Life in the Paleozoic

The Cambrian Revolution
- in the latest Proterozoic hardened worm burrows appear in the fossil record
- hard skeletal parts (the "small shellies") first appeared in the lowermost Cambrian about 543 m.y.; early skeletal materials were chitinophosphatic; calcium carbonate became the predominant skeletal material as the Cambrian progressed into the Ordovician (and atmospheric oxygen increased to near present levels)
- most animal phyla evolved during the Cambrian; all later animal species derive from the basic forms established in the Cambrian
jawless fishare the oldest fossils of vertebrates
- no land plants or land animals

Mid Paleozoic Life

Rise of Fishes
- coiled ammonoid cephalopods (descended from the nautiloids) evolved and became the dominant cephalopod hunter in the Devonian
- The Devonian is known as the Age of Fishes
- jawless fish (agnathans), which had been known only by fossilized scales and bony plates since the Late Cambrian, became common in the middle Paleozoic; they were probably clumsy swimming bottom feeders;
- the first jawed fishes, the acanthodian, arose in the Silurian, probably as fast swimming predators.  They had scales rather than bony plates.
- the placoderms, which had heavy head and body armor, evolved in the Silurian
- the sharks evolved in the Silurian, or no later than the Devonian
      (agnathans, acanthodians, placoderms, and sharks are all cartilaginous fishes)
- the ray fin and lobe fin fishes also arose in the Devonian; they represent the bony fishes
     99% of all bony fish today are ray fins;
     one group of lobe fins later gave rise to tetrapods (amphibians, reptiles, birds, and mammals)

Invasion of the Land (first signs were Late Ordovician spores and burrows)

Plants
- 3 problems for land plants a) waterproof cuticle so it doesn't dry out; b) strong support since it can't rely on water buoyancy; c) need new means of passing sperm to egg or remain dependant upon water in which sperm can swim to egg

Spore Bearing Plants (rely on water for fertilization)
- the first vascular plants (Sil-Dev) were mostly spore-bearing, ground-creepers, with rootless and leafless stalks, that lived near water; vascular system was
- lycopsids evolved in Early Devonian; most were small ground plants like the modern "club moss" but some grew as trees to 30m; lycopsids have leaves attached directly to the trunk
- the modern horsetails are descendants of the joint-stemmed plants (sphenopsids)
- true ferns were a third important spore-bearing plant group in the Devonian; ferns were the most advanced of the spore bearing plants but still relied on water for reproduction/fertillization

Animals
- invertebrates quickly followed the plants onto the land in Late Ordovician
- Silurian millipedes were followed by other arthropods by the Lower Devonian (scorpions, spiders, mites, and wingless insects)
- Late Devonian, first tetrapods (four-limbed vertebrate animals) evolved from lobe fin fishes and adapted to the land
    They continued to lay their eggs in water as their fish ancestors had and as most modern amphibians do.

Late Paleozoic Life

Marine Life
- archaic Devonian fish (Acanthodians, Placoderms) extinct; ray fin bony fish were dominant vertebrate predators

Terrestrial Plant life
- spore bearing plants (require water fertilization)
      lycopsids (scale trees), sphenopsids (scouring rushes and horsetails), tree ferns were all abundant
- gymnosperms have a naked seed attached to a leaf or scale; may be wind fertilized; doesn't require water for fertilization; gymnosperms include:
- first gymnosperms: seed ferns evolved in the Late Devonian
     one of the most important groups was glossopteris, the late Paleozoic seed fern of the southern continents
- conifers: cone-bearing plants (eg. cordaites, late Pz conifers of the northern continents)
- coal deposits were widespread in the Carboniferous as forests spread and termites had not yet evolved to digest all of the new wood
- cooler and dryer global conditions (and glaciation on the southern continents) developed in the Pennsylvanian and Permian periods due to
     spread of forests and preservation as coal and
     weathering of new mountain ranges resulting from the continental collisions that formed the supercontinent Pangea
     both of which removed CO₂ from the atmosphere
- drier conditions led to dominance of the gymnosperms since they didn't rely on water-borne fertilization

Terrestrial Animal life
- Carboniferous, Age of Amphibians; tetrapods of the Carboniferous were like today's amphibians (frogs and salamanders) in that they need to lay their eggs in water so they don't dry out and die; there were many forms including lizard-like, snakelike, crocodile-like; some were quite large
- Age of Cockroaches; huge insects in Pennsylvanian; dragonfly to nearly 1 meter (presumably because of the abundance of oxygen produced by the spread of forests and preservation of the biomass in coal
- mid-Carboniferous, first amniotes, this line of tetrapods evolved the amniotic egg which allows oxygen in and carbon dioxide out but water can't get out so eggs don't need to be laid in the water; the first amniotes gave rise to two important lineages:
1) sauropsids which gave rise to dinosaurs and modern reptiles
        and
2) synapsids, now extinct except for one lineage that evolved into the mammals
- Permian drying allowed the amniotes to take over the leading role among terrestrial animals because of their reproductive advantage (didn't have to lay eggs in the water); the Permian is often called the Age of Reptiles though it is now clear that the dominant reptiles were the synapsids which later gave rise to the mammals, not to a lineage of modern reptiles

