John Philips "Middle Life" era; bounded by PT and KT extinctions; three periods Triassic, Jurassic, Cretaceous

"Age of Reptiles": dinosaurs rule on land; marine reptiles in the sea; pterosaurs in air; Modern fauna takes over and Mesozoic Marine Revolution heats up

Generally, an interval of greenhouse climates; calcite oceans

Late Paleozoic was time of continental assembly; many continent-continent collisions

Triassic: stable Pangea until end

Later Mesozoic, breakup of Pangea in several stages:

a) initial rifting began opening modern north Atlantic in late Triassic-Jurassic; Laurasia pulling back from Gondwanaland

b) rifting  of Gondwanaland:  South America/ Africa pulling apart to form South Atlantic

c) in Cretaceous, India began pulling away from Africa and moving north toward Asia
 
 

TRIASSIC PERIOD ( 250 to 296 Ma)

Name: Trias meant threefold; Von Alberti named the interval for rocks near Hanover, Germany where there is a three-fold development lower redbeds (Bunter), middle shelly limestones (Muschelkalk) and upper redbeds (Keuper); redbeds predominate in many parts of the world

Climate:  Because of the extent of Pangea many areas were far removed from oceans, tended to be warm and arid (at least seasonally) there may have been N and S mega monsoons;  no more glaciers anywhere, as far as known

Sequence Stratigraphy:  sea-level is low globally; led to long regressive phase of Absaroka megasequence;  middle Trias highstand associated with Muschelkalk and development of the Dachstein reefs of Alpine region

Excellent Milankovitch cycles in Triassic lake deposits of eastern US

Redbeds were very extensive; lots of siliciclastic sediment coming out of still high standing Appalachians, Ouachitas, Ancestral Rockies; vast alluvial plains extending into  narrow western interior sea; much of western scenery is red due to Triassic redbeds; Chinle Fm. alluvial sediments; carries coniferous logs; Petrified Forest in northern Arizona; similar facies elsewhere
Moenkopi shows mix of marine non-marine; nice cycles; oldest crayfish burrows; termite nests; Wingate dune sandstones form tops of many buttes
Kayenta river sediments;  all suggest arid or semi-arid conditions

Tectonics: New terranes accreting to western North America; especially Sonomia microcontinent and Golconda accretionary wedge (Sonoma Orogen ended in Early Triassic)

By Middle Triassic a subductional regime was reestablished from Alaska to Chile area; classic Andean type margins forming

Other pieces added farther north in Jurassic (more later)

Extensional tectonics beginning in eastern North America during late Triassic would lead to beginnings of North Atlantic by mid Jurassic; many faulted bounded, half-graben basins in latest Triassic to Jurassic;

Most famous are Connecticut River Valley, Newark, Gettysburg, and Culpepper basins; all have red arkosic sediments  Conn. Valley famous for dinosaur tracks; fossil fish in fresh water lake deposits

Basaltic extrusives and intrusives along sharp bounding faults;  Palisades sill is a famous sillwith columnar jointing basalts in cliff now facing Hudson River

Tethys Ocean opened eastward extensional tectonics in the Triassic of present Alps reflects widening of Tethys as Africa began pulling away from Europe
 

Triassic Life: Slow recovery from greatest of all mass extinctions; more mass extinction late in Triassic decimated several groups; one of the "big five" extinctions

Marine Ecosystem:

By Mid Triassic there were again reefs forming; especially along Tethys Ocean scleractinian (aragonitic hexacorals) stony corals evolved de nova from anemone stocks; took o roles of earlier rugose and tabulate corals

Mollusks, primarily clams and snails made reasonable recovery;

Ammonoids barely pulled through PT with o only ~ 2 genera going through; but rapidly diversified again as ceratites (intermediate sutural complexity); then again ceratites were wiped out by Late Triassic extinctions and ammonoids made it through by a thread into Jurassic

a few brachiopods left; decapod crustaceans; regular echinoids;

Conodonts made it through and were moderately common in Triassic but extinct by end of period

Holostean grade bony fish and sharks

Marine reptiles include the odd placodonts (crushing teeth) and nothosaurs; both extinct by end Triassic;  first ichthyosaurs appear

Terrestrial Ecosystem:

