Mesozoic Era
The Age of Reptiles Mesozoic was the "Age of Reptiles." During Mesozoic, reptiles inhabited the land, the seas, and the air. Dinosaurs appeared during Triassic, and were the dominant land vertebrates until the end of Cretaceous. Marine reptiles - plesiosaurs, ichthyosaurs, and mosasaurs inhabited Mesozoic seas. Reptiles were able to fly by gliding during Triassic, and to fly with flapping wings by Jurassic.
The Appearance of Mammals, Birds and Flowering Plants Mammals first appeared during Triassic, evolving from synapsids or so-called "mammallike reptiles." Early mammals were small and rodent-like. The earliest true bird is Archaeopteryx, which appeared during Jurassic. Flowering plants or angiosperms first appeared during Early Cretaceous and became dominant during all subsequent geologic time.
The Diversity of Life during Mesozoic At the beginning of Mesozoic, diversity was low following Permian extinctions. Recovery from Permian extinctions was slow for many groups. In the oceans, the molluscs re-expanded to become much more diverse than during Paleozoic. Modern reef-building corals, swimming reptiles, and new kinds of fishes appeared.
The Diversity of Life during Mesozoic Extinctions occurred during Late Triassic and Late Cretaceous
Late Triassic Extinction Mass extinction occurred during Late Triassic Affected life on the land and in the sea About 20% of all marine animal families extinct Conodonts and placodonts (marine reptiles) became extinct Bivalves, ammonoids, plesiosaurs, and ichthyosaurs affected but recovered and rediversified during Jurassic. Among the terrestrial organisms affected by the extinction were synapsids ("mammal-like reptiles") and large amphibians.
Diversity Increased during Jurassic and Cretaceous 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. The decline of organisms which lived attached to the seafloor (such as brachiopods and stalked crinoids) may be related to the increase in predators in Cretaceous seas.
Cretaceous Life Life during Cretaceous consisted of a mixture of both modern and ancient forms. Modern types of bivalves, gastropods, and fishes were present along with now-extinct organisms such as ammonoids, belemnoids, and marine reptiles. On the land, the dominant plants changed from gymnosperms to angiosperms (flowering plants).
Late Cretaceous Extinction Mesozoic ended with a major extinction. Affected both vertebrates and invertebrates, on land and in the sea. Extinction of dinosaurs, ammonoids, large marine reptiles (plesiosaurs and mosasaurs), rudists, and others. Drastic reductions in coccolithophores, planktonic foraminifera, radiolarians, and belemnoids.
Climate and Plate Tectonics Overall, climates during Mesozoic were warm. This was in contrast to the cool dry climates which characterized many continental areas near the end of Paleozoic. Evidence for warming includes the disappearance of glaciers which were common in many areas during Permian.
Climatic warming was related to continental drift and the breakup of Pangea during Mesozoic. As the continents moved away from the South Pole, conditions were no longer favorable for glaciers to exist. Blue triangles indicate glacial tillites.
Fossil plants from Jurassic and Cretaceous indicate that tropical climates existed in areas that today have temperate climates. Subtropical plants were living in areas that were 70 o from the equator during Cretaceous - a latitude similar to that of northern Alaska. This suggests that temperatures were much warmer during Cretaceous than they are today. During Jurassic and Cretaceous, the continents were at roughly the same latitudes that they occupy today.
Climate Affects Sea Level With the disappearance of the glaciers, sea level rose through Jurassic, and continued rising to a maximum during Cretaceous.
Plate Tectonics Affects Sea Level Sea level rise was also related to the rifting and fragmentation of Pangea. The mid-atlantic ridge system developed as the Atlantic Ocean widened. The basaltic rocks that were extruded along the midocean ridge system were hot and thermally inflated. As a result, they displaced a considerable volume of sea water onto the continents.
Shallow Seas Flood the Continents Duing Late Jurassic and Cretaceous, epicontinental seas flooded large areas of North America and Europe. These epicontinental seas also contributed to the warmer climate because the water carried heat.
Triassic
Jurassic
REPLACE FIGURE (Fig. 13-12) Cretaceous
Mesozoic Life in the Seas
Foraminifera Calcium carbonate hard parts Paleozoic foraminifera were bottom dwellers (benthic). First planktonic foraminifera appeared during Jurassic Planktonic forams experienced an adaptive radiation during Cretaceous.
