Chapter 34
Given that I do not usually give the taxonomy lectures
(the material is addressed via projects and assignments) these notes will not
have any words removed.-Dr. G.
Deuterostomate Animals
Deuterostomate Animals
Deuterostome Ancestors
Echinoderms: Pentaradial Symmetry
Hemichordates: Conservative Evolution
Chordates: New Ways of Feeding
Colonizing the Land: Obtaining Oxygen from the Air
Birds: More Feathers and Better Flight
The Origin and Diversity of Mammals
Primates and the Origin of Humans
Deuterostomes and Protostomes: Shared Evolutionary Themes
Deuterostome Ancestors
A group of extinct animals known as the yunnanozoans are the likely ancestors of all deuterostomes.
These animals had a large mouth, six pairs of external gills, and a lightly cuticularized, segmented posterior body section.
Deuterostome Ancestors
Modern deuterostomes fall into two major clades.
The echinoderms and hemichordates compose one clade.
This group is characterized by a three-part coelom and bilaterally symmetrical, ciliated larvae.
The other clade includes the chordates.
The ancestors of this clade had nonfeeding, tadpole-like larvae and a unique dorsal supporting structure.
Echinoderms: Pentaradial Symmetry
Two major structural features evolved in the echinoderms (phylum Echinodermata).
One was a system of calcified internal plates covered by thin layers of skin and some muscles.
In some early echinoderm ancestors, these plates became fused inside the entire body, giving rise to an internal skeleton.
The other feature was a water vascular system, a network of calcified hydraulic canals leading to extensions called tube feet.
This system functions in gas exchange, locomotion, and feeding.
Echinoderms: Pentaradial Symmetry
The development of these two structural innovations led to a striking evolutionary radiation.
There have been about 23 echinoderm classes described. Only 6 classes survive today, with a total of about 7,000 species.
Nearly all living species have a bilaterally symmetrical, ciliated larva that feeds for some time as a planktonic organism before transforming into an adult with pentaradial symmetry.
Living echinoderms are divided into two lineages: the subphylum Pelmatozoa and the subphylum Eleutherozoa.
Echinoderms: Pentaradial Symmetry
The only surviving pelmatozoans are the sea lilies and the feather stars (class Crinoidea).
Most sea lilies attach to a substrate by means of a flexible stalk consisting of a stack of calcareous disks.
The main body of the animal is a cup-shaped structure that contains a tubular digestive system. Five to several hundred bendable arms extend outward from the cup.
Echinoderms: Pentaradial Symmetry
The arms are used for feeding. They are oriented in passing water currents so that food particles stick to tube feet on the arms, which transfer them to a groove that runs down the arm to the mouth.
Feather stars are similar to sea-lilies, but they have flexible appendages with which they grasp the substratum.
Echinoderms: Pentaradial Symmetry
Most of the surviving echinoderms are members of the eleutherozoan lineage.
The sea urchins and the sand dollars (class Echinoidea) lack arms but they share a five-part body plan with all other echinoderms.
Sea urchins are hemispherical animals covered with spines attached to an underlying skeleton.
Sand dollars are flattened or disc-shaped animals that feed on algae and fragments of organic matter on the seafloor.
Echinoderms: Pentaradial Symmetry
The class Holothuroidea, the sea cucumbers, have tube feet that are used primarily for attaching to a substrate rather than for moving.
They have anterior tube feet that are modified into tentacles used for feeding.
The body of sea cucumbers is oriented differently from other echinoderms. The mouth is anterior and the anus is posterior, not ventral and dorsal as in other echinoderms.
Echinoderms: Pentaradial Symmetry
The sea stars (class Asteroidea) are the most familiar echinoderms.
Their tube feet serve as organs of locomotion and sites for gas exchange.
Tube feet are moved by expansion and contraction of circular and longitudinal muscles in the tube.
Sea stars are important predators in many marine environments, preying on polychaetes, gastropods, bivalves, and fishes.
