BIO112.02 Laboratory Guide #4
DIVERSITY OF THE VERTEBRATES
INTRODUCTION
Compared to the diversity of invertebrate phyla, the vertebrates are a pretty homogeneous group. Vertebrates are properly a subphylum of Phylum Chordata. All vertebrates evidence each of the following defining characteristics of the chordates at some stage in their development:
1. A hollow dorsal nerve cord, which is a tube composed of neurons that runs beneath the dorsal surface of the animal. The anterior portion of the nerve cord is modified to form a brain.
2. A notochord, which is a flexible connective tissue rod that extends the length of the body between the gut and the nerve cord. In more primitive vertebrates this is maintained in the adult. In more advanced vertebrates, the notochord is an embryological structure which induces development of the neural tube.
3. Pharyngeal gills slits, which are openings from the upper portion of the digestive tract through the body wall. The fate of these slits during development varies in the different chordate classes.
4. A post-anal tail.
Unlike members of the other two chordate subphyla, the Urochordata and Cephalochordata, members of Subphylum Vertebrata have a 'backbone'; i.e., a column of vertebrae composed of bone or cartilage which surrounds and protects the nerve cord. Anteriorly, the vertebral column connects to the cranium, which surrounds the brain. Other characteristics typical of most vertebrates include the following:
1. The presence of skin, composed of dermal and epidermal layers, which protects the body surface.
2. The division of the body into three areas: the head, trunk, and tail.
3. The presence of nephritic kidneys which remove metabolic wastes.
4. A closed circulatory system.
In this exercise, we will see many of the above shared characteristics, as well as those characteristics which differ among the eight major vertebrate classes: the Myxini, Cephalaspidomorphi, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia. We will use a traditional "phenetic" taxonomy for the vertebrates. However, you should recognize three things about the taxonomy of vertebrates:
1. There is no universally accepted taxonomic scheme for the vertebrates. The taxonomy presented here is a widely used classification scheme, but not the only one in common use.
2. Traditional "phenetic" taxonomies are based almost exclusively on morphology. They do not reconcile very well with the actually phylogeny (evolutionary history) of the vertebrates. As an example, birds are phylogenetically a subset of the dinosaurs, which are themselves a subset of the reptiles. However, traditional taxonomy artificially elevates birds to the level of a class, while relegating dinosaurs to a mere order in Class Reptilia. Similarly, traditional taxonomies totally miss the rather close phylogenetic relationship between the birds and the crocodilians; crocodilians are in many ways anatomically and physiologically more similar to birds than to any other reptiles.
3. The alternative "cladistic" taxonomies which rely strictly on monophyletic groups are prohibitively cumbersome to use because they are not strictly hierarchical and they define many logical modern vertebrate groups by exclusion; e.g. dinosaurs become "non-avian dinosaurs". We will use a traditional taxonomy because it is more convenient, straightforward and familiar. We will also study only extant (living) groups of vertebrates, while recognizing that the vast majority of vertebrate species and the majority of higher taxa are, in fact, extinct.
After completing this laboratory you should be able:
1) To describe the distinguishing characteristics of the living vertebrate groups.
2) To be able to classify representative animals from each group of vertebrates.
3) To describe the major adaptive trends in vertebrate evolution, especially those accompanying the movement of some ancestral groups from water to land.
Materials
Representative live, preserved, skeletal, and/or fossil specimens
for most taxonomic groups.
Models for some taxonomic groups
Poster guides to the diversity within some groups.
Procedure
Specimens of each vertebrate class are on display. These specimens include skeletal remains, preserved animals, live animals, and some behavioral artifacts (such as bird's nests).
1. Work through these displays.
a. A dichotomous key which outlines the major distinguishing features of each vertebrate class is provided below.
b. Following the key is additional information about each class, as well as a guide to lower taxa (subclasses, infraclasses, superorders, orders) within each class. On the sample specimens, identify and examine all of the external structures written in bold print in this guide.
c. Additional information will be on display with the specimens.
d. Your textbook has additional information on these vertebrate classes. The Biology Department has several additional vertebrate biology textbooks.
