Chapter 15
Life of the Mesozoic Era
main points…..
1.marine invertebrates that survived end Permian
extinction diversified and repopulated the seas
2. flowering plants evolve in Cretaceous
3. reptile diversification continued…
4. Mesozoic= ‘Age of Dinosaurs’; lasted 140 million
years!!
5. flying reptiles and marine reptiles develop, become
extinct at end Cretaceous
6. birds evolved from reptiles.
7. mammals evolve from early reptile, coexist along
with reptiles…
8. fossil evidence for transition from reptiles to
mammals exists…
9. Mesozoic mammals=small, low diversity
10. proximity of continents and mild climates allowed
plants and animals to spread widely. Plate tectonics
would lead to population isolation……
• Recall the Paleozoic
extinctions
Survivors
– that decimated the marine
invertebrate faunas,
– causing a phenomenal
decrease in biotic diversity
• The survivors of this crisis
in life history
– diversified during the
Triassic
– and repopulated the seas,
accounting
– for the success of several
types of
– cephalopods, bivalves, and
several other invertebrates
Mesozoic Life Fascinates
• The animals existing during the Mesozoic Era
– fascinate nearly everyone
• Ever since Sir Richard Owen
– first used the term dinosaur in 1842,
– dinosaurs have been the objects of intense curiosity
• No other group of animals
– has so thoroughly captured the public imagination,
– but dinosaurs were only one type of Mesozoic
reptile
The Age of Reptiles
• Other Mesozoic reptiles include
– flying reptiles
– marine reptiles,
– as well as turtles, crocodiles, lizards, and snakes
• Geologists informally call the Mesozoic
– "The Age of Reptiles,"
– calling attention to the importance of reptiles
– among land-dwelling animals
Many New Discoveries
• Scientists have investigated Mesozoic animals
– for more than 150 years,
– yet much knowledge of dinosaurs and their
relatives
– comes from studies beginning during the 1970s
• Paleontologists make so many new discoveries
– augmenting knowledge of Mesozoic life
– that it is difficult to keep up with the current
literature
Dinosaurs Movies
• Of course Mesozoic animals,
–
–
–
–
–
especially dinosaurs,
have been popularized in numerous books,
TV specials, and movies
such as Jurassic Park (1993)
and its two sequels,
• The Lost World (1997)
• and Jurassic Park III (2001),
• as well as Dinosaur (2000)
Mammals Too
• The evolution and diversification of Mesozoic
reptiles was certainly important,
– but so were several other events
– such as the origin of mammals during the Triassic
• Thus mammals and dinosaurs were
contemporaries
– throughout the Mesozoic,
– but mammals were not particularly diverse
– and all were small creatures
Birds
• Birds also made their appearance,
– most likely evolving
– from small carnivorous dinosaurs
– during the Jurassic
Land Plants
• Important changes took place
–
–
–
–
in land plant communities
as the flowering plants evolved
during the Cretaceous
and soon became widespread and numerous
• The major groups of Paleozoic land plants
persisted,
– but now they constitute less than 10% of all species
Systems Approach
• Here we continue to emphasize
– the systems approach to Earth and life history
• The distribution of land and sea
– profoundly influences oceanic circulation,
– which in turn partly controls climate
• The proximity or separation of landmasses
– partly determines the geographic distribution of
organisms
Isolation
• Pangaea began fragmenting
– during the Triassic and continues to do so
• Organisms had increasing difficulty
– migrating between continents as a result
• In fact, South America and Australia
– became isolated island continents
– their faunas evolving in isolation
– became quite different from those elsewhere
Mesozoic Mass Extinctions
• Mass extinctions at the end of the Mesozoic,
– second in magnitude only to the Paleozoic
extinctions,
– had a tremendous impact on the biosphere
• But because dinosaurs were among the victims,
– these extinctions have received
– much more attention than any other extinction
• So just as at the end of the Paleozoic Era,
– biotic diversity was sharply reduced,
– but once again many survivors evolved rapidly,
– giving rise to the Cenozoic fauna
Marine Invertebrates and
Phytoplankton
• Following the Paleozoic mass extinctions,
– the Mesozoic was a time
– when marine invertebrates repopulated the seas
• The Early Triassic invertebrate fauna
–
–
–
–
–
was not very diverse,
but by the Late Triassic the seas
were once again swarming with invertebrates
from planktonic foraminifera
to cephalopods
Brachiopods Never
Fully Recover
• The brachiopods,
– that had been so abundant during the Paleozoic,
– never completely recovered from their near
extinction
• Although they still exist
– the bivalves
– have largely taken over their ecological niche
Mollusks
• Mollusks such as
– cephalopods, bivalves, and gastropods
– were the most important elements
– in the Mesozoic marine invertebrate fauna
• Their rapid evolution
– and the fact that many cephalopods were nektonic
– make them excellent guide fossils
• Cephalopods were present throughout the
Mesozoic
– but they were most abundant during the Jurassic
and Cretaceous
Ammonoidea
•
•
•
•
The Ammonoidea,
– cephalopods with wrinkled sutures,
– constitute three groups:
– the goniatites, ceratites, and ammonites
Ammonites, while present during the
entire Mesozoic,
– were most prolific during the Jurassic
and Cretaceous
Most ammonites were coiled,
– some attaining diameters of 2 m,
– whereas others were uncoiled
– and led a near benthonic existence
Cephalopods
– such as the Late Cretaceous ammonoids Baculites
– and Helioceros
– were important predators
– and excellent guide fossils
Surviving Cephalopods
• Ammonites became extinct
– at the end of the Cretaceous,
• but two related groups of cephalopods
– survived into the Cenozoic
– the nautiloids,
• including the living pearly Nautilus,
– and the coleoids, represented by extinct belemnoids
• which are good Jurassic and Cretaceous guide fossils
– as well as by the living squid and octopus
Bivalves
• Two Cretaceous bivalves: Oysters and clams
– epifaunal suspension feeders
• Bivalves were particularly diverse and
abundant during the Mesozoic
– Even today they remain important elements in the
marine invertebrate fauna
Mesozoic Reef-Builders
• Where shallow marine waters were warm and clear,
– coral reefs proliferated, as they do today
• An important reef-builder throughout the Mesozoic
– was a group of bivalves known as rudists
• Rudists are important
– because they displaced corals
– as the main reef-builders during the later Mesozoic
– and are excellent guide fossils
– for the Late Jurassic and Cretaceous
Mesozoic Reef-Builders: Rudists
very large clams…..
