Phylum Chordata
From Sea Otters to Sea Squirts
• The Big Four!
– pharyngeal slits - a series of openings that connect the inside of the
throat to the outside of the "neck". These are often, but not always,
used as gills.
– dorsal nerve cord - a bundle of nerve fibers which runs down the
"back". It connects the brain with the lateral muscles and other
organs.
– notochord - cartilaginous rod running underneath, and supporting,
the nerve cord.
– post-anal tail - an extension of the body past the anal opening.
Chrodate Body Plan
The basic chordate body plan consists of a tube within a tube. The
inner "tube" is the digestive tract and the outer "tube" is the body
wall. The cavity space between the two tubes is the coelom. Note that
the coelom is lined with mesoderm. The major chordate
characteristics, notochord and dorsal nerve cord (shown here as the
neural tube), are also part of the body plan.
Evolutionary Linkages
The evolutionary relationship between the echinoderms and
hemichordates (acorn worms) is indicated by the similarity of their
larvae. Both larval forms are small, motile, and swim by utilizing
bands of cilia.
Weirdos from the Burgess Shale
Some Designs Failed
Chordate Phylogeny
Chordate Characteristics
• The Big Four!
– Bilateral symmetry; segmented body; three germ layers; welldeveloped coelom.
– Notochord (a skeletal rod) present at some stage in life cycle.
– Single, dorsal, tubular nerve cord; anterior end of cord usually
enlarged to form brain.
– Pharyngeal gill slits present at some stage in life cycle.
• Other Characteristics:
– Postanal tail, usually projecting beyond the anus at some stage but
may or may not persist.
– Segmented muscles in an unsegmented trunk.
– Ventral heart, with dorsal and ventral blood vessels; closed blood
system.
– Complete digestive system.
– A cartilaginous or bony endoskeleton present in the majority of
members (vertebrates).
Chordate Subphyla
The Burgess Shale
Chordate Radiation
Subphylum Urochordata
•- The swimming larvae resemble tadpoles and possess all of the four key chordate
characteristics.
•"Tail Cord." There are approximately 1500 described species of these marine
living chordates. They are commonly called tunicates (also sea squirts) because of
the tough, nonliving tunic that surrounds the animal. As sessile adults they lose the
notochord and the tail, their dorsal hollow nerve cord becomes reduced, but they
retain the pharyngeal gill slits.
Example: tunicates, sea squirts
Urochordate Characteristics
Colonial Tunicates
Subphylum Cephalochordata "Head Cord"
• There are only 25 species described in this subphylum of
marine lancelets.
– They are all small, slender, laterally compressed, translucent
animals.
– Textbook example of the four key chordate characteristics. Once
considered to be the ancestor of the chordates, they are still
believed to more closely resemble the earliest prevertebrates than
any other animal known.
– Examples: Amphioxus (the lancelets)
A Typical Cephalochordate Body
Plan
The Lowly Chordate on the
Seafloor--Who would have
guessed?
Subphylum Vertebrata
Subphylum Vertebrata "Backboned"
• This is the largest and most diverse subphylum of
chordates whose evolution has been guided by the basic
adaptations of:
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the living endoskeleton
efficient respiration
advanced nervous system
paired limbs
This group is divided into those without jaws and those with jawed
mouths.
Superclasses and Classes of
Subphylum Vertebrata
•Superclass Agnatha ("Without jaws"): Those vertebrates without
true jaws or appendages including the hagfishes and the lampreys.
•Superclass Gnathostomata ("Jawed mouth"): Those vertebrates with
jaws and usually with paired appendages.
-Class Chondrichthyes - The cartilaginous fishes.
-Class Osteichthyes - The bony fishes.
-Class Amphibia - The amphibians.
-Class Reptilia - The reptiles.
-Class Aves - The birds.
-Class Mammalia - The mammals.
Superclass Agnatha
• Characteristics:
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Slender, eel-like body.
Median fins but no paired appendages.
Fibrous and cartilaginous skeleton; notochord throughout life.
Heart with one atrium and one ventricle.
Dorsal nerve cord with differentiated brain.
Digestive systems without stomach.
Sense organs of taste, smell and hearing.
External fertilization.
