vertebrate-life-9th-edition-by-pough-f-harveypdf [65-70].pdf

10 mm (a) Ant erio SS S S S r S S S S environments. Under what conditions did the first vertebrates evolve? The vertebrate kidney is very good at excreting excess water while retaining biologically important molecules and ions. That is what a freshwater fish must do, because its body fluids are cont...

10 mm (a) Ant erio SS S S S r S S S S environments. Under what conditions did the first vertebrates evolve? The vertebrate kidney is very good at excreting excess water while retaining biologically important molecules and ions. That is what a freshwater fish must do, because its body fluids are continuously being diluted by the osmotic inflow of water and it must excrete that water to regulate its internal concentration (see Chapter 4). Thus, the properties of the vertebrate kidney suggest that vertebrates evolved in freshwater. Despite the logic of that inference, however, a marine origin of vertebrates is now widely accepted. Osmoregulation is complex and fishes use cells in the gills as well as the kidney to control their internal fluid concentration. Probably the structure of the kidney is merely fortuitously suited to freshwater. Two lines of evidence support the hypothesis of a marine origin of vertebrates: The earliest vertebrate fossils are found in marine sediments. Pb All nonvertebrate chordates and deuterostome Pa (b) Figure 3–2 Conodonts. (a) Clydagnathus. (b) Close-up of the feeding apparatus (conodont elements) inside the head of Idiognathodus. The anterior S elements appear to be modified for grasping, and the posterior P elements for crushing. for example, conodont myomeres have the V shape of nonvertebrate chordates. The ability to form mineralized tissues in the skin is a feature of all vertebrates from ostracoderms onward, but the production of teeth, or toothlike structures, as seen in conodonts, may have evolved independently several different times among vertebrates, as discussed later in the chapter. Accepting conodonts as vertebrates has changed our ideas about early vertebrate interrelationships, particularly the importance—in the vertebrate phylogeny— of having mineralized tissues. The toothlike structures of conodonts and the bony dermal skeletons of ostracoderms are now thought to make these animals more derived vertebrates than the soft-bodied, jawless fishes living today (Figure 3–3). The Environment of Early Vertebrate Evolution By the Late Silurian, ostracoderms and early jawed fishes were abundant in both freshwater and marine 50 CHAPTER 3 invertebrate phyla are exclusively marine forms, and they have body fluids with approximately the same osmolal concentration as their surroundings. Hagfishes also have concentrated body fluids, and these high body-fluid concentrations probably represent the original vertebrate condition. 3.2 Extant Jawless Fishes The extant jawless vertebrates—hagfishes and lampreys—once were placed with the ostracoderms in the class “Agnatha” because they lack the gnathostome features of jaws and two sets of paired fins. They also have other ancestral features, such as lack of specialized reproductive ducts, and neither has mineralized tissues. However, it is now clear that the “Agnatha” is a paraphyletic assemblage, and that ostracoderms are actually more closely related to gnathostomes than are living jawless vertebrates. Hagfishes and lampreys have often been linked as cyclostomes because they have round, jawless mouths, but this grouping may also be paraphyletic, since lampreys and gnathostomes share derived features that hagfishes lack (see Figure 3–3 and Figure 3–4 on page 52). Because both hagfishes and lampreys appear to be more distantly related to gnathostomes than were the armored ostracoderms of the Paleozoic, we will look at them before considering the extinct agnathans. The fossil record of the extant jawless vertebrates is sparse. Lampreys are known from the Late Devonian period— Priscomyzon, a short-bodied form, from South Africa; Early Vertebrates: Jawless Vertebrates and the Origin of Jawed Vertebrates Recent Cambrian 488 SARCOPTERYGII ACANTHODII PLACODERMI OSTEOSTRACI PITURIASPIDA GALEASPIDA THELODONTI ANASPIDA HETEROSTRACI ASTRASPIDA 444 ARANDASPIDA 416 STEM CHONDRICHTHYANS “Cephalaspida” ACTINOPTERYGII HOLOCEPHALI “Ostracoderms” 359 Myllokunmingia/Haikouichthys PALEOZOIC Devonian Carboniferous 299 Ordovician Silurian PETROMYZONTIFORMES MYXINIFORMES Permian 251 ELASMOBRANCHII 201.5 CONODONTA Triassic 0.01 15. 