Familiar Coral
• A new and familiar type of coral
– also appeared during the Triassic,
– the scleractinians
• Whether sclaractinians evolved from rugose
corals
– or from an as yet unknown soft-bodied group of
anthozoans
– with no known fossil record is still unresolved
Burrowing Organisms
• One of the major differences
–
–
–
–
between the Paleozoic and Mesozoic
marine invertebrate faunas
was the increased abundance and diversity
of burrowing organisms
• Paleozoic burrowers, with few exceptions,
– were soft-bodied animals such as worms
• The bivalves and echinoids,
– which were epifaunal elements
– during the Paleozoic,
– evolved various means of entering infaunal habitats
Escaping from Predators
• This trend toward an infaunal existence
– may reflect an adaptive response
– to increasing predation
– from the rapidly evolving fish and cephalopods
• Bivalves, for instance,
–
–
–
–
expanded into the infaunal niche
during the Mesozoic,
and by burrowing
they escaped predators
Mesozoic Primary Producers
• The primary producers in the Mesozoic seas
– were various types of microorganisms
1. Coccolithophores are an important group
– of phytoplankton
– that first evolved during the Jurassic
– and became extremely common during the
Cretaceous
Coccolithophores
• Coccolithophores from the Gulf of Mexico
– of Miocene age
– of Miocene-Pliocene age
Diatoms
2. Diatoms
–
–
–
–
which build their skeletons of silica,
made their appearance during the Cretaceous,
but they are more important
as primary producers during the Cenozoic
• Diatoms are presently most abundant
– in cooler oceanic waters
– and some species inhabit freshwater lakes
Diatoms
• Diatoms from Upper Miocene rocks in Java
Dinoflagellates
3. Dinoflagellates were common during the
Mesozoic and today are the major primary
producers in warm water
• One of
Eocene age
from
Alabama
• of MiocenePliocene
age from the
Gulf of
Mexico
Foraminifera
4. The foraminifera
• single-celled consumers
– underwent an explosive diversification
– during the Jurassic and Cretaceous
• They are still diverse and abundant today
• The planktonic forms
– in particular
– diversified rapidly,
– but most genera
– became extinct at the end of the Cretaceous
Increasing Complexity
• The Mesozoic was a time of
– generally increasing complexity
– of the marine invertebrate fauna
• At the beginning of the Triassic,
– diversity was low and food chains were short
• Near the end of the Cretaceous, though,
– the marine invertebrate fauna was highly complex
– with interrelated food chains
• This evolutionary history
– reflects changing geologic conditions
– influenced by plate tectonic activity
Plants—
Primary Producers on Land
• Plants practice photosynthesis
– and thus lie at the base of the food chain on land,
– so we discuss them as a prelude
– to consideration of land-dwelling animals
• Just as during the Late Paleozoic,
– seedless vascular plants and gymnosperms
– dominated Triassic and Jurassic land-plant
communities,
– and, in fact, representatives of both groups
– are still common
Seedless Vascular Plants
and Gymnosperms
• Seedless vascular plants and gymnosperms
– were prolific
• until angiosperms
– replaced many of them
– during the Mesozoic
Gymnosperms
• Among the gymnosperms,
Ginkgo
– the large seed ferns became extinct
– by the end of the Triassic,
– but ginkgos remained abundant
– and still exist in isolated regions,
• Conifers continued to diversify
– and are now widespread in some terrestrial habitats,
– particularly at high elevations and high latitudes
Cycads
•
•
A new group of gymnosperms
– known as cycads made its
appearance
– during the Triassic
These palm-like plants
– became widespread
– and now exist in tropical
– and semi-tropical areas
Angiosperms
• The long dominance of seedless plants and gymnosperms
– ended during the Early Cretaceous,
– perhaps the Late Jurassic,
– when many were replaced
– by angiosperms,
• or flowering plants
• Angiosperms probably evolved
– from specialized gymnosperms
Sapindopsis,
Cecil County, Maryland
Angiosperms’ Success
• Several factors account for their phenomenal
success,
– but chief among them is their method of
reproduction
• Two developments were particularly important
– the evolution of flowers,
• which attract animal pollinators, especially insects
– and the evolution of enclosed seeds
More Than 90%
• Seedless vascular plants and gymnosperms
– are important and still flourish in many
environments
– in fact, many botanists regard ferns
– and conifers as emerging groups
• Nevertheless, a measure of the angiosperms'
success is
– that today with 250,000 to 300,000 species
– they account for more than 90% of all land plant
species,
– and they occupy some habitats
– in which other land plants do poorly or cannot exist
The Diversification of Reptiles
• Reptile diversification began
– during the Mississippian Period
– with the evolution of the protorothyrids,
– apparently the first animals to lay amniotic eggs
• From this basic stock of so-called stem reptiles,
– all other reptiles
– as well as birds and mammals evolved
Reptiles and Birds
• Relationships among fossil and living reptiles
and birds
The Story of Reptile
Diversification
• Recall that pelycosaurs
– were the dominant land vertebrates
– of the Pennsylvania and Permian
• Here we continue our story
– of reptile diversification
– with a group called archosaurs
Dinosaurs Orders
• All dinosaurs possess
– a number of shared characteristics,
– yet differ enough for us to recognize
– two distinct orders,
• the Saurischia
• and Ornithischia
• A distinctive pelvic structure
– characterizes each order
Distinctive Pelvic Structure
• Saurischian dinosaurs
– have a 1izardlike pelvis
– and are thus called lizard-hipped dinosaurs
• Ornithischians
– have a birdlike pelvis
– and are called bird-hipped dinosaurs
Dinosaur Cladogram
• Cladogram showing dinosaur relationships
– showing Pelvises of ornithischians and saurischians
– Among the several subgroups of dinosaurs
• theropods
were
carnivores
• and all
others were
herbivores
Common Ancestor
• For decades, paleontologists thought
–
–
–
–
–
–
–
each order evolved independently
during the Late Triassic,
but they now conclude
that both orders
had a common ancestor
much like archosaurs
known from Middle Triassic rocks in Argentina
Dinosaur Ancestors
• These dinosaur ancestors were small
–
–
–
–
–
–
less than 1 m long
long-legged carnivores that walked
and ran on their hind limbs,
so they were bipedal,
as opposed to quadrupedal animals
that move on all four limbs
Dinosaurs
• Sir Richard Owen
–
–
–
–
–
proposed the term dinosaur in 1842
to mean "fearfully great lizard"
although now "fearfully" has come to mean
"terrible," thus the characterization of dinosaurs as
"terrible lizards"
• But of course they were not terrible,
– or at least no more terrible
– than animals living today,
– and they were not lizards
Misconceptions about Dinosaurs
• It is true that many were quite large,
– in fact, dinosaurs varied from giants
• weighing several tens of metric tons
– to those no larger than a chicken
• To consider them poorly adapted
– is to ignore the fact
– that dinosaurs were extremely diverse
– and widespread for more than 140 million years!