Echinoderms: Pentaradial Symmetry
The brittle stars (class Ophiuroidea) are similar in structure to the sea stars, but they have flexible arms that are composed of jointed hard plates.
Most feed by ingesting particles from the surfaces of sediments and assimilating the organic material from them, although some feed by capturing small animals.
Unlike most other echinoderms, they have only one opening to the digestive tract.
Echinoderms: Pentaradial Symmetry
Little is known about the class Concentricycloidea, the sea daisies, which were discovered in 1986.
They have tiny, disc-shaped bodies with a ring of marginal spines and two ring canals, but no arms.
They apparently eat prokaryotes, which they digest outside their bodies and then absorb.
They may be greatly modified sea stars.
Hemichordates: Conservative Evolution
The acorn worms and pterobranchs (phylum Hemichordata) are probably most similar in form to the ancestor they share with the echinoderms.
They are characterized by a three-part body plan that consists of a proboscis, a collar, and a trunk.
Hemichordates: Conservative Evolution
There are about 70 species of acorn worms which live in burrows in muddy and sandy sediments.
Their proboscis is a digging organ. It is coated with a sticky mucus that allows it to trap prey in the sediment.
This food-laden mucus is conveyed by cilia into the mouth; from there it is moved through the digestive tract by ciliary action.
A pharynx is behind the mouth. It opens to the outside through a number of pharyngeal slits through which water can exit.
Vascularized tissue in the pharyngeal slits serves as a gas exchange apparatus.
Hemichordates: Conservative Evolution
There are 10 living species of pterobranchs.
They are sedentary animals that live in a tube secreted by the proboscis.
Behind the proboscis is a collar with 19 arms with tentacles for for capture and gas exchange.
Chordates: New Ways of Feeding
The phylum Chordata is the second major lineage of deuterostomes.
This phylum evolved several different modifications of the coelomic cavity that provided new ways for capturing and handling food.
They also evolved a different body plan characterized by an internal dorsal supporting structure.
The pharyngeal slits that originally served as sites for gas exchange and eliminating water became enlarged in the chordates.
Chordates: New Ways of Feeding
The chordates are bilaterally symmetrical animals whose body plans are characterized by the following shared features at some stage in their development:
Pharyngeal slits
A dorsal, hollow nerve cord
A ventral heart
A tail that extends beyond the anus
A dorsal supporting rod, the notochord
The phylum has three subphyla: Urochordata, Cephalochordata, and Vertebrata.
Chordates: New Ways of Feeding
The tunicates (subphylum Urochordata) may be similar to the ancestors of all chordates. All are marine and most are sessile as adults.
Their swimming, tadpole-like larvae reveal their close evolutionary relationship with the other chordates.
The larva has pharyngeal slits, a dorsal, hollow nerve cord, and a notochord.
The notochord and nerve chord are lost during metamorphosis into the adult animal.
The adults pharynx is enlarged into a pharyngeal basket lined with cilia, whose beating moves water through the animal.
Chordates: New Ways of Feeding
There are three major urochordate groups: ascidians, thaliaceans, and larvaceans.
About 90 percent of the known species of tunicates are sea squirts (class Ascidiacea).
The thaliaceans (salps) float in tropical and subtropical oceans at varying depths.
Thaliaceans live singly or in chainlike colonies and retain their notochord and nerve cords throughout their lives.
The larvaceans are solitary planktonic animals that retain their notochords throughout their lives.
Chordates: New Ways of Feeding
There are 25 species of lancelets (subphylum Cephalochordata), which are tiny fishlike animals.
The notochord extends the length of the body throughout their lives.
Lancelets live partly buried in soft marine sediments.
Chordates: New Ways of Feeding
In the lineage that gave rise to the vertebrates (subphylum Vertebrata), the enlarged pharyngeal basket came to be used to extract prey from mud.
The vertebrates have a jointed, dorsal vertebral column that replaced the notochord as their primary support.
