines articulate on “ball-and-socket” joints moved by small muscles at the bases. 4. Among the several kinds of pedicellaria, the three-jawed variety on long stalks is most common. 5. Some species have pedicellariae with poison glands that secrete a toxin that paralyzes small prey. 6. Five converging teeth and sometimes branched gills encircle the peristome. 7. The anus, genital pores, and madreporite are aboral and in the periproct region. 8. Sand dollars and heart urchins have shifted the anus to the posterior and can be defined bilaterally. 9. Inside the test is Aristotle’s lantern, a complex set of chewing structures. 10. A ciliated siphon connects the esophagus to the intestine; food can be concentrated in the intestine. 11. Sea urchins are largely omnivorous, but their primary diet consists mainly of algae; sand dollars filter particles through their spines. 12. Hemal and nervous systems resemble those in asteroids. 13. Ambulacral grooves are closed and radial canals run just beneath the test in each radii. 14. In irregular urchins, respiratory podia are arranged in fields called petaloids on the aboral surface. 15. Sexes are separate; both eggs and sperm are shed into the sea for external fertilization. 16. Some, including pencil urchins, brood young in depressions between the spines. 17. Echinopluteus larvae of nonbrooding echinoids live a planktonic existence before becoming urchins. 22.8. Class Holothuroidea (Figures 22.24–22.27) A. Diversity 1. There are approximately 1150 species of holothuroids. 2. As their common name suggests, these animals resemble cucumbers. 3. They are greatly elongated in the oral-aboral axis. 4. Ossicles are greatly reduced and the body is soft. 5. Some species crawl on the ocean bottom, others are found under rocks or burrow. B. Form and Function 1. The body wall is leathery with tiny ossicles buried in it; a few have dermal armor. 2. In some, locomotor tube feet are distributed to all five ambulacral areas; most have them only on the ambulacra that faces the substratum. 3. The side that faces the substratum (the sole) has three ambulacra, adding a secondary bilaterality. 4. All tube feet, except oral tentacles, are absent in burrowing forms. 5. Oral tentacles are 10–30 tube feet surrounding the mouth. 6. The coelomic cavity has many coelomocytes. 7. The digestive system opens into a cloaca; a respiratory tree also empties into the cloaca. 8. A madreporite lies free in the coelom; the hemal system is more developed than in other echinoderms. 9. The respiratory tree also serves for excretion; gas exchange also occurs through the skin and tube feet. 10. Sexes are separate but some are hermaphroditic. 11. Sea cucumbers have a single gonad; this is considered a primitive character. 12. Fertilization is external and produces free-swimming auricularia. 13. A few brood their young inside the body or on the body surface. D. Behavior and Ecology (Figure 22.27) 1. Sea cucumbers use both ventral tube feet and muscular body waves to move. 2. Some trap particles on the mucus of their tentacles, ingesting food particles in their pharynx. 3. Others graze the sea bottom with their tentacles. 4. Sea cucumbers cast out part of their viscera when irritated; they must regenerate these tissues. 5. The organs of Cuvier are expelled in the direction of an enemy; they are sticky and have toxins. 6. One small fish, Carapus, uses the cloaca and respiratory tree of a sea cucumber for shelter. 22.9. Class Crinoidea (Figures 22.28, 22.29) A. Diversity 1. Crinoids include both sea lilies and feather stars; they have primitive characters. 2. Crinoids are far more numerous in the fossil record. 3. They are unique in being attached for most of their life. 4. Sea lilies have a flower-shaped body at the tip of a stalk. 5. Feather stars have long, many-branched arms; adults are free-moving but may be sessile. 6. Many crinoids are deep-water species; feather stars are found in more shallow water. B. Form and Function 1. The body disc or calyx is covered with a leathery skin or tegument of calcareous plates. 2. The five arms branch to form more arms, each with lateral pinnules as in a feather. 3. The calyx and arms form a crown. 4. Sessile forms have a stalk formed of plates; it appears jointed and may bear cirri. 5. A madreporite, spines and pedicellariae are absent. 6. The upper surface has a mouth that opens into an esophagus and intestine; it then exits the anus. 7. Tube feet and mucous nets allow it to feed on small organisms in the ambulacral grooves. 8. It has a water-vascular system, an oral ring and a radial nerve to each arm. 9. Sexes are separate; gonads are merely masses of cells in the genital cavity of the arms and pinnules. 10. Gametes escape through ruptures in the pinnule wall; some brood their eggs. 11. Doliolaria larvae are free-swimming before they become attached and metamorphose. 12. Most living crinoids are 15–30 centimeters long; some fossil species had stalks 20 meters long. 22.10. Phylogeny and Adaptive Diversification (Figure 22.30) A. Phylogeny 1. The fossil record is extensive but there are still many theories about their evolution. 2. From the larvae, we know the ancestor was bilateral and the coelom had three pairs of spaces. 3. Possibly sessile groups derived independently from free-moving adults with radial symmetry. 4. Traditionally, first echinoderms believed to be sessile and radial, giving rise to free-swimming forms. 5. Early forms may have had endoskeletal plates with stereom structure and external ciliary grooves. 6. Carpoids may be an extinct variation, or a separate subphylum. 7. Echinoids and holothuroids are related; the relationship of ophiuroids and asteroids is controversial. B. Adaptive Diversification 1. If their ancestors had a brain and sense organs, these were lost in adoption of radial symmetry. 2. Current evidence suggests that the oral surface is anterior and the aboral surface is posterior; this indicates the arms represent lateral growth zones. 3. The basic body plan has limited their evolutionary opportunities to become parasites. 4. Only the most mobile ophiuroids have any commensal species. C. Classification Subphylum Pelmatozoa Class Crinoidea Subphylum Eleutherozoa Class Asteroidea Class Ophiuroidea Class Echinoidea Class Holothuroidea 22.11. Phylum Hemichordata A. Diversity and Characteristics 1. Formerly considered a subphylum of chordates based on their possession of gill slits and a rudimentary notochord. 2. The “notochord” is really a buccal diverticulum, a stomochord, and not homologous to chordate notochord. 3. Hemichordates are vermiform bottom dwellers, living in shallow waters; most are sedentary or sessile. 