Active and Cared for Their Young
• Although various media
– now portray dinosaurs as more active animals,
– the misconception that they were lethargic,
– dim-witted beasts persists
• Evidence now available indicates
– that some were quite active
– and perhaps even warm-blooded
• Some species probably cared for their young
– long after hatching,
– a behavioral characteristic most often associated
– with birds and mammals
Care of the Young
• Maiasaura, a Late Cretaceous ornithopod,
nested in colonies in northern Montana
– In this scene a
female leads
her young to a
feeding area
Questions Remain
• While many questions remain unanswered
about dinosaurs,
– their fossils and the rocks containing them
– are revealing more and more
– about their evolutionary relationships and behavior
Saurischian Dinosaurs
• The Saurischians,
• or lizard-hipped dinosaurs,
– include two distinct groups
– known as theropods and sauropods
• All theropods
– were carnivorous bipeds
– ranging in size from tiny Compsognathus
– to giants such as Tyrannosaurus
• and similar species
• that might have weighed
• as much as 7 or 8 metric tons
Dinosaur Cladogram
Small Theropod Dinosaur
• Compsognathus weighed only 2 or 3 kg
– Bones found within its ribcage indicate it ate lizards
Tyrannosaurus
• The skull of
Tyrannosaurus,
– another
theropod,
– measured
more
than
1 m long
Velociraptor and Deinonychus
• The movie Jurassic Park and its sequels
– popularized some of the smaller theropods
– such as Velociraptor,
– a 1.8-m-long predator
– with large sickle-like claws on the hind feet
• This dinosaur and its somewhat larger relative
– Deinonychus probably used their claws
– in a slashing type of attack
Deinonychus
• Lifelike restoration of Deinonychus in its
probable attack posture
• It was about 3 m long
Sauropods
• Included among the sauropods
– are the truly giant,
– quadrupedal herbivorous dinosaurs
– such as Apatosaurus, Diplodocus, and
Brachiosaurus,
– the largest known land-animals of any kind
• Brachiosaurus,
• a giant even by sauropod standards,
–
–
–
–
might have weighed as much as 75 metric tons,
and partial remains
indicate that even larger sauropods
may have existed
Ornithischian Dinosaurs
• Scientists recognize five distinct groups of
ornithischians
–
–
–
–
–
ornithopods,
pachycephalosaurs,
ankylosaurs,
stegosaurs,
and ceratopsians
Dinosaur Cladogram
Ornithopod Dinosaurs
• Ornithopods include the duck-billed dinosaurs
– with flattened bill-like mouths
• They were especially numerous during the Cretaceous
– and several genera had head crests
– which might have served a variety of functions
• All ornithopods were herbivores
– and primarily bipedal
– with well-developed forelimbs
– that allowed them to walk
– in a quadrupedal fashion, too
Duck-Billed Dinosaurs
• Two dinosaurs from the Late Cretaceous
– with head crests,
• hollow, bony extensions of the skull
– Parasaurolophus
– Corythosaurus
Pachycephalosaurs
• The most distinctive feature
– of the pachycephalosaurs
– is their thick-boned,
– dome-shaped skull
• Perhaps males used these thick skulls
– in butting contests for dominance and mates
• The few known genera of pachycephalosaurs
– lived during the Late Cretaceous
Horned Dinosaurs
• Ceratopsians or horned dinosaurs
–
–
–
–
–
have a good fossil record
indicating that small Early Cretaceous animals
were ancestors to large Late Cretaceous genera
such as Triceratops
which was a very common dinosaur in North
America
• The later ceratopsians had huge heads,
– a large bony frill over the neck for muscle
attachment,
– and a horn or horns on the skull
Triceratops
• Skeleton of the ceratopsian Triceratops
– in the Natural History Museum, London, England
Triceratops
Fossil Herds
• Fossil trackways
–
–
–
–
–
and bone beds
of ceratopsians
indicate that these large,
quadrupedal herbivores
moved in herds
Stegosaurs
• The stegosaur known as Stegosaurus
– was a medium-sized, quadrupedal herbivore
– from the Jurassic Period
• Its most distinctive features include
– a spiked tail,
• used almost certainly for defense,
– and triangular plates on the back
• The exact arrangement
–
–
–
–
of these plates is debated,
but most paleontologists are convinced
that they functioned
as a device to absorb and dissipate heat
Dinosaur Cladogram
Ankylosaurs
• Finally the ankylosaurs
– were the most heavily armored of all dinosaurs
• All were quadrupedal herbivores
– and some were quite large
• Bony armor
– protected the animal's back, flanks,
– and top of the head
• The tail ended in a bony club
– that could undoubtedly deliver a crippling blow
– to an attacking predator
• Some ankylosaur species
– lacked this bony club
Ankylosaur
• The ankylosaur
Euoplocephalus
– Note the heavy
armor and bony
club at the end
of the tail
• The ankylosaur
Sauropelta
Warm-Blooded Dinosaurs?
• Were dinosaurs endotherms
• warm-blooded
– like today's mammals and birds,
• or were they ectotherms
• cold-blooded
– as are all of today's reptiles?
• Almost everyone now agrees
– that some compelling evidence exists for dinosaur
endothermy,
Opinion Is Divided
• but opinion is still divided among
(1) those holding that all dinosaurs were endotherms;
(2) those who think only some were endotherms; and
(3) those proposing that dinosaur metabolism,
• and thus their ability to regulate body temperature,
• changed as they matured
• Bones of endotherms
–
–
–
–
typically have numerous passageways that,
when the animals are alive,
contain blood vessels,
but considerably fewer passageways are present in
bones of ectotherms
Higher Metabolic Rates
• Endotherms must eat more
– than comparable sized ectotherms
– because their metabolic rates are so much higher
• Consequently, endothermic predators
– require large prey populations
– and thus constitute a much smaller proportion
– of the total animal population than their prey,
– usually only a few percent
Predators to Prey Proportion
• In contrast, the proportion
– of ectothermic predators to prey
– might be as high as 50%
• Where data are sufficient
– to allow an estimate,
– dinosaur predators made up 3% to 5%
– of the total population
• Nevertheless, uncertainties in the data
– make this less than a convincing argument
– for many paleontologists
Large Brain
• A large brain in comparison to body size
– requires a rather constant body temperature
– and thus implies endothermy
• Some dinosaurs were indeed rather brainy,
– especially the small- and medium-sized theropods
Insulation
• So brain size might be a convincing argument
for these dinosaurs,
– but even more compelling evidence for theropod
endothermy
– comes from their probable relationship to birds,
– and the rather recent discoveries in China
– of dinosaurs with feathers or a feather-like covering
• Today, only endotherms have
– hair, fur, or feathers for insulation
Flying Reptiles
• Paleozoic insects
–
–
–
–
–
–
were the first animals to achieve flight,
but the first among vertebrates were pterosaurs,
or flying reptiles,
which were common in the skies
from the Late Triassic
until their extinction at the end of the Cretaceous
Pterodactyls
• Pterodactyls, a long-tailed Late Jurassic
pterosaur
– Among
several
known
species,
– wingspan
ranged from
50 cm to 2.5
m
Adaptations for Flight
• Adaptations for flight include
– a wing membrane
• supported by an elongated fourth finger
– light hollow bones,
– and development of those parts of the brain
• associated with muscular coordination and sight
• Because at least one pterosaur species
– had a coat of hair or hair-like feathers,
– possibly it was an endotherm
• as perhaps all pterosaurs were
Wings
• In all flying vertebrates,
– the forelimb has been
modified into a wing
– A long 4th finger
supports the pterosaur
wing
– whereas in birds the 2nd
and 3rd fingers are fused
together
– and in bats, fingers 2
through 5 support the
wing
– Are these wings
analogous, homologous,
or both?
