R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
CHAPTER 15
LIFE OF THE MESOZOIC ERA
OUTLINE
INTRODUCTION
MARINE INVERTEBRATES AND PHYTOPLANKTON
AQUATIC AND SEMIAQUATIC VERTEBRATES
The Fishes
Amphibians
PLANTS—PRIMARY PRODUCERS ON LAND
THE DIVERSIFICATION OF REPTILES
Archosaurs and the Origin of Dinosaurs
Dinosaurs
Warm-Blooded Dinosaurs?
Flying Reptiles
Mesozoic Marine Reptiles
Crocodiles, Turtles, Lizards, and Snakes
FROM REPTILES TO BIRDS
PERSPECTIVE Mary Anning and Her Contributions to Paleontology
ORIGIN AND EVOLUTION OF MAMMALS
Cynodonts and the Origin of Mammals
Mesozoic Mammals
MESOZOIC CLIMATES AND PALEOBIOGEOGRAPHY
MASS EXTINCTIONS—A CRISIS IN THE HISTORY OF LIFE
SUMMARY
CHAPTER OBJECTIVES
The following content objectives are presented in Chapter 15:
 Marine invertebrates that survived the Paleozoic extinctions diversified and
repopulated the seas.
 Land plant communities changed considerably when flowering plants evolved during
the Cretaceous.
 Reptile diversification began during the Mississippian and continued throughout the
Mesozoic Era.
 Among the Mesozoic reptiles, dinosaurs had evolved by the Late Triassic and soon
became the dominant land-dwelling vertebrate animals.
 In addition to dinosaurs, the Mesozoic was also the time of flying reptiles and marine
reptiles, as well as turtles, lizards, snakes, and crocodiles.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
 Birds evolved from reptiles, probably from some small carnivorous dinosaur.
 Mammals evolved from reptiles only distantly related to dinosaurs, and they existed
as contemporaries with dinosaurs.
 The transition from reptiles to mammals is very well supported by fossil evidence.
 Several varieties of Mesozoic mammals are known, all of which were small, and their
diversity remained low.
 The proximity of continents and generally mild Mesozoic climates allowed many
plants and animals to spread over extensive geographic areas.
 Extinctions at the end of the Mesozoic Era were second in magnitude only to the
Paleozoic extinctions. These extinctions have received more attention than any others
because dinosaurs were among the victims.
LEARNING OBJECTIVES
To exhibit mastery of this chapter, students should be able to demonstrate comprehension
of the following:
 the repopulation of the seas by marine invertebrates following the Paleozoic mass
extinction
 the increasingly diverse and complex Mesozoic marine invertebrate communities
 changes in the composition of land-plant communities, especially with the appearance
of angiosperms
 the archosaurs and the origin of the dinosaurs
 the diversity and adaptations of dinosaurs and other reptiles
 the fossil evidence for the transition from mammal-like reptiles to mammals
 the evolution of birds from reptiles
 the divergence of Mesozoic mammals
 the relationship of plate tectonics, paleoclimates, and the geographic distribution of
plants and animals
 the possible causes and significance of the terminal Cretaceous extinction
CHAPTER SUMMARY
1. Invertebrate survivors of the Permian extinctions repopulated the seas and gave rise
to increasingly diverse marine communities.
Figure 15.1 Cretaceous Seascape
138
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
2. Some of the most abundant invertebrates were cephalopods (especially
ammonoids), foraminifera, and the reef-building rudists.
Figure 15.2 Belemnoids
Figure 15.3 Planktonic Foraminifera
Figure 15.4 Primary Producers
3. Sharks and other cartilaginous fishes became more abundant during the Mesozoic,
but they never came close to matching the diversity of the bony fishes. The bony
fishes belong to three groups. The advanced group, teleosts, became the most
diverse of all bony fishes by the Cretaceous. Frogs and salamanders were present
by the Mesozoic.
Figure 15.5 The Mesozoic Fish Leedsichthys and the short-Necked Plesiosaur
Liopluerodon
4. Land plant communities of the Triassic and Jurassic consisted of seedless vascular
plants and gymnosperms. The angiosperms, or flowering plants, evolved during the
Early Cretaceous, diversified rapidly, and were soon the most abundant land plants.
