Life of the Mesozoic CHAPTER 12
Introduction • The 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.
Overview of Mesozoic Era: “age of reptiles” • Diverse Reptiles, including Dinosaurs • Beginning of Evolution for Birds and Mammals • Expansion of Grasses and Flowering Plants • Climate, a Strong Influence • Locations of continents • Major sea-level changes • Mountain building
Mesozoic Climates • Primary Control: balance of incoming and outgoing solar radiation • Factors affecting balance • configuration and dimension of oceans and continents • development and location of mountain systems and land bridges • changes in snow, cloud, or vegetative cover • carbon dioxide content of atmosphere • location of poles (no ice caps) • amount of radiation-aerosols contributed by volcanoes • astronomic factors: changes in Earth’s orbital parameters
Triassic Climate • Relatively cool • Pangaea continents still clustered • lower sea level • mountain building, many highlands • Paleo-equator: central Mexico to northern Africa • Wind shadow deserts (aridity) in continental interiors • red beds • evaporites
Figure 11-3 (p. 384) Generalized paleogeographic map for the Triassic of North America.
Jurassic Climate • Relatively mild • no glacial deposits • evidence of monsoons and aridity • coals in many spots, including Antarctica • tropical conditions in wide belts • Continents at latitudes of today • Atlantic opening • Tethys was an arm of proto-Pacific: warm ocean currents flowed through Tethys
Figure 11-7 (p. 387) Generalized paleogeographic map for the Jurassic of North America.
Cretaceous Climate • Relatively warm • subtropical flora at ± 70o of equator • high and low latitude coals • high sea-level stand: maximum inundation of Phanerozoic • Continents near today’s position • Arctic Canada near north pole • Antarctica at South Pole • End-Cretaceous change • rapid cooling and temporary vast chilling • vast regression • major mountain building • plankton-produced CO2 shortage • volcanic activity • Terminal Cretaceous climatic event shown by: • tropical cycads sharply reduced • hardy conifers and angiosperms expanded • oxygen isotope studies of shells show ocean temperature decline began 80 m.y. ago • strong connection to global extinctions at 65 m.y. ago?
Figure 11-12 (p. 392) Generalized paleogeographic map for the Cretaceous of North America.
The 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.
The 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.
Mesozoic Vertebrates • Amphibians: rise of modern forms • Triassic Transition: continuity among land animals • Survivors of Permian extinction (245 m.y. ago) • temnospondyl amphibians • mammal-like reptiles (including therapsids, mammal ancestors) • New reptile groups • first turtles (toothed turtles) • tuataran lizards • archosaurs: crocodiles, flying reptiles, thecodonts, dinosaurs • Example “basal archosaur”: Hesperosuchus
Mesozoic Vertebrates (cont) • Diapsid groups • lepidosaurs: snakes, lizards, and their ancestors • archosaurs: ornithischian and saurischian dinosaurs, flying reptiles (pterosaurs), and crocodilians.
Basal Archosaurs • Several 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.
Basal Archosaurs • Basal 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
Dinosaurs • The 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.
Dinosaurs • Basal Archosaurs (Thecodonts) were the ancestors of the dinosaurs • Dinosaurs were composed of two orders: saurischia (lizard-hipped) and ornithischia (bird-hipped) • Saurischia: pelvic bones like thecodonts • Ornithischia: pubis parallel to ischium like birds • Earliest dinosaurs: saurischia (Traissic, 225 m.y. old, Argentina)
Dinosaurs • Saurischiandinosaurs- lizard-hipped • Ornithischiandinosaurs- bird-hipped
Dinosaurs Comparison of the skulls and teeth of saurischian dinosaurs (A) and ornithischian dinosaurs (B).
Saurischian 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.
Saurischian Dinosaurs Two groups: • Theropods - bipedal carnivorous dinosaurs • Sauropods - large quadrupedal herbivorous dinosaurs
Carnivorous saurischians (Theropods) Larger carnivores; hind limbs robust; claws on toes; small fore limbs; serrated teeth • Deinonychus and Velociraptor: Cretaceous predators • Family Allosauridae: Allosaurus (U.S.) • Giant theropods: Tyrannosaurus (13 m, 4 tons, North America); Gigantosaurus (Argentina); Carcharodontosaurus (Africa)
Theropods Bipedal carnivorous dinosaurs • Coelophysis • Ornithomimus • Giganotosaurus • Allosaurus • Tyrannosaurus • Deinonychus • Velociraptor
Large 4-legged herbivorous dinosaurs - “long necks” Seismosaurus Argentinosaurus Nuoerosaurus Diplodocus Sauropods • Apatosaurus • Brachiosaurus • Supersaurus • Ultrasaurus
Sauropods 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.
Herbivorous saurischians or sauropodomorphs (Jurassic-Cretaceous) • Evolved from Late Triassic protosaurapod (Plateosaurus) • Long necks, long tails, four-legged stance • Apatosaurus (formerly Brontosaurus; Jurassic, Colorado): 30 tons • Brachiosaurus: longer fore limbs to reach higher vegetation • Supersaurus: 80 to 100 tons • Sauropodomorphs 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
Ornithischians (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 vegetation • Includes both two-legged (bipedal) and four-legged (quadrupedal) types. • Front legs shorter indicating descent from two-legged forms (in four-legged forms).
Ornithischian Dinosaurs Examples: • Ceratopsians • Stegosaurs • Ankylosaurs • Ornithopods
Ceratopsians Triceratops Styracosaurus Microceratops
Stegosaurs The 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
Ornithopods Bipedal and quadrupedal herbivores • Camptosaurus • Iguanodon • Pachycephalosaurus • Hadrosaurs (or duck-billed dinosaurs) in the Cretaceous, such as Parasaurolophus, Edmontosaurus, Bactrosaurus, and Maiasaura
Figure 12-32 (p. 435) Internal structure of the skull crest of Parasaurolophus cyrtocristatus. (From Hopson, J. A. 1975. Paleobiology 1:24.)
Interesting 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.
Interesting 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.
Interesting 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.
Interesting 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.
Interesting 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.
Interesting 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.
Interesting 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:
EcologyCold/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?
Warm-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.