Chordates: New Ways of Feeding
The following traits characterize the vertebrate body plan:
A rigid internal skeleton, with the vertebral column as the anchor that provides support and mobility
Two pairs of appendages attached to the vertebral column
An anterior skull with a large brain
Internal organs suspended in a large coelom
A well-developed circulatory system, driven by contractions of a ventral heart
Chordates: New Ways of Feeding
Filter-feeding ancestral vertebrates lacked jaws and gave rise to the jawless fishes.
The ostracoderms were a group of jawless fishes that evolved a bony external armor that protected them from predators.
The hagfishes and the lampreys are the only jawless fishes (class Agnatha) to survive beyond the Devonian period. They have tough skins instead of external armor.
They lack paired appendages and have a round mouth that acts as a sucking organ by attaching to their prey and rasping at its flesh.
Chordates: New Ways of Feeding
Many new kinds of fishes evolved during the Devonian period.
Members of one lineage evolved jaws from some of the skeletal arches that supported the gill region.
Jaws allowed fish to catch and subdue relatively large, living prey. The ability to chew aided in chemical digestion.
The heavily armored placoderms (class Placodermi) were the dominant early jawed fishes.
No placoderms survived to the end of the Paleozoic era.
Chordates: New Ways of Feeding
The cartilaginous fishes (class Chondrichthyes) became abundant during the Devonian period.
They include the sharks, skates and rays, and chimaeras.
They have a skeleton composed entirely of a firm but pliable material called cartilage.
Their skin is flexible and leathery; the loss of external armor increased their mobility and their ability to escape from predators.
Chordates: New Ways of Feeding
Pairs of unjointed appendages called fins control swimming.
These fins include the pectoral, pelvic, and dorsal fins.
Sharks move forward by means of their tail and pelvic fins.
Skates and rays propel themselves by means of undulating movements of their enlarged pectoral fins.
Nearly all cartilaginous fishes live in the oceans.
Chordates: New Ways of Feeding
The ray-finned fishes (class Actinopterygii) have internal skeletons of bone rather than cartilage.
Most species are covered with flat, smooth, thin, lightweight scales that provide some protection.
Their gills open into a single chamber covered by a hard flap, whose movements improve the flow of water over the gills.
Gas exchange takes place in the gills.
Chordates: New Ways of Feeding
The early ray-finned fishes evolved gas-filled sacs to supplement the action of gills in respiration, enabling them to live in areas where oxygen was periodically in short supply.
In most species, these lunglike sacs evolved into swim bladders, which serve as organs of buoyancy.
Ray-finned fishes radiated during the Tertiary into about 24,000 species, in a wide variety of sizes, shapes, and lifestyles.
Some fishes form large aggregations, called schools, in open waters.
Many species perform complex behaviors.
Colonizing the Land:
Obtaining Oxygen from the Air
The evolution of lunglike sacs in response to the inadequacy of gills for respiration in oxygen-poor waters set the stage for the invasion of land.
Some bony fishes were able to supplement their gills with lung sacs when oxygen levels were low.
This ability allowed them to breathe air and to leave the water temporarily.
The lobe-finned fishes (class Actinistia) were the first lineage to evolve jointed fins.
Colonizing the Land:
Obtaining Oxygen from the Air
The lungfishes (class Dipnoi) were important predators in shallow-water habitats in the Devonian, but most lineages died out.
The three surviving species live in stagnant swamps and muddy waters in the Southern Hemisphere.
Some descendants of early fishes with jointed fins began to use terrestrial food sources and over time became fully adapted to life on land.
This lineage is believed to have given rise to the tetrapods: the four-legged amphibians, reptiles, birds, and mammals of today.
Colonizing the Land:
Obtaining Oxygen from the Air
The amphibians (class Amphibia) arose during the Devonian period.
The jointed fins of their ancestors evolved into walking legs.
Finlike legs probably allowed Devonian predecessors of amphibians to crawl from one body of water to another.
Eventually they evolved the ability to live on dry land.
Most amphibian species have small lungs and exchange gases through their skin, and are confined to moist environments.
Colonizing the Land:
Obtaining Oxygen from the Air
There are about 4,500 species of amphibians alive on Earth today.