Too Weak for Sustained Flapping
• Experiments and studies of fossils
– indicate that the wing bones of large pterosaurs
• such as Pteranodon
– were too weak for sustained flapping
• These comparatively large animals
– probably took advantage of rising air currents
• to stay airborne,
– mostly by soaring
– but occasionally flapping their wings for
maneuvering
Pteranodon
• The shorttailed
pterosaur
• known as
Pteranodon
• was a large
Cretaceous
animal
• with a
wingspan of
more than 6 m
Mesozoic Marine Reptiles
• Several Mesozoic reptiles
– adapted to a marine environment
– including turtles and some crocodiles,
– as well as the Triassic mollusk-crushing placodonts
• Here, however, we concentrate on
– the ichthyosaurs, plesiosaurs, and mosasaurs
• All of these marine predators
– were thoroughly aquatic,
– but other than all being reptiles
– they are not particularly closely related
Ichthyosaurs
• The streamlined, rather porpoise-like
ichthyosaurs
– varied from species measuring only 0.7 m long
– to 15-m-long giants
• Although details of their ancestry
–
–
–
–
are still not fully known,
they evolved from small animals
whose fossils remind one researcher of
"a lizard with flippers … [that] still retained some
key features of their land-dwelling ancestors”1
1Motani,
R. 2000 Rulers of the Jurassic Seas, Scientific American, v 283, no 6, p 55
Ichthyosaurs
• Restoration showing ichthyosaurs
• fully aquatic animals
• that evolved from
land-dwelling
ancestors
Ichthyosaurs
• They had numerous sharp teeth
• Preserved stomach contents reveal a diet of
– fish, cephalopods, and other marine organisms
• It is doubtful that ichthyosaurs
–
–
–
–
could come onto land,
so females must have retained eggs
within their bodies
and gave birth to live young
• A few fossils with small ichthyosaurs
– in the appropriate part of the body cavity
– support this interpretation
Mary Anning
• An interesting side note in the history of
paleontology
–
–
–
–
–
is the story of Mary Anning (1799-1847),
who when only about 11 years old discovered
and directed the excavation
of a nearly complete ichthyosaur
in southern England
• This and subsequent discoveries
– made her a well-known fossil collector
Plesiosaurs
• The plesiosaurs,
• another well-known group of Mesozoic marine reptiles,
– belonged to one of two subgroups:
– short necked and long-necked
• Most were modest sized animals 3.6 to 6 m
long,
– but one species
– found in Antarctica
– measures 15 m
Plesiosaurs
• Although the
marine reptiles
– plesiosaurs
– were aquatic
animals,
• their fipperlike
forelimbs
– probably
allowed them
to come out
onto land
Mosasaurs
• Mosasaurs were Late Cretaceous marine
lizards
– related to the present-day
– Komodo dragon or monitor lizard
• Some species measured no more than 2.5 m
long,
– but a few such as Tylosaurus were large,
– measuring up to 9 m
• Mosasaur limbs resemble paddles
– and were used mostly for maneuvering
– whereas the long tail provided propulsion
Tylosaurus
• Tylosaurus
was
– a large,
– Late
Cretaceous
– mosasaur
– It measured
up to 9 m
long
Mosasaur Skull
• Mosasaur skull on display
– in the Museum of Geology and Paleontology,
– University of Florence, Italy
Mosasaurs Were Predators
• All mosasaurs were predators,
–
–
–
–
and preserved stomach contents indicate
that they ate fish, birds, smaller mosasaurs,
and a variety of invertebrates
including ammonoids
Crocodiles
• By Jurassic time,
– crocodiles had become
– the most common freshwater predators
• All crocodiles are amphibious,
– spending much of their time in water,
– but they are well equipped for walking on land
• Crocodile evolution has been conservative,
– involving changes mostly in size
• from a meter or so in Jurassic forms
• to 15 m in some Cretaceous species
Turtles
• Turtles, too, have been evolutionarily
conservative
– since their appearance during the Triassic
• The most remarkable feature of turtles
– is their heavy, bony armor;
– turtles are more thoroughly armored
– than any other vertebrate animal, living or fossil
• Turtle ancestry is uncertain
• One Permian animal
– had eight broadly expanded ribs,
– which may represent the first stages
– in the development of turtle armor
Lizards, and Snakes
• Lizards and snakes are closely related,
– and lizards were in fact ancestral to snakes
• The limbless condition in snakes
• some lizards are limbless, too
– and skull modifications
– that allow snakes to open their mouths very wide
– are the main differences between these two groups
• Lizards are known from Upper Permian strata,
– but they were not abundant until the Late
Cretaceous
From Reptiles to Birds
• Scientists were aware of a number of
characteristics
– shared by reptiles and birds
– even before they found fossil evidence
– showing a relationship between the two groups
• For example, birds and reptiles
– lay shelled, yolked eggs,
– and both share a number of skeletal features
– such as the way the jaw articulates with the skull
Birds Do Not Closely Resemble
Living Reptiles
• But of course birds have feathers
– whereas reptiles have scales
– or a tough, beaded skin
• Furthermore, birds do not closely resemble
– any living reptile,
• so why do scientists think they are justified
– when they claim that birds evolved from reptiles?
Archaeopteryx
• Fossil bird,
Archaeopteryx
– from the Jurassic
Solenhofen
Limestone in
Germany
– has feather
impressions in the
wings and long tail
– A wishbone and
feathers made it a
bird
Archaeopteryx
• In most
anatomical
details
Archaeopteryx
more closely
resembled a
small theropod
– It had claws on
its wing and
reptilian teeth
• but was a bird
Archaeopteryx
• These animals, known as Archaeopteryx
–
–
–
–
–
are birds by definition,
but their numerous reptilian features
convince many scientists
that they evolved from
some kind of small theropod
• Even fused clavicles are found in several
theropods,
– and recent discoveries in China of theropods
– with some kind of feathery covering
– provide more evidence of this relationship
Ancestor/Descendant Gap
• Opponents of the theropod-bird scenario
– point out that theropods
– are typically found in Cretaceous-aged rocks,
– whereas Archaeopteryx is Jurassic
• However, some of the fossils
• coming from China
– are about the same age as Archaeopteryx,
– thus narrowing the gap
– between presumed ancestor and descendant
• Nevertheless, opponents also note
– that some of the details
– of the theropod and bird hand differ
Bird Evolution
• A Mesozoic fossil, from China,
– is slightly younger than Archaeopteryx
– and possesses both primitive and advanced features
• It retains abdominal ribs
– similar to those of Archaeopteryx and theropods,
– but it has a reduced tail
– more typical of present-day birds
• Another Mesozoic bird, from Spain
– is also a mix of primitive and advanced
characteristics,
– but it appears to lack abdominal ribs
Chinese Bird Dinosaurs!