Figure 15.6 Mesozoic Vegetation
Figure 15.7 The Angiosperms or Flowering Plants
5.
Dinosaurs evolved from small, bipedal archosaurs during the Late Triassic, but
were most common during the Jurassic and Cretaceous periods.
Figure 15.8 Relationships Among Fossil and Living Reptiles and Birds
6.
Dinosaurs differ enough that two distinct orders are recognized: the Saurischia and
the Ornithischia.
Figure 15.9 Cladogram Showing Relationships Among Dinosaurs
Figure 15.10 Theropod Dinosaurs were Bipedal Carnivores
Table 15.1
Summary Chart for the Orders and Suborders of Dinosaurs
Figure 15.11 Hadrosaurs or Duck-billed Dinosaurs
Figure 15.12 Representatives of Two of the Five Groups of Ornithischians
Enrichment Topic 1. The Tyrannosaurus’ Growth Spurt
New analysis of ribs of Tyrannosaurus rex indicated that the dinosaurs grew quickly in
their “teenage” years. Researchers investigated bones that do not bear weight—such as
ribs and shinbones—and were able to accurately predict the ages of modern alligators and
lizards of known ages. The researchers discovered that T. rex grew up fast, with most of
its growth occurring between the ages of 14 and 18 years. The researchers hypothesized
that as the animal got larger, it probably suffered a decline in its running ability. Nature
News, August 2004, http://www.nature.com/news/2004/040809-7/
Enrichment Topic 2. Coprology
Coprolites, or fossilized fecal material, are providing a multitude of data towards
understanding the biology of the organisms that produced them. A Tyrannosaurus
coprolite, 65 cm in length, contained bone fragments and preserved fibers. Researchers
determined the bone fragments were consistent with pachycephalosaurs.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
This research supports the hypothesis that the tyrannosaurs did not retain their food in
their guts for long periods of time. Geology Today, July-Aug. 2004, v.20 n.4 p.129
7. 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.
8. That some theropods had feathers indicates that they were warm-blooded and
provides further evidence of their relationship to birds
Figure 15.13 Feathered Dinosaur
Enrichment Topic 3. Parental Care in Dinosaurs
Researchers assessed parental care of Cretaceous toodontid and oviraptorid dinosaur by
examining clutch volume and the bone histology of brooding adults. The large clutch
volumes of Troodon, Oviraptor, and Citipati most closely compared with a bird-paternal
care model. Paternal care in both troodontids and oviraptorids indicates that the paternal
care system evolved before the emergence of birds, and represents birds’ ancestral
condition. Varricchio, Moore, Erickson, Norell, Jackson, and Borkowski, “Avian Paternal
Care Had Dinosaur Origin,” Science, 2008, v. 322, issue 5909, p. 1826-1828.
9. Pterosaurs, the first flying vertebrates, varied from sparrow size to comparative
giants. The larger pterosaurs probably depended on soaring to stay aloft, whereas
smaller ones flapped their wings. At least one species had hair or hairlike feathers.
Figure 15.14 The Pterosaurs (Flying Reptiles)
Enrichment Topic 4. Pterosaur and Bird Embryos Indicate Flight.
A fossilized embryo of a pterosaur that was discovered in China dates to 121 million
years ago. The embryo was identified as a pterosaur through its sturdy upper arm bone
and elongated fourth finger. The surprising discovery was that the embryo was unusually
advanced with the membranes of its five-inch wings already developed. The researchers
believed that the pterosaurs could fend for themselves after hatching, and could probably
fly immediately. Discover, Jan. 2005 v.26 n.1
In a parallel discovery of an early bird embryo, scientists noted that the complete feather
structures, the embryo’s large skull, and ossified bones indicated that the baby birds were
able to fend for themselves soon after hatching. In addition, the curved claws of the
embryo indicated that the birds would have spent much of their time in the trees. Science
News, Oct. 23, 2004 v.166 n.17 p.261.
10. 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. The marine reptiles known as mosasaurs
were most closely related to lizards.