2. Be able to describe the distinguishing features of each class of vertebrates. Within each class, be able to recognize the major subdivisions.
3. Be able to use the distinguishing features to reliably classify these sample vertebrates. Pay particular attention to animals from different classes which look superficially similar. For example, think about how could you reliably distinguish a salamander (Amphibia) from a lizard (Reptilia), a shark (Chondrichtyes) from a sturgeon (Osteichthyes), or a lamprey (Cephalaspidomorphi) from an eel (Osteichtyes) or snake (Reptilia).
Study Suggestions
1. Make detailed sketches and notes on specimens. This will help you to look at the specimens more closely, as well as to help you study later.
2. Plan to come view the specimens once or twice more before the lab test. Test yourself by attempting to identify the specimens as accurately as possible by their common names, as well as to classify them without first looking at their labels.
3. The words in bold print in the extended guide below are words you should know and/or structures you should be able to identify or describe.
A Dichotomous Key to the Vertebrate Classes
Note: This key is based, in some cases, on secondarily derived characteristics of adult animals. This key roughly follows actual phylogenetic relationships. Notice that this produces an "unbalanced" key with lots of exceptions (e.g. snakes are reptilian tetrapods but do not have four legs) and omissions (e.g. non-avian dinosaurs).
1. Organisms are without jaws CEPHALASPIDOMORPHI, MYXINI[also: lack paired appendages (fins)]
1. Organisms have jaws
2. Organisms have fins
3. Organisms have cartilaginous skeletons CHONDRICHTHYES[Also: most have no operculum covering gills slits; have 5-7 gills with
separate openings; have non-overlapping, placoid (bony) scales with
projecting points; have subterminal mouth; tail is either heterocercal
or whip-like; have no swim bladder but use oil instead for flotation]
3. Organisms have partially bony skeletons OSTEICHTHYES
[Also: have an operculum over gill slits; have thin, overlapping dermalscales; most use swim bladder for flotation]
2. Organisms are tetrapods ('four-footed') as adults or embryologically
3. Organisms have moist skin AMPHIBIA
[Also: terrestrial but remain tied to aquatic habitats; usually haveexternal fertilization; eggs have jelly-like membrane coverings;
development includes metamorphosis from aquatic larval form
to lung breathing adult (usually); are ectothermic
3. Organisms have dry skin
4. Organisms are ectothermic or heterothermic REPTILIA[Also: have scales; have amniotic egg with leathery shells;
have internal fertilization; have homodont dentition or
are adontoid]
4. Organisms are endothermic or homeothermic
5. Organisms have feathers AVES
[Also: have front limbs modified for flight (usually); scaleson feet; amniotic egg with calcareous shell; have beak are
devoid of teeth]
5. Organisms have fur (hair) MAMMALIA
[Also: have mammary glands; have heterodont dentition(usually); have suprahepatic diaphragms; have well-developed
brains]
CLASSES CEPHALASPIDOMORPHI, MYXINI (jawless fishes)
Members of superclass Agnatha are primitive, jawless fish. Their endoskeletons are composed almost exclusively of cartilage and fibrous tissue, with virtually no bone. They have no true lateral appendages. Extinct groups of agnathans were ancestral to all modern vertebrates. Modern agnathans include the hagfishes (class Myxini) and lampreys (class Cephalaspidomorphi). Both hagfishes and lampreys are eel-like in form and have a single dorsal nostril. Hagfishes are scavengers; they attach to the flesh of dead fish with their mouths and use their rough tongues to scrape away tissue. Most lampreys are parasites which attach with their mouths to living fish, rasp away enough tissue to maintain a blood flow, and then ingest the blood.
CLASS CHONDRICTHYES (cartilagenous fishes)
Chondrichthyans are the most primitive living form of gnathostomes (jawed animals). They are exclusively marine fish. Their endoskeletons are composed entirely of cartilage. They have placoid scales, composed of a bony dermal plates which project through the epidermis, giving the skin its characteristic "sandpaper" feel. They are negatively buoyant and must actively swim to stay off the bottom. Chondrichthyans are generally divided into two subclasses: Subclass Elasmobranchii (sharks, rays, skates, and sawfishes) and subclass Holocephali (chimeras and ratfishes). Some species of sharks have remained essentially unchanged since the Jurassic Period, more than 65 million years ago.
