MESOZOIC LIFE - OVERVIEW

1. PLANTS DIVERSIFIED; MODERN GROUPS (PHYTOPLANKTON & ANGIOSPERMS) EVOLVED
2. ANIMALS DIVERSIFIED & IMPORTANT GROUPS EVOLVED OR FLOURISHED
   • Marine Animals
     • planktonic foraminifera & radiolarians, scleractinian (modern) corals, & rudist pelecypods evolved
     • crustaceans diversified & ammonoid cephalopods flourished
     • echinoids became the dominant echinoderm group
   • Terrestrial Animals
     • Reptiles, especially Dinosaurs, diversified
     • Birds & Mammals evolved
3. MASS EXTINCTION at the END of the CRETACEOUS
   • Extraterrestrial & terrestrial causes have been suggested

MESOZOIC PLANTS

1. MARINE PLANTS
   • Groups of phytoplankton (floating photosynthesing protists) that are important members of modern ocean ecosytems (most marine plants are protists) evolved
     • Dinoflagellates (cellulose) in Triassic; Coccolithophores (calcium carbonate) in Jurassic; Silicoflagellates (silica) & Diatoms (silica) in Cretaceous
2. TERRESTRIAL PLANTS
   • Cycads (along with other gymnosperms) - dominant during most of the Mesozoic
   • Angiosperms (flowering seed plants) evolved in the Late Jurassic or Early Cretaceous & became dominant starting in the Late Cretaceous
     • Angiosperm seeds are carried in fruit
   • Seed Ferns - extinct in Jurassic; True Ferns, Scale trees & sphenopsids - greatly diminished in importance compared to the Paleozoic

MESOZOIC INVERTEBRATES

1. TERRESTRIAL INVERTEBRATES
   • Continued expansion of groups evolved during the Carboniferous
     • Insects - important pollinators for Cycads, Ginkoes & Angiosperms
2. MARINE INVERTEBRATES
   • Protista (Protozoa) - Class Sarcodina became important
     • Radiolarians - shells of silica; Foraminifera - shells of calcium carbonate
     • Planktonic families became important in the Cretaceous
   • Coelenterata - Scleractinian corals evolved & formed large reefs
     • live in extremely shallow warm water - contain symbiotic algae
     • scleratinian coral reefs became important marine habitat
   • Arthropoda - Crustaceans (lobsters/crabs/shrimp/barnacles) became numerous
   • Mollusca
     • Pelecypods evolved the siphon & replace brachiopods as the dominant bivalve
       • Rudists evolved & became important reef builders during the Cretaceous
     • Cephalopods (particularly Orders Belemnoidea & Ammonoidea) flourished during the Mesozoic & ammonoids are useful in Mesozoic biostratigraphy
       • Belemnite shells - entirely within animal
       • Ammonites = dominant ammonoids, particularly during Jurassic & Cretaceous
     • Gastropods continued to flourish
   • Echinodermata - Echinoids became the dominant echinoderm