Figure 15.15 Mesozoic Marine Reptiles
140
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Enrichment Topic 5. Aquatic Reptiles of the Mesozoic
A discovery in China of an aquatic reptile, Dinocephalosaurus orientalis, has led
researchers to propose that the animal’s throat could open and create a suction to
“vacuum” in its prey. The ribs of the neck appear to have made the neck stiff and
inflexible, but protusions on the cervical ribs suggested to the paleontologists that the ribs
were used in hunting. By contracting the neck vertebrae, the ribs would have splayed,
increasing the diameter of the esophagus. The created suction would have pulled water—
and prey—into the animal’s mouth. Although some fish and turtles capture prey by
suction, the mechanism proposed for this reptile would have created a greater force.
Science News, Sept. 25, 2004, v.166 n.13 p.166.
11. During the Jurassic, crocodiles became the dominant freshwater predators. Turtles
and lizards were present during most of the Mesozoic. By the Cretaceous, snakes
had evolved from lizards.
12. Jurassic-aged Archaeopteryx, the oldest known bird, possesses so many theropod
characteristics that it has convinced most paleontologists the two are closely
related.
Figure 15.16 Archaeopteryx from the Jurassic-age Solnhofen Limestone of
Germany
Enrichment Topic 6. The Bird-Brained Archaeopteryx
Researchers confirmed that Archaeopteryx had a brain and body geared for flight.
Researchers used computed tomography (CT) to investigate the brain case of the first
Archaeopteryx specimen. The scientists believed that the bird had enlarged brain regions
for vision and movement control, parallel to modern birds. The inner ear structure was
also bird-like, as well as its brain-to-body size ratio. The researchers concluded that
Archaeopteryx was more bird-like than first believed. Nature News, August 2004,
http://www.nature.com/news/2004/040802/
Researchers also recently hypothesized that Archaeopteryx used hind leg feathers to
improve its flight. Although scientists originally thought these hind leg feathers were for
insulation and streamlining, reanalysis of the structures indicated that they functioned like
small auxiliary wings. Science News Sept. 23, 2006 v.170 p.197.
13. 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
differentiate one group of fossils from the other.
Figure 15.17 Evolution of the Mammal Jaw and Middle Ear
Table 15.2
Summary Chart Showing Some Characteristics and How They
Changed during the Transition from Reptiles to Mammals
Figure 15.18 Comparison of the Teeth of a Mammal and a Reptile
14. Several types of Mesozoic mammals existed, but most were small, and their
diversity was low. Both marsupial and placental mammals evolved during the
Cretaceous from a group known as eupantotheres.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Figure 15.19 Relationships Among the Early Mammals and their Descendants
Figure 15.20 Restorations of the Oldest Known Marsupial and Placental
Mammals
Enrichment Topic 7. Dinosaur-Eating Mammals.
Before the discovery of mammalian Mesozoic fossils in China, most paleontologists
believed that mammals of the Mesozoic were small and insectivorous. However, one
fossil, Repenomamus robustus, was found to contain the remains of its last meal—a
young juvenile Psittacosaurus. The mammal was probably 9 to 13 pounds in size and
less than 1.5 feet in length when it was alive. The discovery was important because it
offered proof that mammals also preyed upon dinosaurs. Geotimes, March 2005 v.50 n.3
p.8-9.
15. Because during the Mesozoic the continents were close together and climates were
mild, plants and animals occupied much larger geographic ranges than they do
now.
16. 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
other factors were important, too.
Figure 15.21 End of Mesozoic Extinctions
Enrichment Topic 8. Arctic Dinosaur Fossils Contradict Dinosaur Decline
Fossils uncovered in the Arctic support a sudden dying of dinosaurs. This contradicts
some paleontologists’ beliefs that extinction of dinosaurs was gradual and in decline
when a meteorite hit. Pascal Godefroit and his colleagues described fossils from
northeastern Russia that suggested that dinosaurs were not in decline at all. These fossils
are dated between 68 and 65 million years old, just before the extinction event. Dinosaur
eggshells and fossils of duck-billed hadrosaurs as well as theropods indicate a healthy
polar population. Not all scientists are in agreement with the interpretation of the data,
however. Kaplan, “Dinosaur Fossils Suggest Speedy Extinction,” Nature News, January
20, 2009.