4. Members of Class Enteropneusta are called acorn worms. 5. Members of Class Pterobranchia are smaller, usually 1 to 12 mm. B. Class Enteropneusta (Figures 22.31–22.33) 1. Form and Function a. The wormlike acorn worms have a mucus-covered body and an active proboscis that collects food in mucous strands. b. Cilia carry particles to the groove at the edge of the collar, then to the mouth. c. They thrust the proboscis into the mud and ingest mud to extract the organic matter. d. A buccal diverticulum connects the protocoel with a proboscis pore to the outside. e. Contraction of body musculature then forces the excess water out through the gill slits. f. A roll of gill pores is part of the branchial system that connects with a series of gill slits in the sides of the pharynx. g. Hemichordates are largely ciliary-mucus feeders using their U-shaped gill slits. h. Have middorsal vessel that expands into a sinus and heart vesicle above the buccal diverticulum. i. Blood enters a network of blood sinuses called glomeruli and then through an extensive system of sinuses to the gut and body wall. j. The nervous system consists mostly of a sub-epithelial plexus, reminiscent of that of cnidarians and echinoderms. k. Dorsal nerve cord (neurochord) formed by invagination of ectoderm; is hollow in some species. l. Sexes are separate; fertilization is external, and in some species a ciliated tornaria larva develops similar to that of echinoderm larva. m. At least one species undergoes asexual reproduction. C. Class Pterobranchia (Figures 22.34, 22.35) 1. Form and Function: a. The basic plan of Class Pterobranchia is similar to that of Enteropneusta. b. Pterobranchs are small animals, usually 1 to 7 mm in length. c. Many individuals may live together in collagenous tubes; however, zooids are not connected. d. The body is divided into three regions—proboscis, collar, and trunk. e. As in a lophophore, ciliated grooves on the tentacles and arms collect food. f. Some species are dioecious, others monoecious; asexual reproduction is by budding. g. The fossil graptolites of the middle Paleozoic are often placed as an extinct class under the hemichordates, and the discovery of an organism that seems to be a living graptolite suggests that this may be a controversial but correct alignment. D. Phylogeny and Adaptive Diversification (Figures 22.1, 22.3) 1. Phylogeny a. Hemichordate phylogeny is far from being completely understood, although it is known that they share characters with both echinoderms and chordates. b. With chordates they share pharnygeal slits. c. The Ambulacraria hypothesis unites echinoderms and hemichordates on the basis of a shared diffuse epidermal nervous system. d. Another phylogenetically important character is the shared tripartite coelom in hemichordates and echinoderms. e. The buccal diverticulum is now believed to be a synapomorphy of hemichordates only. f. Early embryogenesis of hemichordates is remarkably like that of echinoderms suggesting that echinoderms are a sister group of hemichordates. g. The early tornaria larva is almost identical to the bipinnaria larva of asteroids. h. Sequence analysis of the gene encoding the small subunit of rRNA supports a deuterostome clade (Echinodermata, Hemichordata, and Chordata). i. Sequence analysis of the gene encoding small-subunit rRNA supports placement of Chaetognaths among protostomes; however, it is possible that Chaetognaths are neither protostomes nor deuterostomes but originated independently from an early coelomate lineage. 2. Adaptive Diversification a. Possibly because of their sedentary lifestyles, pterobranchs have undergone little adaptive divergence and have retained a tentacular type of ciliary feeding. b. Enteropneusts, although sluggish, are more active than pterobranchs, having lost their tentaculated arms, use a proboscis to trap small organisms in mucus, and in the process have diversified only slightly more. c. Molecular evidence suggests pterobranchs are derived from within the enteropneust lineage. Lecture Enrichment 1. Sea stars, sea urchins, fossil crinoids and some other specimens are hard and durable and readily passed around to provide a meaningful feel of their nature. 2. Ask a student who has experience with farm or factory equipment to explain how a “hydraulic” system works based on fluid pressure. 3. The size and color of many asteroids and the strangeness of holothuroids as “cucumber animals” make this a naturally interesting group to describe in lecture. 4. While inland states lack any living echinoderms, they may have plentiful fossil crinoids in creek beds or roadcuts. 5. Echinoderms have many lead-ins to ecological stories, some of which are featured in blocks in this chapter: the decline in Diadema sea urchins and the surge in the crown-of-thorns starfish. Commentary/Lesson Plan Background: Modern echinoderms are today only marine, but are relatively common along coastlines. Coastal students may have substantial experience with them. Echinoderms will, at most, be vacation artifacts to inland students. Some students may have played with crinoids as limestone “beads” from streambeds without knowing what they were. We assume that most students will understand a “hydraulic” system but student experience with farm equipment is dramatically less than a generation ago; this may need to be explained. Misconceptions: Students are beginning to build a mental image of living tissue of invertebrates as “soft machinery” with recognizable vascular and muscular components, and any mineral shell is an external crust. Many echinoderms have a mineral- or cartilage-like internal appearance that raises doubt that they can have the same life functions as soft animals. Schedule: HOUR 1 22.1. Phylum Chaetognatha A. Diversity B. Form and Function 22.2. Clade Ambulacraria A. Diversity 22.3. Phylum Xenoturbellida A. Characteristics 22.4 Phylum Echinodermata A. Diversity B. Form and Function C. Reproduction 22.5. Class Asteroidea A. Diversity B. Form and Function C. Sea Daisies 22.6. Class Ophiuroidea A. Form and Function B. Biology 22.7. Class Echinoidea A. Diversity B. Form and Function HOUR 2 22.8. Class Holothuroidea A. Diversity B. Form and Function C. Reproduction D. Biology 22.9. Class Crinoidea A. Diversity B. Form and Function C. Reproduction 22.10. Phylogeny and Adaptive Diversification A. Phylogeny B. Adaptive Diversification C. Classification 22.11. Phlyum Hemichordata A. Diversity B. Phylogeny C. Adaptive Diversification ADVANCED CLASS QUESTIONS: 1. The incongruous appearance of echinoderms makes it difficult to associate them with soft-bodied animals that have “normal” tissues and blood. If a non-scientist observer suggests that they might have originated separate from other animal life—an “alien life form”—what evidence can you provide that they actually are a continuation of the animal lineage? Answer: While the appearance of echinoderms may indeed seem incongruous compared to other animals, particularly due to their radial symmetry and unique anatomical features, there is substantial evidence supporting their classification as part of the animal lineage rather than as separate, "alien" life forms. Here are some key pieces of evidence: 1. Molecular Phylogenetics: Molecular phylogenetic analyses, which compare genetic sequences among different organisms to infer evolutionary relationships, consistently place echinoderms within the animal kingdom. Studies examining the DNA and protein sequences of echinoderms and other animals reveal shared genetic similarities and phylogenetic relationships that place echinoderms firmly within the broader animal lineage. 