MESOZOIC VERTEBRATES

1. REPTILES
   • Different subclasses are distinguished by skull structure (See Appendix A)
     • Anapsids - no holes on side of skull (turtles, extinct primitive reptiles)
     • Synapsids - one hole on lower side of skull (extinct pelycosaurs & therapsids)
     • Euryapsids - one hole on upper side of skull (most extinct marine reptiles [icthyosaurs, plesiosaurs, nothosaurs, placodonts])
     • Diapsids - two holes on side of skull (most living reptiles [lizards, snakes, crocodiles, tuatara], extinct dinosaurs, extinct pterosaurs [flying], & extinct mosasaurs [marine])
   • Many new reptile types appeared in the Triassic
     • Turtles (evolved from primitive reptiles; Mesozoic turtles had teeth)
     • Tuataras (evolved from primitive reptiles)
     • Crocodiles (Cretaceous forms got to be as long as 15 meters)
     • Lizards & snakes (snakes evolved from lizards by the Cretaceous)
     • Marine & Flying reptiles & Dinosaurs (see below)
     • Thecodonts (teeth set in sockets) - a mostly bipedal group that gave rise to crocodiles, lizards, snakes, flying reptiles & dinosaurs
   • The Triassic land fauna was dominated by therapsids & thecodonts
   • Primitive reptiles & labyrinthodonts survived into the Mesozoic
2. DINOSAURS
   • Expanded & diversified in the Jurassic & Cretaceous
   • Traditionally divided into two orders based on hip structure: Saurischia (lizard-hipped) & Ornithischia (bird-hipped)
   • Early dinosaurs were mostly Saurichians, which are divided into 2 suborders
     • Bipedal carnivorous Theropods are divided into 4 families
       • Coelurosaurs - small-medium forms of the Triassic-Jurassic & from which other theropods & birds are thought to have evolved
       • Ornithomimusaurs - small-medium forms of the Cretaceous
       • Deinonychosaurs - small-medium forms of the Cretaceous with slashing toe claws & with which birds are thought to have the closest common ancestor
       • Carnosaurs - medium-large forms of the Jurassic (including Allosaurus) & the Cretaceous (including Tyrannosaurus)
     • Giant, 4-footed herbivorous Sauropods are divided into 2 families
       • Diplodocids - (including Brontosaurus [Apatosaurus]) probably reared up on their back legs & balanced on their tails
       • Brachiosaurids - (including Supersaurus, Ultrasaurus & Seismosaurus) had long necks
       • The dominant herbivores during the Jurassic & were high browsers
   • Ornithischians are divided into 4 suborders
     • All were herbivorous with the front teeth replaced by a beak & cheek teeth adapted for crushing coarse vegetation
     • Stegosaurs - medium, 4-footed dinosaurs with a row of pentagonal plates on their backs & 2 pairs of spikes mounted on their tails
       • Lived mostly during the Jurassic; thought to have reared up on back legs & balanced on tail
     • Ankylosaurs - medium, 4-footed dinosaurs covered with closely fitted bony plates & with a bone ball at the end of their tails
       • Evolved from & replaced stegosaurs during the Cretaceous
     • Ceratopsians - small-medium, 4-footed dinosaurs with horns on their face & shieldlike bony frill at back of skull
       • Evolved from ornithopods (see next suborder) during the Cretaceous
       • Dominant herbivours along with hadrosaurs during the Late Cretaceous
     • Ornithopods - small-medium, bipedal, duck-billed dinosaurs
       • Hadrosaurs (many had bony skull crests) were especially numerous during Late Cretaceous
     • Ornithopods, Ankylosaours & Ceratopsians were low browsers and were the dominant herbivores during the Cretaceous
       • Robert Bakker has suggested that replacement of high browsing sauropods by low browsing ornithischians aided fast growing angiosperms in replacing slow growing gymnosperms as the dominant land plant
   • Arguments in favor of warm-bloodedness (Endothermy)
     • predator to prey ratios; erect stance; richly vascularized bones; growth rates; social behaviour & migration of herds; feathers on new find
     • hair on flying reptiles
     • complete dominance over mammals
   • Arguments in favor of cold-bloodedness (Ectothermy)
     • dinosaurs were reptiles & modern reptiles are cold-blooded
     • erect stance & vascularized bones were responses to large size
     • large size itself
   • Still an open question
     • it is no longer accepted that dinosaurs were slow & ponderous; dinosaur behaviour probably like bird & mammal behaviour
     • theropods were quick & agile, large sauropods & ornithischians assembled in social herds, & smaller sauropods & ornithischians had birdlike behaviour
3. FLYING REPTILES
   • First flying reptiles - gliders
     • wings were skin supported by greatly elongated ribs
   • Pterosaurs - active flyers with maneuvering ability
     • wings were skin stretched between elongated 4th finger, sides of the body & rear limbs (other 3 fingers - ordinary length & terminated in claws)
       • relatively large heads with long jaws
     • Rhamphorhynchoids - primitive pterosaurs with long tails
     • Pterodactyloids - advanced pterosaurs with no tails
       • had enormous wingspans - Pteranodon = 7 meters; Quetzalcoatlus northropi = 15.5 meters
     • Pterosaurs were warm-blooded - fine hair covers well-preserved pterosaurs
       • active flyers must maintain constant high internal temperature
4. MARINE REPTILES
   • Several groups with marine adaptations, including paddle-shaped limbs, streamlined bodies, & reproductive adaptations for birth of young at sea
   • Euryapsids
     • Placodonts - distinctive teeth to catch & eat shellfish
     • Nothosaurs - long necks; short, broad bodies; tails; webbed feet
       • gave rise to Plesiosaurs by the Jurassic
     • Plesiosaurs - long or short necks; short, tailless bodies; flippers
     • Icthyosaurs - most fish-like marine reptile
       • analogous to modern dolphins & porpoises, but with shark-like tail (vertebrae extending into lower lobe), 4 flippers & dorsal fin
   • Anapsids - Sea turtles
   • Diapsids
     • Marine crocodiles occurred mostly during the Jurassic
     • Mosasaurs - short necks; long bodies & tails
5. BIRDS
   • Many reptilian structural characteristics & probably descended from coelurosaurs during the Jurassic
   • Feathers are probably modified reptilian scales
   • Mesozoic birds are toothed
   • Fossil record is poor
     • mostly terrestrial with delicate & hollow bones
   • Archeopteryx - first fossil bird to be discovered
     • well-preserved in Jurassic limestone, with the imprint of feathers
     • except for its feathers, it was distinctly reptilian - thecodont teeth, long tail, claw-bearing free fingers, unkeeled sternum
   • Marine birds - no tail but with teeth
     • Duck-, pelican-, tern- & gull-like forms in the Cretaceous
     • Wingless swimmers as well
6. MAMMALS - (see Cenozoic Life notes)