- Late Permian Mass Extinction

A two-step mass extinction.  The first extinction, 7-8 m.y. before the terminal Permian mass extinction killed off 70% of marine species, but had little effect on the land.  The Terminal Permian mass extinction was much worse.  All rugose and tabulate corals, and the last of the trilobites became extinct.  Most ammonoids, crinoids, and bryozoans became extinct.  On land, therapsid (the dominant) synapsids were hard hit.   Forests were decimated - woody plants were replaced by non-woody lycopods.  Large concentrations of fungi are found in shallow marine sedimentary strata, remnants of huge volumes of decaying wood washed into the sea.

All together, 90-95% of all marine species (80-85% of all marine and terrestrial species) and 75% of all vertebrate families became extinct by the end of the Permian.

What Caused the Permo-Triassic Mass Extinctions?
The cause is apparently related to massive volcanic activity.  The Siberian traps, the largest outpouring of flood basalt in the Phanerozoic coincides with the Terminal Permian mass extinction. A smaller flood basalt province in China erupted several million years earlier, approximately conicident with the Guadalupian mass extinction. Some deposits from explosive, felsic volcanoes are also found.  Perhaps sulfur aerosols and volcanic ash darkened the sky causing a sudden, short-term “nuclear winter” or some other environmental consequence of these extraordinary volcanic events.

Life in the Mesozoic

Terrestrial Life

- the dominant Permian fauna of synapsids and amphibians was replaced in the Mesozoic by archosaurs: which included the lineages of crocodilians, dinosaurs, pterosaurs, and others
-therapsids (synapsid reptiles) remained the dominant land animal in the first part of the Triassic.  However, therapsids became extinct at the end of the Triassic coincident with a large increase in atmospheric CO₂ in the beginning of the Jurassic caused by the onset of rifting of Pangea and formation of the North Atlantic Igneous Province.  Extinction of the therapsids made way for the dinosaurs to become the dominant land animals.
- mammals descended from last synapsids (therapsids) in Late Triassic
     remained shrew-like or mouse-like for ~150 m.y.
- primitive pterosaurs (flying reptiles) and dinosaurs appeared in the Late Triassic
- a key adaptation of the dinosaurs was their "hole in the hip socket" that allowed them to place all of their weight directly over their hind limbs rathern than having their legs splayed out to the side like most reptiles.
- ornithischian ("bird-hipped") dinosaurs -> stegosaurs, ceratopsids (triceratops; Late Cretaceous), hadrosaurs (duck-billed; Jurassic Park's poison spitters)
- saurischian ("lizard-hipped) dinosaurs included sauropods (brontosaurus - now called apatosaurus) and tyrannosaurs (e.g., T. rex)
- Birds arose during the Jurassic; Archaeopteryx (the oldest fossil birds; Late Jurassic)
they evolved from small carnivorous saurischian (lizard-hipped) dinosaurs (raptors of Jurassic Park fame)
-fish-eating toothed birds were common in the Late Cretaceous
- great diversification of herbivorous (plant-eating) dinosaurs in Late Cretaceous with the rise of angiosperms (flowering plants) [to delight of T. rex]
- modern types of amphibians: frogs and salamanders appeared in Triassic
- reptiles: crocodiles and turtles, Triassic; lizards, Jur-Cret; first snakes (constrictors), Cretaceous
- primitive marsupials and placental mammals were already established by Late Cretaceous

Cretaceous-Paleogene (~65 m.y.) Mass Extinction
- many marine microplankton, some brachiopods, mollusks, echinoids, fish, and land plants went extinct
- all dinosaurs (except for birds!) went extinct
- iridium anomaly, glass spherules, shocked quartz found at K/T stratigraphic horizon: all consistent with an asteroid impact
- Chicxulub crater found buried beneath younger sediments along the north coast of the Yucatan Peninsula, Mexico
     the 125 km wide crater implies impact of an asteroid body of about 10 km across

Life in the Cenozoic 

- Paleocene: explosive evolutionary radiation of mammals filling the ecological niches vacated by the dinosaurs
from shrew-like mammals of the Cretaceous, 22 mammalian orders (almost all the modern orders) evolved by the Eocene, including the familiar groups: bats, whales, hoofed animals, carnivores, rodents, and primates

As the repitles before them had done, the mammals recolonized the land, the oceans (whales and dolphins), and the air (bats).

The one remaining branch of the dinosaurs, the birds, are today one of the most diverse and successful animal groups, inhabiting air, land (ostriches, moas, kiwis), and sea (penguins).