Note the 4 wings!!...the preserved imprints of feathers
(Xu, Xing; et al 2003. Four winged Dinosaurs from China, Nature, vol 421, p. 335-340)
Bird-Reptile Relationship
• Another possible candidate for bird ancestry
– is a small lizardlike reptile
– known as Longisquama
– that was discovered during the 1960s in Kyrgystan
• Longisquama is probably an archosaur,
– and it too appears to have feathers,
– but some paleontologists
– think these structures are actually long scales
• In any event, the bird-reptile relationship is
firmly established,
– but disagreement exists on the exact bird ancestor
Protoavis
• Archaeopteryx's fossil record is not good
enough
– to resolve whether it is the actual ancestor of
today's birds
– or an animal that died out without leaving
descendants
• Of course, that in no way diminishes the fact
– that it had both reptile and bird characteristics
• However, some claim that fossils of two crowsized individuals
– known as Protoavis
– represent an even earlier bird than Archaeopteryx
Cretaceous Climates
• Cretaceous climates were more strongly zoned
by latitude,
– but they remained warm and equable
– until the close of that period
• Climates then became more seasonal and
cooler,
– a trend that persisted into the Cenozoic
• Dinosaur and mammal fossils
– demonstrate that interchange was still possible,
– especially between the various components of
Laurasia
Mass Extinctions—A Crisis in
the History of Life
• The greatest mass extinction took place
–
–
–
–
–
at the end of the Paleozoic Era,
but the one at the close of the Mesozoic
has attracted more attention
because among its casualties were
dinosaurs, flying reptiles, and marine reptiles
• Several kinds of marine invertebrates
– also went extinct, including ammonites,
• which had been so abundant through the Mesozoic,
– rudistid bivalves, and some planktonic organisms
Mesozoic Extinctions
• Numerous hypotheses proposed
– to explain Mesozoic extinctions
– have been dismissed as improbable,
– untestable, or inconsistent with the available data,
• A proposal that has become popular since 1980
–
–
–
–
–
is based on a discovery
at the Cretaceous-Tertiary boundary in Italy
a 2.5-cm-thick clay layer
with a remarkably high concentration
of the platinum group element indium
• High iridium concentrations
– have now been identified
– at many other Cretaceous-Tertiary boundary sites
Cretaceous-Tertiary Boundary
• At this CretaceousTertiary boundary site
– in Italy,
– a 2.5-cm-thick clay
layer (1 inch!!)
– shows a concentration
– much higher than
expected
– of the platinum-group
element iridium
• Closeup view of the boundary clay in the Raton
Basin, New Mexico
Iridium Anomaly
• The significance of this iridium anomaly
– is that iridium is rare in crustal rocks
– but is found in much higher concentrations
– in some meteorites
• Accordingly, some investigators propose
–
–
–
–
a meteorite impact to explain the anomaly,
and further postulate that the meteorite,
perhaps 10 km in diameter,
set in motion a chain of events leading to
extinctions
Impact Consequences
• According to the impact hypothesis,
–
–
–
–
–
–
–
about 60 times the mass of the meteorite
was blasted from the crust high
into the atmosphere,
and the heat generated at impact
started raging forest fires
that added more particulate matter
to the atmosphere
• Sunlight was blocked for several months,
– causing a temporary cessation of photosynthesis,
– food chains collapsed,
– and extinctions followed
Boundary Sites
•
Some Cretaceous-Tertiary boundary sites
– also contain soot
– and shock-metamorphosed quartz grains,
– both of which are cited
– as additional evidence of an impact
DSDP Site 718 US E Coast
Acid Rain
• Furthermore, with sunlight greatly diminished
• Earth's surface temperatures
– were drastically reduced,
– adding to the biologic stress
• Another proposed consequence of an impact
– is that sulfuric acid (H2SO4) and nitric acid (HNO3)
– resulted from vaporized rock and atmospheric
gases
• Both would have contributed
– to strongly acid rain that might have had
– devastating effects on vegetation
– and marine organisms
Impact Site
• Some now claim that a probable impact site
– centered on the town of Chicxulub
– on the Yucatan Peninsula of Mexico
– has been found
• The 180-km diameter structure
– lies beneath layers of sedimentary rock
– and appears to be the right age
Meteorite Impact Crater
• Proposed
meteorite impact
crater
– centered on
Chicxulub
– on the Yucatán
Peninsula
– of Mexico
Evidence at Chicxulub
• Evidence supporting the conclusion
–
–
–
–
that the Chicxulub structure
is an impact crater includes shocked quartz,
what appear to be the deposits of huge waves,
and tektites,
•
•
•
•
small pieces of rock
that were melted
during the proposed impact
and hurled into the atmosphere
Terrestrial and Marine Extinctions
• Even if a meteorite did hit Earth,
– did it lead to these extinctions?
• If so, both terrestrial and marine extinctions
– must have occurred at the same time
• To date, strict time equivalence
– between terrestrial and marine extinctions
– has not been demonstrated
• The selective nature of the extinctions
– is also a problem
Impact Hastened Extinction?
• In the terrestrial realm,
–
–
–
–
large animals were the most drastically affected,
but not all dinosaurs were large,
and crocodiles, close relatives of dinosaurs
survived although some species died out
• Some paleontologists think that dinosaurs,
–
–
–
–
some marine invertebrates,
and many plants were already on the decline
and headed for extinction
before the end of the Cretaceous
• A meteorite impact
– may have simply hastened the process
Satisfaction?
• In the final analysis,
– Mesozoic extinctions
– have not been explained to everyone’s satisfaction
• Most geologists now concede
– that a large meteorite impact occurred
– but we also know that vast outpourings of lava
– were taking place in what is now India
• Perhaps these brought about
– detrimental atmospheric changes
Shallow Seas Withdrew
• Furthermore, the vast shallow seas
– that covered large parts of the continents
– had mostly withdrawn by the end of the
Cretaceous,
– and the mild equable Mesozoic climates
– became harsher and more seasonal
– by the end of that era
• But the fact remains
– that these extinctions were very selective,
– and no single explanation accounts
– for all aspects of this crisis in life history
Cynodonts and the
Origin of Mammals
• The transition from cynodonts to mammals
–
–
–
–
is so well documented by fossils
that classification of some
as either reptile or mammal
is difficult
• We can easily recognize living mammals
–
–
–
–
as warm-blooded animals with hair or fur
that have mammary glands and,
except for the platypus and spiny anteater,
give birth to live young
Skeletal Modifications
• Obviously these criteria are inadequate
– for classifying fossils
– for them, we must use skeletal structure only
• Several skeletal modifications
– characterize the transition
– from mammal-like reptiles to mammals;
• but distinctions between the two groups
– are based largely on details of the middle ear,
– the lower jaw,
– and the teeth
Reptile and Mammal Jaws
• Reptiles have only one small bone
– in the middle ear — the stapes
– while mammals have three
• the incus, the malleus, and the stapes
• Also, the lower jaw of a mammal
– is composed of a single bone called the dentary,
– but a reptile's jaw is composed of several bones
• In addition, a reptile's jaw
– is hinged to the skull at a contact
– between the articular and quadrate bones,
• while in mammals the dentary
– contacts the squamosal bone of the skull
Transition From Cynodonts to
Mammals
