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BIOLOGICAL DIVERSITY: ANIMALSIII Table of Contents Evolutionary Historyof Animal Groups |Deuterostomesand Protostomes | PhylumEchinodermata Phylum Hemichordata |Phylum Chordata |SubphylumUrochordata | SubphylumCephalochordata Subphylum Vertebrata| ClassChondrichthyes | ClassOsteichthyes | ClassAmphibia | ClassReptilia Class Aves | ClassMammalia | Links EvolutionaryHistory of Animal Groups | Backto Top Coelomates are animals that have internal bodycavities, or coeloms.Humans are coelomates, since we have an abdomenal cavity containingdigestive organs, some of the excretory and reproductive organs, anda thoracic cavity that contains the heart and lungs. Coelomates alsoform a variety of internal and external skeletons. External skeletonsand coeloms appeared during the Cambrian-Ordovician time. Theseskeletons offered several advantages to their producers: Secretion of a mineral shell that allowed the animal to use the shell as a mineral repository. Protection from drying out in the intertidal zone during low tides. Protection from predators. Sites for anchoring muscle attachments, offering new patterns of locomotion and increased strength. First appearances and relative diversity (width ofshaded area) for major groups of animals. Image from Purves etal., Life: The Science of Biology, 4th Edition, by SinauerAssociates (www.sinauer.com) andWH Freeman (www.whfreeman.com),used with permission. Deuterostomesand Protostomes | Backto Top Protostomes(mollusks, annelids, and arthropods) develop so that the firstopening in the embryois the mouth (protostome = first mouth). Protostomes arebilaterallysymmetrical, have three germ layers, theorgan level of organization, the tube-within-a-tubebody plan, and a true coelom. The coelom,a body cavity between the digestive tract and body wall completelylined by mesodermallows the digestive system and body wall to move independently.Because of this, internal organs can be more complex. Coelomic fluidassists respiration and circulation by diffusing nutrients, andexcretion by accumulating wastes. This fluid functions in place ofseveral organ systems in the higher animals. The coelom may serve asa storage area for eggs and sperm, facilitating development of thesegametes within the animal body. Coelomic fluid protects internalorgans and also serves as a hydrostaticskeleton. Protostomes develop their embryoby spiral cleavage. Deuterostomes(as exemplified by the echinoderms and chordates) develop the anusfirst, then the mouth at the other end of the embryo. Deuterostomesare coelomate animals these embryological characteristics: Radial cleavage in embryonic cell division: the daughter cells sit on top of previous cells. Fate of cells is indeterminate; if embryonic cells are separated, each one develops into a complete organism. The blastopore is associated with the anus, and the second embryonic opening is associated with the mouth. Differences in cleavage between the embryos ofprotostomes and deuterostomes. Images from Purves et al.,Life: The Science of Biology, 4th Edition, by SinauerAssociates (www.sinauer.com) andWH Freeman (www.whfreeman.com),used with permission. The previous chapter looked at the protostomes(annelids, molluscs, and arthropods). This chapter will examine thedeuterostomes, primarily the echinoderms and chordates. PhylumEchinodermata | Backto Top There are 6000 species of echinoderms; all extantspecies are marine. The phylum Echinodermataincludes the sea urchins, sea stars, sea cucumbers, andstarfish. Most adults have radialsymmetry, while their larvae arebilaterallysymmetrical.Echinoderms have an endoskeletonconsisting of calcareous plates bearing spines. Radial symmetryappears to be an advantage to the mostly bottom-dwelling echinoderms,who can thus feed in every direction. Adultechinoderms have no brain. Members of thephylum have a water vascular system that powers their multitude oftube feet. Anatomy of an adult sea star. Lower (right) imageis a cross section through an arm of the adult sea star.Images from Purves et al., Life: The Science of Biology, 4thEdition, by Sinauer Associates (www.sinauer.com)and WH Freeman (www.whfreeman.com),used with permission. Classification of Echinodermata There are several taxonomic classes of echinodermsof varying familarity to the general public. The Class Crinoidea The class Crinoidea includes about 600 species ofcrinoids, the stalked sea "lilies" and the motile feather stars.Their branched arms are used for filter-feeding and give the animalsa flowerlike or plantlike appearance (hence the term sea lily).Crinoid stalks and blastoid heads are common fossils in certain partsof North America. Specimen of the Carboniferous crinoidParadichocrinus planus. Image is from http://www.tyrrellmuseum.com/tour/crinoid.jpg. The Class Holothuroidea The class Holothuroidea has 1,500 species of seacucumbers. Sorry, these are NOT good eating as a general rule!Holothuroideans have a long leathery body, and feed by tentacleslocated around their mouth. The Class Echinoidea Scientists generally place about 950 species ofsea urchins and sand dollars are in the class Echinoidea. Both seaurchins and sand dollars have spines thar they use for locomotion,defense, and burrowing. Sea urchins have long, blunt spines. Sanddollars are flattened with a five-part flowerlike pattern of poresfor skin projections. This image of a pencil sea urchin (left) is from http://www.seashells.org/identcatagories/seaurchins.htm;the right image of a bed of sea urchins is from http://seaurchin.org/. The image of a sand dollar is from http://www.weblync.com/ccbeach/. The