• Examples of living “Agnaths”:
– Class Myxini - The hagfishes
– Class Cephalaspidomorphi - The lampreys
Sea Lampreys
Gross!!!
Fossil Agnaths
The first vertebrates to evolve were the agnathans. Their traces are found in rocks of the
Late Cambrian period, more than 520 million years old. These first fishes had no jaws
in which to seize and manipulate prey. Nor did they have paired fins, to stabilize their
bodies in the water. The bony armor has been preserved in the rocks, and gives the
fossil agnathans their collective name of ostracoderms, meaning "shell-skins."Despite
their lack of jaws, ostracoderms dominated the seas and freshwaters of the northern
hemisphere for about 130 million years, from Early Ordovician to Late Devonian times.
Silurian Predators-Eurypterids
Major Evolutionary Step #1
• The Evolution of Jaws--Why is this so
important?
Jaw Evolution
1) ancestral fishes were jawless and somewhat similar to today's agnathans
2) anterior gill arches began to be modified into jawlike structures
3) functional jaws were first exhibited in the Placodermi
4) shows the configuration of jaws in most modern fishes
Placoderms
The placoderms, or "flat-plated skins," were a strange assemblage of heavily
armored jawed fishes. Several large, interlocking plates formed a bony head shield,
while another series of plates encased the front part of the body in a trunk shield.
The rest of the body was usually naked, with no scaly covering. This group was the
first to exhibit jaws, which exhibited broad dental plates instead of teeth. They first
appeared in the Early Devonian and had died out by the Early Carboniferous.
Huge Jawed Predators
These fish are common in Silurian Rocks in Ohio
Class Chondrichthyes
Members have a cartilaginous skeleton, pectoral fins and pelvic fins,
probably evolved in the sea from ancestors of Placoderms during the
Devonian or earlier.
•These possess a lateral line system.
•By lacking the armor of their ancestors, this group exhibits a
greater degree of mobility.
•Also, the appendages' bone types are featured in the tetrapod
vertebrates. Extant representatives are sharks and rays.
Major Characteristics of Class
Chondrichthyes:
• Characteristics:
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A heterocercal caudal fin.
Mouth ventral.
Skin with placoid scales.
Endoskeleton entirely cartilaginous.
Placoid Scales!
Two-chambered heart.
Respiration by means of 5 to 7 pairs of gills with separate and
exposed gill slits, no operculum.
– No swim bladder or lung.
– Internal fertilization and separate sexes.
• Examples
– Sharks, Skates and Rays
– Chimaera or ratfish
Cartilaginous Body Plans
Cartilaginous Wonders
•Agnatha- jawless, armored protovertebrates without paired appendages
•Placodermi- jawed armored extinct fishes with paired appendages
•Chondrichthyes- cartilaginous fishes
•Osteichthyes- bony fishes with an operculum and swim bladder. This
group diversified into lung- ray- and lobe-finned fishes, the latter of
which gave rise to the amphibians.
Class Osteichthyes
The "bony fishes" evolved in freshwater, apparently diverging from an
ancestor common to placoderms and chondrichthyes about 425 or 450
MYA. In members of this group, calcium phosphate reinforces the
skeleton.
Other characteristics of bony fishes include:
• mucus production
• a very well developed lateral line system, a swim bladder (which
evolved from lung-like sacs derived from the pharynx and used for
respiration in many early bony fishes) and an operculum.
Both bony and cartilaginous fishes diversified extensively in the
Devonian and Carboniferous, leading to ray-finned fishes, lobe-finned
fishes, and lungfishes. Lobe-finned fishes (similar to the coelocanth)
gave rise to amphibians.
Ray-Finned Fishes
Interestingly, although the ray-finned fishes include the modern fishes
and are the most diverse group of vertebrates, they are a "dead-end" in
terms of evolution, since there are no "more advanced" groups which
evolved from them.
Lobe-Finned Fish
Although among the fishes the lobe-finned fishes were not as successful
as the ray finned fishes, they proved to be in the mainstream of evolution
because they gave rise to the other groups of vertebrates.
One of the members of the lobefinned group provided the ancestor of the
first land animals, the amphibians. This transition was accomplished
relatively quickly in their evolution; the first lobe-finned fishes appeared
in the Early Devonian, and by the end of that period, some 20 million
years later, the amphibians had set foot on dry land.