• During the transition from cynodonts to
mammals,
–
–
–
–
–
the quadrate and articular bones
that had formed the joint
between the jaw and skull in reptiles
were modified into the incus and malleus
of the mammalian middle ear
• Fossils document the progressive enlargement
– of the dentary
– until it became the only element
– in the mammalian jaw
Mammal and Reptile Skulls
Transitional Cynodonts
• Likewise, a progressive change
– from the reptile to mammal jaw joint
– is documented by fossil evidence
• In fact, some of the most advanced cynodonts
– were truly transitional because they had a
compound jaw joint consisting of
(1) the articular and quadrate bones typical of reptiles
and
(2) the dentary and squamosal bones as in mammals
Differentiated Teeth
• Several other aspects of cynodonts
– also indicate that they were ancestors of mammals
• Their teeth were somewhat differentiated
– into distinct types
– in order to perform specific functions
• In mammals the teeth are fully differentiated
– into incisors, canines, and chewing teeth,
– but typical reptiles do not have differentiated teeth
Sets of Teeth
• In addition, mammals have
– only two sets of teeth during their lifetimes
– a set of baby teeth and the permanent adult teeth
• Typical reptiles have teeth
– replaced continuously throughout their lives,
– the notable exception being in some cynodonts
– who in mammal fashion had only two sets of teeth
Tooth Occlusion
• Another important feature of mammal teeth is
occlusion;
– that is, the chewing teeth meet surface to surface
– to allow grinding
• Thus, mammals chew their food,
– but typical reptiles, amphibians, and fish do not
• However, tooth occlusion
– is known in some advanced cynodonts
Secondary Palate
• Another mammalian feature,
– the secondary palate,
– was partially developed in advanced cynodonts
• This bony shelf
– separating the nasal passages from the mouth
cavity,
– is an adaptation for eating and breathing at the
same time,
– a necessary requirement for endotherms
– with their high demands for oxygen
Secondary Palate
• Views of the bottoms of skulls
– showing the progressive development
– of the bony secondary palate
– an early therapsid
– a cynodont
– an early mammal
Mosaic Evolution
•
•
•
•
In short,
some mammalian features
evolved more rapidly than others
Recall the concept of mosaic evolution: major
evolutionary changes tend to take place in
stages, not all at once…
– from earlier in the term
Mammals Diverged
• The early mammals diverged
– into two distinct branches
• One branch includes the triconodonts
– and their probable evolutionary descendants,
– the monotremes,
• or egg-laying mammals,
– which includes the platypus
– and spiny anteater of the Australian region
Early Mammals and Descendants
• Mammal evolution proceeded along two
branches
• egg-laying mammals
– one leading to today’s monotremes
– and the other led to
– marsupials and placental mammals
Second Branch
• The second evolutionary branch
– included the marsupial mammals,
• pouched mammals
– and the placental mammals and their ancestors,
• the euphantotheres
• All living mammals
– except monotremes
– have ancestries that can be traced
– back through this branch
Insectivora
• Placental mammals of the order Insectivora, in
a scene from the Late Cretaceous
– None of
these
animals
measured
more than
a few
centimeters
long
Placental Mammals
• Placentals,
–
–
–
–
–
other than bats and a few rodents,
never reached Australia,
thus explaining why marsupials
continue to dominate the continent's fauna
even today
Mesozoic Climates and
Paleogeography
• Fragmentation of the supercontinent Pangaea
–
–
–
–
began by the Late Triassic,
but during much of the Mesozoic,
close connections existed
between the various landmasses
• The proximity of these land-masses alone,
–
–
–
–
however, is not sufficient
to explain Mesozoic bio-geographic distributions,
because climates are also effective barriers
to wide dispersal
Extensive Geographic Ranges
• During much of the Mesozoic,
– climates were more equable
– and lacked the strong north and south zonation
– characteristic of the present
• In short, Mesozoic plants and animals
– had greater opportunities
– to occupy much more extensive geographic ranges
Triassic Climate
• Pangaea persisted as a single unit
– through most of the Triassic
• The Triassic climate
–
–
–
–
was warm-temperate to tropical,
although some areas,
such as the present southwestern United States,
were arid
• Mild temperatures
– extended 50° north and south of the equator,
– and even the polar regions
– may have been temperate
Paleogeography of the World
• During the Triassic Period
Mild Jurassic Climate
• The mild Triassic climate persisted into the
Jurassic
• Ferns, whose living relatives
– are now restricted to the tropics of southeast Asia,
– lived as far as 63° south latitude
– and 75° north latitude
Late Jurassic
• By the Late Jurassic,
– Laurasia had become partly fragmented
– by the opening North Atlantic,
– but a connection still existed
• The South Atlantic had begun to open
– so that a long, narrow sea
– separated the southern parts of Africa and South
America,
– otherwise the southern continents were still close
together
Dinosaurs Roamed Widely
• Dinosaurs roamed widely across Laurasia and
Gondwana
• For example, the giant sauropod Brachiosaurus
– is found in western North America and eastern
Africa
• Stegosaurs
– and some families of carnivorous dinosaurs
– lived throughout Laurasia and in Africa
Late Cretaceous
• By the Late Cretaceous,
– the North Atlantic had opened further,
– and Africa and South America
– were completely separated
• South America remained an island continent
– until late in the Cenozoic,
– and its fauna became increasingly different
– from faunas of the other continents
• Marsupial mammals reached Australia
– from South America via Antarctica,
– but the South American connection
– was eventually severed
Paleogeography of the World
• During the Late Cretaceous Period
Summary
• Among the invertebrates,
– survivors of the Paleozoic extinctions
– diversified and gave rise
– to increasingly diverse marine communities
• Some of the most abundant invertebrates
– were cephalopods, especially
– ammonoids, foraminifera,
– and the reef-building rudists
• Land-plant communities
– of the Triassic and Jurassic
– consisted of seedless vascular plants and
gymnosperms
Summary
• The angiosperms, or flowering plants,
– evolved during the Early Cretaceous,
– diversified rapidly,
– and were soon the most abundant land-plants
• Dinosaurs evolved from small,
– bipedal archosaurs during the Late Triassic
– but they were most common
– during the Jurassic and Cretaceous periods
Summary
• All dinosaurs evolved from a common
ancestor,
– but differ enough so that two distinct orders are
recognized, the Saurischia and the Ornithischia
• Bone structure, predator-prey relationships,
– and other features have been cited
– as evidence of dinosaur endothermy
• Although there is still no solid consensus,
– many paleontologists think some dinosaurs
– were indeed endotherms
Summary
• The fact that some theropods had feathers
– indicates they were warm-blooded
– and provides further evidence of their
relationship to birds
• Pterosaurs, the first flying vertebrates,
– varied from sparrow-sized to comparative
giants
• The larger pterosaurs probably depended on
soaring
– to stay aloft whereas smaller ones flapped their
wings
Summary
• At least one species had hair or hairlike
feathers
• The fish-eating, porpoiselike ichthyosaurs
– were thoroughly adapted to an aquatic
environment,
– whereas the plesiosaurs
– with their paddle-like limbs