Uses of Foraminifera Planktonic foraminifera became useful for biostratigraphy during mid-cretaceous Forams are valuable for biostratigraphy because of: Abundance Short geologic ranges (rapid evolution) Geographically widespread Found in sediments of varying water depths
Uses of Foraminifera Petroleum exploration - Because of their small size and hard shells, large numbers of forams can be recovered while drilling for oil. They are used to correlate and trace stratigraphic units between wells. Sensitive indicators of water temperature and salinity Useful for interpreting ancient environmental conditions
Mesozoic Invertebrates Mesozoic benthic marine invertebrates include: corals bivalves crinoids gastropods sponges sea urchins bryozoans starfish brachiopods, barnacles
Mesozoic Invertebrates Overall during Mesozoic there was a decline in sessile benthos. The ability to swim or burrow may have been the best defense against increasingly diverse predators.
Corals Scleractinian corals or hexacorals appeared during Triassic. Reef-building corals are restricted to clear, warm, shallow waters of normal marine salinity, in part because they have a symbiotic relationship with algae that live within the coral polyp. The symbiotic algae are photosynthetic and dependant on sunlight, which necessitates the clear, shallow water.
Molluscs The molluscs include: Bivalves Gastropods (snails) Cephalopods (ammonoids, belemnoids, squids, etc.) Molluscs diversified following Permian extinctions, and became more diverse than during Paleozoic. During Mesozoic, the molluscs surpassed the brachiopods (which had dominated the Paleozoic seafloor).
Bivalves Oysters were among the most successful bivalves, including such genera as Exogyra and Gryphaea.
Rudist Bivalves Rudists were bivalves that built reefs during Jurassic and Cretaceous One of their two valves (or shells) was enlarged and conical in shape, up to 1m tall. The other valve was small and served as a lid on top of the other large, conical valve. Rudists became extinct at or near the end of Cretaceous.
Gastropods Predatory gastropods appeared during Cretaceous. They were able to drill circular holes in shells in order to extract the soft parts of the organism for food. This was a new mode of predation not seen before. A common living example of a carnivorous gastropod is the moon snail.
Cephalopods Mesozoic cephalopods include: Ammonoids Nautiloids Belemnoids Squids Most were nektonic (swimmers).
Ammonoids Ammonoids were among the dominant swimming invertebrates in Mesozoic seas. Ammonoids were so abundant and varied that Mesozoic could be called the "Age of Ammonoids." The geologic range of ammonoid cephalopods is Devonian to Cretaceous.
Ammonoids Useful in biostratigraphy and worldwide correlation of Mesozoic rocks because they were abundant, morphologically variable, widely distributed, and had short geologic ranges (evolved rapidly). Distinctive features is the character of the sutures seen on the outside of the fossils. Sutures are the seam where internal partitions called septa intersect the outside wall of the shell.
Suture Patterns Three suture patterns of the ammonoids are goniatite, ceratite, and ammononite. Nautiloid cephalopods have smoothly curved septa.
Belemnites Belemnites have an internal calcareous shell (which resembles a cigar) called a rostrum. The rostrum is made of fibrous calcite, arranged in concentric layers The front part of this shell is chambered, as in the nautiloids and ammonoids. Geologic range: Mississippian to Eocene The belemnites were highly successful during Jurassic and Cretaceous.
Squids Squids were numerous during Jurassic and Cretaceous. Soft parts are preserved in a few rare specimens. Squids may have evolved from Triassic belemnites.
Arthropods Modern types of marine crustaceans and other types of arthropods appeared during Mesozoic, including crabs, shrimp, crayfish, lobsters, and ostracodes.
Echinoderms All echinoderms have 5-part symmetry. Echinoids include the sand dollars, sea biscuits, and sea urchins. Echinoids became more diverse during Mesozoic than they had been during Paleozoic. Other echinoderms included starfish and ophiuroids (or brittle stars). Crinoids were not as common as they had been during Paleozoic.
Mesozoic Vertebrates Fishes Two major types of fishes existed during Mesozoic: Chondrichthyes - cartilaginous fishes or sharks Osteichthyes - ray-finned bony fishes Both of these groups existed during Paleozoic, and experienced a decline during Permian extinction, and both groups rediversified during Mesozoic. Jawless fishes (Agnatha) were also present.
Evolutionary Changes in the Fishes Many evolutionary changes occurred among the fishes during Mesozoic. By the end of Mesozoic, few fishes remained with primitive characteristics. The swim bladder appeared during Mesozoic. It is a sac of gases used for buoyancy regulation. The swim bladder evolved from the lungs that were present in Paleozoic fishes.