The elasmobranchs have multiple exposed gill slits. The most anterior gill slit on each side is reduced into a small, dorsally located spiracle. Sharks are free-swimming, have lateral gill slits and a heterocercal tail (dorsal lobe is larger than ventral lobe). Buoyancy is provided by the heterocercal tail, hydroplaning action of the flat pectoral (anterior) fins, and oil produced by the liver. Skates, rays, and sawfish are compressed dorsoventrally, have ventrally located gills, and pectoral fins flattened into broad, wing-like structures. They are predominantly bottom-feeders.
Look closely at the shark jaws on display . Notice that both the upper and lower jaws are only loosely attached to the skull. This allows sharks to extend both jaws during feeding to tear flesh from their prey. Notice also the multiple rows of teeth. As individual teeth are lost during the feeding process, new teeth move forward to replace them.
CLASS OSTEICHTHYES (bony fishes)
Osteichthyans inhabit both marine and fresh-water environments. In osteichthyans the endoskeleton is composed partly to primarily of bone. There are more species of these bony fish than of all of other vertebrate classes combined. They are mostly free-swimming, with buoyancy provided by a gas-filled swim-bladder (see the fish skeleton on display), derived as dorsal outpocketing of the digestive system. The gills are covered with a bony plate, called an operculum, which increases the efficiency of ventilation (movement of water across the gills).
Most bony fish are suction-feeders, they eat by rapidly expanding the pharynx to suck water, food items, and/or prey into the mouth. The skin is generally covered by overlapping dermal scales which do not project through the epidermis. The two main groups of bony fish are the Subclass Actinopterygii (ray-finned fish) and the much smaller Subclass Sarcopterygii (lobe-finned fish).
Actinopterygians may be divided into three superorders. Order Chondrostei (sturgeons, paddlefish, bichirs) have skeletons composed mostly of cartilage. Living species are exclusively fresh-water. They have heterocercal (uneven lobes) or holocercal (single lobe) tails. Order Holostei (gars, and bowfins), have largely bony skeletons, but the cranium is cartilaginous. They are fresh-water fish with holocercal tails. Order Teleostei (all other ray-finned fish) is by far the most speciose and, in many ways, the most modern group. Teleost fish skeletons are composed almost exclusively of bone, and they generally have homocercal (same size lobes) tails.
Sarcopterygians are "lobe-finned" fish, with a characteristic fleshy lobe at the base of each lateral fin. They tend to be bottom-dwelling or shallow water fish, in which the fins are used to maneuver along the substrate. There are two orders of living sarcopterygians. Order Crossopterygii includes the extinct ancestor(s) of all of the modern terrestrial vertebrates, but is currently represented by only a single marine species, the coelocanth, a so-called "living fossil". Order Dipnoi includes the fresh-water lungfishes. Lungfishes have some "primitive" features, including a prominent notochord and a mostly cartilaginous skeleton. They also have some "modern" features similar to amphibians, such as paired atria in the heart, and paired ventral lungs, which allow them to breath air.
CLASS AMPHIBIA (amphibians)
Amphibians (literally "dual life") are a group of terrestrial and aquatic (fresh-water) vertebrates. Modern amphibians all have moist, smooth, scale-less skins, with numerous mucous and poison glands. Terrestrial amphibians generally must return to the water or to very moist locations to lay eggs. Terrestrial amphibians have paired lungs. Respiration in aquatic amphibians and aquatic larval forms is via either external gills or the skin (cutaneous respiration). Amphibians have a heart with two atria and a single ventricle. Modern amphibians have kinetic skulls, meaning that there are multiple jaw hinges. Feeding is generally by suction in aquatic forms and by "lingual feeding" (capturing prey with an extended tongue) for terrestrial forms. Amphibians are "cold-blooded" ectotherms whose body temperature is largely determined by the surrounding environmental temperature.