12. 16. Osteichthyes 14. Chondrichthyes ? 13. Eugnathostomata 11. Gnathostomata ? ? 10. 9. 8. 7. Myopterygii 6. Pteraspida 4. 542 5. 3. 2. 1. Millions of years Figure 3–3 Phylogenetic relationships of early vertebrates. This diagram depicts the probable relationships among the major groups of “fishes,” including living and extinct jawless vertebrates and the earliest jawed vertebrates. The black lines show relationships only; they do not indicate times of divergence or the unrecorded presence of taxa in the fossil record. Lightly shaded bars indicate ranges of time when the taxon is believed to be present, because it is known from earlier and later times, but is not recorded in the fossil record during this interval. The hatched bar shows probable occurrence based on limited evidence. Only the best-corroborated relationships are shown and question marks indicate uncertainty about relationships. The numbers at the branch points indicate derived characters that distinguish the lineages—see the Appendix for a list of these characters. Extant Jawless Fishes 51 GNATHOSTOMES (jawed vertebrates) Pla co rm s”† de co “O str a Ea ve rly C rte am bra b tes rian Ha † gfi sh es La mp rey s Co no do nt s *† CYCLOSTOMES* OSTEICHTHYANS (bony vertebrates) de rm s*† Ho l o (ch ce rat ime pha fis ras lan he or s Ela s) s (sh mo ark bra s a nc nd hs Ac ray an s) tho dia ns Ac *† (ra tinop y-f inn teryg ed ian fis s he s) CHONDRICHTHYANS (cartilaginous fishes) ? Sa (lo rcop an be-fi tery d t nn gia etr ed ns ap fis o d he s) s “AGNATHANS” (jawless vertebrates) Endochondral bone Teeth on jaws Jaws, pelvic fins Pectoral fins ? Dermal bone Mineralized tissues W-shaped myomeres Distinct head with cranium Figure 3–4 Simplified cladogram of vertebrates. Only living taxa and major extinct groups are shown. Quotation marks indicate paraphyletic groups. An asterisk indicates possibly paraphyletic groups. A dagger indicates extinct groups. the Late Carboniferous period—Hardistiella from Montana and Mayomyzon from Illinois; and the Early Cretaceous period—Mesomyzon from southern China, the first known freshwater form. All these fossil lampreys appear to have been specialized parasites similar to the living forms. Myxinikela (an undisputed hagfish) and a second possible hagfish relative, Gilpichthys, have been found in the same Carboniferous deposits as Mayomyzon. Hagfishes—Myxiniformes There are about 75 species of hagfishes in two major genera (Eptatretus and Myxine). Adult hagfishes (Figure 3–5) are elongated, scaleless, pinkish to purple in color, and about half a meter in length. Hagfishes are entirely marine, with a nearly worldwide distribution except for the polar regions. They are primarily deepsea, cold-water inhabitants. They are the major scavengers of the deep-sea floor, drawn in large numbers by their sense of smell to carcasses. Structural Characteristics Large mucous glands that open through the body wall to the outside are a unique feature of hagfishes. These so-called slime glands secrete enormous quantities of mucus and tightly 52 CHAPTER 3 coiled proteinaceous threads. The threads straighten on contact with seawater to entrap the slimy mucus close to the hagfish’s body. An adult hagfish can produce enough slime within a few minutes to turn a bucket of water into a gelatinous mess. This obnoxious behavior is apparently a deterrent to predators. When danger has passed, the hagfish makes a knot in its body and scrapes off the mass of mucus, then sneezes sharply to blow its nasal passage clear. Hagfishes lack any trace of vertebrae, and their internal anatomy shows many additional unspecialized features. For example, the kidneys are simple, and there is only one semicircular canal on each side of the head. Hagfishes have a single terminal nasal opening that connects with the pharynx via a broad tube, and the number of gill openings on each side (1 to 15) varies with the species. The eyes are degenerate or rudimentary and covered with a thick skin. The mouth is surrounded by six tentacles that can be spread and swept to and fro by movements of the head when the hagfish is searching for food. Two horny plates in the mouth bear sharp toothlike structures made of keratin rather than mineralized tissue. These tooth plates lie to each side of a