2. Developmental Biology: Echinoderms undergo embryonic development that shares many features with other animals, including bilateral symmetry during early stages of development. Despite their adult radial symmetry, echinoderms exhibit bilateral symmetry in their larvae, which undergo metamorphosis into the characteristic radial body plan. This developmental process provides evidence of shared ancestry with other animals and supports their classification within the animal kingdom. 3. Fossil Record: Fossilized remains of early echinoderms and transitional forms dating back hundreds of millions of years provide evidence of their evolutionary history and gradual divergence from other animal groups. These fossils show intermediate stages in the evolution of echinoderm characteristics and demonstrate their continuity with other animals over geological time scales. 4. Anatomical Homologies: Despite their unique anatomical features, echinoderms share fundamental anatomical and physiological characteristics with other animals. For example, they possess specialized tissues, organs, and organ systems, including a digestive system, nervous system, and reproductive system, that are characteristic of animal life. Comparative anatomy reveals homologous structures between echinoderms and other animals, supporting their common ancestry. 5. Genetic Toolkit: Echinoderms possess many of the same genes and genetic regulatory networks that are conserved across the animal kingdom and play essential roles in development, growth, and physiology. Studies of gene expression patterns and developmental processes in echinoderms reveal similarities with other animals, indicating their shared genetic toolkit and evolutionary heritage. Overall, the convergence of evidence from molecular phylogenetics, developmental biology, the fossil record, anatomical homologies, and genetic studies supports the conclusion that echinoderms are a continuation of the animal lineage rather than separate "alien" life forms. While their unique appearance may be striking, echinoderms exhibit numerous similarities with other animals and share a common evolutionary history within the animal kingdom. 2. A zoological set of charts produced in the 1940s places the echinoderms immediately following the coelenterates, an older taxon including what we now recognize as cnidarians and ctenopores. Now we are discussing them at the end of the invertebrates and just before considering the chordates. Why might they have been considered more primitive a half century ago? This includes both issues of radial symmetry and a sessile lifestyle similar to that of coral. Why do we now place them in a much more “advanced” position? Answer: The shifting perceptions of echinoderms from being considered more primitive to being placed in a more "advanced" position in the classification hierarchy over the past half-century can be attributed to advancements in our understanding of their evolutionary relationships, developmental biology, and ecological roles. Here's why echinoderms were once considered more primitive and why they are now recognized as occupying a more advanced position: 1. Perception of Radial Symmetry: In the past, the radial symmetry exhibited by echinoderms may have led to their classification as more primitive organisms. Radial symmetry was traditionally associated with simpler body plans and was seen as a characteristic of more primitive animals. As a result, echinoderms, with their distinctive radial symmetry, were often grouped with other organisms exhibiting similar features, such as coelenterates (cnidarians and ctenophores). 2. Sessile Lifestyle: Echinoderms include both sessile (stationary) and motile (moving) species, with some echinoderms, such as sea stars and sea cucumbers, being more mobile while others, like sea lilies and sea urchins, are sessile or have limited mobility. The sessile lifestyle of some echinoderms may have contributed to their perception as more primitive organisms, similar to coral and other sedentary animals. 3. Advancements in Evolutionary Biology: Over the past few decades, advancements in evolutionary biology, including molecular phylogenetics and comparative genomics, have provided new insights into the evolutionary relationships among different animal groups. Molecular studies have revealed that echinoderms are more closely related to chordates (including vertebrates) than to coelenterates, indicating a more complex evolutionary history than previously thought. 4. Developmental Biology: Studies of echinoderm embryonic development have uncovered complex regulatory mechanisms and genetic pathways involved in their development, indicating greater developmental complexity than previously appreciated. Echinoderms undergo bilateral symmetry during early embryonic stages, suggesting a closer evolutionary relationship with other bilaterally symmetrical animals, including chordates. 5. Ecological Roles and Adaptations: Echinoderms exhibit a wide range of ecological roles and adaptations, including sophisticated feeding strategies, complex behaviors, and diverse habitats. Their ecological diversity and adaptive radiation into various marine environments suggest a higher degree of ecological complexity and evolutionary innovation than previously recognized. Overall, the reevaluation of echinoderms' position in the classification hierarchy reflects a deeper understanding of their evolutionary history, developmental biology, and ecological significance. While they were once considered more primitive due to their radial symmetry and sessile lifestyle, modern research has revealed their evolutionary complexity and importance in marine ecosystems, leading to their recognition as occupying a more advanced position among invertebrates. CHAPTER 23 CHORDATES CHAPTER OUTLINE 23.1. The Chordates A. Structural Plan (Figure 23.1) 1. The name Chordata comes from the notochord, an internal, rodlike, semirigid, fluid-engorged (thus utilizes hydrostatic pressure) structure enclosed in a sheath (see 23.1A for further discussion). 