Life of the Mesozoic Era

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Life of the Mesozoic Era

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Title: Life of the Mesozoic Era

1
Chapter 15
Life of the Mesozoic Era
2
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

3
Baryonyx

This Mesozoic scene shows the 10 m-long
carnivorous dinosaur Baryonyx
and two large herbivorous dinosaurs
in the background.
Dinosaurs are the most popular of all extinct
organisms.

4
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

5
Dinosaur 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)

6
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 none were very large

7
Birds

Birds also made their appearance,
most likely evolving
from small carnivorous dinosaurs
during the Jurassic
Remarkable discoveries of feathered dinosaurs in
China
have important implications about dinosaur
biology
and are important for evaluating dinosaur
relationships with birds

8
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

9
Systems Approach

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

10
Isolation

Pangaea began fragmenting
during the Triassic and continues to do so
Organisms had increasingly difficulty
migrating between continents as a result
In fact, South America and Australia
became isolated island continents
and their faunas evolving in isolation
became quite different from those elsewhere

11
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
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

12
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

13
Brachiopods Never Fully Recover

The brachiopods,
that had been so abundant during the Paleozoic,
never completely recovered from their near
extinction
Although brachiopods still exist
the bivalves
have largely taken over their ecological niche

14
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

15
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 nearly benthonic existence

16
Cephalopods

Cephalopods
such as the Late Cretaceous ammonoids Baculites

and Helioceros

were important predators
and excellent guide fossils

17
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 were squidlike in appearance
and are good Jurassic and Cretaceous guide
fossils
as well as by the living squid and octopus

18
Belemnoids

These extinct squidlike cephalopods
were abundant during the Cretaceous
and are excellent guide fossils
for the Jurassic and Cretaceous

19
Cretaceous Hard Chalk Seafloor

Mollusks were major elements of the Mesozoic
marine invertebrate fauna, particularly during
the Cretaceous

20
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

21
Cretaceous Bivalves

Bivalves were particularly diverse and abundant
during the Mesozoic
Even today they remain important elements in the
marine invertebrate fauna

22
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

23
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

24
Familiar Coral

A new and familiar type of coral
also appeared during the Triassic,
the scleractinians
Whether scleractinians evolved from rugose corals
or from an as yet unknown soft-bodied ancestor
with no known fossil record is still unresolved

25
Echinoids

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
Both groups began a major adaptive radiation
during the Late Triassic
that continued throughout the remainder
of the Mesozoic and Cenozoic

26
Foraminifera

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

27
Planktonic Foraminifera

Many planktonic foraminifera
are excellent guide fossils for the Cretaceous,
such as species of the genus Globotruncana,
which is restricted to the Upper Cretaceous.

28
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

29
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 escaped predators by burrowing

30
Mesozoic Primary Producers

The primary producers in the Mesozoic seas
were various types of microorganisms
Coccolithophores are an important group
of calcareous phytoplankton
that first evolved during the Jurassic
and became extremely common during the Cretaceous

31
Coccolithophores

Coccolithophores from the Gulf of Mexico
of Miocene age

of Miocene-Pliocene age

32
Diatoms

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

33
Diatoms

Diatoms from Upper Miocene rocks in Java

34
Dinoflagellates

Dinoflagellates
Which are organic-walled phytoplankton,
were common during the Mesozoic and today are the
major primary producers in warm water

An Eocene dinoflagellate from Alabama

A Miocene-Piocene dinoflagellate from the Gulf of
Mexico

35
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

36
Aquatic and Semiaquatic VertebratesFish 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

37
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

38
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

39
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

40
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

41
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

42
PlantsPrimary Producers on Land

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
Among the gymnosperms,
the large seed ferns became extinct
by the end of the Triassic,
but ginkgos remained abundant
and still exist in isolated regions

43
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

44
Gymnosperms

Conifers continued to diversify
and are now widespread in some terrestrial
habitats,
particularly at high elevations and high
latitudes
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

45
Mesozoic Plants

The Jurassic landscape was dominated by
seedless vascular plants,
particularly ferns,
as well as gymnosperms
such as conifers,
tree ferns,
and cycads

46
Cycads

Gymnosperms such as these cycads
are still quite common

47
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
Studies of fossil and living gymnosperms
show that some have a close relationship with
angiosperms,
but precise ancestors remain obscure

48
Fossil Angiosperms

Archaefructus sinensis
from Lower Cretaceous rocks in China
is among the oldest known angiosperms

49
Fossil Angiosperms

Reconstruction of Archaefructus sinensis

50
Angiosperms Evolved and Adapted

Since they evolved, angiosperms have adapted
to nearly every terrestrial habitat
from mountains to deserts
and some have even adapted
to shallow coastal waters
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

51
Reproductive Cycle of Angiosperms
52
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

53
The Diversification of Reptiles

Reptile diversification began
during the Mississippian Period
with the evolution of the protorothyrids,
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

54
Reptiles and Birds

Relationships among fossil and living reptiles
and birds

55
The Story of Reptile Diversification

Recall that pelycosaurs
were the dominant land vertebrates
of the Pennsylvanian and Permian periods
Here we continue our story
of reptile diversification
with a group called archosaurs

56
Reptiles and Birds
57
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

58
Archosaurs Teeth

For instance, all have teeth set in individual
sockets,
except today's birds,
but even the early birds had this feature,
and they all lay amniotic eggs
One of the most important features is the limbs
that indicate semi-upright or upright posture
We now turn to a discussion of dinosaurs.