protrusible tongue and spread apart when the tongue is protruded. When the Early Vertebrates: Jawless Vertebrates and the Origin of Jawed Vertebrates (a) Lateral view External gill opening 10 mm (b) Sagittal section of head region Nostril Olfactory sac Brain Nasopharyngeal duct Velum Notochord Spinal cord Excurrent branchial duct Barbel Mouth Branchial pouch Teeth on tongue Tongue muscle Pharynx Internal openings to branchial pouches Figure 3–5 Hagfishes. tongue is retracted, the plates fold together and the teeth interdigitate in a pincerlike action. Hagfishes have large blood sinuses and very low blood pressure. In contrast to all other vertebrates, hagfishes have accessory hearts in the liver and tail regions in addition to the true heart near the gills. These hearts are aneural, meaning that their pumping rhythm is intrinsic to the hearts themselves rather than coordinated via the central nervous system. In all these features, hagfishes resemble the condition seen in amphioxus—although, like other vertebrates, their blood does have red blood cells containing hemoglobin, and the true heart has three chambers. Feeding Hagfishes attack dead or dying vertebrate prey. Once attached to the flesh, they can tie a knot in their tail and pass it forward along their body until they are braced against their prey and can tear off the flesh in their pinching grasp. They often begin by eating only enough outer flesh to enter the prey’s coelomic cavity, where they dine on soft parts. Some recent, but controversial, research suggests that hagfishes can actually absorb dissolved organic nutrients through their skin and gill tissues. Reproduction In most species, female hagfishes out- number males by a hundred to one; the reason for this strange sex ratio is unknown. Examination of the gonads suggests that at least some species are hermaphroditic, but nothing is known of mating. The yolky eggs, which are oval and more than a centimeter long, are encased in a tough, clear covering that is secured to the sea bottom by hooks. The eggs are believed to hatch into small, completely formed hagfishes, bypassing a larval stage. Unfortunately, almost nothing is known of the embryology or early life history of any hagfish because few eggs have been available for study. However, some hagfish embryos have recently been examined, and it has been determined that hagfishes possess neural crests like other vertebrates. Hagfishes and Humans We still know very little about hagfish ecology: we do not know how long hagfishes live; how old they are when they first begin to reproduce; exactly how, when, or where they breed; where the youngest juveniles live; what the diets and energy requirements of free-living hagfishes are; or virtually any of the other information needed for good management of commercially exploited populations of hagfishes. And, strangely enough, hagfishes do have an economic Extant Jawless Fishes 53 importance for humans. Almost all so-called eel-skin leather products are made from hagfish skin. Worldwide demand for this leather has eradicated economically harvestable hagfish populations in Asian waters and in some sites along the West Coast of North America. Lampreys—Petromyzontiformes There are around 40 species of lampreys in two major genera (Petromyzon and Lampetra); the adults of different species range in size from around 10 centimeters up to 1 meter. Structural Characteristics Although lampreys are similar to hagfishes in size and shape (Figure 3–6), they have many features that are lacking in hagfishes but shared with gnathostomes. Traditionally it has been assumed that only lampreys had structures homologous with the vertebrae of jawed vertebrates: minute cartilaginous elements called arcualia, homologous with the neural arches of vertebrae. However, recent work has shown that hagfishes also have vertebral rudiments in the ventral portion of their tails (homologs of the hemal arches). Lampreys are unique among living vertebrates in having a single nasal opening situated on the top of the head, combined with a duct leading to the hypophysis (pituitary) and known as a nasohypophysial opening. Development of this structure involves distortion of the front of the head, and its function is not known. Several groups of ostracoderms had an apparently similar structure, which evidently evolved convergently in those groups. The eyes of lampreys are