A Living Colecanth
Lungfish
Paleontologists agree that the amphibians must have evolved from one of
the 3 groups of lobe-finned fishes (lungfish, coelacanths, or extinct
rhipidistians). However, there is disagreement on which group is the
most likely ancestor. Most paleontologists consider that amphibians
evolved from the rhipidistian fishes, based on the remarkable similarity
in the pattern of bones in their skulls and fins/limbs. Other
paleontologists, however, believe that the lungfish were ancestral to the
amphibians, since the development of the lungs, nostrils and limbs of
living lungfish is strikingly similar to those of living amphibians.
Major Evolutionary Step #2
• The Move to Land
Why Make that First Step????
Class Amphibia
Age of Amphibians
This figure illustrates the similarities in bone structure between the lobe-finned
fishes (such as coelacanths) and amphibians. The muscles and bony axis of the
paired fleshy fins of these fishes provide a structure that could easily have
evolved into the limbs of an early amphibian. Similarly, there seems little
doubt that these fishes possessed lungs like those of amphibians. Living
lungfish, for example, have them, and a similar structure (though single) is
present in the living coelacanths.
That First Step!
What Makes an Amphibian
an Amphibian ?
Ichthyostega-That First Step!!
Fossils of the earliest amphibian, Ichthyostega, were found in Greenland
and were dated about 370 MYA when Greenland was part of a
Euramerican continent that lay near the equator, and stretched from
today's western North America to eastern Europe. Members of this genus
are most often considered to represent the type of animal which first
exhibited a terrestrial existence, unlike its ancestors, the lobe finned
fishes.
Pangaea Revisited
A remarkable feature of the distribution of these early amphibians,
and of their relatives, the reptiles, is that until the middle of the
Permian period (about 100 million years later), nearly every one of
them has been found only on this former Euramerican continent.
This strongly suggests that this continent was the homebase in
which they first evolved and diversified. Only after the middle of
the Permian--when Asia and the southernlandmass of Gondwana
had become attached to Euramerica to form the supercontinent of
Pangaea--did the amphibians and reptiles spread throughout the
world.
Implications for Biodiversity
Major Step#3-The Amniotic Egg
The most important modifications which enabled subsequent vertebrates
to become independent of the aquatic environment were 1) modifications
of the outer covering into a multilayered skin which is fairly impermeable
to water and 2) evolution of an amniote egg.
The evolution of this type of egg was a major step in the conquest of
the terrestrial environment. Probably, it was first exhibited in stem
reptiles.
What Makes a Reptile a
Reptile??
Reptilian Evolution
Class Reptilia includes 7,000 species of lizards, snakes, turtles and
crocodilians. They exhibit important characteristics for terrestrial life
(keratinized scales, lungs, amniote egg, et al.). Ectothermy allows them
to survive using only 10% of the energy expended by mammals of
similar size. Reptiles first arose during the Carboniferous, about 300
MYA.
Class Reptilia
Stem Reptiles
From among their members came 2 major
evolutionary lines--one led to the mammals, and the
other to the archosaurs, "ruling reptiles".
Plesiosaurs
Plesiosaurs were large marine reptiles of the Mesozoic. They exhibited
long narrow flippers which aided locomotion of their sturdy, deep
bodies and short tails. There were two major groups (or superfamilies)
of plesiosaurians, which differed in the lengths of their necks and in
their feeding habits. The plesiosaurs had long necks and short heads,
and fed on smaller sea creatures. The pliosaurs had short necks and
large heads to enable them to bite and swallow larger prey.
Ichthyosaurs
The Ichthyosaurs, “fish lizards” flourished throughout the Jurassic Period and
into the Cretaceous. They had a streamlined, torpedo-shaped body with a
stabilizing dorsal fin on the back; short, paired paddles for steering; and a
strong, fishlike tail with 2 equal lobes for swimming. Ball-and-socket joints
between the tail vertebrae allowed for powerful strokes from side to side. The
tail, together with the great flexibility of the backbone, propelled the animal
rapidly through the water- the swimming method used by modern, fast-moving
fishes.
