Chapter 15 Life of the Mesozoic Era Mesozoic

Chapter 15 Life of the Mesozoic Era Mesozoic

Chapter 15 Life of the Mesozoic Era Mesozoic Life Fascinates Ever since Sir Richard Owen first used the term dinosaur in 1842, these animals have been the objects of intense curiosity Current interest in dinosaurs was fueled by the movies Jurassic Park and its sequels

No other group of animals has so thoroughly captured the public imagination, but dinosaurs were only one type of Mesozoic reptile Cretaceous Scence In this scene from the Late Cretaceous, Ankylosaurus is defending itself from the large predator Tyrannosaurus. 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," although there were far more species of insects and fishes at this time The Age of Reptiles The Mesozoic was in important time in the evolution of reptiles. Recent discoveries have added much to our knowledge:

Remarkable discoveries of feathered dinosaurs in China have important implications about the warm-blooded-cold-blooded dinosaur debate, and the relationships of dinosaurs to birds. Birds & Mammals Birds first appeared during the Jurassic, probably having evolved from small carnivorous dinosaurs

Mammals evolved from mammal-like reptiles during the Triassic Mammals and dinosaurs were contemporaries but mammals were not nearly as diverse and all were small Land Plants & Invertebrates Important changes took place

in Cretaceous land plant communities when flowering plants evolved and soon became the most numerous and diverse of all land plants The invertebrate animals that survived the Paleozoic mass extinctions diversified during the Triassic and repopulated the seas. Mesozoic Extinction

Biotic diversity once again increased in all realms of the organic world, only to decrease again at the end of the Mesozoic. The Mesozoic extinction was second in magnitude to the one at the end of the Paleozoic, but it is more widely known. 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 of continents partly determines the geographic distribution of organisms Isolation Pangaea began fragmenting during the Triassic and continues to do so

Intercontinental interchange among faunas became increasingly difficult for most organisms In fact, South America and Australia were isolated continents and their faunas evolving in isolation became increasingly different from those elsewhere 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 brachiopods 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 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

Cephalopods Cephalopods such as the Late Cretaceous ammonoids Baculites and Helioceros were important predators and excellent guide fossils Surviving Cephalopods

Ammonites went 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

Belemnoids These extinct squidlike cephalopods were abundant during the Cretaceous and are excellent guide fossils for the Jurassic and Cretaceous 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 Cretaceous Bivalves 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 Reefs did not rebound from the Permian extinctions until the Middle Triassic Mesozoic Reef-Builders 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 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 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 Burrowing Organisms A major difference between Paleozoic and Mesozoic marine invertebrate faunas was the increased abundance and diversity of burrowing organisms.

With few exceptions, Paleozoic burrowers were soft-bodied animals such as worms. The bivalves and echinoids evolved various means of entering infaunal habitats This may have been an adaptive response to increasing predation from the rapidly evolving cephalopods Foraminifera The foraminifera

single-celled consumers diversified rapidly 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 The planktonic foraminifera are excellent guide fossils for the Cretaceous

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. 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 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 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 MiocenePiocene dinoflagellate from the Gulf of Mexico

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 Fish Today, Earths oceans, lakes, and rivers are populated by about 24,000 species of bony fish whereas only 930 species of cartilaginous fish exist all of them confined to the seas 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 Sharks, an evolutionarily conservative group, were and remain important in marine fauna as predators Lungfishes and Crossopterygians Few species of lungfishes and crossopterygians persisted into 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 known as coelacanths 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 Superficially they resemble each other but important changes took place as one group replaced another The internal skeleton of the primitive and intermediate

varieties was partly cartilaginous but in the advanced group it was completely bony Advanced Bony Fish The primitive bony fishes existed mostly during the Paleozoic, but by Middle Mesozoic time, the intermediate group predominated The advanced group, more formally known as teleosts,

became the most diverse of all bony fishes by Cretaceous time in both fresh and saltwater environments and now are the most varied and numerous of all vertebrate animals Mesozoic Fish Mesozoic fish Leedsichthys and the shortnecked plesiosaur Liopluerodon The fish (background), from the intermediate group was one of the largest ever It was probably a plankton feeder 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 diverged during the Late Permian or Early Triassic