Class Ophiuroidea The class Ophiuroidea consists of approximately2,000 species of brittle stars. Brittle stars have a central diskfrom which long, flexible arms radiate. These long arms allow them tomove rapidly. The Class Asteroidea The class Asteroidea contains about 1,500 speciesof sea stars (commonly known as starfishes). Most starfish have adorsoventrally flattened body. Starfish have a central disk to whichfive, or a multiple of five, sturdy arms are attached. Sea stars arecommon along rocky coasts where they eat clams, oysters, and otherbivalves. The five-rayed body has an oral (mouth) and aboral(upper) side. Spines project from the endoskeletal plate through thethin dermis. Pincerlike pedicellarie keep the surface free fromparticles. Gas exchange is conducted by skin gills. On the oralsurface, each arm has a groove lined with tube feet. A sea star feeds by everting its stomach. Itpositions itself over a bivalve and attaches tube feet to each sideof the shell. By working tube feet in an alternating fashion, itopens the shell open. Only a small crack is needed to insert itscardiac stomach into the prey. Stomach enzymes begin digesting thebivalve as it is trying to close its shell. Partially digested foodis then taken into the pyloric stomach for complete digestion. Ashort intestine opens at the anus on the aboral side. Each arm has a well-developed coelom containing apair of digestive glands and male or female gonads. The nervoussystem is a central ring with radial nerves in each arm. Alight-sensitive eyespotis at the end of each arm, facilitating coordinated but slowresponses. Locomotion depends upon the water vascular system.Water enters on the aboral side through the sieve plate, which isalso known as the madreporite. Water passes through a stone canal toa ring canal and into the radial canals in each arm. The radialcanals feed into lateral canals extending into tube feet, each ofwhich has an ampulla. Contraction of an ampulla forces water into thetube foot, expanding it; when the foot touches a surface, the centerwithdraws forming a suction and adhering to surfaces. Echinoderms lack complex respiratory, excretory,and circulatory systems. Fluids within the coelomic cavity carry outthe function of diffusing substances and gases. Gas exchange occursacross the skin gills and tube feet. Nitrogenous wastes diffusethrough coelomic fluid and across the body wall. Cilia on theperitoneum lining the coelom keep the coelomic fluidmoving. Sea stars reproduce both sexually and asexually.If the body is fragmented, each fragment can regenerate a wholeanimal. Sea stars spawn and release either eggs or sperm at the sametime. The bilateral larvae undergoes a metamorphosis to become aradially symmetrical adult. PhylumHemichordata | Backto Top The Phylum Hemichordata includes about 90 speciesof acorn worms. Some acorn worms living on tidal mud flats have aproboscis, a collar, and a trunk. The dorsal nerve cord in the collarand trunk resembles the nerve cord of chordates. The pharynx belowthe collar has gill slits. Larva of hemichordates resembles the larvaof echinoderms. These facts cause some scientists to believeechinoderms and hemichordates share a common ancestor and thathemichordates and chordates are related by a commonancestor. Hemichordates are classified into three classes,two with living members, one consisting of only fossil forms. TheEnteropneusta includes the acorn worms; the Pterobranchia includes 20species of colonial bottom dwelling worms. The fossil (extinct)group, the Graptolithina (graptolites), are common fossils in theOrdovician and Silurian. They have recently been placed in thisphylum. Specimen of acorn worm (L), a member of the PhylumHemichordata, and a fossil of a graptolite (R, Monograptus), amember of an extinct group of hemichorates. Images fromhttp://www.ucmp.berkeley.edu/chordata/hemichordata.html. PhylumChordata | Backto Top The Phylum Chordata includes about 45,000 speciesthat occupy nearly all environments. All chordates at sometime duringtheir life history have: a notochord: a dorsal supporting rod located dorsally just below the nerve cord; it provides support and is replaced by the vertebral column in vertebrates a dorsal hollow nerve cord: a fluid-filled canal; spinal cord is protected by vertebrae pharyngeal gill pouches: openings that function in feeding, gas exchange, or both These features are seen only during embryonicdevelopment in most vertebrates. Not all chordates arevertebrates.In the invertebrate chordates, fish, and amphibian larvae, pharyngealgill pouches become functioning gills. Terrestrial vertebrates havetheir pouches modified for various purposes; in humans, the firstpouches become the auditory tubes, the second become tonsils, and thethird and fourth pairs become the thymus and parathyroidglands. Most chordates have an internal skeleton againstwhich muscles work. Most have a postanal tail that extends beyond theanus; in some (like humans), this may only appear in embryos. The evolutionary origin of chordates remains amystery, although biochemistry and comparative embryology indicatesechinoderms and chordates share a common ancestry. Although scanty,fossil finds from the Cambriansuggest chordates were present in the Burgess Shale deposits. Invertebrate Chordates Not all chordates are vertebrates. Some chordatesare invertebrates, lacking a vertebral column. In these invertebratechordates, the notochord persists and is never replaced by thevertebral column. SubphylumUrochordata The subphylum Urochordata contains 1,250 speciesof tunuicates that have gill slits. Adults have a body composed of