– could most likely come out of the water
– to lay their eggs
• Marine reptiles known as mosasaurs
– are most closely related to lizards
Summary
• Crocodiles became the dominant freshwater
predators during the Jurassic
• Turtles and lizards were present during most
of the Mesozoic
• Snakes evolved from lizards by the
Cretaceous
• The fact that Jurassic-aged Archaeopteryx,
– the oldest known bird,
– possesses so many theropod characteristics
– has convinced most paleontologists that the two
are closely related
Summary
• Mammals evolved by the Late Triassic,
– but they differed little from their ancestors,
– the cynodonts
• Minor differences in the lower jaw, teeth,
and middle ear
– serve to differentiate one group from the other
• Several types of Mesozoic mammals
existed,
– but all were small,
– and their diversity was low
Summary
• Both marsupial and placental mammals
evolved
– during the Cretaceous
– from a group known as eupantotheres
• Because the continents
– were close together during much of the
Mesozoic
– and climates were mild, plants and animals
– occupied much larger geographic ranges
– than they do now
Summary
• Among the victims
–
–
–
–
–
of the Mesozoic mass extinctions were
dinosaurs,
flying reptiles,
marine reptiles,
and several groups of marine invertebrates
• A huge meteorite impact
– may have caused these extinctions,
– but some paleontologists
– think that other factors were important, too
Cephalopods
• Cephalopods
– such as the Late
Cretaceous
amminoids
Baculites
– and Helioceros
– were important
predators
– and excellent guide
fossils
Sauropod History
• Sauropods were preceded in the fossil record
– by considerably smaller dinosaurs
– known as prosauropods
• These Late Triassic and Early Jurassic
dinosaurs
– were certainly closely related to sauropods
– but probably were not their ancestors
• Sauropods were particularly common during
the Jurassic,
– but only a few genera existed during the Cretaceous
Pteranodon
• The shorttailed
pterosaur
• known as
Pteranodon
• was a large
Cretaceous
animal
• with a
wingspan of
more than 6 m
Young Opossum’s Jaw
• View of the inside of a young opossum’s jaw
– showing that the elements of the middle ear
– are still attached to the dentary
– during early development
• This is the same
arrangement
– of bones in the
adults
– of the ancestors of
mammals
Meteorite Impact Crater
• Proposed
meteorite impact
crater
– centered on
Chixulub
– on the Yucatán
Peninsula
– of Mexico
Cycads
• Gymnosperms such as these cycads
– are still quite common
Reproductive Cycle of Angiosperms
Misconceptions about Dinosaurs
• Nevertheless, dinosaurs
• more than any other kind of animal have inspired awe,
–
–
–
–
but unfortunately, their popularization
in many cartoons, books, and movies
has commonly been inaccurate
and has contributed to many misunderstandings
• For instance, many people think
– that all dinosaurs were large,
– and because they are extinct
– they must have been poorly adapted
Eoraptor
• A 1-m-long theropod known as Eoraptor
– discovered in Argentina
– in Upper Triassic rocks
– is too specialized
– to be the ancestor of all dinosaurs,
– but it was no doubt
– quite close to that ancestry
Disagreement Still Exists
• In general, the case for endothermy
– in many theropods and in some ornithopods
– is fairly convincing
• Nevertheless, disagreement still exists
– and for some dinosaurs the question is still open
Ichthyosaur
• Life-sized frieze of a 18.2-m-long ichthyosaur
– known as Shonisaurus popularis
– at Berlin-Icthyosaur State Park, Nevada
Opossum Skull
• Skull of an opossum showing the typical
mammalian dentary-squamosal jaw joint
Cynodont Skull
• The skull of a cynodont
– shows the articular-quadrate jaw joint of reptiles
Adult Opossum’s
Middle Ear Bones
• Enlarged view of an adult opossum’s middle
ear bones
Ankylosaurs
• This 6-m-long dinosaur known as Sauropelta
– lived in western North America during the Early
Cretaceous
– It belongs to a group of dinosaurs called
ankylosaurs
– many of which had a large bony club at the end of
the tail
Echinoids
• In addition, another invertebrate group
– that prospered
– during the Mesozoic
– was the echinoids
• Echinoids were exclusively epifaunal
–
–
–
–
during the Paleozoic,
but branched out
into the infaunal habitat
during the Mesozoic
Reptiles and Birds
Mongolian Double Fossil
• A particularly interesting fossil from Mongolia
– shows a Velociraptor grasping
– a herbivorous dinosaur called Protoceratops
• It seems that both animals perished
– when the Velociraptor attacked
Arguments for Endothermy
• Good arguments for endothermy
–
–
–
–
–
exist for several types of dinosaurs,
although the large sauropods
were probably not endothermic
but nevertheless were capable
of maintaining a rather constant body temperature
• Large animals heat up and cool down
– more slowly than smaller ones
– because they have a small surface area
– compared to their volume
Less Heat Loss
• With less heat loss
– across their comparatively smaller surface area
– sauropods probably retained heat more effectively
– than their smaller relatives
• In general, the case for endothermy
– in many theropods and in some ornithopods
– is fairly convincing
• Nevertheless, disagreement still exists
– and for some dinosaurs the question is still open
Four-Chambered Heart
• Some scientists point out
–
–
–
–
that certain duck-billed dinosaurs
grew and reached maturity much more quickly
than would be expected for ectotherms
and conclude that they must have been warmblooded
• Furthermore, a recently prepared fossil
ornithopod
– discovered in 1993
– has a preserved four-chambered heart
– much like that of living mammals and birds
Convincing Evidence
• Three-dimensional imaging of this heart,
–
–
–
–
now on display
at the North Carolina Museum of Natural Sciences,
has convinced many scientists
that this animal was an endotherm
Snake Families
Differentiated Early
• Snakes first appear during the Cretaceous,
– but the families to which most living snakes belong
– differentiated since the Early Miocene
• One Early Cretaceous genus from Israel
– appears to show characteristics intermediate
– between snakes and their lizard ancestors
Fossil Feathers
• Several fossils with feather impressions
– have been discovered
– in the Solnhofen Limestone of Germany
• They definitely have feathers
–
–
–
–
–
and a wishbone, consisting of fused clavicle bones,
so typical of birds,
and yet in most other skeletal features
they most closely resemble
small theropod dinosaurs
Embryos
• Recall that the study of embryos
– provides evidence for evolution
• Opossum embryos show
– that the middle-ear bones of mammals
– were originally part of the jaw
• In fact, even when opossums are born,
–
–
–
–
–
the middle-ear elements are still attached
to the dentary
but as they develop further,
these elements migrate to the middle ear,
and a typical mammal jaw joint develops
Angiosperms Evolved
and Adapted
• Recent studies
– have identified both fossil and living gymnosperms
– that show close relationships to angiosperms
• Angiosperms have adapted
–
–
–
–
to nearly every terrestrial habitat
from mountains to deserts
and some have even adapted
to shallow coastal waters
Aquatic and Semi-aquatic
Vertebrates—Fish and Amphibians
• Sharks and the other cartilaginous fishes
– became more abundant during the Mesozoic,
– but even so they never came close
– to matching the diversity of the bony fishes
• Although an evolutionarily conservative group,
– sharks, were and still are important members
– of the marine fauna, especially among predators
Lungfishes and Crossopterygians
• Few species of lungfishes and crossopterygians
– existed during the Mesozoic,