2. All chordates have five basic characteristics: 1) dorsal, tubular nerve cord overlying 2) a supportive notochord; 3) pharyngeal slits; 4) an endostyle for filter feeding, and 5) a postanal tail for propulsion. 3. Chordates share features with some invertebrates: bilateral symmetry, anterioposterior axis, coelom, tube-within-a-tube body plan, metamerism and cephalization. 4. However, the evolutionary position of Chordates is uncertain. 5. Earlier theories were based on a relationship with the protostome branch; this is considered unlikely. 6. The echinoderm-hemichordate assemblage or deuterostomes are considered the chordate sister group. 7. The important common features are: radial cleavage, anus derived from the blastopore, mouth derived from a secondary opening, and a coelom formed by fusion of enterocoelous pouches. 8. Chordates have more structural unity in body plan than is found in many other phyla. 23.2. Traditional and Cladistic Classification of the Chordates (Figures 23.2, 23.3; Table 23.1) A. Traditional and Cladistic Systems Diverge 1. Most people recognize the general traditional groups; but cladists no longer use Agnatha and Reptilia. 2. Reptiles are paraphyletic because they do not contain all of the descendants. a. Reptiles, birds and mammals compose a monophyletic clade called Amniota. b. By cladistics, reptiles can only be grouped as amniotes that are not birds or mammals. c. There are no positive features that group only reptiles to the exclusion of birds and mammals. 3. For the same reasons, agnathans (hagfishes and lampreys) are paraphyletic because their most common recent ancestor is also an ancestor of all remaining vertebrates. 4. In contrast, the branches of a phylogenetic tree represent real lineages with geological information. 5. Traditional classification makes certain distinctions. a. Protochordata (or Acraniata) are separated from the Vertebrata (or Craniata) that have a skull. b. Vertebrates may be divided into Agnatha (jawless) and Gnathostomata (having jaws). c. Vertebrates are also divided into Amniota, having an amnion, and Anamniota lacking an amnion. d. Gnathostomata is subdivided into Pisces with fins and Tetrapoda, usually with two pair of limbs. e. Many of these groupings are paraphyletic; alternative monophyletic taxa are suggested. f. 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. 23.3. Five Chordate Hallmarks A. Notochord (Text-art 23.1) 1. In most cases, the notochord extends the length of the body and lies between the gut tract and the nervous system. 2. Stiffens the body, providing skeletal scaffolding for attachment of swimming muscles. 3. Notochord is fluid-filled, not within a uniform cavity, but rather within cells or within compartments between cells. 4. This feature as well as the other three is always found at some embryonic stage of all chordates; the notochord is the first part of the endoskeleton to appear in the embryo. 5. In protochordates and jawless vertebrates, the notochord persists throughout life. 6. In vertebrates, a series of cartilaginous or bony vertebrae form from mesenchymal cells derived from blocks of mesodermal cells lateral to the notochord. 7. In most vertebrates, the notochord is entirely displaced by vertebrae but persists as intervertebral discs. B. Dorsal Tubular Nerve Cord (Text-art 23.2) 1. In most invertebrate phyla, the nerve cord is ventral to the alimentary canal and solid. 2. In chordates, the single cord is dorsal to the alimentary canal and is tubular. 3. The anterior end enlarges to form the brain. 4. The cord is produced by the infolding of ectodermal cells on the dorsal side of the body. C. Pharyngeal Pouches and Slits (Text-art 23.3) 1. Pharyngeal slits lead from the pharyngeal cavity to the outside. 2. They form by the inpocketing of the outside ectoderm and the evagination of the pharynx endoderm. 3. In aquatic chordates, the two pockets break through to form the pharyngeal slit. 4. In amniotes these pockets may not break through and only grooves are formed. 5. In tetrapods, the pharyngeal pouches give rise to a variety of structures, including the Eustachian tube, middle ear cavity, tonsils and parathyroid glands. 6. The perforated pharynx functions as a filter-feeding apparatus in protochordates. 7. Pharyngeal pouches or slits are not unique to chordates; hemichordates also have pharyngeal slits. 8. Fishes added a capillary network with gas-permeable walls; this network evolved into gills. D. Endostyle or Thyroid Gland (Text-art 23.4) 1. Recently, the endostyle was recognized as a shared chordate character. 2. The endostyle or its derivative, the thyroid gland, is found in all chordates. 3. Some cells in the endostyle secrete iodinated proteins homologous with the iodinated-hormone-secreting thyroid gland of adult lampreys and the remainder of vertebrates. E. Postanal Tail (Text-art 23.5) 1. The postanal tail, plus musculature, provided motility for larval tunicates and Amphioxus to swim. 2. This was increased in fishes but became smaller or vestigial in later lineages. 23.4. Ancestry and Evolution A. History 1. The earliest protochordates were soft-bodied and would not have left many fossils. 2. Most work has been conducted on early developmental stages where early features are conserved. 3. A theory that chordates evolved within the protostome lineage was discarded due to embryo evidence. 4. The deuterostomes are a natural grouping that has a common origin in Precambrian seas. 5. Anatomical, developmental, and molecular evidence indicate that chordates arose about 570 million years ago from a lineage related to echinoderms and hemichordates. 6. Molecular data suggest that a clade containing both echinoderms and hemichordates is the sister group of chordates. (Figure 22.34). 23.5. Subphylum Urochordata: Tunicata (Figures 23.4–23.8) A. Diversity 1. There are about 3000 species of tunicates identified. 2. They occur in all seas and at all depths. 3. Most are sessile as adults although a few are free-living. 4. The tunic is the tough, nonliving test that surrounds them and contains cellulose. 5. In most species, only larval forms bear all chordate hallmarks; adults lose many of these characters. 6. During adult metamorphosis, the notochord and tail disappear; the dorsal nerve cord is reduced. 7. Urochordata is divided into Ascidiacea, Appendicularia and Thaliacea. B. Form and Function of Ascidians 1. They are called sea squirts because they discharge a jet of water when disturbed. 