anouter tunic with an incurrent and excurrent siphon. When they aredisturbed, tunicates tend to squirt water out. Water passes into apharynx and out numerous gill slits, the only chordate characteristicthat remains in adults. Microscopic particles adhere to a mucoussecretion in the pharynx and are eaten. The larvae are bilaterallysymmetrical and have the three chordate characteristics. Tunicatelarva metamorphose into the sessile adult. Beating of numerous cilialining the inside of the pharynx creates a current to move waterthrough a tunicate. Some suggest larvae became sexually maturewithout developing tunicate characteristics; thus, a urochordatelarva was ancestral to vertebrates; or a cephalochordate larva mayhave been ancestral to vertebrates. Habit photograph of an unidentifiedurochordate. Image is from http://www.ucmp.berkeley.edu/chordata/urochordata.html. SubphylumCephalochordata The lancelets have three chordate characteristics.The 23 species of lancelets are in the genus Branchiostoma inthe subphylum Cephalochordata. Their elongated, lance-shaped bodyresembles the lancelet, a two-edged surgical knife. They inhabitshallow coastal waters; they lie partly buried in sandy substratesand filter feed. Lancelets feed on microscopic particles filteredfrom the constant stream of water that enters the mouth and exitsthrough the gill slits into an atrium that opens at the atriopore.Lancelets retain the three chordate characteristics as an adult. Thenotochord extends from head to tail, accounting for the name"Cephalochordata." Lancelets have segmented muscles and their dorsalhollow nerve cord has periodic branches. Anatomy of Ascaris, a typicalchordate. Image from Purves et al., Life: The Science ofBiology, 4th Edition, by Sinauer Associates (www.sinauer.com)and WH Freeman (www.whfreeman.com),used with permission. Images of Pikaia, a fossil chordate fromthe Burgess Shale (Cambrian) of British Columbia, Canada.Left, reconstruction diagram from http://www.nmnh.si.edu/paleo/ppikaia.htm;right (fossil), from http://www.nmnh.si.edu/paleo/fpikaia.gif. Vertebrate Chordates The vertebrates comprise a large group ofchordates, and are subdivided into seven classes (3 classes of fish,amphibians, reptiles, birds, and mammals).Vertebrates have an internal skeleton of cartilage orbone, with vertebrae surrounding the dorsal nerve cord. SubphylumVertebrata The subphylum Vertebrata consists of about 43,700species of animals with backbones. Vertebrates exhibit all three ofthe chordate characteristics at some point during their lives. Theembryonic notochord is replaced by a vertebral column in the adult.The vertebral column is made of individual hard segments(vertebrae)surrounding the dorsal hollow nerve cord. The nerve cord is the onechordate feature present in the adult phase of allvertebrates. The vertebral column, part of a flexible but strongendoskeleton,is evidence that vertebrates are segmented. The vertebrate skeletonis living tissue (either cartilage or bone) that grows as the animalgrows. The endoskeleton and muscles form an organ system(the musculoskeletal system) that permits rapid and efficientmovement. The pectoral and pelvic fins of fishes evolved into jointedappendages that allowed vertebrates to move onto land. The skull, themost anterior component of the main axis of the vertebrateendoskeleton, encases the brain.The high degree of cephalizationin vertebrates is accompanied by complex sense organs concentrated inthe head region. Eyes developed as outgrowths of the brain. Ears wereequilibrium devices in aquatic vertebrates that function assound-wave receivers in land vertebrates. Vertebrates have a completedigestive system and a large coelom. Their circulatory system isclosed, with respiratory pigments contained within blood vessels. Gasexchange is efficiently accomplished by gills, lungs, and in a fewcases, moist skin. Kidneys are efficient in excretion of nitrogenouswaste and regulation of water. Reproduction is usually sexual withseparate sexes. Classification of the Vertebrata The first vertebrates were fishlike. Fishes areaquatic, gill-breathing vertebrates that usually have fins and skincovered with scales. The larval form of a modern-day lamprey, whichlooks like a lancelet, may resemble the first vertebrates: it has thethree chordate characteristics (like the tunicate larva), as well asa two-chambered heart, a three-part brain, and other internal organsthat are like those of vertebrates. Small, jawless, and finless ostracoderms were theearliest vertebrates. They were filter feeders, but probably werealso able to move water through their gills by muscular action.Ostracoderms have been found as fossils from the Cambrian throughDevonian periods, when the group finally went extinct. Althoughextant jawless fishes lack protection, many early jawless fishes hadlarge defensive head shields. Class Petromyzontiformes, Lampreys These long, eel-like, jawless fish arefree-swimming predators on other fish. Lampreys hatch in freshwaterand many live their lives entirely in freshwater. Some lampreysmigrate to the sea, but must return to freshwater to reproduce.Lampreys have a sucker-like mouth that lacks a jaw. Drawing of several lampreys. From the ImageGallery at lycos.com. (Five Lampreys, Lydon, A. F., 1879 AnimalKingdom: Vintage Illustration). Sea lamprey on lake trout. Image courtesyof Great Lakes Fishery Commission (http://www.glfc.org/slft.htm),used by permission. Sea lamprey mouth, close-up. Image courtesyof Great Lakes Fishery Commission (http://www.glfc.org/slft.htm),used by permission. Class Myxini, Hagfish Members of the class Myxini have a partial