59
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

60
Misconceptions about Dinosaurs

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

61
Misconceptions about Dinosaurs

It is true that many were quite large,
but dinosaurs varied from giants
weighing several tens of metric tons
to those that weighed no more than 2 or 3 kg
To consider them poorly adapted
is to ignore the fact
that dinosaurs were extremely diverse
and widespread for more than 140 million years!

62
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

63
Questions Remain

Many questions remain unanswered about dinosaurs,
their fossils and the rocks containing them
are revealing more and more
about their evolutionary relationships and
behavior

64
Dinosaurs Orders

All dinosaurs possess
a number of shared characteristics,
such as full and upright posture
with limbs directly beneath their bodies
yet differ enough for us to recognize
two distinct orders,
the Saurischia
and Ornithischia
A distinctive pelvic structure
characterizes each order

65
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

66
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

67
Common Ancestor

Paleontologists are convinced
that both orders of dinosaurs
share a common ancestor,
much like archosaurs
from the Middle Triassic rocks
in Argentina.

68
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

69
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 but even larger species

70
Dinosaur Cladogram
71
Small Theropod Dinosaur

Compsognathus weighed only 2 or 3 kg
Bones found within its ribcage indicate it ate
lizards

72
Tyrannosaurus

The skull of Tyrannosaurus,
another theropod,
measured more than 1 m long

73
Theropods with Feathers

Beginning in 1996,
Chinese scientists
have made remarkable discoveries
of theropods with feathers
Molecular analysis shows
that they were indeed composed
of the same material as bird's feathers

74
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

75
Deinonychus

Lifelike restoration of Deinonychus in its
probable attack posture
It was about 3 m long

76
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

77
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

78
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

79
Ornithischian Dinosaurs

Scientists recognize five distinct groups of
ornithischians
ornithopods,
pachycephalosaurs,
ankylosaurs,
stegosaurs,
and ceratopsians

80
Dinosaur Cladogram
81
Ornithopod Dinosaurs

Ornithopods include the duck-billed dinosaurs
with flattened bill-like mouths
Hadrosaurs were especially numerous during the
Cretaceous
and several species had head crests
which might have served a variety of functions

82
Duck-Billed Dinosaurs

Two dinosaurs from the Late Cretaceous
with head crests,
hollow, bony extensions of the skull

83
Duck-Billed Dinosaurs

Edmontosaurus had no crest

84
Ornithopods Were Herbivores

All ornithopods were herbivores
and primarily bipedal
with well-developed forelimbs
that allowed them to walk
in a quadrupedal fashion also

85
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

86
Pachycephalosaurs

The most distinctive feature
of the pachycephalosaurs
is their thick-boned,
dome-shaped skull
Paleontologists note that the thick skull bones
are found in juveniles but not adults
These bipedal herbivores
are known from Late Cretaceous-aged rocks

87
Ceratopsians

The fossil record of Ceratopsians,
or horned dinosaurs,
shows that small Early Cretaceous animals
were the ancestors of large Late Cretaceous
genera
such as Triceratops
and related genera with huge heads,
a large bony frill over the neck,
and a horn or horns on the skull
They were quite common in North America.

88
Triceratops

Skeleton of the ceratopsian Triceratops

89
Fossil Herds

Fossil trackways
and bone beds
of ceratopsians
indicate that these large,
quadrupedal herbivores
moved in herds

90
Stegosaurs

The most distinctive features of Stegosaurus
a medium-sized, quadrupedal herbivore
from the Jurassic Period
were a spiked tail,
used almost certainly for defense,
and plates on the back
The exact arrangement
of these plates is uncertain,
although they are usually depicted in two rows
with plates on one side offset from those on the
other
Most paleontologists are convinced
that plates functioned
to absorb and dissipate heat

91
Stegosaurus

Stegosaurus from the Late Jurassic was about 9 m
long, and had plates on its back and bony spikes
on its tail.

92
Ankylosaurs

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 of some species ended in a bony club
that could undoubtedly deliver a crippling blow
to an attacking predator

93
Ankylosaur

The ankylosaur Euoplocephalus
Note the heavy armor and bony club at the end of
the tail

The ankylosaur Sauropelta

94
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

95
Opinion Is Divided

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,
contain blood vessels,
but considerably fewer passageways are present in
bones of ectotherms

96
Endothermic Bone

Proponents of dinosaur endothermy
note that dinosaur bones
are 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

97
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

98
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

99
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

100
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 discoveries in China
of dinosaurs with feathers or a feather-like
covering
Today, only endotherms have
hair, fur, or feathers for insulation

101
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
warm-blooded
Furthermore, a recently prepared fossil
ornithopod
discovered in 1993
has a preserved four-chambered heart
much like that of living mammals and birds

102
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

103
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

104
Less Heat Loss

With less heat loss
across their comparatively smaller surface area,
sauropods probably retained heat more effectively
than their smaller relatives

105
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

106
Pterodactyls

Pterodactyls, a long-tailed Late Jurassic
pterosaur

Among several known species,
wingspan ranged from 50 cm to 2.5 m

107
Pteranodon

The short-tailed pterosaur
known as Pteranodon
was a large Cretaceous animal
with a wingspan of more than 6 m

108
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 hairlike feathers,
possibly it was an endotherm
as perhaps all pterosaurs were

109
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?

110
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

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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

112
Smaller Pterosaurs

Smaller pterosaurs,
in contrast,
probably stayed aloft
by vigorously flapping their wings
just as present-day small birds do

113
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

114
Ichthyosaurs

The streamlined, rather porpoiselike 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
ancestors1
1Motani, R. 2000 Rulers of the Jurassic Seas,
Scientific American, v 283, no 6, p 55

115
Ichthyosaurs