Amphibians are generally divided into three orders. Order Urodela (or Caudata) is comprised of the salamanders. Salamanders have four legs (usually) and a prominent tail. The legs do not support the trunk off the ground. Salamanders move by undulating the body, with the legs serving merely as "pivot-points". Salamanders all have aquatic larval forms. Some species remain aquatic throughout their lives, some species have a terrestrial "eft" stage then return to the water as adults, and some species metamorphose directly into fully terrestrial adults. Salamanders have internal fertilization; sperm are passed from male to female in a spermatophore sack.
Order Anura (literally "no tail") is comprised of the frogs and toads. Anuran eggs and sperm are shed into the water, where external fertilization takes place. These eggs hatch into tadpole larvae. Tadpoles grow and eventually metamorphose into tail-less adults. Additional diagnostic features of frogs and toads are the relatively long hind legs used for jumping and/or swimming, and a prominent external eardrum or tympanum associated with each ear.
Order Gymnophiona (or Apoda) is comprised of the caecilians. These amphibians have no legs and no pelvic or pectoral girdles. They are burrowing animals of tropical rainforests.
CLASS REPTILIA (reptiles)
Reptiles are a diverse group of vertebrates. Primitive reptiles descended from primitive scaled amphibians, and in turn, gave rise to modern reptiles, birds, and mammals, as well as several extinct groups. Reptiles have a dry skin, covered with cornified scales which prevent desiccation. Reptiles and most of their descendants have amniotic eggs, with a delicate sac-like membrane which surrounds the embryo and maintains a fluid environment within the egg. Reptile eggs are also covered with a leathery shell which prevents desiccation (see display egg). This, and internal fertilization, frees many modern reptiles from a dependence on water. The reptilian heart has a ventricle which is partially or completely partitioned into two separate chambers for pulmonary vs. systemic circulation. Reptiles, like all of the groups discussed so far, are heterotherms, meaning that their body temperature fluctuates with the environmental temperature. However, most reptiles generate at least some internal heat by metabolic processes, making them at least partially endothermic.
Classical taxonomy divides modern reptiles into three major orders. Order Chelonia (sometimes called Testudinata) contains the turtles. Turtles are characterized by their shell, which consists of a dorsal carapace and a ventral plastron. Each is constructed of elaborated dermal plates. The ribs and vertebrae are fused to the shell, and the pelvic and pectoral girdles are relocated inside the rib cage. Turtles have no teeth; feeding is by a combination of biting beak-like jaws, and suction-feeding in aquatic forms. Turtle cardiovascular anatomy is clearly adapted for diving.
Order Squamata consists of the lizards (Suborder Lacertilia), snakes (Suborder Serpentes), and tuataras (Suborder Sphenodontia). Squamates have kinetic skulls and feed by grasping the prey and swallowing it whole. Lizards are the ancestral group. Most lizards are quadrupeds, but look closely at the display samples of the legless lizards or "glass-snakes". They may be distinguished from both salamanders and snakes by the presence of external ear openings and eyelids. Lizards also tend to hold the trunk slightly off of the ground, allowing somewhat more rapid and efficient terrestrial locomotion than in salamanders. Snakes are a group of legless descendants of lizards and move by undulating the body against the substrate. Most snakes are terrestrial, but some sea snakes are marine and return to the land only to breed. Snakes provide an extreme example of a kinetic skull with multiple jaw bones and hinges. A snake can generally swallow whole a prey which is larger than the apparent size of the snake's head. Most snakes are oviparous (lay eggs), but a few species are ovoviviparous (eggs hatch in the oviducts and live young are born). Tuataras are primitive lizard-like animals. The two surviving species of tuatara are both native to New Zealand.
Order Crocodilia includes crocodiles, caimans, alligators, and gavials. Crocodilians are distinguished from other reptiles by the elongated snout, the akinetic skull with a single pair of sturdy jaw hinges, and by the two postorbital temporal fenestrae (diapsid = two skull openings behind the eye socket). Crocodilians also have a true four-chambered heart, however there are vascular shunts between the systemic and pulmonary circulations. Crocodilians generally feed by ripping chunks of flesh from their prey.