cranium(skull), but no vertebrae. Their skeleton is made of of cartilage, asis that of sharks. Hagfish lack jaws, and for this reason used to beclassified with the lampreys in a group called the Agnatha ("nojaws") or the Cyclostomata ("round mouth"). This image of a hagfish is from http://oberon.educ.sfu.ca/splash/wednesday/hag1.jpg.The yellow in the background is the slime that hagfish secrete. Fish: Vertebrates With Jaws The fish first appeared during the CambrianPeriod. Whether fish first evolved in fresh or salt water is unclearfrom the fossil record. The jawless fish arethe most primitive group, although they were a very important groupduring the Silurian and Devonian periods.Hagfish and lampreys are the only living members ofthis class today. They have long, cylindricalbodies with cartilage skeletons and no paired fins. The first jawed fish were the Placoderms, anextinct group of Devonian-aged jawed fishes. Placoderms were armoredwith heavy plates and had strong jaws and paired pectoral and pelvicfins. Paired fins allow fish to balance and to maneuver well inwater, which facilitate both predation and escape. The fossil on the right is a cast of the placoderm,Bothriolepis, is from http://www.toyen.uio.no/palmus/galleri/montre/english/x18.htm.The fossil on the left is a model of the placoderm Coccosteusand is from http://www.toyen.uio.no/palmus/galleri/montre/english/x19.htm. The evolution of jaws is an example ofevolutionary modification of existing structures to perform newfunctions. Jaws are modified gill arches, andallowed the exploitation of new roles in the habitats: predators withpowerful jaws. There are two classes of jawedfish: the cartilaginous fish and the bony fish. Steps in the evolution of jaws by modification ofgill arches. Images from Purves et al., Life: The Scienceof Biology, 4th Edition, by Sinauer Associates (www.sinauer.com)and WH Freeman (www.whfreeman.com),used with permission. ClassChondrichthyes: Cartilaginous Fish The class Chondrichthyes contains approximately850 species of skates, rays, andsharks. They have jaws, lots of teeth, pairedfins, and a cartilage endoskeleton.Cartilaginous fish first appeared during the DevonianPeriod and expanded in diversity during the Carboniferous and Permianbefore nearly disappearing during the great extinction that occurrednear the end of the Permian. A large group ofcartilagenous fish still survives today and is an important part ofthe marine fauna. These fish have five to seven gill slits on bothsides of the pharynx, and lack the gill covers found in bony fish.The chondrichthyian body is covered epidermal placoid (or toothlike)scales. Developmental studies show the teeth of sharks are enlargedscales. The largest sharks are filter feeders, not thepredators of Hollywood movies. Basking and whale sharks eat tons ofcrustaceans (small krills, etc.) filtered from the water. Most sharksare fast-swimming, open-sea predators. The great white shark feeds ondolphins, sea lions and seals (and people sometimes). In other words,anything is WANTS to! Shark ansd Ray GIFs from http://www.postmodern.com/~fi/sharkpics/clipart/large1.htmand http://www.postmodern.com/~fi/sharkpics/clipart/rays.htm. Rays and skates live on the ocean floor; theirpectoral fins are enlarged into winglike fins; they swim slowly.Stingrays have a venomous spine. The electric ray family can feed onfish that have been stunned with electric shock of over 300 volts.Sawfish rays have a large anterior "saw" that they use to slashthrough schools of fish. (sharkanimated GIF from http://www.postmodern.com/~fi/sharkpics/clipart/animated.htm) ClassOsteichthyes, the Bony Fish There are about 20,000 species of bony fish, foundboth in marine and freshwater, comprising the classOsteichthyes. This class is divided into twogroups: the lobe-finned (Sarcopterygii) and ray-finned fish(Actinopterygii). The bony fish have a bony skeleton. Most species inthis class are ray-finned with thin, bony rays supporting the fins. Afew fishes are lobe-finned and are thought to be related to theancestors of amphibians. Cross section of a fish. Note the location ofvarious internal structures such as the vertebra, coelom, andgut. Image from Purves et al., Life: The Science ofBiology, 4th Edition, by Sinauer Associates (www.sinauer.com)and WH Freeman (www.whfreeman.com),used with permission. Ray-finned Fish (Actinopterygii) The ray-finned fish include familiar species suchas tuna, bass, perch, and trout. Ray-finnedfish are the most successful and diverse of the vertebrates (morethan half of all vertebrate species belong to this group). Thin, bonysupports with radiating bones (hence the term ray-finned) hold thefins away from the body. Ray-finned fish obtain their food by filterfeeding and by preying on insects and other animals. Their skin iscovered by scales formed of bone. These scales are homologous to ourown hair (and the feathers of birds), being derived from the sameembryonic tissues. The gills in this group of fish do not openseparately and are covered by an operculum. Ray-finned fish have aswim bladder, a gas-filled sac, that regulates buoyancy and depth.Sharks lack this feature, which enables fish to "sleep" withoutsinking. The swim bladder acts much the way a ballast tank does on asubmarine to control buoyancy. Salmon, trout, and eels can migrate from freshwater to salt water, but must adjust kidney and gill function to thetonicity of their environments. In freshwater, the fish ishyoptonicrelative to its aqueous (watery) environment. Water is constantlyflooding into the fish, and must be removed by the fish'sexcretorysystem. In seawater, the fish is