large and well developed, as is the pineal body, which lies under a pale spot just posterior to the nasal opening. In contrast to hagfishes, lampreys have two semicircular canals on each side of the head—a condition shared with the extinct ostracoderms as well as in gnathostomes. In addition, the heart is innervated by the parasympathetic nervous system (the vagus nerve, X), as in gnathostomes, but not in hagfishes. Chloridetransporting cells in the gills and well-developed kidneys regulate ions, water, and nitrogenous wastes, as well as overall concentration of body fluids, allowing lampreys to exist in a variety of salinities. Lampreys have seven pairs of gill pouches that open to the outside just behind the head. In most other fishes and in larval lampreys, water is drawn into the mouth and then pumped out over the gills in continuous or flow-through ventilation. Adult lampreys spend much of their time with their suckerlike mouths affixed to the bodies of other fishes, and during this time they cannot ventilate the gills in a flow-through fashion. Instead, 54 CHAPTER 3 they use a form of tidal ventilation by which water is both drawn in and expelled through the gill slits. A flap called the velum prevents water from flowing out of the respiratory tube into the mouth. The lampreys’ mode of ventilation is not very efficient at oxygen extraction, but it is a necessary compromise given their specialized mode of feeding. Feeding Most adult lampreys are parasitic on other fishes, although some small, freshwater species have nonfeeding adults. The parasitic species attach to the body of another vertebrate (usually a bony fish that is larger than the lamprey) by suction and rasp a shallow, seeping wound through the integument of the host. The round mouth is located at the bottom of a large fleshy funnel (the oral hood), the inner surface of which is studded with keratinized conical spines. The oral hood, which appears to be a hypertrophied upper lip, is a unique derived structure in lampreys. The protrusible tonguelike structure is also covered with spines, and together these structures allow tight attachment and rapid abrasion of the host’s integument. This tongue is not homologous with the tongue of gnathostomes because the tongue muscle is innervated by a different cranial nerve (the trigeminal nerve, V, rather than the hypoglossal nerve, XII). An oral gland secretes an anticoagulant that prevents the victim’s blood from clotting. Feeding is probably continuous when a lamprey is attached to its host. The bulk of an adult lamprey’s diet consists of body fluids of fishes. The digestive tract is straight and simple, as one would expect for an animal with a diet as rich and easily digested as blood and tissue fluids. Lampreys generally do not kill their hosts, but they do leave a weakened animal with an open wound. At sea, lampreys feed on several species of whales and porpoises in addition to fishes. Swimmers in the Great Lakes, after having been in the water long enough for their skin temperature to drop, have reported initial attempts by lampreys to attach to their bodies. Reproduction Lampreys are primarily found in northern latitude temperate regions, although a few species are known from southern temperate latitudes. Nearly all lampreys are anadromous; that is, they live as adults in oceans or big lakes and ascend rivers and streams to breed. Some of the most specialized species live only in freshwater and do all of their feeding as larvae, with the adults acting solely as a reproductive stage in the life history of the species. Little is known of the habits of adult lampreys because they are generally observed only during reproductive activities or when captured with their host. Early Vertebrates: Jawless Vertebrates and the Origin of Jawed Vertebrates (a) Lateral view of an adult Dorsal fin Nasohypophyseal opening Eye Oral hood 10 mm Gill openings (b) Sagittal section of head region Nasohypophyseal Olfactory Nasal duct sac opening Mouth Pineal eye Hypophyseal pouch Brain Spinal cord Notochord Dorsal aorta Esophagus Ventral aorta Oral hood Tongue muscle Velum Respiratory tube Internal openings to branchial ducts (c) Larval lamprey (ammocoete) Region of mandibular arch Otic capsule Brain Gill pouch Eye spot Spinal cord Notochord Nostril Upper lip Liver Heart Velum Gill opening Endostyle Figure 3–6 Lampreys. Extant Jawless Fishes 55