The living amphibians are divided into three orders:
Gymnophiona (the wormlike, burrowing caecilians)
Anura (frogs and toads)
Urodela (salamanders)
Most species live in water at some time in their lives.
Colonizing the Land:
Obtaining Oxygen from the Air
In a typical amphibian life cycle, adults spend part of all of their time on land, but return to fresh water to lay eggs.
Amphibian eggs can only survive in moist environments.
Fertilized eggs develop into larvae that live in water until they undergo metamorphosis to an adult.
Some amphibians are entirely aquatic, others entirely terrestrial.
Colonizing the Land:
Obtaining Oxygen from the Air
Two morphological changes contributed to the ability of one vertebrate lineage to control water loss and exploit a wider variety of terrestrial habitats:
The evolution of an egg with a shell impermeable to water
A combination of traits that reduced water loss, such as skin that is impermeable to water and kidneys that could excrete concentrated urine
The vertebrates that evolved these traits are called amniotes.
Colonizing the Land:
Obtaining Oxygen from the Air
Amniote eggs have a calcium-impregnated shell that prevents the evaporation of fluids inside but allows O2 and CO2 to pass through.
These eggs store large quantities of yolk that allow the embryo to attain a relatively advanced state of development before it hatches.
Colonizing the Land:
Obtaining Oxygen from the Air
The reptiles (class Reptilia) are an early amniote lineage that arose from the tetrapods during the Carboniferous period.
Although called a class, the reptiles are a paraphyletic group.
Some species have eggs that do not develop shells and are retained inside the females body until they hatch. Some of these species evolved placentas that nourish the developing embryos.
Reptiles have skin covered with horny scales that reduce water loss, exchange gas by the lungs, and have a heart divided into chambers that separate oxygenated from unoxygenated blood.
Colonizing the Land:
Obtaining Oxygen from the Air
The lineages leading to modern reptiles began to diverge about 250 mya.
The turtle lineage (subclass Testudines) has changed very little.
Turtles have a combination of ancestral traits and highly specialized characteristics that they do not share with any other vertebrate group. Therefore, their phylogenetic relationships are uncertain.
Colonizing the Land:
Obtaining Oxygen from the Air
The subclass Squamata includes lizards, snakes, and the amphisbaenians, a group of legless burrowing animals with greatly reduced eyes.
The tuataras (subclass Sphenodontida) are a sister group to the lizards. This group was diverse in the Mesozoic, but only two species exist today.
Sphenodontids resemble lizards but differ in anatomical features.
Colonizing the Land:
Obtaining Oxygen from the Air
The crocodilians (subclass Crocodylia) are confined to tropical and warm temperate environments.
They spend much of their time in water but build nests on land or on floating piles of vegetation.
Colonizing the Land:
Obtaining Oxygen from the Air
The dinosaurs dominated terrestrial environments from 215 mya until about 65 mya.
The ability to breathe and run simultaneously was a major innovation in the evolution of terrestrial vertebrates.
In the lineages leading to the mammals, dinosaurs, and birds, the legs assumed a vertical position.
Muscles that enabled the lungs to be filled and emptied while the limbs moved also evolved.
These muscles are present in living birds and mammals, and their existence can be inferred in dinosaurs from their fossils.
Colonizing the Land:
Obtaining Oxygen from the Air
Dinosaur fossils recently discovered in China show that in some small predatory dinosaurs, the scales had been highly modified to form feathers.
Microraptor gui had feathers on all four limbs, very similar in structure to modern bird feathers.
Birds: More Feathers and Better Flight
A dinosaur lineage gave rise to the birds (subclass Aves) during the Mesozoic era.
The oldest known avian fossil is Archaeopteryx, (150 mya) which had feathers virtually identical to those of modern birds.
Archaeopteryx also had well-developed wings, a wishbone, and typical perching bird claws.
Another early bird known only from fossils is Confuciuornis sanctus; fossilized remains were discovered in 120125-million-year-old fossil beds in China.
