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C O U R S E
Paleontology: Early Vertebrate Evolution
Alison Murray, University of Alberta
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C O U R S E L E C T U R E Vertebrate Environments Notes taken on February 24, 2017 by Edward Tanguay |
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what kinds of environments the earliest vertebrates inhabited
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paleontology is blending of
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biology
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geology
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to understand how and why certain features evolved, we need to interpret the rocks that fossils are found in
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this tells us what kinds of environments the animals lived in
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depositional environments on land are more common
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depositional environments in the ocean are less common
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since life began in the oceans, we need to understand this habitat and its depositional environments
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most common place to find fossils of marine organisms like fish, corals, crustaceans, or clams
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the continental shelf
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in the shallow waters off the coasts of continents
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beaches are high energy environments
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delicate fossils can be broken and eroded
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continental slopes and the deep ocean are hard for us to reach
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how to tell what the environment was like when the fossil animals were alive
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geologists interpret rocks in two ways
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what the rock is made of
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what kind of sedimentary structures were preserved when the rock was deposited
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there are also fossils in carbonate rocks
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made up of at least 50% calcium carbonate
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mostly limestone
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limestone is made up of the shells of marine animals such as mollusks
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key fossil localities
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rivers
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most energetic near their sources in highlands
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most rivers transport and deposit sand and mud
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some with strong currents move gravel
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lens shaped sand or gravel channel deposits are an indicator of a fluvial environment
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as rivers travel over gradually flatter land, change to meandering rivers with shallower, wider channels
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able to transport less material
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rivers end up at an ocean, sea, or large lake
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empties into sea at a delta or an estuary
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here you find carbonate sediments
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water here is brackish, i.e. not as salty as ocean water
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some animals and plants are adapted to live in this type of water
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heavily influenced by actions of the tidal current which produces sedimentary structures
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herringbone cross-stratification
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alternating parallel layers
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this means the sediments were probably deposited in a tidal environment
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horizontal beds are called cross beds
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inclined layers are cross-strata
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sediment was being deposited fast in order to preserve the cross-stratification
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waves also affect cross-stratificaton
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wave base
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deeper during storms
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shallower during fair weather
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many fossil localities were deposited below storm wave base
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usually have shales
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rocks with the most ripples are from the shallowest environment
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barrier islands
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spits
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small point of land running into the sea, or a long, narrow shoal extending from the shore into the sea
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lagoons
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areas of quiet, low-energy water
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fine sediments can be deposited here
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water evaporates and concentrates minerals
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can become saltier than ocean water
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forms evaporate
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calcite
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calcium carbonate
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gypsum
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anhydrite
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calcium sulfate
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halite
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sodium chloride
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usually what we mean when we say salt
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interbedded with mud
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if we find these minerals, there is a good chance we are looking at what used to be a shallow lagoon
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good environments for preserving fossils
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because the water is still
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are still and salty
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not a lot of things can live in them
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becomes anoxic, i.e. little oxygen
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this means if an animal falls into the lagoon and sinks to the bottom, there aren't many animals living their to scavenge the carcass
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small grain size tells us that the waters were very still
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good for preserving details
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here we find some of the most exquisite fossils every found have come from lagoon or lagoon-like environments
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Archaeopteryx
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deeper continental shelf environments
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Man on the Hill
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preserves 415 million year old vertebrate fossils
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accumulated in deep water on the continental shelf
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the environment was close enough to land for fine sediments to be deposited
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but far enough offshore that most of the sedimentation was carbonate
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ripples became less common
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carbonate platforms
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where you find marine fossils
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the photic zone
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where light penetrates
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a few centimeters to 200 meters deep, depending on the clarity of the water
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plankton
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float and form the basis of the marine food chain
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here we find reefs
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corals thrive in nutrient-rich water
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the steeper the slope, the less stable
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plate corals
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make the most use of the least light
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in deeper waters, chances of finding vertebrate fossils becomes relatively small
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turbidites
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deposits that can form in many environments
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occur in deep water
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Burgess Shale
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most famous Cambrian fossil locality area in the world
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famous for the exceptional preservation of the soft parts of its fossils
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likely deposited by a turbidity current
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ocean floor
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here we don't find many vertebrates
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find mostly skeletons of plankton
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these animals are pelagic
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spend their lives floating in the water column
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chert
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fine-grained silica-rich microcrystalline or cryptocrystalline sedimentary rock that may contain small fossils
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most of the sea floor in earth's history has been melted and recycled into new crust, taking its fossils with it
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the old ocean crust still in today's ocean is about 150 million years old
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not that old compared to the oldest continental rocks
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over 4 billion years old
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when an oceanic plate means a continental plate, the oceanic plate sinks
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the oceanic crust is subducted beneath the continental crust
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most marine fossils come from continental shelf deposits
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when oceanic plates meat a continental plate, some contents get scraped off