Also recovering but also hard hit by late Triassic extinctions

Plant life dominated by conifers and ferns; fewer seed plants or coal swamp floras' seed ferns did come through but were decimated by Triassic extinctions

Labyrinthodont amphibians still present but became extinct in Trias; possible forerunners of Lissamphibia (frogs) appear

First turtles appear; ancestors maybe in the pareiosaurs

Therapsids made it through and were well represented in Early Triassic by Lystrasaurus and others; lots of mammal-like forms in Karoo beds of South Africa

Mammals evolved from therapsids mid way through Triassic; see excellent intermediate series; evolution of more upright posture, heterodont teeth, hair (whisker pits); inner ear bones (exapted from old parts of back of jaw articulation (hyomandibular); mammary glands; remain small overshadowed by dinosaurs

In a totally different lineage the archosaurs i n the diapsid reptiles evolved into diverse forms; lizard-like forms; crocodile group appears,  thecodonts were forerunners and early analogs of dinosaurs (armored forms, crocodilians. analogs; carnivores (Rauisuchians) and partly bipedal lagosuchians (perhaps the immediate ancestors of dinosaurs: very similar but differences in  ankle bone

Dinosaurs first appear in mid Triassic; South American form recently found is oldest lies outside two major orders: Ornithischia and Saurischia; they both appeared in Middle Triassic; began to take over; very common trackways in  Triassic of Conn. Valley

Pterosaurs; including Rhamphorhynchus appear first in Triassic; first flight in vertebrates (there were also gliding lizard like forms; but probably no birds yet)
 
 

JURASSIC PERIOD  (206 to 142 Ma)

Name: The great explorer Humboldt named this system informally for exposures in the Jura Mountains of the France-Swiss border area in 1799; name only formalized later; an Age of Dinosaurs; in Europe the Jura was divided into Liassic (Lower), Middle, and Upper;  classic exposures of Jurassic in Britain especially in Dorset Coast and Yorkshire; famous Lyme Regis  with succession of Lias ammonites; in Tethys large sponge reefs Alpine area

Climate:  Moderating and becoming moist, greenhouse phase; glaciers not known; climate gradients slight

Sequence Stratigraphy: End of Absaroka Supersequence early in period; unconformity and the beginning of Sloss's Zuni Sequence; early  or Jurassic phase sometimes called the Sundance phase;  in western US see a change from early Jurassic sand dunes: famed Navajo Formation of Zion National Park; trough crossbedded huge ergs or dune fields along coastal plains reworking of older quartz sand

Upward change to peritidal carbonates and then fully open marine shales and limestone's of the Sundance-Twin Creek Formation;  famous for belemnites;  then change back to non-marine Morrison molasse

Tectonics:  Continued rifting in eastern North America; opening of north Atlantic underway;  reefs developing on continental shelf;
Gulf area rifting leading to a restricted basin in which evaporites of Louann salt formed; Smackover limestone shows transition to normal marine conditions associated with Sundance transgression

In western area:  addition of more exotic terranes in Pacific NW; Cache Creek and Stikina terranes of British Columbia and Yukon; maybe originated in a larger continent in Pacific Ocean which broke up

Subduction under present California led to emplacement of Great Valley ophiolite slab; the development of accretionary wedge-(Coast Range- Franciscan melange)- forearc basin (Great Valley)- and magmatic arc (Sierra Nevada and Idaho batholith; series of granitic batholiths

Uplift of "Mesocordillera" leads to retroarc thrusting (beginnings of Sevier thrust belt)  and retroarc foreland basin; Sundance basin; the rapid erosion n of Mesocordillera yielded wedge of molasse -Morrisson Delta; most famous dinosaur deposits in the world; progradation infilled Sundance sea

Jurassic Life

Marine Ecosystem:
Pelagic life: some of first coccoliths, dinoflagellates,

Benthic life includes diverse gastropods and bivalves; common oysters Gryphea ("devil’s toenails);

Ammonoids rediversify with a vengeance; now complex ammonitic sutures predominate; key to Jurassic biostratigraphy (Wm. Smith)
Belemnites become very abundant; calcitic guards occur in masses (belemnite "battle fields")

First crabs; crushing claws, part of "Mesozoic Marine Revolution " duraphagous (shell crushing)