nowhypertonicor isotonicrelative to the seawater, requiring conservation of body water. Bony fishes depend on color vision to detect bothrivals and mates. Sperm and eggs are released into the water, withnot much parental care for the newborn. Most fish have fertilizationand embryonic development taking place outside the female'sbody. Lobe-finned Fish (Sarcopterygii) This group includes six species of lungfishes andone species of coelacanth that has muscular fins with large, jointedbones attaching the fins to the body. Lobe-finned fish have fleshyfins supported by central bones, homologous to the bones in your armsand legs. These fins underwent modification,becoming the limbs of amphibians and their evolutionary descendantssuch as lizards, canaries, dinosaurs, and humans. The lungfish are a small group found mostly infreshwater stagnant water or ponds that dry up in Africa, SouthAmerica, and Australia. Australian lungfish. Image is fromhttp://www.burkesbackyard.com.au/facts/1998/pets/images/lungfish_16.gif. Coelacanths live in deep oceans. They were onceconsidered extinct, although more than 200 have been captured since1938. Mitochondrial DNA analysis supports the hypothesis thatlungfish are probably the closest living relatives ofamphibians. Coelacanth, a living fossil. Image is fromhttp://www.dierentuin.net/pictures/coelacanth.jpg. The crossopterygian fish (represented by themarine extant deep-living coelacanth and extinct freshwater forms)are regarded as ancestors of early amphibians.Extinct crossopterygians had strong fins, lungs, anda streamlined body capable of swimming as well as traveling shortdistances out of water. Comparison of the skeletons of a crossopterygianlobe-finned fish and an early amphibian. Image from Purves etal., Life: The Science of Biology, 4th Edition, by SinauerAssociates (www.sinauer.com) andWH Freeman (www.whfreeman.com),used with permission. The "Tetrapods" The term "tetrapod" (meaning four-limbed orfour-footed) has historically been applied to the land vertebrates(amphibians, reptiles, dinosaurs, birds, and mammals). There haverecently been proposals to restrict its use to a more cladisticallysound definition, summarized by TheDefinition of the Taxon Tetrapoda. Allother animals from this point have four limbs and are calledtetrapods. I use the term here not in the strict cladistic sense, butin a more inclusive sense to include the living forms as well asfossil vertebrates that had free digits. A discussion of tetrapodphylogeny is available at Phylogenyof stegocephalians, from theTreeof Life Page. Most zoologists would accept that the Devonianlobe-finned fishes were ancestral to the amphibians. Animals (bothvertebrate as well as many invertebrates such as insects) that liveon land use limbs to support the body, especially since air is lessbuoyant than water. Lobe-finned fishes and early amphibians also hadlungs and internal nares to respire air. Two hypotheses have been proposed to explain theevolution of amphibians from lobe-finned fishes. Lobe-finned fishes capable of moving from pond-to-pond had an advantage over those that could not. The supply of food on land, and the absence of predators, promoted adaptation to land. The first amphibians diversified duringCarboniferous Period (commonly known as the Age of Amphibians). Class Amphibia:Animals Move Ashore This class includes 4000 species of animals thatspend their larval/juvenile stages in water, and their adult life onland. Amphibians must return to water to mateand lay eggs. Most adults have moist skin that functions in helpingtheir small, inefficient lungs with gas exchange.Frogs, toads, newts, salamanders, and mud puppies arein this transitional group between water and land. Amphibian features not seen in bony fishinclude: Limbs with girdles of bone that are adapted for walking on land. A tongue that can be used for catching prey as well as sensory input. Eyelids that help keep the eyes moist. Ears adapted for detecting sound waves moving through the thin (as compared to water) medium of the air. A larynx adapted for vocalization. A larger brain than that of fish, and a more developed cerebral cortex. Skin that is thin, smooth, nonscaly, and contains numerous mucous glands; the skin plays an active role in osmotic balance and respiration. Development of a lung that is permanently used for gas exchange in the adult form, although some amphibians supplement lung function by exchange of gases across a porous (moist) skin. A closed double-loop circulatory system that replaces the single-loop circulatory path of fish. Development of a three-chambered heart that pumps mixed blood before and after it has gone to the lungs. Reproduction involves a return to the water. Therterm "amphibian" refers to two life styles, one in water, the otheron land. Amphibians shed eggs into the water where externalfertilization occurs, as it does in fish. Generally, amphibian eggsare protected by a coat of jelly but not by a shell. The young hatchinto aquatic larvae with gills (tadpoles). Aquatic larvae usuallyundergo metamorphosis to develop into a terrestrial adult. Amphibians, like fish, are ectothermic;they depend upon external heat to regulate body temperatures. If theenvironmental temperature becomes too low, ectotherms becomeinactive. Salamanders more likely resemble earliestamphibians due to their S-shaped movements. Salamanders practiceinternal fertilization; males produce a spermatophore that femalespick up. Frogs and toads are tailless as adults, with their hindlimbs specialized for jumping. Vintage illustration of frogs (left), from the Internet. Closeupof a frog, from