Birds: More Feathers and Better Flight
Most paleontologists believe that birds evolved from terrestrial bipedal dinosaurs that used their forelimbs for capturing prey.
These dinosaurs may have initially developed feathers for insulation or display, and eventually were able to become airborne for short distances.
There are about 9,600 species of birds today, ranging in size from the 2-gram bee hummingbird to the 150-kilogram ostrich.
Birds: More Feathers and Better Flight
As a group, birds eat almost all types of animal and plant material.
They serve as a major agent of seed dispersal by eating the seeds of plants.
Feathers function not only in flight, but also in thermoregulatory and display functions.
The bones of birds are modified for flight; they are hollow and have internal struts for strength.
The breastbone forms a large, vertical keel to which pectoral muscles are attached. These muscles pull the wings downward during the propulsive movement in flight.
Birds: More Feathers and Better Flight
Flight is metabolically expensive, and thus birds have very high metabolic rates.
As a consequence, they generate large amounts of heat, whose loss is controlled by feathers that can trap or release warm air.
Birds have an enlarged cerebellum, the center of sight and muscular coordination.
Most birds lay eggs in a nest, where the young are usually cared for by the parents after hatching.
The Origin and Diversity of Mammals
Mammals (class Mammalia) appeared in the early part of the Mesozoic era.
Small mammals coexisted with reptiles and dinosaurs for at least 150 million years.
Todays mammals range in size from tiny shrews and bats that weigh only 2 grams to the endangered blue whale, which can measure up to 33 meters long and weigh up to 160,000 kilograms.
The Origin and Diversity of Mammals
Skeletal modifications accompanied the evolution of the small mammals from their larger reptilian ancestors:
Bones from the lower jaw were incorporated into the middle ear, leaving a single bone in the lower jaw.
The number of bones in the skull was decreased.
The bulk of the limbs and the bony girdles from which they are suspended were reduced.
Mammals have fewer teeth than reptiles, but mammal teeth are more differentiated.
The Origin and Diversity of Mammals
Skeletal features are readily preserved as fossils, so these developments can be traced in the fossil record.
Soft parts of animals are seldom fossilized; thus, it is difficult to tell when mammalian features such as mammary glands, sweat glands, hair, and a four-chambered heart evolved.
The mammals are unique in providing their young with milk secreted by mammary glands.
Mammalian eggs are fertilized within the females body, and prior to birth, embryos undergo a period of development called gestation within a uterus.
The Origin and Diversity of Mammals
The approximately 4,000 species of living mammals are divided into two major subclasses: Prototheria and Theria.
The subclass Prototheria contains a single order, the Monotrema, which contains only three species.
The monotremes differ from other mammals in that they lack a placenta, lay eggs, and have legs that poke out to the side.
The Origin and Diversity of Mammals
Two major groups of mammals, the marsupials and the eutherians, are members of the subclass Theria.
Females of the group Marsupialia have a ventral pouch in which they carry and feed their offspring.
Gestation in marsupials is short; early stages of offspring development take place in the pouch.
There are about 240 living species of marsupials.
The Origin and Diversity of Mammals
Most living mammals are eutherians.
Eutherians are more highly developed at birth than marsupials, and no external pouch houses them after birth.
There are about 4,000 living species of eutherians in 16 different groups.
Primates and the Origin of Humans
Humans belong to another eutherian lineage, the primates.
The primates likely descended from small tree-living insectivores during the Cretaceous period.
A nearly complete fossil found in Wyoming of an ancient primate species, Carpolestes, dates to 56 mya.
This early primate had grasping feet with an opposable big toe that had a nail rather than a claw.
These grasping limbs are one of the major adaptations that distinguish primates from other animals.
Primates and the Origin of Humans
The primate lineage split into two main branches early in its evolutionary history: the prosimians and the anthropoids.
The prosimians include the lemurs, pottos, and lorises.
The mainland prosimian species are arboreal and nocturnal.
Diurnal and terrestrial prosimian species are present on the island of Madagascar.