LECTURE SUGGESTIONS
Mary Anning and the History of Science
Mary Anning, as well as many of the other pioneering geologists during the Golden Age
of Geology (1798-1840) investigated Mesozoic fossils. The history of science can be
brought into the classroom by using selected readings, research initiatives, and/or
Interactive Historical Vignettes (IHVs). In the IHV format, the instructor and/or students
present brief skits that reveal the history and nature of science. Two IHVs on Mary
Anning’s life have been written. Clary & Wandersee, “Mary Anning: She’s More than
‘Seller of Sea Shells at the Seashore,” The American Biology Teacher Mar. 2006, v.139
n.3 p.153-157; Wandersee & Clary, “Interactive Historical Vignette: The Scientific
142
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Endeavors of Mary Anning, the First Woman Paleontologist, Teaching and Learning
Science: A Handbook, K. Tobin, Ed. (2007).
The Ruling Reptiles
1. Introduce students to the dominating roles that reptiles played during the
Mesozoic in the seas, the skies, and on the land. Stress that not all of the reptiles
that lived during the Mesozoic were dinosaurs.
2. The premier soap opera of paleontology may be the saga of the dinosaur
searchers, particularly Edward Drinker Cope and Othneil Charles Marsh. In the
rush to discover dinosaur fossils and name new species, competition was fierce
and each man publicly tried to out-maneuver the other. In spite of the
competition, mistakes were eventually corrected, and science advanced. A brief
background history of the feud, as well as suggestions for incorporating the Bone
Wars into the classroom, are included in Clary, Wandersee, & Carpinelli, 2008.
“The Great Dinosaur Feud: Science Against All Odds,” Science Scope, v. 32, no.
2, p. 34-40.
3. The affiliation of birds with reptiles cannot be denied, especially with the new
fossil discoveries in China of feathered dinosaurs. Discuss with students whether
the relationship between amphibians and reptiles, reptiles and birds, or reptiles
and mammals is closest. Some paleontologists state that the “non-flying
dinosaurs went extinct at the end of the Cretaceous.” Is this a valid statement?
Why or why not?
The Terminal Mesozoic Extinction
1.
There is much information written at an introductory level about the Cretaceous
mass extinction. Students can be divided into groups to investigate various causes,
including a) meteorite impact, b) voluminous volcanism in the Deccan Traps, c)
global cooling, and other hypotheses. All groups should be prepared to debate this
topic and present sound scientific arguments for their cases, as well as against the
other possible hypotheses. An impartial outside committee, such as a college
speech class, may be brought in to evaluate the debate.
2.
Students may also predict which niches will be available at the beginning of the
Cenozoic Era. How is the world different—regardless of the cause for the
extinctions? Students may hypothesize why some organisms survived (such as
crocodiles) while other species went extinct.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
CONSIDER THIS
1. Because so much controversy surrounds the Cretaceous mass extinction, have
students formulate a hypothesis as to what would happen on Earth if similar-sized
meteorite to the Yucatan bolide impacted today. Would humans survive? What
habitats would be destroyed? Would there be global warming or cooling?
2. How would the world be different today if some dinosaurs survived the Mesozoic
mass extinction? Would mammals have risen to a dominating position in the
Cenozoic?
3. How do you think natural selection in the marine community was affected during
the Cretaceous with the Cretaceous Interior Seaway connecting the Arctic and the
Gulf? How might natural selection have proceeded on the western margin of
North America as compared to the eastern margin?