http://www.conservation.state.mo.us/conmag/1997/03/52.jpg. Class Reptilia:Reproducton Without Water This class of 6000 species includes the snakes,lizards, turtles, alligators, and crocodiles.Reptiles that lay eggs lay an egg surrounded by athick protective shell and a series of internal membranes.Reptiles have internal fertilization: their gametesdo not need to be released into water for fertilization tooccur. The amnioticegg is a superb adaptation to life onland. While amphibians need to lay their eggs in water, theirdescendants (reptiles) were not as strongly tied to moistenvironments and could truly expand into more arid areas. Reptileswere the first land vertebrates to practice internal fertilizationthrough copulation and to lay eggs that are protected by a leatheryshell with food and other support for the growing embryo. The amniote egg contains extraembryonic membranesthat are not part of the embryo and are disposed of after the embryohas developed and hatched. These membranes protect the embryo, removenitrogenous wastes, and provide the embryo with oxygen, food, andwater. The amnion, one of these extraembryonic membranes, creates asac that fills with fluid and provides a watery environment in whichthe embryo develops. The embryo develops in a "pond within theshell". Structure of the amniote egg, a definingcharacteristic for reptiles, birds, and mammals. Image fromPurves et al., Life: The Science of Biology, 4th Edition, bySinauer Associates (www.sinauer.com)and WH Freeman (www.whfreeman.com),used with permission. Evolutionary History of Reptiles Reptiles first evolved during the Carboniferoustime and partly displaced amphibians in many environments. The firstreptiles (often referred to as the stem reptiles) gave rise toseveral other lineages, each of which adapted to a different way oflife. Reptilian success was due to their terrestrial (amniotic) eggand internal fertilization, as well as their tough leathery skin,more efficient teeth and jaws, and in some, bipedalism (traveling ontheir hind legs, allowing the forelimbs to grasp prey or food, orbecome wings). One group, the Pelycosaurs (fin-backed or saillizards) are related to therapsids, mammal-like reptiles ancestral tomammals. Other groups returned to aquatic environments. Ichthyosaurswere fishlike (or dolphin-like) free-swimming predators of theMesozoic seas. The plesiosaurs had a long neck and a body adapted tpswimming though use of flippers (legs that evolutionarily reverted toa flipper-like shape). These free-swimmers also adapted to live birthof their young (since they could not return to the land to lay eggs).Thecodonts were the reptiles that gave rise to most of the reptiles,living and extinct. Pterosaurs were flying reptiles that dominatedthe Mesozoic skies. They had a keel for attachment of flight musclesand air spaces in bones to reduce weight. Dinosaurs (descended from some thecodonts) andmammal-like reptiles' had their limbs beneath the body providingincreased agility and facilitating gigantic size. Lizards have theirelbows out (like you do when you do a push-up). By having theirelbows in, dinosaurs and mammals place more of the weight of the bodyon the long bones instead of the elbows, ankles, and knees. Relationship between limbs and body. Note thatreptiles have their upper limbs jutting out from the body, whilemammals have their limbs in line with the body, supporting and moreeasily raising the body mass off the ground. Image from Purveset al., Life: The Science of Biology, 4th Edition, by SinauerAssociates (www.sinauer.com) andWH Freeman (www.whfreeman.com),used with permission. Reptiles dominated the earth for about 170 millionyears during the Mesozoic Era. The mass extinction of many reptilegroups at the close of the Mesozoic (the Cretaceous Period) has beenwell documented and the subject of many hypotheses. The 1980hypothesis by Luis and Walter Alvarez and others proposes the impactof a large meteorite at the end of the Cretaceous period caused acatastrophic environmental collapse that led to the extinction ofnearly 50% of all species of life on Earth. The survivors, birds andmammals, reaped the spoils and diversified during the Cenozoic Era.Three groups of reptiles remain: turtles, snakes/lizards, andcrocodiles/alligators. About 6,000 species of reptiles comprise the ClassReptilia. Most live in tropics or subtropics. Lizards and snakes liveon land, while turtles and alligators live in water for much of theirlives. Reptiles have a thick, scaly skin that is keratinized andimpermeable to water. This same keratin is a protein found in hair,fingernails, and feathers. Protective skin prevents water loss butrequires several molts a year. Reptilian lungs are more developedthan those of amphibians. Air moves in and out of the lungs due tothe presence of an expandable rib cage in all reptiles exceptturtles. Most reptiles have a nearly four-chambered heart. Thecrocodile has a completely four-chambered heart that more fullyseparates oxygen-rich blood from from deoxygenated or oxygen-poorblood. The well-developed kidneys excrete uric acid; less water islost in excretion. Reptiles are ectothermic; they require a fractionof the food per body weight of birds and mammals, but arebehaviorally adapted to warm their body temperature by sunbathing. Photograph of a lizard (L) and a gavial (R), by Bill Everitt,obtained from PicturesNOW! Snakes and lizards live mainly in the tropics anddesert. Lizards have four clawed legs and are carnivorous; marineiguanas on the Galapagos are adapted to spend time in the sea;frilled lizards have a collar to scare predators, and blind wormlizards live