2. Most are attached to rocks or pilings when adults, and among the most abundant intertidal animals. 3. Colonial and solitary ascidians have their own test; compound forms share a common test. 4. In some compound ascidians, each has its own incurrent siphon but they share the excurrent siphon. 5. The mantle lines the tunic. 6. The incurrent or oral siphon marks the anterior; the excurrent or atrial siphon marks the dorsal side. 7. Water entering the incurrent siphon passes through a ciliated pharynx with an elaborate basketwork. 8. Feeding depends on the formation of a mucous net that is secreted by the endostyle. 9. Cilia on gill bars of the pharynx pull the mucus into a sheet; particles trapped in the sheet are worked into a rope and carried back to the esophagus and stomach. 10. The heart drives blood first in one direction, then in reverse. 11. Many concentrate rare chemical elements, such as vanadium, in dramatically high concentrations. 12. The nervous system has one nerve ganglion and a plexus of nerves on the dorsal side of the pharynx. 13. The subneural gland samples incoming water and may have an endocrine function. 14. Sea squirts are hermaphroditic with a single ovary and a single testes; fertilization occurs in the water. 15. Adult sea squirts only have one of the five chordate features: pharyngeal slits. 16. The tadpole larvae, however, have all five chordate characteristics. (Figure 23.6) 17. The larva does not feed, but swims awhile before attaching and developing into a sessile adult. C. Form and Function of Thalacians 1. Salps in the class Thaliacea are pelagic and have a lemon-shaped body that is transparent. (Figure 23.7) 2. Salps pump water through the body by muscular contraction rather than ciliary action. 3. Salps alternate sexual and asexual generations and respond rapidly to increases in food supply. D. Form and Function of Appendicularia (Larvacea) 1. These animals resemble the larval stages of other tunicates. 2. Each builds a delicate hollow sphere of mucus interlaced with passages for water to enter. 3. Phytoplankton and bacteria trapped on a feeding filter inside this sphere are drawn into the mouth through a tube. 4. After the filters become clogged with wastes, they are left behind and a new sphere is built. 5. They are paedomorphic, sexually mature individuals that retain the larval body form of ancestors. 23.6. Subphylum Cephalochordata (Figure 23.9) A. Diversity 1. Lancelets are slender, laterally flattened, translucent animals about 5–7 centimeters long. 2. They live in sandy bottoms of coastal waters around the world. 3. Originally labeled in the genus Amphioxus, they are by priority now in the genus Branchiostoma. 4. About 25 species of amphioxus are described; five occur in North American coastal waters. B. Form and Function 1. Amphioxus has the four distinctive characteristics of chordates in simple form. 2. Water enters the mouth driven by cilia in the buccal cavity and pharynx. 3. Water passes through pharyngeal slits where food is trapped in mucus secreted by the endostyle. 4. Food is moved through the gut via cilia which are concentrated in areas called the ileocolic ring. 5. Food particles separated from the mucus are passed into a hepatic cecum where they are phagocytized. 6. Filtered water leaves the body by an atriopore. 7. The closed circulatory system is complex but lacks a heart. 8. Blood is pumped by peristaltic contractions in the ventral aorta, passes upward through branchial arteries in the pharyngeal bars to paired dorsal aortas. 9. Their blood moves by microcirculation through tissues and returns to the ventral aorta. 10. Blood lacks erythrocytes and hemoglobin and mainly transports nutrients. 11. A hollow nerve cord lies above the notochord. 12. Pairs of spinal nerve roots emerge at each trunk segment. 13. Sense organs are simple, including an unpaired ocellus that functions as a photoreceptor. 14. The anterior nerve cord is not enlarged yet is homologous to the vertebrate brain. 15. Sexes are separate. 16. Gametes are set free in the atrium and pass through the atriopore where fertilization occurs outside. 17. Cleavage is holoblastic and a gastrula forms by invagination. 18. The larvae soon hatch and gradually become the shape of adults. D. Basic Plan 1. Amphioxus possesses features that suggest the vertebrate plan. a. A hepatic cecum, which is a diverticulum resembling the vertebrate pancreas that secretes digestive enzymes. b. The segmented trunk muscles resemble vertebrate patterns. c. They possess the basic circulatory plan that advanced chordates elaborate. 23.7. Subphylum Vertebrata (Craniata) A. Adaptations That Guided Vertebrate Evolution 1. The earliest vertebrates were substantially larger than the protochordates. 2. The earliest vertebrates were also considerably more active than the protochordates. a. The earliest vertebrates where characterized by increased speed and mobility resulting from modifications of the skeletal structures and muscles. b. The higher activity level and size of vertebrates also requires structures specialized in the location, capture, and digestion of food and adaptations designed to support a high metabolic rate. B. Musculoskeletal Modifications (Text-art 23.6) 1. Most vertebrates possess both an exoskeleton and endoskeleton of cartilage or bone. 2. The endoskeleton permits almost unlimited body size with much great economy of building materials. 3. The endoskeleton forms excellent jointed scaffolding for the attachment of segmented muscles. 4. The segmented body muscles (myomeres) changed from the V-shaped muscles of cephalochordates to the W-shaped muscles of vertebrates. 5. Also unique to vertebrates are the presence of fin rays of dermal origin in the fins, aiding in swimming. 6. The endoskeleton probably was composed initially of cartilage and later gave way to bone. 7. The endoskeleton of living hagfishes, lampreys, sharks and their kin, and even in some “bony” fishes, such as sturgeons, is mostly composed of cartilage. 8. The structural strength of bone is superior to cartilage, making it ideal for muscle attachment in areas of high mechanical stress. 9. Perhaps bone evolved, in part, as a means of mineral regulation (since phosphorus and calcium are used for many physiological processes and are in particularly high demand in organisms with high metabolic rates). 10. Some of the most primitive fishes, including Ostracoderms and placoderms were partly covered in a bony, dermal armor (this armor is modified in later fishes as scales). 11. Many of the bones encasing the brain of advanced vertebrates develop from cells that originate from the dermis. 