– and the latter declined
– and was nearly extinct by the end of the era
• Only one crossopterygian species exists now
– and the group has no known Cenozoic fossil record
Living Fossil
• Latimeria
– belongs to a group of fish once thought to have
gone extinct at the end of the Mesozoic Era
– A specimen was caught off the coast of East Africa
in 1938
– Since then
many more
have been
captured
Bony Fish
• All bony fish,
• except lungfishes and crossopterygians,
– belong to 3 groups, which for convenience we call
– primitive, intermediate, and advanced
• The primitive bony fishes
– existed mostly during the Paleozoic,
• but by Middle Mesozoic time,
– the intermediate group predominated
Advanced Bony Fish
• The advanced group,
– more formally known as teleosts,
– was dominant by Cretaceous time
– in both marine and freshwater environments
• With about 20,000 living species
– they are by far the most diverse
– and numerous of all living vertebrate animals
Labyrinthodont Amphibians
• The labyrinthodont amphibians
– were common during the latter part of the
Paleozoic,
– but the few surviving Mesozoic species died out
– by the end of the Triassic
• Since their greatest abundance
– during the Pennsylvanian Period,
– amphibians have made up
– only a small part of the total vertebrate fauna
• Frogs and salamanders evolved
– during the Mesozoic,
– but both have poor fossil records
Archosaurs and the
Origin of Dinosaurs
• Reptiles known as archosaurs
• archo meaning "ruling" and sauros meaning "lizard”
– include crocodiles, pterosaurs (flying reptiles),
dinosaurs, and the ancestors of birds
• Including such diverse animals
– in a single group implies
– that they share a common ancestor
– and indeed they possess several characteristics that
unite them
Archosaurs Teeth
• For instance, all have teeth set in individual
sockets,
– except today's birds,
– but even the earliest birds had this feature,
– and they all lay amniotic eggs
• We now turn to a discussion of dinosaurs,
– but will have more to say
– about the other archosaurs later
“Endothermic Bone”
• Proponents of dinosaur endothermy
– note that dinosaur bones
– are more similar to those of living endotherms
• Yet crocodiles and turtles
– have this so-called endothermic bone,
– but they are ectotherms,
• and some small mammals
– have bone more typical of ectotherms
• Perhaps bone structure is related
– more to body size
– and growth patterns than to endothermy,
– so this evidence is obviously not conclusive
Pterosaurs
• Pterosaurs are generally depicted in movies
– as large, aggressive creatures,
– but some were no bigger than today's
– sparrows, robins, and crows
• However, a few species had
– wingspans of several meters,
– and one Cretaceous pterosaur found in Texas had
– a wingspan of at least 12 m!
• Nevertheless, even the very largest species
– probably weighed less than a few tens of kilograms
Smaller Pterosaurs
• Smaller pterosaurs,
–
–
–
–
in contrast,
probably stayed aloft
by vigorously flapping their wings
just as present-day small birds do
Mary Anning
• Mary Anning
– lived in Lyme Regis
– on England’s south
coast
• where she began
collecting
– and selling fossils
– when she was 11
years old
Mary Anning
• Unfortunately,
–
–
–
–
many scientists of her time
could not accept that an untutored girl
could possess such knowledge and skill
and attributed her discoveries to others
Mammal Divergence
• The divergence of marsupials and placentals
– from a common ancestor
– probably occurred during the Early Cretaceous
– and a recently discovered fossil in china
substantiates this
• The earliest known placental mammals
– were members of the order Insectivora
– an order represented today
– by shrews, moles, and hedgehogs
Iridium Anomaly Origin
• The iridium anomaly is real,
– but its origin and significance are debated
• We know very little
– about the distribution of iridium in crustal rocks
– or how it may be distributed and concentrated
• Some geologists suggest
– that the iridium was derived
– from within Earth by volcanism,
– but this idea is not very well supported by evidence
Cephalopods
• Cephalopods
– such as the Late
Cretaceous
ammonoids
Baculites
– and Helioceros
– were important
predators
– and excellent guide
fossils
Mesozoic Bivalves
• Mesozoic bivalves diversified
– to inhabit many epifaunal and infaunal niches
• Oysters and clams
–
–
–
–
–
epifaunal suspension feeders
became particularly diverse and abundant
and despite a reduction in diversity
at the end of the Cretaceous,
remain important animals in the marine fauna today
Rudist Bivalves
• Two genera of Cretaceous reef-building
bivalves known as rudists
• These and other rudists replaced corals as the
main reef-building animals of the Mesozoic
Ginkgos
• Ginkgos
– have changed very
little
– for millions of years
• They were found
– living in some
isolated habitats in
Asia
– and have been
transplanted
elsewhere
Planktonic Foraminifera
• Globotruncana calcarata
– from the Cretaceous Pecan Gap Chalk of Texas
• Planktonic
forams
– became diverse
during the
Jurassic and
Cretaceous,
– Many died out at
the end of the
Cretaceous
Fossil Angiosperms
• From the lower
Cretaceous
– Potomac Group
– of the eastern United
States
• Sapindopsis,
– Cecil County,
Maryland
Fossil Angiosperms
• From the lower
Cretaceous
– Potomac Group
– of the eastern United
States
• Aralia
– from New Jersey
Theropods with Feathers
• Beginning in 1996,
– Chinese scientists
– have made remarkable discoveries
– of small theropods with feathers
• Molecular analysis shows
– that they were indeed composed
– of the same material as bird's feathers
• We will discuss the significance of these fossils
– more fully in the section
– on Warm-Blooded Dinosaurs
Ornithopods Were Herbivores
• All ornithopods were herbivores
–
–
–
–
and primarily bipedal
with well-developed forelimbs
that allowed them to walk
in a quadrupedal fashion, too
Ichthyosaurs’ Tail and Forelimbs
• Ichthyosaurs used
– their powerful tail
• for propulsion
– and their flipperlike forelimbs
• for maneuvering
Short-Necked Plesiosaurs
• Short-necked plesiosaurs might have been
bottom feeders,
–
–
–
–
but their long-necked cousin
may have used their necks
in a snakelike fashion
to capture fish with their numerous sharp teeth
• These animals
– probably came ashore to lay their eggs
Origin and Early Evolution
of Mammals
• Therapsids,
• or the advanced mammal-like reptiles
– diversified into numerous species
– of herbivores and carnivores
• These terrestrial vertebrates
– were the most diverse and numerous
– during the Permian
• One particular group of carnivorous therapsids
– called cynodonts was the most mammallike of all
– and by the Late Triassic gave rise to mammals
Small Theropods?
• These Late Triassic fossils have hollow bones
– and the breastbone structure of birds,
– but because no feather impressions were found,
– many paleontologists think they are small
theropods
Mesozoic Mammals
• Mammals evolved during the Late Triassic,
– but for the rest of the Mesozoic Era
– they remained small and not very diverse
• Furthermore, they still retained several reptilian
characteristics,
– but had mammalian features as well
• For instance, the Triassic-aged triconodonts
– had the fully differentiated teeth typical of
mammals,
– but they also had both the reptile and mammal
types of jaw joints
Worldwide Distribution of Fauna
• Triassic fauna had a truly worldwide
distribution
• Some dinosaurs had continuous ranges
–
–
–
–
–
across Laurasia and Gondwana,
the peculiar gliding lizards
lived in New Jersey and England,
and reptiles known as phytosaurs
lived in North America, Europe, and Madagascar