underground. Snakes evolved from lizards and lost theirlegs as an adaptation to burrowing. Their jaws can readily dislocateto engulf large food. The snake's tongue collects airborne moleculesand transfers them to the Jacobson's organ for tasting. Somepoisonous snakes have special fangs for injecting theirvenom. Turtles have a heavy shell fused to the ribs andthoracic vertebrae; they lack teeth but use a sharp beak; sea turtlesmust leave the ocean to lay eggs onshore. Galapagos tortoises, image from http://home.capp.ch/marcel/Gal_Turt.htm. Crocodiles and alligators are largely aquatic,feeding on fishes and other animals. They both have a muscular tailthat acts as a paddle to swim and a weapon. The male crocodilebellows to attract mates. In some species the male also protects theeggs and young. The Archosauria: Birds, Dinosaurs, andMore Cladistic analyses place the birds, alligators,and dinosaurs in the same clade, the Archosauria (or "rulingreptiles"). This group is a major group of diapsids (vertebrates thathave two openings in their skulls) that have single openings in eachside of the skull, in front of the eyes (antorbital fenestrae), amongother characteristics. This helps to lighten the skull, provides moreroom for muscles and other tissues, and allows more skull flexibilitywhen eating. Other typical archosaurian characteristics includeanother opening in the lower jaw (the mandibular fenestra), a highnarrow skull with a pointed snout, teeth set in sockets, and amodified ankle joint. The ancestral archosaurs probably originated some250 million years or so ago, during the late Permian period. Theirdescendants (such as the dinosaurs) dominated the realm of theterrestrial vertebrates for a most of the Mesozoic Era. The birds andcrocodilians are the last living groups of archosaurs. Class Aves: Birds of aFeather The class Aves (birds) contains about 9000species. Birds evolved from either a dinosaurian or other reptiliangroup during the Jurassic (or possibly earlier). The earliest birdfossils, such as the Jurassic Archaeopteryx or TriassicProtavis, display a mosaic of reptilian and bird features(teeth in the bill, a jointed tail, and claws on the wing arereptilian; feathers and hollow bones are bird-like). Archaeopteryx, once considered the firstbird. The fossil is from the Solenhoefen Limestone (Jurassic) ofGermany. Image is from http://www.ucmp.berkeley.edu/diapsids/birds/. The distinguishing feature of birds is feathers:which provide insulation as well as aid in flight. Structure of a feather. Image from Purveset al., Life: The Science of Biology, 4th Edition, by SinauerAssociates (www.sinauer.com) andWH Freeman (www.whfreeman.com),used with permission. Hummingbird Feather Shaft, Barbs and Barbules (SEMx185). This image is copyright Dennis Kunkel at www.DennisKunkel.com,used with permission. Remember, not all animals that fly have feathers,but all almost every endothermic animal (warm-blooded) has a coveringof hair or feathers for insulation. The recent (1999) discovery of a"feathered" dinosaur adds credence to this speculation. The dinosaurcould not fly, so of what use would feather be but insulation (orpossibly mating). Modern birds appeared during the early Tertiary,and have adapted to all modes of life: flying (condors, eagles,hummingbirds), flightless-running (ostriches, emus), and swimming(penguins). Birds exhibit complex matingrituals as well as social structure (a pecking order!). Images of birds from a rich media search at lycos.com. Class Mammalia: GotMilk? Class Mammalia contains around 5000 species placedin 26 orders (usually). The three unifyingmammalian characteristics are: hair the presence of three middle ear bones the production of milk by mammary glands Milk is a substance rich in fats and proteins.Mammary glands usually occur on the ventralsurface of females in rows (when there are more than two glands).Humans and apes have two mammary glands (one right, one left), whileother animals can have a dozen or more. All mammals have hair at somepoint during their life. Mammalian hair is composed of the proteinkeratin.Hair has several functions: 1) insulation; 2) sensory function(whiskers of a cat); 3) camouflage, a warning system to predators,communication of social information, gender, or threats; and 4)protection as an additional layer or by forming dangerous spines thatdeter predators. Modifications of the malleusand incus(bones from the jaw in reptiles) work with the stapesto allow mammals to hear sounds after they are transmitted from theoutside world to their inner ears by a chain of these threebones. Mammals first evolved from the mammal-likereptiles during the Triassicperiod, about the same time as the firstdinosaurs. However, mammals were minor playersin the world of the Mesozoic,and only diversified and became prominent after the extinction ofdinosaurs at the close of the Cretaceous period. Mammals have since occupied all roles once held bydinosaurs and their relatives (flying: bats; swimming: whales,dolphins; large predators: tigers, lions; large herbivores:elephants, rhinos), as well as a new one (thinkers and tool makers:humans). There are 4500 species of livingmammals. Mammalian Adaptations Mammals developed several adaptations that help explain their success. Teeth are specialized for cutting, shearing or grinding; thick enamel helps prevent teeth from wearing out. Mammals are capable of rapid locomotion. Brain sizes are larger per pound of body weight than most other animals'. Mammals have more efficient control over their body temperatures than do birds. Hair provides insulation. Mammary glands provide milk to nourish