First irregular burrowing echinoids (response to MMR)
Long stemmed crinoids beginning to retreat to deeper water

Fishes include diverse holosteans; sharks less common

Marine reptiles; ichthyosaurs (Lyme Regis: Mary Anning); plesiosaurs
Terrestrial Ecosystem:

Age of Cycads; cycads/cycadeoids ("pineapple palms") and other gymnosperms (conifers, ginkgos); ferns; dinosaur food

Labyinthodonts gone; but lissamphibia becoming common

Therapsids all but gone; small mammals around

Archosaurs (ruling diapsid reptiles) rule; crocodilians;
Dinosaurs: common, (duh!) include large representatives of both orders:
Ornithischia: stegosaurs common; typical of Morrison
Saurischia: sauropods (brontosaurs, Diplodocus, Brachiosaurus)
Theropods: allosaurs; coelurosaurs: gave rise to first bird (Archeopteryx) feathered dinosaurs; feathers for insulation; Aves are Dinosaurs!!
CRETACEOUS PERIOD (142-65 Ma)

Name : D’Halloy (1822) applied name to denote a time of abundant chalk deposits (Latin creta= chalk; these fine grained porous limestones made of coccoliths, planktonic forams

A long period (77 million years) with many very important events in tectonic, climatic, and life history

Climate:  Cretaceous is commonly known as the greenhouse period, though it may not have been much warmer than Jurassic;  correlated with high rates of sea-floor spreading; high CO2; but possibly greenhouse heating was mitigated by burial of organic matter and carbonate deposition;  very equitable; because of deflected currents warm temperatures extended into Arctic circle (dinosaurs and crocodiles. this far north; (how did they deal with the darkness of winter?)

Stagnation of deep ocean circulation; warm saline bottom water vs. cold bottom water; global oceanic anoxic events (OAEs);
 

Sequence Stratigraphy and Sea Level:

Again, correlated with high rates of seafloor spreading, absence of glaciers; minor lowstands in early part of period, maximum highstands near end of period; but drop at end of period

Second and stronger phase of Zuni Supersequence: Mancos Phase; strong flooding in Western Interior Seaway, stretching from Gulf of Mexico to Alaska; many areas flooded for first time; onlap of Cretaceous onto Precambrian basement in places

Also see smaller scale cycles, several million years in duration; cyclothems; also superb Milankovitch cycles of limestone and marl (muddy limestone);
Second great age of coals (but at opposite  climatic phase)

Early Cretaceous Dakota Sandstone is basal transgressive unit in western Interior Seaway; famous aquifer; contains early angiosperm leaves

Transgression culminates in dark, organic rich shales; Mowry, Mancos  to the east, the dark shales interfinger with Niobrara Chalk
Regressive sandstone tongues in Book Cliffs sandstones record progradation and/or sea level fall;

Tectonic History:

Major seafloor spreading; fastest rates in known history
Rifting in south Atlantic; entire Atlantic opening; rift pulls Greenland off from North America

Antarctica-Australia rift away from Africa

India rifts from Africa and moves north toward Asia

In western North America continued accretion of terranes (Wrangellia collides with northwest)

Continued subduction beneath western margin of North America but; accretionary prism of Franciscan melange building up with broad Great Valley and Olympic foreland basins filling with flysch
 

Subduction proceeding at decreasing angle through Jurassic-Cretaceous this means that magmatic arc (Sierra Nevada arc-Idaho batholith) migrate increasingly eastward into craton

Behind this arc is the famed Sevier fold and thrust belt; thrusting episodes of lower to mid Cretaceous is referred to as Sevier Orogeny; thrust push eastward from Nevada into Utah and along Idaho-Montana border;

In turn, the Sevier foreland basin: Western Interior Seaway migrates eastward and is infilled with flysch and molasse
 
 

Cretaceous Life:

Marine ecosystem:

Pelagic realm changing; dominance of two new phytoplankton groups:
First diatoms; siliceous tests beginning to accumulate as oozes,
Coccoliths are the formers of chalk; calcite oceans

Planktonic foraminifera appear; radiolarians flourishing (Franciscan cherts) but declining some as diatoms increase (silica competition)

the marine ecosystem geared up; increasing nutrient input due to angiosperm related weathering??
Mesozoic Marine Revolution heated up; predatory crustaceans and vertebrates and highly predaceous neogastropods