CLASS AVES (birds)
Birds have numerous adaptations for flight, although some ("ratite" birds such as the ostrich, kiwi, and penguins) have secondarily lost this capacity. The most distinguishing features of birds are feathers and the modification of the forelimb into wings. Feathers are keratinized elaborations of basic reptilian scales, which originally evolved to serve an insulating function.
Bird skulls are largely akinetic. Birds are toothless; the horny beak is used to grasp or tear food objects. The anterior part of the stomach is modified into a muscular gizzard for grinding up ingested food. The respiratory system of birds is elaborated to include multiple passages, air sacs, and hollow spaces within the long bones. All flying birds also have a prominent keel or carina (derived from the sternum - see sample skeletons) which anchors the flight muscles. Birds are completely homeothermic and endothermic, have true four-chambered hearts, and have separated systemic and pulmonary circulations. Bird eggs have calcified shells. Extensive parental care of infant offspring is also characteristic of birds.
CLASS MAMMALIA (mammals)
The two most obvious distinguishing features of mammals are hair and mammary glands. Hairs are keratinized structures derived from scales, and provide an insulating coat for most mammals. Mammary glands are modified sweat glands which provide nourishment for infant offspring. In terrestrial mammals, the limbs are positioned directly under the trunk, and the vertebral column is braced to hold both the trunk and the head up. This allows very rapid and efficient locomotion, either by running or hopping. Mammals have akinetic skulls, comparatively large craniums, and large, elaborate brains.
Most mammals (with the exception of the edendates and cetaceans) have heterodont teeth, that is, several kinds of teeth which serve specialized functions. Mammals feed by biting and chewing. Chewing is uniquely mammalian, and is made possible by lateral mobility of the jaw joints, as well as cheeks and lips which keep food in the mouth. Mammals are endothermic and most are fully homeothermic. They have true four-chambered hearts with separate systemic and pulmonary circulatory loops. Most mammals (with the exception of the monotremes) bear live young and provide extensive parental care.
The three major groups of mammals are the monotremes (Subclass Protheria - platypuses and spiny anteaters), marsupials (Subclass Theria, Infraclass Metatheria - pouched animals such as kangaroos and opossums) and the placental mammals (Subclass Theria, Infraclass Eutheria - all other mammals).
Monotremes are indigenous to Australia and unique among the mammals in that they have a common urogenital opening or cloaca, lay leathery eggs, and lack nipples on the mammary glands. Marsupials are indigenous to both Australia and the New World.
In marsupials the young are "born" at an embryonic level of development, migrate across the abdomen of the mother, enter a marsupium or pouch, and attach to a mammary nipple. Subsequent development takes place in this pouch, until the young become independent. This is actually a very efficient system, which allows a female marsupial to simultaneously have three litters of offspring - one "on foot", one in the pouch, and one in the uterus.
Placental mammals are the most speciose, widely distributed, and varied group. They are characterized by the development of a placenta, which nourishes the developing fetus within the uterus and allows live birth of relatively developed offspring. We have samples on display for most of the following prominent groups: the edentates (Order Edentata - such as armadillos), insectivores (Order Insectivora - such as shrews and moles), bats (Order Chiroptera), primates (Order Primates - prosimians, monkeys, and apes; the latter including people), rodents (Order Rodentia - rats, mice, squirrels and their allies), lagomorphs (Order Lagomorpha - rabbits, picas, cavies, and their allies), ungulates (Orders Artiodactyla and Perissodactyla - hoofed grazing or browsing herbivores), and carnivores (Order Carnivora - cats, dogs, bears, raccoons, mustelids, seals, sea lions, etc.). Most mammals are terrestrial quadrupeds (locomote on four legs). Some primates are bipedal (locomote on two legs) or, more commonly bibrachial (swing on two arms). Bats (Order Chiroptera) constitute an insectivore offshoot that has independently evolved true flight. Cetaceans (Order Cetacea - porpoises and whales) and sirenians (Order Sirenia - manatees and dugongs) are two groups of ungulates which have independently returned to completely aquatic lives.