Synapsids
Therapsids (reptilian ancestors of the mammals) and the mammals
themselves all exhibit a skull like that of the synapsids. It has an opening
behind each eye socket which allowed the development of longer jaw
muscles. This resulted in stronger jaws that could be opened wide and
closed forcefully, to deal with large prey.
Therapsids
Therapsids were descended from the stem reptiles and are presumed to
be the ancestors of the mammals. "Non-reptilian" characteristics of
members of this group were thoracic and pelvic skeletal features which
permitted the legs to be positioned directly underneath the body.
The Evolution of Birds
Archaeopteryx is considered by many to be a "missing link" because it
exhibits both reptilian and avian characteristics. It dates from the
Jurassic, about 150 MYA. It had teeth and a long pointed tail (like
reptiles but unlike birds), but also had feathers and skeletal elements
(like birds but unlike reptiles).
South American Phororhachids-35 MYA
Flightless, fast-running predatory birds, some nearly three meters tall,
armed with enormous beaks and claws. These birds were the dominant
South American land carnivores for much of the Cenozoic; recently,
phororhachid bones have been found in Florida, suggesting that
phororhachids were not confined to South America.
Class Mammalia
Mammals evolved from reptilian stock (therapsids) before birds. Fossils
220 MY old have been found which are believed to be those of early
mammals. Mammal descendents coexisted with dinosaurs during the
Mesozoic. Early mammals were small shrew-like organisms which are
presumed to have been insectivorous nocturnal forms. Most agree that
the extinction of dinosaurs opened adaptive zones which could
subsequently be taken over by mammals. The Cenozoic, 63 MYA to
present is sometimes called the Age of Mammals.
Class Mammalia
What Makes a Mammal a Mammal?
Mammalian Characteristics
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Hair composed of keratin;
endothermy;
4 chambered heart;
diaphragm;
mammary glands produce a nutritionally balanced
product, milk, to nourish neonates;
most are born rather than hatched (eggs are
fertilized internally and in placentals, embryos
develop in utero
many have large brains and are capable learners;
long duration of parental care;
dentition specialization.
Extinctions Led the Way!
Explosive Adaptive Radiation
Mammal Phylogeny
South American Mammals
The Great Faunal Exchange!
Primate Taxonomy
New World and Old World Monkeys
Two distinct modern groups of monkeys exist, the "new world
monkeys" and "old world monkeys". Ancestors of these groups existed
around 40 million years ago in Africa and/or Asia.
How did new world monkeys got to South America?
New world monkeys (including spider monkeys, capuchins, and
squirrel monkeys) are all:
•Arboreal
•have prehensile tails
•Have nostrils which open to the side.
Old World Monkeys
Old world monkeys include monkeys (mandrills, baboons, macaques and
rhesus monkeys) and apes (chimpanzees, gorillas, orangutans, and
gibbons).
•Both are arboreal and ground dwelling forms among old world
monkeys
•None have prehensile tails.
•Their nostrils open downward.
Humans & Ape Distinctions
In searching for fossil relatives of humans, we need to be aware of the
differences between men and apes. These fall into three functional
categories that have evolved separately from one another:
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Cranial differences
Dental differences
Locomotion differences
Cranial Differences
• By comparison to apes, humans have:
– Larger brains (1200 cc vs. 500 cc)
• The larger brain creates a number of
differences in other cranial features:
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flatter face
smaller brow ridges
no post-orbital constriction
no sagittal crest
Skull Shapes
Ape
Human
Dentition
• By comparison to apes, humans have:
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smaller canines
no gap between upper canine and premolar
no sectorial (modified) first lower premolar
relatively small incisors and large molars
very thick molar enamel
a parabolic rather than U-shaped dental arch
Dentition
Apes
Human
Hominid Radiation & Climate
Hominid Phylogeny 1-5 MYA
The Cradle of Humanity
Hominid Adaptive Radiation
WHY?
Locomotion
• By comparison to apes, humans have:
– a foramen magnum that points down
– a curved lumbar spine
– a short, flared (versus long and thin) ilium (the
upper most section of the hip bone or pelvis)
– a strong, robust talus (ankle bone)
– a strong, non-opposable big toe
– a complex two-way arch system in the foot
Pelvis Structure
Hominid Foot Prints
Hominoid Taxonomy
Vertebrate Diversity Over Time