Marine Reptiles Adaptations to a marine environment included: Paddle-shaped limbs Streamlined bodies Modified lungs for greater efficiency Reproductive modifications for birth at sea in some groups. Others, like the sea turtles, return to land to lay eggs
Plesiosaurs Fed on fish using slender curved teeth Short, broad body and extraordinarily long neck with small head Up to 40 feet long (12 m) Large, many-boned, paddle-like limbs
Ichthyosaurs ("fish-lizards") The most fish-like of Mesozoic reptiles Resemble dolphins, but with upright rather than horizontal tail fins Top predators Large eyes to pursue prey Had live young, not eggs Became rare and disappeared during Cretaceous
Mosasaurs Cretaceous only Up to 50 ft long (15 m) Probably top predators Attacked ammonoids, as evidenced by bite marks on ammonoid shells.
Crocodiles Crocodiles evolved during Triassic as terrestrial animals This was the last important group of Early Mesozoic marine reptiles to evolve Some adapted to marine environment by earliest Jurassic. Rare by Cretaceous Rapid swimmers. Evolved from archosaurs (related to dinosaurs)
Sea Turtles Evolved during Cretaceous Grew to 12 feet long (4 m) Genus Archelon and others
Dinosaurs The name "dinosaur" comes from the Greek deinos = "terrifying" and sauros = "lizard." Dinosaurs appeared during Late Triassic, about 225 m.y. ago. The earliest dinosaurs were small. Many were less than 3 ft long. By the end of Triassic, dinosaurs were up to 20 feet long. They became much larger later during Jurassic and Cretaceous.
Dinosaurs Two Orders of dinosaurs are distinguished on basis of hip or pelvic structure: Saurischian dinosaurs - lizard-hipped Ornithischian dinosaurs - bird-hipped
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.
Two groups: Saurischian Dinosaurs 1. Theropods - bipedal carnivorous dinosaurs 2. Sauropods - large quadrupedal herbivorous dinosaurs
Sauropods Large 4-legged herbivorous dinosaurs - "long necks" Apatosaurus Brachiosaurus Supersaurus Ultrasaurus Seismosaurus Argentinosaurus Nuoerosaurus Diplodocus Seismosaurus Supersaurus (right two)
Ornithischian Dinosaurs Evolved near the end of Triassic "Bird-hipped" - Pelvic structure resembles that of modern birds. Includes both two-legged and four-legged types. All herbivores. Lacked teeth in the front of the upper and lower jaws. Beak for cropping vegetation. Teeth were limited to the sides of the jaws, and were well adapted for crushing and grinding vegetation. Front legs shorter, indicating descent from two-legged forms.
Triceratops Styracosaurus Protoceratops Monoclonius Ceratopsians
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.
Interesting Facts about Dinosaurs Fossil dinosaur eggs with embryos inside have been found in Upper Cretaceous rocks of the Gobi Desert, 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 Some duck-billed dinosaurs (hadrosaurs) had bony skull crests with hollow tubes which may have served as vocal resonating chambers for producing sounds. 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: The stance and gait of dinosaurs (as judged from trackways), The microscopic structure of the bones, Isotopic analyses of bones, and Proportions of predators to prey.
Flying Archosaurs or Pterosaurs The word pterosaur means "winged lizard." The earliest flying reptiles were probably gliders. Later forms were active fliers, with flapping wings, rather than gliders. There were two groups of pterosaurs: Rhamphorhyncoids, which had long tails with a diamond-shaped tip. Pterodactyls, which were more advanced and tailless. Pteranodon.
Pterosaurs Pterosaurs dominated the skies for more than 100 million years. They existed from Late Triassic to Late Cretaceous. Jurassic and Cretaceous pterosaurs had large heads and eyes, and long jaws with thin slanted teeth. The bones of the fourth finger were elongated to support the wing membrane.
Pterosaurs Eudimorphodon (right front), a 60-cm pterosaur with sharp teeth for catching fish, and Peteinosaurus (left)
Pterosaurs The largest pterosaur was Quetzalcoatlus which had a 50 foot wingspan. Late Cretaceous of western Texas. It was the largest known flying vertebrate that ever lived on the Earth. Some Mesozoic flying reptiles were covered with hair or fur, indicating that they were warm blooded. An example is Sordes pilosus, whose name means "hairy devil."
Pterosaurs Quetzalcoatlus