L13- Cretaceous Terrestrial Ecosystems

The Cretaceous World-

• continents still splitting apart, high sea levels, greenhouse world, shallow continental seas

  

• CO2 still high but gradually declining to modern levels (icehouse world now), O2 gradually rising

  

• climatically sensitive fossils→ difficult to interpret when continents are separate, no current between australia and antarctica, cool temperate at poles, latitudinal banding but gets dragged around by currents

  

Plants:

• Early Cretaceous→ no angiosperms

  • bryophytes

  • lycopsids, ferns, sphenopsids→ fern prairies, all shrubs (undergrowth)

  • pteridosperms (bennettites and caytoniales) go extinct

  • cycads

  • gingkos

  • conifers→ all groups

  • gnetales appear→ last of sea plants (potential ancestor of angiosperm)

    

  → plants resemble the jurassic world

• angiosperms appear→

  

  • first ones were small shrubs→ dark and disturbed hypothesis

    

  • pollen are found at equatorial latitudes→ originally at the equator, diversify north and south

    • can trace migration of angiosperms through their pollen grains and megafossil evidence

    

  • phylogeny of modern angiosperms→ are sister group to gynosperms (seed plants)

    

  • most present day angiosperms are tropical in their requirements:

    • 50% are confined to tropical regions

    • 75% have optimal development in tropical environments

      → modern angiosperm biota adds more evidence to the fact that it was tropical in origin

• biomes→

  • the most diversity is in the angiosperm biotas e.g. tropical summerwet:

    

  • get polar forests→ evidence around northern and southern ice sheets

    

Insects:

• there are at least 25 major Cretaceous deposits

• diversification of insects is associated with angiosperm diversification, but insects did not drive the origin of angiosperms→ insects and plants are associated

• the more diverse an angiosperm assemblage is, the more diverse insect assemblages are e.g. in the tropics

• all three major groups of insects evolved advanced sociality (ants-bees-vespid wasps, termites and aphids)

• get first amber deposits

• crabs appear but are aquatic

Biotic Interactions:

• first angiosperms are pollinated by pollen collecting or pollen eating insects

• more derived angiosperms are pollinated by nectar later on

• not much pollen and food dispersal at this time

• plant-insect interactions can’t be linked to origin of angiosperms because these diversifications happened before

  

Fish:

• new groups→ all modern groups of teleosts appear, huge diversification

• there are still some basal actinopterygians

• this is when fish become modern in aspect

  

Amphibians:

• last of temnospondyls come through but disappear and are left with lissamphibians→ modern groups

  

Reptiles:

Synapsids:

• only remaining synapsids are mammals→ diversify widely throughout jurassic-cretaceous but are not dominant in the biota (dinosaurs still dominating)

  

Anapsids:

• only remaining groups are turtles→ debate if they are anapsids or not

Diapsids:

• huge radiation of modern groups of snakes and lizards

• ichthyosaurs and plesiosaurs persist in the shallow continental seas

• crocodiles persist→ huge diversity

• pterosaurs persist in the sky

• dinosaurs continue to persist and dominate

• taxonomy of dinosaurs

  • get two new groups→ get complete phylogeny of dinosaurs now

    

    • pachycephalosurs→

      

    • ceratopsids→

      

• diet changes in the cretaceous→

  • mesophtyic diet changes to a cenophytic diet due to angisoperm diversifciation

  • no grass→ understory is ferns, lycopsids and horsetails, some dycots, dominated by fern priaries

Birds:

• get ancient groups, diversify

• birds evolved but lost ability to fly due to pterosaurs dominating

• mass extinction wipes out most of the old groups→ birds diversify in pterosaur niche

• there were flying birds that coexist with pterosaurs but there are much less of them→ found in china deposits

  

K/T Mass Extinction:

• 5th mass extinction

• dinosaurs disappear on the land, ammonites disappear in the oceans

• boundary layer has high iridium layers, globally and at high and low sedimentation rates→ only from outer space

  • bolide hit planet

  • hit at the chixulub site→ can learn about the nature of the impact, material gets thinner away from this site

  • lots of basalt released from earth cracking open- outgassing, blocks out sunlight, no photosynthesising in the ocean, plants can’t grow back on land, nuclear winter, shut down of primary productivity→ increased extinction but bolide impact was the main cause

• find layers of charcoal→ fires

• finds layers of ferns→ grow back after fires

• is the shortest mass extinction→ impact, aftereffects (earthquakes, tsunamis)

• forest reestablish, survivor animals e.g. crocodiles, birds, repopulate planet