Primates and the Origin of Humans
The anthropoids include the tarsiers, monkeys, apes, and humans.
They evolved from an early primate lineage about 55 mya in Africa or Asia.
The New World monkeys probably reached South America from Africa when those two continents were still touching.
All New World monkeys are arboreal; many having long, prehensile tails.
Old World monkeys are terrestrial and arboreal, but none have prehensile tails. They often live in social groups.
Primates and the Origin of Humans
About 22 mya, the lineage leading to modern apes separated from other Old World primates.
About 9 mya, members of one European ape lineage migrated to Africa and became ancestors of modern African apes and of humans.
Primates and the Origin of Humans
The hominids separated from other ape lineages about 6 mya in Africa.
The earliest protohominids are known as the ardipithecines.
These apes had morphological adaptations for bipedalism.
Bipedal locomotion frees the hands to manipulate objects and carry them while walking.
It also elevates the eyes, enabling animals to spot predators and prey.
Bipedal movement is more energetically economical than quadruped movement.
Primates and the Origin of Humans
The ardipithecines gave rise to the australopithecines.
The most complete fossil skeleton of an australopithecine was discovered in Ethiopia in 1974 and is known as Lucy.
Lucy is about 3.5 million years old and belongs to the species Australopithecus afarensis.
Experts disagree over how many species are represented by the different australopithecine fossils that have been found, but it is clear that at least two distinct types lived together over much of eastern Africa.
Primates and the Origin of Humans
Early hominidsmembers of the genus Homolived contemporaneously with australopithecines for about a half million years.
The oldest fossils belong to the extinct species Homo habilis, estimated to have lived about 2 mya.
Homo erectus evolved in Africa about 1.6 mya and may have exterminated H. habilis.
H. erectus used fire and made tools that were probably used for digging, capturing animals, cleaning and cutting meat, scraping hides, and cutting wood.
H. erectus was replaced in tropical regions by Homo sapiens about 200,000 years ago.
Primates and the Origin of Humans
Large brain size evolved in the Homo lineage as social lives became increasingly complex.
The human brain has large amounts of omega-3 and omega-6 fatty acids, which cannot be synthesized; they must be obtained from the diet.
Access to fat-rich foods from aquatic environments may have been the key factor that supported the expansion of the human brain.
The archeological record of the past 100,000 years includes large piles of mollusk shells and fish bones, as well as carved points used for fishing, supporting this idea.
Primates and the Origin of Humans
Several Homo species existed during the mid-Pleistocene epoch and were skilled hunters of large mammals.
H. neanderthalensis was widespread in Europe and Asia between 75,000 and 30,000 years ago. They hunted large mammals and made a variety of tools.
Many scientists believe that they were exterminated by the H. sapiens known as the Cro-Magnons.
Cro-Magnon
people spread across Asia and reached
Primates and the Origin of Humans
The evolution of larger brains increased the behavioral capacity of our ancestors, especially the capacity for language.
Our expanded mental abilities are largely responsible for the development of culture, the process by which knowledge and traditions are passed from one generation to another by teaching and observation.
Cultural learning greatly facilitated the spread of domesticated plants and animals.
Human societies converted from those that were based on hunting and gathering to those that were pastoral and agricultural.
Deuterostomes and Protostomes:
Shared Evolutionary Themes
Deuterostome evolution paralleled protostome evolution in several ways.
Both groups exploited abundant food in soft marine sediments attached to rock or suspended in water.
In lineages of both groups, the body became compartmentalized.
Planktonic larval stages evolved in both groups.
Both groups colonized the land, but the internal skeletons of deuterostomes were able to support much larger animals.
The terrestrial deuterostomes recolonized aquatic environments a number of times.
Animation 34.1 Life Cycle of a Frog
Video 34.1 An echinoderm
Video 34.2 Lungfish
Video 34.3 From egg to tadpole: Embryonic development in a frog
Video 34.4 The amniote egg
Video 34.5 Pollination of a night-blooming cactus by a bat