but both have poor fossil records Plants Primary 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 Ginkgos These fan-shaped leaves from a present-day gingko tree

look much like those of their ancient ancestors 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 Mesozoic Plants The Jurassic landscape was dominated by seedless vascular plants, especially ferns, as well as

gymnosperms including conifers and cycads Cycads These living cycads look much like the vegetation of the Jurassic 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 The fossil record of the earliest angiosperms is sparse so their precise ancestors remain obscure Nevertheless, studies of living plants and the fossils

indicate close relationships with the gymnosperms. Fossil Angiosperms Archaefructus sinensis from Lower Cretaceous rocks in China is among the oldest known angiosperms Fossil Angiosperms Restoration of Archaefructus sinensis

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, including enclosed seeds and above all the origin of flowers which attract animal pollinators especially insects Reproductive Cycle of Angiosperms

More Than 90% The interrelationships among flowering plants and insects are so close that biologists refer to changes in one induced by the other as coevolution The 250,000 to 300,000 species of angiosperms that now exist account for more than 90% of all land plant species. They inhabit some habitats hostile to other plants, a testimony to their success

Nevertheless, seedless vascular plants and gymnosperms remain important in the worlds flora The Diversification of Reptiles Reptiles first appeared during the Mississippian Period and when they first evolved from amphibians they did not look very different from their ancestors This group of so-called stem reptiles, gave rise to all other reptiles, birds, and mammals All living reptiles are cold-blooded,

lay amniotic eggs, practice internal fertilization, and have a tough scaly skin In addition, living reptiles and dinosaurs with the exception of turtles have two openings on the side of the skull in the temporal area Fossil and Living Reptiles and Birds Relationships among fossil and living reptiles and birds Dinosaurs Dinosaurs are included among the reptiles

but they possess several characteristics that set them apart They had teeth set in individual sockets, a reduced lower leg bone (fibula), a pelvis anchored to the vertebral column by three or more vertebrae, a ball-like head on the upper leg bone (femur), and elongate bones in the palate Dinosaurs Dinosaurs had a fully upright posture with the limbs directly beneath their bodies,

rather than the sprawling stance or semi-erect posture as in other reptiles Their upright posture and other limb modifications may have been partly responsible for their incredible success Dinosaur Misconceptions Contrary to popular belief there were no flying dinosaurs or fully aquatic ones

although there were Mesozoic reptiles that filled these niches Nor were all dinosaurs large even though some certainly were Dinosaurs lived only during the Mesozoic Era unless we consider the birds, which evolved from one specific group of dinosaurs 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 Reptiles and Birds

Archosaurs All archosaurs have teeth set in individual sockets, except today's birds, but even the early birds had this feature Additionally, these animals have a single skull opening in front of the eye that is not found in other reptiles Dinosaur Orders Dinosaurs share many characteristics

yet differ enough for us to recognize two distinct orders based primarily on their type of pelvis: the Saurischia and Ornithischia 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 relationships among dinosaurs Pelvises of ornithischians and saurischians are shown for comparison theropods were carnivores and all others were herbivores Common Ancestor Paleontologists are convinced

that both orders of dinosaurs had a common ancestor, much like archosaurs from the Middle Triassic rocks in Argentina. These long-legged, small less than 1 m long dinosaur ancestors 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"

Of course we have no reason to think that they were any more terrible than animals living today and they were not lizards Misconceptions about Dinosaurs Nevertheless, these ideas persisted and even their popularization in cartoons and movies has commonly been inaccurate and contributed to misunderstandings For instance, many people think

that all dinosaurs were large, and they were poorly adapted because they went extinct Misconceptions about Dinosaurs Many were large, but dinosaurs varied from giants weighing several tens of metric tons to those that weighed only 2 or 3 kg To consider them poorly adapted is to ignore the fact that as a group

they 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 brainy animals at least by reptile standards

and more active than formerly thought It seems that some species cared for their young long after they hatched, a behavior that is found mostly in birds and mammals Questions Remain Although many questions about dinosaurs remain unanswered, their fossils and the rocks containing them are revealing more and more about their evolution and behavior

