The Mesozoic Era

Geology 103

Key theme: Pangea breaks up

• Process will take 150 my and will extend into the Cenozoic

• Laurentia/Baltica and Australia/Antarctica are still joined at the end of the Mesozoic

After the Permian extinction…

Life came back in 10 to 20 million years. No tabulate or rugose corals, so reefs were made of different organisms. Many niches on land were repopulated by much different creatures.

Angiosperms appear in the Cretaceous

• Angiosperm plants are those that have flowers• Major change in plant life – insects are now the pollinators• These are generally broad-leaf plants

Diversification of Reptiles

Marine ReptilesIchthyosaurs

Flying ReptilesPterosaurs

The DinosaursSaurischianOrnithischian

Reptiles gain flight in Triassic

• Pteranodon – much different wing structure and musculature than modern flying mammals like bats, or even non-mammals like birds.

The Dinosaurs

Approximately 700 species in 300 generaMostly warm-blooded (homeostatic)

Rapid metabolism; prey-predator ratio; many blood vessels pores in the bones

Reproduction and HabitsNesting behavior and social behavior (herds)

CharacteristicsEoraptor earliest thecodont (“socket-teeth”), related to crocodilesSauropods long necks and large body quadrupeds; Ornithopods are bipedal herbivores (Camptosaurus); Stegosaurs and Ceratopians are quadruped herbivores

Extinctions: Late Jurassic-Early Cretaceous, and, of course, End Cretaceous

Differences based on pelvic bone arrangement: Late Triassic EvolutionSaurischian

“Lizard-hipped”, earliest groupSimilar to thecodontsTheropods (carnivorous dinosaurs) Prosauropods (herbivores)Sauropods (large herbivores)

Ornithischian“Bird-hipped”, herbivorous dinosaurs evolved from Prosauropods

The Dinosaurs: Middle Triassic

True birds appear in the Jurassic

• Archaeopteryx is the first recognized bird

• Most specimens are from the Solnhofen lagenstatten in Germany

• True feathers, wishbone• However, this creature was

descended from one clade of coelurosaurian theropods; a different clade is believed to be the lineage for birds

True mammals appear in the Triassic

• Evolved from a lineage of mammal-like reptiles

• Hair, mammary glands homeostasis (= “warm-blooded”, though some of the dinosaurs may have been)

Mammal-like reptiles therapsids (cynodonts)Early Triassic: small cynodont gave raise to medium-size carnivores and herbivores that are ancestral to mammalsLate Triassic: a small cynodont gave rise to the earliest mammal, the morganucodontidsMost Triassic and Jurassic mammals were insectivores and very small

The mammals

About twelve inches long; less than 2 kg

Key point: warm climate is the driver of biodiversity/distribution in Mesozoic

Keep an eye on: Tethys Sea

Opens from east in Permian, greatest extent in Jurassic, closes from west in Cretaceous

Triassic PeriodFirst large rifts in Pangea appear

Characterized worldwide by:Nonmarine red bedsArkosic sandstonesLake shales

All these are deposited at the rift margins

Rifts will become modern oceans

Absaroka transgression is ending

Mountains of Taconic orogeny erode into newly-forming rift basins (circled area is New England, arrows show direction of transport)

PALISADES SILL, NJEvidence of rifting of Atlantic Ocean

The “fall line”Interesting aspect of eroding the Taconic: the Cretaceous sediment is eroded back on US east coast rivers until it gets to Taconic crystalline rocks – waterfalls are formed

Red beds

• Red beds are sandstones, siltstones and shales that are, well, “red” due to iron oxide which was formed either due to erosion of red soils or exposure of iron-rich sediments to oxygen

Gulf of Mexico opens (restricted basin)

• Restricted basin means that marine waters evaporate and leave lots of salt

• Tropical marine water also is quite bioproductive – organisms get trapped in salt and eventually turn into petroleum (oil and gas)

• Salt domes here protect the oil and therefore are exploited

Sonoma orogeny ends on US west coastSubduction zone switches from westward dipping to eastward dipping – finally, denser oceanic seafloor to the west – all these mountains are gone, except for basin sediments

End of Triassic map

Mass extinction at the end of the Triassic – the least understood of the “Big Five”

Causes of the end of Triassic mass extinction

Not really known, but may have to do with the end of the Absaroka transgression (sea level was falling through the Triassic) and the huge volumes of ocean floor basalt erupted when the Americas rifted from Africa/Europe.

Jurassic PeriodBeginning of Zuni transgression

On US mainland, epeiric Sundance Sea in Canada; due to low sea levels, significant nonmarine sand and silt deposition – Morrison Fm in northern US is a good source of dinosaur bones

Nevada orogeny is responsible for emplacement of many batholiths

Obduction occurs when low-density rocks ride up over the other plate at a subduction zone

Jurassic age batholiths

• All that remains of the Nevadan orogeny are these batholiths which have been obducted (in some cases) on the North American plate

Early Jurassic of the western US

Chinle Formation at Petrified Forest National Park, Arizona

Cretaceous Period“Creta” means chalk

Reflects the Zuni transgression; worldwide warm seas that allows coccolithophores to thrive and their shells to deposit and form chalk

Rudist (bivalve) reefs are now oil and gas traps

During the Cretaceous…

Sea level were higher; epeiric seas covered continentsDivergence of planktic organisms led to large coal and oil depositsThe Atlantic Ocean continued to open while the Tethys Ocean closedIndia migrated northwardOxygen levels neared 35% (modern = 21%); polar areas are warm

Why was the Cretaceous warm?The deep ocean was much warmer than today. This implies that more hot rocks were making up the sea floor – greater sea floor spreading!

This may have been caused by increased mantle plume activity

CoccolithsA coccolithophore is a calcium carbonate-secreting green algae. Its shell comprises round coccolith plates, whose function is not well understood. First appearing in the Traissic, coccolithophores are an important part of how calcium is cycled to and from organisms.

Rudist bivalves formed reefs – went extinct at K/T boundary

Which leaves the niche wide open for the scleractinian corals

Sevier (130-80 My) and Laramide (80-50 My) orogenies

Both orogenies are due to the subduction of the Farallon Plate; the difference is timing (Sevier came first) and the angle of subduction

All that remains of the Sevier are folds and thrusts in Nevada and Utah; Laramide are some of the Rockies

Cretaceous/Tertiary (K/T) boundary: the second largest extinction in the record

Causes of the K/T extinction event –– Chicxulub impact

Stratigraphy of an impact

Evidence for impact

Iridium is an extremely rare metal on the Earth’s surface – some asteroids (via meteorites) have high concentrations of iridium. This iridium “spike” seen in Montana is seen worldwide.

Further evidence for impact

Shocked Quartz

Spherulesglass beads, felsic, melting of crustal rocks

Sootcarbonaceous particles, wildfires

Shocked QuartzContains lamelle (little lines), evidence of a high pressure shock wave

Stishovitehigh pressure form of quartz

Causes of the K/T extinction event –– Deccan traps (2nd largest flood basalt)

Some magma sources are contain high concentrations of iridium.

Ammonites and nautiloids flourish in the oceans – ammonites go extinct at the K/T

boundary, nautiloids survive

The reason for this is unclear

In fact, only 15% of terrestrial genera go extinct, while 70% of marine genera do

At the end of the Cretaceous