IMPORTANT TERMS
angiosperm
Archaeopteryx
archosaur
bipedal
cynodont
dinosaur
ectotherm
endotherm
ichthyosaur
iridium anomaly
marsupial mammal
monotreme
mosasaur
Ornithischia
placental mammal
plesiosaur
pterosaur
quadrupedal
Saurischia
therapsid
SUGGESTED MEDIA
Videos
1. PaleoWorld, Flight of the Pterosaurs, The Learning Channel
2. PaleoWorld, Dinos in the Air, The Learning Channel
3. PaleoWorld, Early Birds, The Learning Channel
4. PaleoWorld, The Legendary T. Rex, The Learning Channel
5. PaleoWorld, Carnosaurs: The Giant Predators, The Learning Channel
6. PaleoWorld, Dino Doctors, The Learning Channel
7. NOVA: The Four Winged Dinosaur, WGBH
8. PaleoWorld, Ancient Crocodiles, The Learning Channel
9. PaleoWorld, Earthshakers, The Learning Channel
10. PaleoWorld, Flesh on the Bone, The Learning Channel
11. PaleoWorld, Dino Sex, The Learning Channel
12. PaleoWorld, Horns and Herds, The Learning Channel
13. PaleoWorld, Monsters on the Move, The Learning Channel
14. PaleoWorld, Mystery of Dinosaur Cove, The Learning Channel
15. PaleoWorld, Troodon Dinosaur Genius, The Learning Channel
16. PaleoWorld, Sea Monsters, The Learning Channel
17. Chased by Dinosaurs, BBC
18. Walking with Dinosaurs, BBC
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
19. Chased by Sea Monsters, The Discovery Channel
20. When Dinosaurs Roamed America, The Discovery Channel
21. Life on Earth: Lords of the Air, BBC
22. Life on Earth: The Rise of the Mammals, BBC
23. Lost Worlds, Vanished Lives, The Rare Glimpses, BBC
24. Lost Worlds, Vanished Lives, Dinosaur, BBC
25. Utah’s Dino Graveyard, Discovery Channel
26. The Wollemi Pine: A Tree from the Age of Dinosaurs, Science Screen Report
27. Death of the Dinosaurs, BBC
28. Fossils: Reptiles and Mammals, Films for the Humanities and Sciences
29. Dinosaur: Fossils and Paleontology in Dinosaur National Monument,
Intermountain Natural History Association
30. Life in the Mesozoic Times, Physical Geology II Series
Slides and Demonstration Aids
1. Fossils, Dinosaurs, and Their World, Educational Images, Ltd.
2. Mesozoic Fossil Collection, Science Stuff
CHAPTER 15 - ANSWERS TO QUESTONS IN TEXT
Multiple Choice Review Questions
1.
2.
3.
4.
a
d
b
b
5.
6.
7.
8.
d
e
b
d
9. c
10. a
Short Answer Essay Review Questions
11. Cynodont jaw and skull bones modified into two of the three mammalian ear bones.
One of the reptilian jaw bones became the single mammalian dentary bone, and
teeth differentiated more extensively. The secondary palate completely separated
the mouth from the nasal passages to facilitate breathing while eating.
12.
In Archaeopteryx, bird-like features include feathers and a wishbone, consisting of
fused clavicles. Although it also has reptilian features (a tail and teeth), the
combination of the wishbone and feathers make it a bird by definition.
13. The dominant geological event of the Mesozoic was the breakup of Pangaea.
However, for much of the Mesozoic, close connections still existed between the
various landmasses. Also, the Mesozoic climate tended to be mild, lacking the
strong north and south zones that Earth currently has. Mesozoic plants and animals
had greater opportunities to occupy much more extensive geographic ranges than
today.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
14. The two primary lines of evidence for a large meteorite impact at the end of the
Cretaceous are the worldwide distribution of the iridium layer at many CretaceousPaleogene boundary sites and the 65 million year old Chicxulub meteorite impact
site in the Yucatan, which has shocked quartz, tektites, and chaotic sedimentary
deposits.
15. The early mammals diverged into two distinct branches. One branch includes the
triconodonts, and their probable descendants, the monotremes, or egg-laying
mammals. The other branch includes the marsupial mammals and the placental
mammals. The ancestor of these mammals is the eupantotheres.
16. Pterosaurs have an extended 4th finger to support the wing membrane, hollow
bones, and a keeled breastbone for the attachment of flight muscles. Their brains
were modified for improved vision and muscle control.
17.
In the Triassic, the large seed ferns were around but became extinct by the end of
the period. Ginkgos remained abundant, and conifers continued to diversity. The
cycads made their appearance during the Triassic. The Jurassic landscape was
dominated by seedless vascular plants, especially ferns, as well as gymnosperms
including conifers and cycads. In the Cretaceous, flowering plants replaced many
of the seedless vascular plants and gymnosperms. They predominate even today,
and are successful because they have evolutionary advantages of having enclosed
seeds and flowers, which attract pollinators.