Bivalves; lots of oysters; brachiopod decline; cheilostome bryozoans

Reefs formed by odd rudistid bivalves; robust calcitic shells; pseudocolonial; dominate over corals

Ammonites prominent and some are huge; very complex sutures;  heteromorphs (odd asymmetrical forms)

Giant fish include some of first teleosts with swim bladders; many modern families appear before period is over

Neoselachian sharks and rays evolve

Marine reptiles; huge sea turtles; few ichthyosaurs, but short and long necked plesiosaurs common and the giant  mosasaurs (aquatic monitor lizards)

Terrestrial Ecosystem:

Plants show decline of cycads, but conifers continue dominance; most significant event on land is evolution of angiosperms; flowering plants with double fertilization (provides nutrient for developing plant embryos; begin in marginal swampy environments

Insects diversifying and co-evolving with angiosperms; lepidopterans (butterflies) , hymenoptera (social insects, bees), coleopetera (beetles)

Frogs diversifying; snakes evolve in response

Archosaurs still dominating

Huge crocodilians and crocodile analogs
Dinosaurs: doing very well:
Saurischia:  sauropods declined substantially; theropods very prolific include some of largest  predators ever 5-6 ton Tyrannosaurs; also famed raptors and bird-mimics

Ornithischia: very diverse: ornithopod (hadrosaurs); ankylosaurs; ceratopsians; include some of the last of the dinosaurs

Mammals still small and waiting in the wings

KT Extinction

Actually there were at least three other extinctions in Cretaceous Albian-Cenomanian (98 Ma), Cenomanian-Turonian (91 Ma), and Coniacian-Santonian (88 Ma) ; each associated with widespread anoxia, change in temperature, sea-level fluctuation; lots of ammonites and other mollusks wiped out;  in some ways the K/T is one more of these in pattern

But most famous is the end Cretaceous (Maastrichtian) Cretaceous-Tertiary boundary because this one is associated with dinosaur and pterosaur  extinctions and end of mosasaurs and plesiosaurs, ammonoids, rudistids and others

Over 100 scenarios have been proposed for K-T; but only a  few get real support

Facts about K-T Extinction

Some groups of organisms experienced decline well before terminal event; e.g. rudistids were almost gone and even dinosaurs may have declined. But some of this pattern may have been more apparent than real;  and some organisms do go extinct  suddenly

Major regression taking place in latest Cretaceous

Perhaps temperature change

Very large basaltic extrusions (Deccan traps) almost exactly at K-T (note similarity  with P-T extinction; but many other large flood basalts are NOT associated with episodes of extinction);  Officer and Drake believe that this was THE major cause of the extinction

There is now overwhelming evidence of a major bolide impact associated with this extinction; postulated by Alvarezes:

a)  Ir anomaly in boundary clays
b) shocked quartz grains
c) ash from burned plants
d) microspherules (melt droplets)
e) probable tsunamites
f) crater(s) found: 1) Manson, Iowa;  2) Chixulub,  Gulf of Mexico; partly on Yucatan Peninsula; 200 km outer ring and ~100 km

Link to sudden extinction: a) "nuclear winter scenario": b) darkening by dust; c) extreme cold for some months; then possible warming; d) acid rain; e) wild fires

Selectivity of extinctions:

Plants especially hard hit in area of American west NW of Chixulub; fern spike follows mass extinction of gymnosperms and angiosperms; but seed plants do come back; seeds nay have gone dormant

Large vertebrates hardest hit; the only dinosaurs that made it through were small birds;  fresh water reptiles and amphibians may have made it through the extinction by hibernating

In sea, animals dependent upon plankton were hardest hit; coccoliths down to just one "disaster" species; ammonites wiped out, large reptiles gone

Reef ecosystem hard hit, as usual

Specialized organisms as a whole fared poorly, e.g., rudist, ammonites ("set up for kill"; stable environments preceding a catastrophe promote specialized organisms that do not withstand change well)

Aftermath of some groups of mollusks, crustaceans, fishes;

On land fern spike followed by rediversification of angiosperms; insects; frogs, snakes, birds AND mammals (contingency!)