Saurischian Dinosaurs Paleontologists recognize two groups of saurischians, known as theropods and sauropods Theropods were carnivorous bipeds that varied from tiny Compsognathuis to giants such as Tyrannosaurus and similar but even larger genera Beginning in 1996, Chinese scientists have found several genera of theropods with feathers

Molecular analysis shows that they were composed of the same material as bird's feathers Dinosaur Cladogram Small Theropod Dinosaur Compsognathus weighed only 2 or 3 kg Bones found within its ribcage indicate it ate lizards Velociraptor and Deinonychus The movie Jurassic Park 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, likely used their claws in a slashing type of attack Deinonychus Lifelike restoration of Deinonychus It was about 3 m long and may have weighed 80 kg

Theropods Like predators today, theropods probably avoided large, dangerous prey and went for the easy kill, preying on the young, old, or disabled, or they dined on carrion No doubt the larger theropods simply chased smaller predators away from their kill From evidence available, some theropods such as diminuitive Coelophysis

and medium-sized Deinonychus hunted in packs Sauropods Included among the sauropods are the truly giant, quadrupedal herbivorous dinosaurs

such as Apatosaurus, Diplodocus, and Brachiosaurus Among the sauropods were the largest land animals ever Brachiosaurus, a giant even by sauropod standards, may have weighed 75 metric tons, and partial remains indicate

that even larger sauropods may have existed Trackways show sauropods moved in herds Sauropod History Sauropods were preceded in the fossil record by smaller, Late Triassic to Early Jurassic

prosauropods which were undoubtedly related to sauropods, but were probably not their ancestors Sauropods were most common during the Jurassic, only a few genera existed during the Cretaceous Ornithischian Dinosaurs The distinguishing features of ornithischians is their bird-like pelvis However, they also differ

in that they have no teeth in front of the mouth as saurichians do, and they also have ossified (bone-like) tendons in the back region Scientists recognize five distinct groups of ornithischians ornithopods, pachycephalosaurs, ankylosaurs, stegosaurs, and ceratopsians Dinosaur Cladogram

Ornithopod Dinosaurs In 1822, Gideon Mantell and his wife Mary Ann discovered some teeth that he later named Iguanadon, which proved to be a member of the ornithscian subgroup, ornithopods Hadrasaurus was discovered in North America in 1858, and was the first dinosaur to be assembled and displayed in a museum

Ornithopod Dinosaurs Among the several varieties of ornithopods, duck-billed dinosaurs or hadrosaurs were especially numerous during the Cretaceous and several had crests on their heads that may have been used to amplify bellowing, for sexual display, or for species recognition Duck-Billed Dinosaurs

This group of ornithopods had flattened bill-like mouths and some had crests or other ornamentation Shantungosaurus had the typical flattened bill-like mouth, but no crest Corythosaurus had a crest, a bony extension of the skull Duck-Billed Dinosaurs Parasaurolophus was a crested dinosaur Tsintaosaurus had no crest, but a bony projection It is called the unicorn dinosaur Ornithopods Were Herbivores All ornithopods were herbivores

and primarily bipedal with well-developed forelimbs that could also walk in a quadrupedal manner Maisaura Maiasaura (good-mother lizard) nested in colonies

and used the same nesting area repeatedly Furthermore, their 2-m-diameter nests were spaced 7 m apart or about the length of an adult Some nests contain the remains of juveniles up to 1m long so they must have stayed in the nest area where adults protected and perhaps fed them Maisaura

A bone bed in Montana has the remains of an estimated 10,000 individuals The evidence indicates they were overcome by volcanic gases and later buried by flood deposits Pachycephalosaurs The most distinctive feature of the bipedall, herbivorous pachycephalosaurs is their thick-boned, domed skull although the earliest ones were flat headed

They varied from 1.0 to 4.5 m long and have only been found in Cretaceous-age rocks in the Northern Hemisphere continents The traditional view although not accepted by all paleontologists is that these animals butted heads for dominance or competition for mates Ceratopsians The fossil record of Ceratopsians, or horned dinosaurs,

indicates that small Jurassic bipeds were the ancestors of large Late Cretaceous quadrupeds such as Triceratops Triceratops, with three horns on its skull and related genera had a huge head and a bony frill over the top of the neck. They were especially common in North America during the Cretaceous Triceratops

Skeleton of the ceratopsian Triceratops Fossil Herds Fossil trackways and bone beds of ceratopsians show that these large herbivores moved in herds Bone beds with dozens of individuals

of a single species indicate that large numbers of animals perished quickly, probably during river crossings Stegosaurs The most distinctive features of Stegosaurus were the well-known plates along its back and spikes at the end of its tail for defense The arrangement of the plates is not precisely known, but most restorations show two rows

with plates on one side offset from those on the other Most paleontologists think the plates functioned to absorb and dissipate heat These medium-size, quadrupedal herbivores lived during the Jurassic Other genera did not have broad plates but rather had spikes Stegosaurus Stegosaurus from the Late Jurassic was about 9 m long, and had plates on its back and bony spikes on its tail.

Ankylosaurs The ankylosaurs were quadrupedal herbivores that were more heavily armored than any other dinosaur As a result, they were not very fast The animal's back, flanks, and top of the head were protected by bony armor The tail of some species ended in a bony club that could no doubt deliver a crippling blow to an attacking predator

If the tail proved inadequate, the animal probably simply hunkered down Aggressive, Dangerous Dinosaurs? Typically, dinosaurs have been depicted as aggressive, dangerous beasts, but they probably behaved much as land animals do now. Certainly some lived in herds

and no doubt interacted by bellowing, snorting, grunting, and foot stomping in defense, territorial disputes, and attempts to attract mates 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

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 the bones of ectotherms have considerably fewer passageways

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 not conclusive Higher Metabolic Rates Endotherms must eat more than comparably 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 argument less than convincing 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 recent discoveries in China of dinosaurs with feathers or a featherlike covering Today, only endotherms have hair, fur, or feathers for insulation Feathered Dinosaurs Restoration of the Early Cretaceous feathered dinosaur Caudipteryx from China

The fact that Caudipteryx had short forelimbs, symmetric feathers and was larger than the oldest known bird indicate that it was flightless 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 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 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 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 Pteranodon

The shorttailed pterosaur known as Pteranodon was a large Cretaceous animal with a wingspan of more than 6 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 that controlled muscular coordination and sight 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 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 the wingspan of one Cretaceous pterosaur was at least 12 m! Nevertheless, even the very largest species probably weighed no more than a few tens of kilograms 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 Smaller Pterosaurs Smaller pterosaurs,

in contrast, probably stayed aloft by vigorously flapping their wings just as present-day small birds do Endothermy? At least one small pterosaur called Sordes pilosus (hairy devil)

found in 1971 in what is now Kazahkstan had a coat of hair or hairlike feathers. This outer covering and the fact that wing flapping requires a high metabolic rate and efficient respiratory and circulatory systems as in present-day birds indicates that some, or perhaps all, pterosaurs were warm blooded. Mesozoic Marine Reptiles

Several types of Mesozoic reptiles adapted to a marine environment including turtles and some crocodiles, as well as the Triassic mollusk-crushing placodonts Here, though, 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 giants more than 15 m long Details of their ancestry are still not clear,

but Utatsusarus, a fossil ichthyosaur from Japan, resembled a lizard with legs Ichthyosaurs Ichthyosaurs were fully aquatic reptiles that looked and probably lived much like todays porpoises Ichthyosaurs Tail and Forelimbs Ichthyosaurs used their powerful tail

for propulsion and maneuvered with their flipperlike forelimbs 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 Mary Anning Mary Anning lived in Lyme Regis on Englands south coast where she began collecting

and selling fossils when she was 11 years old Plesiosaurs The plesiosaurs, 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 Plesiosaurs were also aquatic, but their fipperlike forelimbs probably allowed them to come out onto land

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 and their numerous sharp teeth to capture fish

These animals probably came ashore to lay their eggs 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 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 All crocodiles are amphibious, spending much of their time in water, but they are well equipped for walking on land By Jurassic time, crocodiles had become the most common freshwater predators 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 difference between these two groups Lizards are known from Upper Permian strata, but they were not abundant until the Late Cretaceous Snakes 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 showed characteristics intermediate between snakes and their lizard ancestors From Reptiles to Birds Long ago, scientists were aware of the probable relationships between reptiles and birds Birds and reptiles both lay shelled, yolked eggs, and both share a number of skeletal features

such as the way the jaw attaches to the skull Fossil Feathers Since 1860, about 10 fossils have been recovered from the Solnhofen Limestone of Germany that provide evidence for reptile-bird relationships The fossils definitely have feathers

and a wishbone, consisting of fused clavicle bones, so typical of birds, and yet, in most other physical characteristics they most closely resemble small theropod dinosaurs Archaeopteryx These animals, known as Archaeopteryx

are birds by definition, but their numerous reptilian features convince scientists that their ancestors were among theropods Even fused clavicles (wishbone) are found in several theropods, and paleontologists in China have discovered theropods with feathers

providing more evidence for this relationship Archaeopteryx Fossil bird, Archaeopteryx from the Jurassic Solnhofen Limestone in Germany had feathers and a wishbone so it is a bird

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

Bird Evolution Another fossil bird from China, is slightly younger than Archaeopteryx retains ribs in the abdominal region just as Archaeopteryx and small theropods, but it has a reduced tail more like present-day birds More fossils found in China in 2004 and 2005

of five specimens of an Early Cretaceous bird indicate that todays birds may have had an aquatic ancestor With few exceptions, the bones of these birds, Gansus yumenesis are much like those of living birds Protoavis The fossils of Archaeopteryx are significant but there are not enough of them or of other early birds

to resolve whether it is the ancestor of today's birds or an early bird that died out without leaving descendants Of course, that in no way diminishes the fact that it had both reptile and bird characteristics However, there is another candidate for the earliest bird Some claim crow-sized Protoavis from Upper Triassic rocks in Texas, are birds Small Theropods?

Protoavis has hollow bones and a wishbone as todays birds do, but because no feather impressions were found, many paleontologists think they are small theropods Origin of Flight: Two Hypotheses One hypothesis for flight, from the ground up: holds that bird ancestors were bipedal,

fleet-footed ground dwellers that used their wings to leap into the air to catch insects or escape predation The from the trees down hypothesis holds that bird ancestors were bipeds that climbed trees and used their wings for gliding or parachuting Origin of Flight: Two Hypotheses The from-the-ground-up hypothesis

is better supported in that a bipedal theropod ancestor is reasonable because small theropods had forelimbs much like those of Archaeopteryx However, the from-the-trees-down

has an advantage because takeoff from an elevated position is easier, though landing is a challenge Origin and Evolution of Mammals Mammal-like reptiles called therapsids diversified into many species of herbivores and carnivores during the Permian These terrestrial vertebrates

were the most numerous and diverse land-dwelling vertebrates at that time One group of therapsids called cynodonts was the most mammal-like of all and by the Late Triassic time mammals evolved from them Cynodonts and the Origin of Mammals 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 However, these criteria are not sufficient for recognizing fossil mammals for them, we must use skeletal structure only

Several skeletal modifications took place during the transition from mammal-like reptiles to mammals but distinctions between the two groups are based mostly on details of the middle ear, the lower jaw, and the teeth Skeletal Modifications Fortunately, the evolution of mammals

from cyanodonts is so well documented by fossils that classification of some fossils as reptile or mammal is difficult Reptile and Mammal Jaws Reptiles have 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, whereas in mammals the dentary contacts the squamosal bone of the skull

Mammalian Jaw Skull of a mammal showing the typical mammalian dentary-squamosal jaw joint Cynodont Skull The skull of a cynodont shows the articular-quadrate jaw joint of reptiles Mammalian Middle Ear Bones Enlarged view of a mammals middle ear bones

Reptilian Ear Bone Enlarged view of a reptilian ear bone 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 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 becoming double-rooted as they are in mammals, and they were somewhat differentiated into distinct types that performed specific functions In mammals the teeth are fully differentiated into incisors, canines, and chewing teeth, but typical reptiles do not have differentiated teeth