12. Most vertebrates are further protected with keratinized structures derived from the epidermis, such as reptilian scales, hair, feathers, claws, and horns. C. Physiology Upgrade (Text-art 23.7) 1. Vertebrates have modifications to the digestive, respiratory, circulatory, and excretory systems that meet an increased metabolic demand. 2. The perforated pharynx evolved as a filter-feeding device in early chordates. 3. Water with suspended food particles was drawn through the pharynx by ciliary action and trapped by mucus secreted by the endostyle. 4. In the larger, predatory vertebrates, the pharynx was modified into a muscular apparatus that pumped water through the pharynx. 5. With origin of highly vascularized gills, the function of the pharynx shifted to primarily gas exchange. 6. Changes in gut, including a shift from movement of food by ciliary action to muscular action and addition of accessory digestive glands, the liver and pancreas allow more food to be ingested. 7. A ventral three-chambered heart consisting of a sinus venosus, atrium, and ventricle, and erythrocytes with hemoglobin enhanced transporation of nutrients, gases, and other substances. 8. Protochordates have no distinct kidneys, but vertebrates possess paired, glomerular kidneys that remove metabolic waste products and regulated body fluids and ions. D. New Head, Brain, and Sensory Systems (Text-art 23.8) 1. When vertebrate ancestors shifted from filter feeding to active predation, new sensory, motor, and integrative controls became essential for location and capture of larger prey. 2. The anterior end of the nerve cord became enlarged as a tripartite brain (forebrain, midbrain, and hindbrain) and was protected by a cartilaginous or bony cranium. 3. Paired special sense organs such as eyes evolved, along with paired inner ears designed for equilibrium and sound reception. 4. Many other receptors also evolved: mechanoreceptors, chemoreceptors, electroreceptors, and olfactory receptors. E. Neural Crest, Ectodermal Placodes, and Hox Genes 1. Development of the vertebrate head and special sense organs was largely the result of two embryonic innovations present only in vertebrates: the neural crest and ectodermal placodes. 2. The neural crest (derived from a population of ectodermal cells lying along the length of the embryonic neural tube) contributes to the formation of many different structures, among them most of the cranium, pharyngeal skeleton, teeth dentine, some cranial nerves, ganglia, Schwann cells, and endocrine glands. 3. The neural crest cells may also regulate the development of adjacent tissue, such as tooth enamel and pharyngeal muscles (branchiomeres). 4. The ectodermal placodes are plate-like ectodermal thickenings that appear on either side of the neural tube and give rise to the olfactory epithelium, lens of the eye, inner ear epithelium, some ganglia, some cranial nerves, lateral-line mechanoreceptors, and electroreceptors. 5. The placodes also induce the formation of taste buds. 6. The vertebrate head with its sensory structures located adjacent to the mouth (later equipped with prey-capturing jaws) stemmed from the creation of new cell types. 7. Recent studies of the distribution of homeobox-containing 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. 8. One copy of Hox genes is found in Amphioxus and other invertebrates, whereas living gnathostomes have four copies. 9. It may be that the additional copies of genes that control body plan provided genetic material free to evolve a more complex kind of animal. 23.8. Evolutionary History A. The Search for the Vertebrate Ancestral Stock (Figures 23.10, 23.11) 1. The jawless ostrocoderms from the early Paleozoic vertebrate fossil record, share organ system development with living vertebrates indicating organ systems must have originated in early vertebrate and invertebrate lineage. 2. Haikouella lanceolata, a small fishlike creature known from over 300 fossil specimens provides a wealth of information on the evolution of vertebrates. a. It possessed a notochord, pharynx, and dorsal nerve cord. b. It also had pharyngeal muscles, paired eyes, and an enlarged brain. c. However, it is not a vertebrate because it lacks distinctive vertebrate characteristics such as a cranium, an ear, and a telencephalon (anterior lobe of the brain). d. Thus is possesses the transitional morphology between cephalochordates and vertebrates. e. Some researchers hypothesize Haikouella is the sister taxon of vertebrates. B. Chordate Evolution and the Position of Amphioxus (Figure 23.12) 1. The chordates have pursued two paths in their early evolution: one path led to the sedentary urochordates; the other to active, mobile cephalochordates and vertebrates. 2. In 1928, Walter Garstang of England, however, suggested that the chordate ancestral lineage retained into adulthood the larval form of sessile tunicate-like animals. 3. He termed this paedomorphosis, the evolutionary retention of larval traits in an adult body. 4. Paedomorphosis occurs in some amphibians. 5. Garstang’s hypothesis has been challenged recently; molecular data suggest that ancestor of deuterostomes was free-swimming, that the sessile ascidians represent a derived body form, and that free-swimming appendicularians are most similar in body form to ancestral chordates. 6. This has long been considered the closest living relative to the earliest vertebrates. 7. It is not now considered a direct ancestor although it may closely resemble the ancestor. 8. It lacks a brain and the specialized sensory equipment of vertebrates. 9. There are no gills in the pharynx and no mouth for pumping water. 10. Recent studies of homeobox containing genes suggest that the ancestor of both amphioxus and vertebrates was cephalized. 11. Many zoologists still consider the cephalochordates the living closest relative of vertebrates. 12. However, as noted in the prologue to the chapter, Amphioxus is unlike the most recent common ancestor of vertebrates because it lacks the tripartite brain, chambered heart, special sensory organs, muscular gut and pharynx, and neural crest tissue inferred to have been present in that ancestor. 13. In addition, the larger fins of some extinct cephalochordates suggest that they were more free-swimming than modern Amphioxus. D. The Ammocoete Larvae of Lampreys as a Model of the Ancestral Vertebrate Body Plan (Figure 23.13) 1. Lampreys have a larval stage called the ammocoete that closely resembles the amphioxus. 2. Ammocoete larvae were originally considered petromyzontidan adults. 3. The ammocoete larval mouth resembles the amphioxus but draws water in by muscular pumping. 