the young. Mammalian Classification Subclass Prototheria: Order Monotremata:Monotremes(typified by the platypus and echinda) lay eggs that have similarmembranes and structure to reptilian eggs.Females burrow in ground and incubates their eggs.Both males and females produce milk to nourish the young There aretwo families living today and quite a few known from the fossilrecord of Gondwana.Monotremes are today restricted to Australia and New Guinea. Theearliest fossil monotreme is from the early Cretaceous, and youngerfossils hint at a formerly more widespread distribution for thegroup. While their fossilrecord is scarce, zoologists believe thatmonotremes probably diverged from other mammals during the Mesozoic.Monotremes have many differences with other mammals and are oftenplaced in a separate group, the subclass Prototheria. They retainmany characters of their therapsid ancestors, such as laying eggs,limbs oriented with humerus and femur held lateral to body (morelizard-like), a cloaca, skulls with an almost birdlike appearance,and a lack of teeth in adults. This suggests that monotremes are thesister group to all other mammals. However, monotremes do have all ofthe mammalian defining features of the group. The above image of a platypus (L) is from http://www.australian.com/CairnsWildlifeInWild.html,image of echidna is from http://www.chowan.edu/acadp/science/courses/comparativeanatomy/monotremata/monotrematahtm1.htm. Subclass Metatheria: Marsupials(such as the koala, opossum, and kangaroo) are born while in anembryonic stage and finish development outside the mother's body,often in a pouch. Marsupial young leave theuterus,crawl to the pouch, and attach to the nipple of a mammary gland andcontinue their development. Marsupials were once widespread, buttoday are dominant only in Australia, where they underwent adaptiveradiation in the absence of placental mammals. The Metatheriacontains 272 species classified in several orders. Metatheresdiverged from the lineage leading to the eutherian (placental)mammals by the middle of the Cretaceous period in North America. Theearliest marsupial fossils resemble North American opossums.Marsupial fossils are found on other northern hemisphere continents,although they seem not to have been prominent elements of thosefaunas.On the other hand, in South America and Australia, marsupialscontinued to be dominant faunal elements. The marsupials of SouthAmerica began to go extinct in the late Miocene and Early Pliocene(Cenozoicera) when volcanic islands grew together and formed the Isthmus ofPanama, allowing North American placental mammals to cross into SouthAmerica. Australian marsupials remain diverse and dominant nativemammals of the fauna. During the Cenozoic Era many marsupials inSouth America and Australia underwent parallel(or convergent) evolution with placentalmammals elsewhere, producing marsupial "wolves", "lions", andsaber-toothed marsupial "cats". A Red Kangaroo (Megaleia rufa), mother withjoey, at the Smithsonian's National Zoological Park in Washington,D.C. Smithsonian Photo by Jessie Cohen. ©1992 SmithsonianInstitution. Image of a koala (top) and a tasmanian devil(bottom), from Australian Tourist Com. Subclass Eutheria: There are 4000 describedspecies of placentalmammals, a group that includes dogs, cats, and people. The subclassis defined by a true placentathat nourishes and protects the embryos held within the mother's bodyfor an extended gestationperiod (nearly two years for an elephant, and nine very longmonths for a human). The eutherian placenta has extraembryonicmembranes modified for internal development within the uterus. Thechorionis the fetal portion of placenta, while the uterine wall grows thematernal portion. The placenta exchanges nutrients, oxygen, andwastes between fetal and maternal blood. There are 12 orders of placental mammals.Classification is based on the mode of locomotion and methods ofobtaining food. Prominent orders include the bats (order Chiroptera),horses (order Perissodactyla), whales (order Cetacea), mice (orderRodentia), dogs (order Carnivora), and monkeys/apes/humans (orderPrimates). Image of a bengal tiger (L), from the India Tourist Ofc LA. Imageof a drill, from http://www.wqe.com/zoonet/atlanta/drill.jpg. Links | Back toTop The Five Kingdoms A table summarizing the kingdoms of living things. The Radiation of the First Animals Dr. Jere Lipps presents a well illustrated look at early animal evolution. Introduction to the Metazoa: Animals, Animals, Animals! This University of California Berkeley Museum of Paleontology site offers excellent information about the evolution and diversity of various animal groups. Animal Diversity Web A superb site from the University of Michigan's zoology folks that organizes and presents information and media about the various groups of animals. One problem I have with this site is that most of the images of animals are not visible off campus. However, as a primarily text-based site this is well worth the visit. The Virtual Silurian Reef Harley Davidson's aren't the only cool things to come out of Milwaukee. The exhibit from the Milwaukee Public Museum takes you through a reconstruction of an ocean reef as it existed several hundred million years ago during the Silurian Period. The Great White Shark Learn the truth abour the great white shark (no, not golfer Greg Norman). How do they live, reproduce, feed? All text contents ©1995, 2000, 2001, by M.J. Farabee. Usefor educational purposes is heartily encouraged. Back to Table ofContents Email: mj.farabee@emcmail.maricopa.edu Last modified: Tuesday May 18 2010 The URL of this page is: