2IntroductionThe Mesozoic Era is referred to as the "Age of Reptiles". During the Mesozoic, reptiles inhabited the land, the seas, and the air. The Mesozoic is the time in which the dinosaurs lived.The Mesozoic is also the time in which mammals and birds first appeared on Earth.
3Overview of Mesozoic Era: “age of reptiles” Diverse Reptiles, including DinosaursBeginning of Evolution for Birds and MammalsExpansion of Grasses and Flowering PlantsClimate, a Strong InfluenceLocations of continentsMajor sea-level changesMountain building
4Mesozoic ClimatesPrimary Control: balance of incoming and outgoing solar radiationFactors affecting balanceconfiguration and dimension of oceans and continentsdevelopment and location of mountain systems and land bridgeschanges in snow, cloud, or vegetative covercarbon dioxide content of atmospherelocation of poles (no ice caps)amount of radiation-aerosols contributed by volcanoesastronomic factors: changes in Earth’s orbital parameters
5Triassic Climate Relatively cool Pangaea continents still clusteredlower sea levelmountain building, many highlandsPaleo-equator: central Mexico to northern AfricaWind shadow deserts (aridity) in continental interiorsred bedsevaporites
6Figure 11-3 (p. 384) Generalized paleogeographic map for the Triassic of North America.
7Jurassic Climate Relatively mild Continents at latitudes of today no glacial depositsevidence of monsoons and ariditycoals in many spots, including Antarcticatropical conditions in wide beltsContinents at latitudes of todayAtlantic openingTethys was an arm of proto-Pacific: warm ocean currents flowed through Tethys
8Figure 11-7 (p. 387) Generalized paleogeographic map for the Jurassic of North America.
9Cretaceous Climate Relatively warm Continents near today’s position subtropical flora at ± 70o of equatorhigh and low latitude coalshigh sea-level stand: maximum inundation of PhanerozoicContinents near today’s positionArctic Canada near north poleAntarctica at South PoleEnd-Cretaceous changerapid cooling and temporary vast chillingvast regressionmajor mountain buildingplankton-produced CO2 shortagevolcanic activityTerminal Cretaceous climatic event shown by:tropical cycads sharply reducedhardy conifers and angiosperms expandedoxygen isotope studies of shells show ocean temperature decline began 80 m.y. agostrong connection to global extinctions at 65 m.y. ago?
10Figure 11-12 (p. 392) Generalized paleogeographic map for the Cretaceous of North America.
11The Diversity of Life in the Mesozoic At the beginning of the Mesozoic Era, diversity (as indicated by the number of genera) was low, following the Permian extinctions. Recovery from the Permian extinctions was slow for many groups.In the oceans, the molluscs re-expanded to become much more diverse than in the Paleozoic, and modern reef-building corals, swimming reptiles, and new kinds of fishes appreared.A mass extinction occurred at the end of the Triassic Period. The Triassic extinction affected life on the land and in the sea, causing about 20% of all marine animal families to become extinct.
12The Diversity of Life in the Mesozoic (cont) Diversity increased in the Jurassic, and rose quickly during the Cretaceous to higher levels than had existed previously.Much of this expansion in diversity was related to the appearance of new types of marine predators, including advanced teleost fishes, crabs, and carnivorous gastropods.Life in the Cretaceous consisted of a mixture of both modern and ancient forms.A major extinction event occurred at the end of the Cretaceous Period, affecting both vertebrates and invertebrates, on land and in the sea.
13Mesozoic Vertebrates Amphibians: rise of modern forms Triassic Transition: continuity among land animalsSurvivors of Permian extinction (245 m.y. ago)temnospondyl amphibiansmammal-like reptiles (including therapsids, mammal ancestors)New reptile groupsfirst turtles (toothed turtles)tuataran lizardsarchosaurs: crocodiles, flying reptiles, thecodonts, dinosaursExample “basal archosaur”: Hesperosuchus
14Mesozoic Vertebrates (cont) Diapsid groupslepidosaurs: snakes, lizards, and their ancestorsarchosaurs: ornithischian and saurischian dinosaurs, flying reptiles (pterosaurs), and crocodilians.
15Basal ArchosaursSeveral groups of archosaurs were present during the Triassic, and they are referred to as "basal archosaurs" because they are at the starting point (or base) of archosaur evolution.
16Basal ArchosaursBasal archosaurs (formerly called thecodonts) were small, agile reptiles with long tails and short fore-limbs.Many were bipedal (walked on 2 legs). This freed their fore-limbs for other tasks such as catching prey, and later, flight.Hesperosuchus
17Relationships among fossil and living reptiles and birds
18DinosaursThe name "dinosaur" comes from the Greek deinos = "terrifying" and sauros = "lizard".Dinosaurs appeared in the Late Triassic, about 225 m.y. ago.The earliest dinosaurs were small. Many were less than 3 ft long.By the end of the Triassic, dinosaurs were up to 20 feet long.They became much larger later in the Jurassic and Cretaceous.
19DinosaursBasal Archosaurs (Thecodonts) were the ancestors of the dinosaursDinosaurs were composed of two orders: saurischia (lizard-hipped) and ornithischia (bird-hipped)Saurischia: pelvic bones like thecodontsOrnithischia: pubis parallel to ischium like birdsEarliest dinosaurs: saurischia (Traissic, 225 m.y. old, Argentina)
21DinosaursComparison of the skulls and teeth of saurischian dinosaurs (A) and ornithischian dinosaurs (B).
22Figure 12-18 (p. 428) Major groups of dinosaurs.
23Saurischian Dinosaurs "Lizard-hipped". Pelvic structure like lizards.Both two-legged and four-legged types.Both herbivores and carnivores.Teeth extended around entire margin of jaws, or were limited to the front.Teeth adapted to cutting and tearing, but not chewing.Food was ground up in the gizzard, probably aided by stones the dinosaurs swallowed, called gastroliths.The earliest dinosaurs and their basal archosaur ancestors were saurischians.
24Saurischian Dinosaurs Two groups:Theropods - bipedal carnivorous dinosaursSauropods - large quadrupedal herbivorous dinosaurs
25Coelophysis. One of the earliest known theropods (about 6 feet long)
26Carnivorous saurischians (Theropods) Larger carnivores; hind limbs robust; claws on toes; small fore limbs; serrated teethDeinonychus and Velociraptor: Cretaceous predatorsFamily Allosauridae: Allosaurus (U.S.)Giant theropods: Tyrannosaurus (13 m, 4 tons, North America); Gigantosaurus (Argentina); Carcharodontosaurus (Africa)
30Sauropods and Prosauropods The prosauropods were the likely ancestors of the sauropods, and lived from Late Triassic to Early Jurassic.The front legs of the prosauropods were shorter than the hind legs, although they walked on four legs.The prosauropods were replaced by the giant sauropods in the Early Jurassic.
31Herbivorous saurischians or sauropodomorphs (Jurassic-Cretaceous) Evolved from Late Triassic protosaurapod (Plateosaurus)Long necks, long tails, four-legged stanceApatosaurus (formerly Brontosaurus; Jurassic, Colorado): 30 tonsBrachiosaurus: longer fore limbs to reach higher vegetationSupersaurus: 80 to 100 tonsSauropodomorphs left extensive footprint record (“track ways” of Colorado, for example)Advantages of size: avoid predators, slow temperature changes due to surface-to-mass ratio (gigantothermism)Expansion during Early Jurassic; lasted until Early Cretaceous; southern hemisphere sauropodomorphs contined into Late Cretaceous
34Ornithischians (Late Triassic-Cretaceous) Evolved near the end of the Triassic"Bird-hipped"Pelvic structure resembles that of birds.All herbivores.Front teeth replaced by a beak for cropping vegetationIncludes both two-legged (bipedal) and four-legged (quadrupedal) types.Front legs shorter indicating descent from two-legged forms (in four-legged forms).
37StegosaursThe plates on the backs of stegosaurs may have served as body temperature-regulating devices. They may have been used as "radiators" to dissipate body heat, or as "solar panels" to catch the sun's rays.Stegosaurus
39Ornithopods Bipedal and quadrupedal herbivores Camptosaurus Iguanodon PachycephalosaurusHadrosaurs (or duck-billed dinosaurs) in the Cretaceous, such as Parasaurolophus, Edmontosaurus, Bactrosaurus, and Maiasaura
41Figure (p. 435) Internal structure of the skull crest of Parasaurolophus cyrtocristatus. (From Hopson, J. A Paleobiology 1:24.)
42Interesting Facts about Dinosaurs The oldest dinosaurs were discovered in Late Triassic beds in Argentina by paleontologist Paul Sereno. They are about 225 million years old. Eoraptor, one of the oldest dinosaurs, was only about 1 m long; its teeth indicate it was carnivorous.Some dinosaurs apparently roamed in herds.
43Interesting Facts about Dinosaurs Dinosaurs showed sexual dimorphism. Skeletons of females may be distinguished from skeletons of males.Fossil dinosaur eggs with embryos inside have been found in the Gobi Desert of Mongolia.
44Interesting Facts about Dinosaurs The jaws of Tyrannosaurus could exert more than 3000 pounds of biting force (compared with the lion, at "only" 937 pounds of biting force).Its tail was held out horizontally to the back, serving as a counterbalance to the forward part of the body.
45Interesting Facts about Dinosaurs Sauropods, with their long necks, apparently fed on vegetation high in the treetops. Their heads were relatively small, which avoided a heavy burden on the long necks.The large size of the sauropods provided an advantage in dealing with predators, and served to prevent body heat loss. (Large animals lose body heat slower than small animals.) Animals which preserve body heat as a result of their large size are called homeotherms.
46Interesting Facts about Dinosaurs Their footprints suggest that they walked on four legs--able to support weight on land.The rear feet rested on large "pads" like those of elephants.
47Interesting Facts about Dinosaurs Nests of dinosaur eggs suggest that some groups of dinosaurs cared for their young. The Maiasaura were apparently one group of dinosaurs which nurtured their young, as their babies stayed in the nests and grew after hatching.
48Interesting Facts about Dinosaurs Were dinosaurs warm blooded? Paleontologist Robert Bakker has argued since 1968 that dinosaurs were warm blooded like birds. If so, they would no longer be classified as reptiles.Lines of evidence for warm bloodedness include:
49Ecology Cold/Warm Blooded Debate All living reptiles are ectotherms. That is they are cold-blooded animals whose body temperature varies with the outside temperatures. Mammals and birds are endotherms. That is they are warm-blooded and maintain a constant body temperature, regardless of the outside temperature.Some dinosaurs may have been warm-blooded. What is the evidence for this?
50Warm-Blooded Evidence Brain size - endothermy necessary for having a large brain because a complex nervous system requires constant body temperature. Small carnivorous dinosaurs had relatively large brains, and are also clearly related to birds.Predator-prey ratios - Endotherms have a higher metabolism so they need to eat more than ectotherms. Thus in endothermic populations, the predator/prey ratio is lower than in ectothermic populations. Fossil evidence suggests that predator/prey ratios among dinosaurs are similar to that seen in present day mammal populations.
51Warm-Blooded Evidence (cont) Bone histology - Bones of some dinosaurs have numerous passageways that once contained blood vessels. This is more typical of endotherms than ectotherms.Apparent activity rates - High activity rates require endothermy. Dinosaurs are now known to have been a lot more active than previously thought.Stance
52Warm-Blooded Evidence (cont) Presence of feathers and hair – Feathers and hair insulate and help endotherms maintain body temperature. Several dinosaur fossils show evidence of feathers or a down-like material. At least one species of pterosaur had hair or hair-like feathers.Isotopic analysis of bones
53BirdsBirds are warm-blooded (endothermic), have wings and feathers, have a toothless beak, and lay eggs. Most can fly, but some are flightless.Bird bones are hollow, and not easily preserved.Birds may have evolved from basal archosaurs, or from small Triassic theropod dinosaurs. Both groups were bipedal and birdlike in the structure of their limbs, shoulder girdles, and skulls.Several theropod dinosaurs had feathers, hollow bones, and keeled breastbones.
54BirdsFeathers evolved from reptilian scales. The earliest feathers may have been used for insulation, camouflage, or display, rather than flight.The best known bird fossil is Archaeopteryx
55Archaeopteryx Bird-like features of Archaeopteryx: FeathersWingsReptile-like features of Archaeopteryx:Dinosaur-like skeletonTeethLarge tailForelimbs with clawsNo breast bone (meaning that it would have been a weak flier)
57Origin of BirdsBird-like features are found in some dinosaurs, including feathers or protofeathers, in Sinosauropteryx prima, more than 120 million years old, and Caudipteryx zoui, a dinosaur with a feathered tail.The line between dinosaurs and birds has blurred with the new discoveries, so it is difficult to say when the first bird appeared.Birds probably appeared near the end of the Jurassic.Many different types of birds lived during the Cretaceous Period.
58MammalsMammals evolved from mammal-like reptiles in the Late Triassic. Early mammals were rodent-like, and remained small throughout Mesozoic (smaller than housecats).Among the earliest mammals were Megazostrodon, Eozostrodon, and Morganucodon
59Figure 12-50 (p. 450) Restoration of Morganucodon, an early mammal from the Late Triassic of Wales.
60Mammal Characteristics Mammals are warm-blooded (endotherms), and are distinctive because they:Have hair or furFemales have mammary glands that secrete milk to feed their youngFossils of early mammals, like the cynodonts (mammal-like reptiles), show evidence of "whisker pits" on the snout region of the skull, indicating that they were covered with hair or fur.
61Geographic Distribution Of Life Through Mesozoic Pangea existed as a large landmass through Triassic. Climate was relatively similar over a wide latitude range. These factors allowed for wide distributions of species over many different continentsSouth America/Antarctica/Australia became island continents in Late MesozoicFaunas on these continents began to develop independently of other continentsMarsupials remained the dominant mammals in both South America and AustraliaLaurasian continents continued to have strong faunal interchanges until Cenozoic
62A mass extinction occurred at the end of the Cretaceous Period that caused the disappearance of about 1000 genera of marine animals, and about 25% of all known families of animals.
63Many groups died out gradually, and others disappeared suddenly. The extinctions did not all happen simultaneously.On land, only small (less than 50 lb) animals survived.Of the reptiles, only turtles, snakes, lizards, crocodiles, and the tuatara (a reptile from New Zealand) survived the extinction.More than 75% of the marine plankton species disappeared at the end of the Cretaceous.
64Animals both on land and in the sea were affected Animals both on land and in the sea were affected. The extinction at the end of the Cretaceous totally wiped out these groups:DinosaursPterosaurs (flying reptiles)Ammonoids (cephalopod molluscs)Large marine reptiles (ichthyosaurs, plesiosaurs & mosasaurs)Rudists (bivalve molluscs)and many other invertebrate taxa
65There were drastic reductions of these groups, wiping out entire families. Some of these groups had very few survivors:Coccolithophores (calcareous phytoplankton)Planktonic foraminiferaRadiolariansBelemnoids (cephalopod molluscs)EchinoidsBryozoans
66What caused the extinctions? There are many hypotheses to attempt to explain the cause of these extinctions. They can be divided into two groups:Catastrophic external or extraterrestrial triggers for the event (comet, asteroid)Events occurring on the Earth, without outside influences
67Evidence for extraterrestrial causes? A thin layer of clay with a concentration of iridium is found at the boundary at the end of the Cretaceous Period (the boundary clay).Since iridium is more abundant in meteorites than in normal Earth's surface rocks, it was proposed that a large impact of an extra terrestrial object with the Earth at the end of the Cretaceous might have spread iridium around the globe.Other things may also have been responsible for the presence of the iridium, and all possibilities must be considered.
69Asteroid impactAlvarez’s theory (1977): elemental iridium enrichment 30 times normal in terminal Cretaceous clay indicates vaporization of asteroid on impact with Earth; iridium enrichment too great for terrestrial-source explanation; bolide (comet, asteroid, meteorite) impact
70Other evidence for extraterrestrial causes? Shocked quartz (from an impact?)Tiny glass spherules or tektites (cooled droplets of molten rock from an impact?)Carbon soot (remnants of forests burned in a firestorm caused by an impact?)Antarctic fish kill180 km diameter crater in Yucatan, Mexico
71If a bolide (large extraterrestrial object) collided with the Earth, where is the impact crater? The most likely location of an impact structure of the proper age is the Chicxulub structure, a buried circular crater-like structure on the Yucatan Peninsula of Mexico.
72Figure (p. 456) Occurrences of the iridium-rich sediment layer of the Cretaceous-Tertiary boundary. (From Alvarez, W. et al Geol. Soc. Am. Special Paper 190: )
75Possible Terrestrial Causes Volcanic eruptions causing ash and aerosols in atmosphere leading to a drop in temperature. Volcanism was widespread toward the end of the Cretaceous, and volcanic ash can be a source of iridium.Other elements in the boundary clay like antimony and arsenic are common in volcanic ash but not in meteorites.
76Volcanic eruptions releasing sulfur dioxide, leading to sulfuric acid in the atmosphere and acid rain, changing the alkalinity of the oceans, and placing lethal stress on plankton at the base of the food chain, and indirectly affecting the organisms that depended on them for food.
77Decrease in rate of seafloor spreading, leading to a sea level drop, which eliminated the epicontinental seas. There is evidence for a global lowering of sea level at the end of the Mesozoic. Disappearance of the epicontinental seas would have meant a habitat loss for many shallow water species.
78Climatic change as a result of the lowering of sea level and disappearance of the epicontinental seas? Would have caused a harsher climate and more extreme seasonality.Change in atmospheric CO2 levels and O2 levels, as a result of the appearance of new types of plants, or the proliferation of photosynthetic plankton that formed the Cretaceous chalk deposits?
79Appearance of angiosperms changed food chain on land Appearance of angiosperms changed food chain on land? (Many of the dinosaurs ate gymnosperms.)Disease? Viruses?Melting of Arctic Ocean and spillover of freshwater cap onto world oceans, killing marine plankton?Magnetic reversal?Other?
80Extinct!Whatever the cause, changing environmental conditions at the end of the Mesozoic Era led to the disappearance of many kinds of organisms in what may have been a domino effect, as organisms at the base of the food chain were killed, sending waves of extinctions through species higher on the food chain that depended on them.