18. Ornithischian dinosaurs were mostly quadupeds with bird-like pelvises, and all
were herbivores. Armored forms were common. Ornithischians included
pachycephalosaurs, ceratopsians, ankylosaurs, stegosaurs, and ornithopods.
Saurischians, dinosaurs with lizard-like pelvises, included both herbivores (mostly
sauropods) and carnivores (theropods).
19. There is evidence for endothermy in at least some dinosaurs, especially the
theropods and some ornithopods. This evidence includes (a) a similarity of their
bones to those of endotherms, although they are also similar to those of some
ectotherms; (b) a predator to prey ratio that more closely resembles that of
endotherms, although again this is uncertain since the fossil record is not always
representative of who and how many were around; (c) an endotherm-like brain
size, particularly in the small- and medium-sized theropods; (d) the relationship of
theropods to birds, since only endotherms have hair, fur, or feathers for insulation;
(e) the growth rate of some duck-billed dinosaurs, which resembles that of
endotherms; and (f) a fossil ornithopod, which has a four-chambered heart, much
like living birds and mammals. Large sauropods were probably not endothermic;
they likely retained heat effectively because of their size.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
20. Following the Paleozoic mass extinction, the invertebrate fauna of the Early
Triassic was not very diverse. However, mollusks, such as cephalopods, bivalaves,
and gastropods became important elements of the Mesozoic marine invertebrate
fauna. The ammonoids are excellent guide fossils for this era. Mesozoic bivalves
diversified to inhabit many epifaunal and infaunal niches. Oysters and clams
became diverse and abundant. Rudist bivalves were important reef-builders,
although schleractinian corals also evolved. A major difference is the increased
abundance and diversity in the Mesozoic of burrowing organisms. Bivalves and
echinoids were the primary burrowers.
Apply Your Knowledge
1.
The similarity of shapes is a result of convergent evolution, and ichthyosaurs are
not the ancestors of porpoises. Ichthyosaurs were extinct marine reptiles, and the
fossils of ichthyosaurus Utatsusaurus resemble a lizard with legs, providing
evidence of its evolution from reptiles. Porpoises, however, are mammals. As
mammals, they have hair or fur, mammary glands, and give birth to live young.
(Although ichthyosaurs are suspected of giving birth to live young, the females
retained eggs within their bodies; they were not placental mammals as porpoises
are.) There is further evidence showing the evolution of aquatic mammals from
land-dwelling mammalian ancestors. So, although the two organisms resemble each
other in form, it is strictly caused from convergent evolution as natural selection
favored these similar characteristics in very different organisms.
2.
The limestone beds with trilobites and brachiopods were deposited during the
Paleozoic Era. (Trilobites were extinct by the Mesozoic.) The beds were originally
deposited horizontally, but some tectonic event was later responsible for their tilt
(using the principle of original horizontality). The surface between the tilted beds
and the overlying volcanic ash represents an unconformity, specifically an angular
unconformity. The ash was deposited after an erosional event occurred, and this
was followed by deposition in the Mesozoic of the sandstone beds. (The sandstone
beds must be Mesozoic because of the dinosaur fossils). These beds were originally
deposited horizontally as well. The basalt dike is the youngest rock body in these
rocks (principle of cross-cutting relationships). It is also possible to determine
absolute ages for the ash layer and the basalt dike because igneous rocks can be
radiometrically dated.
3.
Student responses will vary, but should consider the similarities between
saurischians and ornithischians (as shown in Figure 15.8, p. 307), which arose from
an ancestral archosaur. Birds and the suarischians should be more closely related,
as bird evolved from theropod dinosaurs (Figure 15.8, p. 307). The relationship
between cynodonts and mammals should resemble Figure 15.19, p. 320. Placement
of the cynodonts (therapsids) and the archosaur (thecodonts) should resemble
Figure 13.17, p. 270.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
4.
The most obvious features of living mammals are endothermy (warm-bloodedness),
hair or fur, and mammary glands. Living crocodiles have none of these features.
Crocodiles are ectothermic, hairless, and produce via amniote egg. Additionally,
crocodiles have one small bone in the middle ear (as opposed to three for
mammals), and while a mammal’s jaw has one bone, a crocodile’s jaw is composed
of several bones. Additionally, crocodiles’ teeth are not differentiated as mammal
teeth are.
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