4. The endostyle, mucus, body muscles, notochord and circulatory system closely resemble amphioxus. 5. In contrast to amphioxus, ammocoetes have a two-chambered heart, a three-part brain, a median nostril, auditory vesicles, a thyroid and pituitary gland. 6. More extensive pharyngeal filaments serve in respiration. 7. The ammocoete has a true liver, gallbladder and pancreatic tissue. 8. In total, the ammocoete larva has the most primitive condition of this set of vertebrate structures. E. The Earliest Vertebrates (Figure 23.14) 1. Until recently, ostracoderms are the earliest articulated vertebrate skeletal fossils. 2. They are found in the late Cambrian deposits in the United States. 3. They were small, heavily armored, jawless, and lacked paired fins. 3. During the last 10 years, researchers have discovered several 530-million-year-old fossils in the Chengjiang deposits belonging to one or two fishlike vertebrates: Myllokunmingia and Haikouichthys. 4. These fossils push back the origin of vertebrates to at least the early Cambrian. 5. The fossils showed many vertebrate characteristic including a heart, paired eyes, otic capsules, and rudimentary vertebrae. 6. The earliest Ostracoderms were armored with bone in their dermis and lacked paired fins that later fishes used for stability. 7. The ostracoderms are 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. 9. Heterostracans a. The heterostracans represent an awkward design that probably filtered particles from the bottom. b. Unlike ciliary filter-feeding protochordates, ostracoderms sucked in water by muscular pumping. c. Therefore, a few authorities believe they may have been able to feed on soft-bodied animals. d. The Devonian saw a major radiation of heterostracans that never evolved jaws or paired fins. 10. Osteostracans (Figure 23.14) a. Coexisting with heterostracans, this group developed paired pectoral fins to stabilize movement. b. Their jawless mouth was toothless. c. They had a sensory lateral line, paired eyes, and inner ears with semicircular canals. d. Although the head was well armored, they lacked any axial skeleton or vertebrae. 11. A typical osteostracan was Cephalaspis, a small marine animal covered with a heavy, dermal armor of cellular bone, including a single-piece head shield. 12. Cephalaspis likely had a sophisticated nervous system and sense organs, similar to those of modern lampreys. 13. Another group of Ostracoderms, the anapsid, were more streamlined than other Ostracoderms. 14. These and other Ostracoderms enjoyed an impressive radiation in the Silurian and Devonian periods. 15. All Ostracoderms became extinct by the end of the Devonian period. 16. Anaspids were streamlined and more closely resembled the modern lamprey. 17. Paleozoic sediments were dated using microscopic, tooth-like fossils called conodonts. 18. Complete conodont animals have been discovered; we do not yet know how to classify them, although with their phosphatized toothlike elements, W-shaped myomeres, cranium, notochord, and paired eye and otic capsules, conodonts clearly belong to the vertebrates. (Figure 23.15) F. Early Jawed Vertebrates (Figure 23.16, 23.17) 1. All living and extinct jawed vertebrates are called gnathostomes in contrast to agnathans. 2. Living agnathans, the lampreys and hagfishes, are often called cyclostomes. 3. Gnathostomes constitute a monophyletic group; all derived organisms share these features. 4. Agnathans, defined by the absence of jaws, may be paraphyletic. 5. “Evo-devo” is a term used by evolutionary biologists to refer to the ability of postulating evolutionary history by considering developmental characteristics of a taxonomic group. 5. Evidence indicates that jaws arose by modification of the first two cartilaginous gill arches. a. Both gill arches and jaws form from upper and lower bars that bend forward and are hinged. b. Both are derived from neural crest cells rather than from mesodermal tissue as are most bones. c. The jaw musculature is homologous to the musculature that originally supported gills. d. The mandibular arch may have first become enlarged to assist gill ventilation, perhaps to meet the increasing metabolic demands of early vertebrates. e. Loss of expression of Hox6 in mandibular gill arch may have led to the evolution of jaws. 6. Placoderms appeared in the early Devonian and were heavily armored; some were large. 7. Acanthodians are included in a clade that underwent a great radiation into the bony fishes that dominate the waters today. G. Classification of the Phylum Chordata Phylum Cordata Subphylum Urochordata Subphylum Cephalochordata Group Craniata Subphylum Vertebrata Superclass Agnatha Class Myxini Class Petromyzontida Superclass Gnathostomata Class Chondrichthyes Class Actinopterygii Class Sarcopterygii Class Amphibia Class Reptilia Class Aves Class Mammalia Lecture Enrichment 1. We begin to see the dissonance between common word usage and strict cladistics in the comparisons of vertebrate classification systems (i.e., we do not consider birds to be “reptiles” or humans to be “fish”). This may be a time to explain to students that the careful and precise usage of words is part of the advancement of science understanding. 2. The concept of paedomorphosis will have substantial implications in other sections of animal evolution including recent human evolution. Commentary/Lesson Plan Background: Specimens of sea squirts, amphioxus, lamprey and hagfish, and (if available) fossils or fossil casts of agnathans will help illustrate these organisms that are otherwise not in the students’ experience base. [Students in the Great Lakes region may be familiar with the lamprey as a pest species.] Misconceptions: There is a tendency to assume that representatives of ancient lineages will also be rare or endangered; the populations of amphioxus, hagfish and lampreys are quite abundant. Because almost all discussion of evolutionary features in earlier biology classes has focused on adult features, students will not find selection on other stages (as in paedomorphosis) intuitive. Schedule: HOUR 1 23.1. The Chordates Characteristics A. Structural Plan 23.2. Traditional and Cladistic Classification of the Chordates A. Traditional and Cladistic Systems Diverge 23.3. Five Chordate Hallmarks A. Notochord B. Dorsal Tubular Nerve Cord C. Pharyngeal Pouches and Slits D. Endostyle E. Postanal Tail 23.4. Ancestry and Evolution A. History 23.5. Subphylum Urochordata (Tunicata) A. Diversity B. Form and Function of Ascidians C. Form and Function of Thalacians D. Form and Function of Larvacea HOUR 2 23.6. Subphylum Cephalochordata A. Diversity B. Form and Function C. Reproduction D. Basic Plan 23.7. Subphylum Vertebrata (Craniata) A. Adaptations That Guided Vertebrate Evolution B. Living Endoskeleton C. Pharynx and Efficient Respiration D. Advanced Nervous System E. Paired Limbs 23.8. Evolutionary History A. Search for the Ancestral Vertebrate B. Chordate Evolution and the Position of Amphioxus C. Ammocoete Larvae of Lampreys as a Model of Primitive Vertebrate Body Plan D. Earliest Vertebrates E. Early Jawed Vertebrates ADVANCED CLASS QUESTIONS: 1. The larval ascidians, appendicularians, amphioxus and larval ammocoetes all provide such a continuum of features of evolutionary change. Why do researchers still remain uncertain about this scenario? Answer:The uncertainty surrounding the evolutionary scenario involving larval ascidians, appendicularians, amphioxus, and larval ammocoetes stems from several factors: 1. Complexity of Evolutionary Relationships: Evolutionary relationships among these organisms are complex and may not fit neatly into a linear continuum. While they share certain morphological and developmental features, they also exhibit significant differences in anatomy, physiology, and genetic makeup. Resolving their precise evolutionary relationships requires comprehensive analyses of multiple lines of evidence, including morphological, developmental, molecular, and ecological data. 2. Evolutionary Convergence and Parallelism: Convergent evolution and parallelism can obscure evolutionary relationships by causing unrelated organisms to develop similar traits independently in response to similar ecological pressures. In the case of ascidians, appendicularians, amphioxus, and ammocoetes, similarities in larval morphology and developmental patterns may reflect convergent evolution rather than shared ancestry, leading to challenges in reconstructing their evolutionary history accurately. 3. Incomplete Fossil Record: The fossil record of these organisms is incomplete, making it difficult to trace their evolutionary trajectories over geological time scales. Fossilized remains of early chordates, including ascidians, appendicularians, and amphioxus, are relatively rare and poorly preserved, limiting our understanding of their evolutionary origins and early diversification. 4. Morphological Plasticity: Some of these organisms exhibit considerable morphological plasticity throughout their life cycles, with larval and adult stages often displaying distinct anatomical features adapted to different ecological niches. This morphological variability can complicate efforts to infer evolutionary relationships based solely on morphological characteristics. 5. Taxonomic Uncertainty: Taxonomic classification of these organisms has undergone revisions over time as new evidence emerges and our understanding of their biology improves. Changes in taxonomic placement and nomenclature can impact interpretations of evolutionary relationships and complicate efforts to reconstruct their evolutionary history accurately. 6. Evolutionary Processes: Evolutionary processes such as gene duplication, gene loss, and genomic rearrangements can further obscure the evolutionary relationships among these organisms by introducing genetic variability and complexity. Understanding how these processes have shaped the genomes and phenotypes of ascidians, appendicularians, amphioxus, and ammocoetes requires detailed genomic and functional analyses. In summary, the uncertainty surrounding the evolutionary scenario involving larval ascidians, appendicularians, amphioxus, and larval ammocoetes reflects the complexities of evolutionary biology, including convergent evolution, incomplete fossil records, morphological plasticity, taxonomic revisions, and genetic variability. Resolving these uncertainties requires interdisciplinary approaches that integrate morphological, developmental, molecular, and ecological data to reconstruct their evolutionary history accurately. 2. Why are researchers so certain about the origin of jaws from gill arches? Answer:Researchers are confident about the origin of jaws from gill arches based on several lines of evidence from comparative anatomy, embryology, paleontology, and molecular biology. Here's why researchers are certain about this evolutionary transition: 1. Anatomical Homology: Jaws in vertebrates exhibit structural and developmental similarities to the gill arches found in jawless fishes, such as lampreys and hagfishes. During embryonic development, the same embryonic tissue that forms gill arches in jawless fishes gives rise to the jaws in jawed vertebrates, providing evidence of anatomical homology between these structures. 2. Fossil Evidence: Fossilized remains of early jawed vertebrates, such as placoderms and early sharks, provide direct evidence of the evolutionary transition from gill arches to jaws. These fossils show intermediate stages in the transformation of anterior gill arches into functional jaws, including modifications in bone structure, tooth development, and muscle attachment points. 3. Comparative Morphology: Comparative studies of jawed and jawless vertebrates reveal similarities in the organization and arrangement of skeletal elements associated with feeding apparatuses. For example, the jaws of modern jawed vertebrates consist of modified derivatives of the same skeletal elements present in the gill arches of jawless fishes, supporting their evolutionary connection. 4. Gene Expression Patterns: Molecular studies have identified genetic pathways and regulatory networks involved in the development of gill arches and jaws in vertebrates. These studies have shown that many of the same developmental genes and signaling pathways are active during the formation of gill arches and jaws, providing molecular evidence of their evolutionary continuity. 5. Evolutionary Theory: The concept of evolutionary theory, including natural selection and genetic variation, provides a theoretical framework for understanding the origin of jaws from gill arches. By natural selection acting on genetic variation within populations, structures like gill arches could have been co-opted and modified over time to give rise to new adaptive features like jaws. Overall, the convergence of evidence from comparative anatomy, embryology, paleontology, molecular biology, and evolutionary theory supports the conclusion that jaws evolved from modified anterior gill arches in jawed vertebrates. This evolutionary transition represents a key innovation in vertebrate evolution that has played a fundamental role in shaping the diversity and success of jawed vertebrates. Instructor Manual for Integrated Principles of Zoology Cleveland Hickman, Jr., Susan Keen, Allan Larson, David Eisenhour, Helen I'Anson, Larry Roberts 9780073524214