Mammal and Reptile 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 reptiles, amphibians, and fish do not However, tooth occlusion is known in some advanced cynodonts Occipital condyle Reptiles and mammals

have a bony protuberance from the skull that fits into a socket in the first vertebra: the atlas This structure, called the occipital condyle is a single feature in typical reptiles, but in cynodonts it is partly divided into a double structure

typical of mammals 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 Embryos In 1837, Karl Reichert discovered that embryos of mammals have an extra bone, the articular, in the lower jaw whereas adult mammals have one bone, the dentary He also found an extra bone in the upper jaw called the quadrate Furthermore, these two bones formed the jaw skull joint

just as they do in reptiles However, as the embryo matured, the articular and quadrate moved to the middle ear where they because the incus and malleus, and the jaw-skull joint typical of mammals developed Opposums In fact, when opposums are born they have this reptile-type jaw-skull joint between the articular and quadrate, but as they develop further,

these bones move to the middle ear and the typical mammal-like jaw-skull forms Mesozoic Mammals Mammals evolved during the Late Triassic, not long after the first dinosaurs appeared but for the rest of the Mesozoic Era most of them were small Exceptions: the Middle Jurassic-aged aquatic mammal from China, about 50 cm long, and Repenomamus giganticus,

also from China, about 1 m long and 12-14 kg, which had the remains of a juvenile dinosaur in its stomach! Mesozoic Mammals Most other Mesozoic mammals were about the size of mice and rats and were not very diverse Furthermore, they still retained several reptilian characteristics, but had mammalian features, too

The Triassic triconodonts had the fully differentiated teeth typical of mammals, but they also had both the reptile and mammal types of jaw joints Mosaic Evolution

In short, some mammalian features evolved more rapidly than others Recall the concept of mosaic evolution 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 todays monotremes and the other led to

marsupials and placental mammals Second Branch The second evolutionary branch includes the marsupial mammals, pouched mammals and the placental mammals and their ancestors, the eupantotheres Divergence of marsupials and placental mammals from a common ancestor

took place during the Early Cretaceous Insectivora Restoration of the oldest known marsupial mammal, Sinodelphys, which measures 15 cm long Insectivora Restoration of the oldest known placental mammal, Eomania, which only measured 1213 cm long 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 landmasses alone,

however, is not sufficient to explain Mesozoic biogeographic 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 supercontinent 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 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 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 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

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 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 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 ExtinctionsA 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 Many hypotheses have been proposed to account for Mesozoic extinctions but most have been dismissed as improbable, or inconsistent with the available data In 1980, a proposal was made that has gained wide acceptance

It is based on a discovery at the Cretaceous-Paleogene boundary in Italy of a 2.5-cm-thick clay layer with a notable concentration of the platinum-group element indium High iridium concentrations have now been identified at many other Cretaceous-Paleogene boundary sites

Boundary Clay Close-up 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 Boundary Sites Some Cretaceous-Paleogene boundary sites

also contain soot and shock-metamorphosed quartz grains, both of which are cited as additional evidence of an impact 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, temporarily halting photosynthesis, food chains collapsed, and extinctions followed 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 strong acid rain that might have had devastating effects on vegetation and marine organisms Impact Site Some geologists point to a probable impact site centered on the town of Chicxulub on the Yucatan Peninsula of Mexico

The 170 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 Chixulub on the Yucatn Peninsula

of Mexico Evidence at Chicxulub Evidence supporting the conclusion that the Chicxulub structure is an impact crater includes shocked quartz, the deposits of huge waves, and tektites,

small pieces of rock that were melted during the 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 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 Conclusive Evidence? In the final analysis,

Mesozoic extinctions have not been explained to everyones 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 Nevertheless, these extinctions were selective, and no single explanation accounts for all aspects of this crisis in life history Summary

Invertebrate 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 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 of pterosaur 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 were 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 Jurassic-aged Archaeopteryx, the oldest known bird, possesses so many theropod characteristics

that it 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 of fossils from the other

Several types of Mesozoic mammals existed, but most were small, and their diversity was low Summary Both marsupial and placental mammals evolved during the Cretaceous from a group known as eupantotheres Because during much of the Mesozoic

the continents were close together 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

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