Life of the Mesozoic Era

Chapter 15 Life of the Mesozoic Era

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

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

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 Plant • 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

Survivors • Recall the Paleozoic extinctions • 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

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 increasingly 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

Cephalopods • Cephalopods • such as the Late Cretaceous amminoids Baculites • and Helioceros • were important predators • and excellent guide fossils

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 • Cephalopods • such as the Late Cretaceous amminoids 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 • Bivalves were particularly diverse and abundant during the Mesozoic • Even today they remain important elements in the marine invertebrate fauna

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

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

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

Familiar Coral • A new and familiar type of coral • also appeared during the Triassic, • the sclaractinians • 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

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

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 • 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 • 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 • Dinoflagellates were common during the Mesozoic and today are the major primary producers in warm water • One of Eocene age from Alabama • of Miocene-Piocene age from the Gulf of Mexico

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

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

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

Aquatic and Semiaquatic 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

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, • 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

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

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

Cycads • Gymnosperms such as these cycads • are still quite common

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

Reproductive Cycle of Angiosperms

Life of the Mesozoic Era