R.M. Clary, Ph.D., F.G.S.

Department of Geosciences

MississippiStateUniversity

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

PerspectiveMary 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.

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 15.1Cretaceous Seascape

2.Some of the most abundant invertebrates were cephalopods (especially ammonoids), foraminifera, and the reef-building rudists.

Figure 15.2Belemnoids

Figure 15.3Planktonic Foraminifera

Figure 15.4Primary 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 15.5The 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 15.6Mesozoic Vegetation

Figure 15.7The Angiosperms or Flowering Plants

5. Dinosaurs evolved from small, bipedal archosaurs during the Late Triassic, but weremost common during the Jurassic and Cretaceous periods.

Figure 15.8Relationships Among Fossil and Living Reptiles and Birds

6.Dinosaurs differ enough that two distinct orders are recognized: the Saurischia and the Ornithischia.

Figure 15.9Cladogram Showing Relationships Among Dinosaurs

Figure 15.10Theropod Dinosaurs were Bipedal Carnivores

Table 15.1Summary Chart for the Orders and Suborders of Dinosaurs

Figure 15.11Hadrosaurs or Duck-billed Dinosaurs

Figure 15.12Representatives 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,

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.

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.

  1. That some theropods had feathers indicates that they were warm-blooded and

provides further evidence of their relationship to birds

Figure 15.13Feathered 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 15.14The 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 15.15Mesozoic Marine Reptiles

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 15.16Archaeopteryxfrom 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,

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 NewsSept. 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 15.17Evolution of the Mammal Jaw and Middle Ear

Table 15.2Summary Chart Showing Some Characteristics and How They Changed during the Transition from Reptiles to Mammals

Figure 15.18Comparison 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.

Figure 15.19Relationships Among the Early Mammals and their Descendants

Figure 15.20Restorations 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 15.21End 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 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.
  1. The premier soap opera of paleontology may be the saga of the dinosaur searchers, particularlyEdward 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.
  1. 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.

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?
  1. 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?
  1. 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 / endotherm / placental mammal
Archaeopteryx / ichthyosaur / plesiosaur
archosaur / iridium anomaly / pterosaur
bipedal / marsupial mammal / quadrupedal
cynodont / monotreme / Saurischia
dinosaur / mosasaur / therapsid
ectotherm / Ornithischia
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
  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 DinosaurNational 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.a / 5.d / 9.c
2.d / 6.e / 10.a
3.b / 7.b
4.b / 8.d
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.