MESOZOIC MASS EXTINCTIONS

1. END OF TRIASSIC
   • Extinction of:
     • All but 1 family of Ammonoid cephalopods
     • Conodonts
     • Labyrinthodont amphibians
     • Many reptile orders:
       • Primitive stem reptiles (Cotylosaurs) [Anapsids, leaving only turtles]
       • Advanced mammal-like reptiles (Therapsids) [Synapsids]
       • Placodonts & Nothosaurs (marine reptiles) [Euryapsids]
       • Thecodonts [Diapsids]
2. END OF CRETACEOUS
   • Extinction of:
     • Many families of calcareous coccoliths, planktonic foraminifera, bryozoans & echinoids
     • Rudists, ammonoids & belemnites
     • Many reptile orders:
       • Plesiosaurs & Ichthyosaurs (marine reptiles) [the remaining Euryapsids]
       • Dinosaurs, Pterosaurs & Mosasaurs [Diapsids, leaving only snakes, lizards, crocodiles & Tuatara]
3. EXTRATERRESTRIAL CAUSES
   • Meteorite or asteroid impacts
     • High concentrations of Iridium (rare in crust, relatively abundant in meteorites), shocked quartz & soot (from immense fires)
     • Deposition from tsunamis
     • Resulted in acid rain, global darkness & greenhouse effect
     • Also associated with Late Devonian mass extincition
   • Planet X & The Companion Star Nemesis
     • periodicity in extinction rate of 26-30 my may indicate disturbance of the Oort cloud of comets
     • consistent with multiple peaks in Iridium
4. TERRESTRIAL CAUSES
   • Volcanism - abundant during the Cretaceous (Deccan Traps & Pacific)
     • possible source of Iridium from deep in Earth
   • Climate Change - mostly getting colder (warmer climates are more stable & less stressful)
   • Sea Level Change - sea level drops due to a decrease in seafloor spreading rates
   • Disease - dinosaurs from different land masses mix

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