Paleontology: Early Vertebrate Evolution is a four-lesson course teaching a comprehensive overview of the origin of vertebrates. Students will explore the diversity of Palaeozoic lineages within a phylogenetic and evolutionary framework. This course examines the evolution of major vertebrate novelties including the origin of fins, jaws, and tetrapod limbs. Students also explore key Canadian fossil localities, including the Burgess Shale (British Columbia), Miguasha (Quebec), and Man On The Hill (Northwest Territories). Watch a preview of the course here: https://uofa.ualberta.ca/courses/paleontology-vertebrate-evolution
1.2 Vertebrate Environments
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From the course by University of Alberta
Paleontology: Early Vertebrate Evolution
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From the lesson
The Phanerozoic Begins
In this lesson we take you back to the beginning of the Phanerozoic Eon to learn what it truly means to have backbone, as we encounter the key anatomical features of vertebrates and their closest chordate relatives. We’ll also introduce the language of evolution – phylogenetics – as we examine some of the contenders for the title of ‘The Earliest Vertebrate’, and give you a crash course in sedimentology, so you can begin to piece together the spectacular environments that were home to our early aquatic ancestors. Just a quick note before you get started: 'Palaios' is the Greek word for 'ancient', so palaeontology or paleontology is the study of ancient life. Both spellings are correct, with palaeontology used in Britain, and paleontology more common in the US.
Meet the Instructors
Alison Murray, Ph.DAssociate Professor
Department of Biological Sciences
Now that we're more familiar with what makes a vertebrate a vertebrate, let's
talk about what kinds of environments the earliest vertebrates inhabited.
Paleontology is a combination of many scientific disciplines, but
in particular, it's a blending of the sciences of biology and geology.
In order to understand how and why certain features evolved,
we need to be able to interpret the rocks that fossils are found in.
From them, we know what kinds of environments the animals lived in.
You may be familiar with some of the depositional environments we find on land.
But you're probably less acquainted with most of the ones we find in the oceans.
Life began in the sea, so for this course, we need to understand
the different habitats and depositional environments that are found in the ocean.
Where do you think you would be most likely to find fossils
of marine organisms like fish, corals, crustaceans, or clams?
Would it be A, on a beach.
B, on a continental shelf.
C, on a continental slope.
Or D, in the deep ocean.
Although there might be fossils in all of these areas, we're most likely to find
fossils of marine organisms in the shallow waters off the coast of continents.
So B is the correct answer. Beaches are high energy environments,
and delicate fossils can be broken and eroded here.
Whereas continental slopes and the deep ocean are hard for
us to reach in order to look for fossils.
But how can you tell what the environment was like
when the fossil animals were alive?
We'll explore that next.
Geologists and paleontologists interpret rocks to
figure out the ancient environments the rocks were formed in.
They determine this in two main ways.
What the rock is made of.
And what kind of structures were preserved when the rock was deposited.
These structures are called sedimentary structures.
In this course, we will also be looking at fossils in carbonate rocks.
Carbonate are any rocks that are made up of least 50% calcium carbonate.
Most carbonate rock is limestone,
and most limestone is made up of the shells of marine animals like mollusks.
Let's explore some depositional environments and
what kinds of rocks in sedimentary structures we find in them,
by taking a look at some key fossil localities.
We'll be looking at these localities throughout the course.
Since we live on land,
we're generally more familiar with terrestrial depositional environments.
Let's start there, with a brief review of rivers.
Generally, rivers are most energetic near their sources, in highlands.
Most rivers transport and deposit sand and mud while some,
with very strong currents, may also move gravel.
Lens shaped sand or gravel channel deposits
are good indicators of a fluvial or river environment.
As rivers travel over gradually flatter land,
they lose energy and change from straight fast deep channels
to meandering rivers with shallower, wider channels.
Because they have less energy, they are able to transport less material and
so, they deposit more of the sediment they were carrying.
If we follow a river all the way to its end,
we often end up at an ocean or sea, or maybe a very large lake.
For this course, we'll mostly be focusing on marine environments.
So for now, we'll look at rivers that empty into large, saltwater bodies.
The area where a river empties into the sea may be a delta or an estuary.
Both of the environments will show a mix between marine and
continental sediments, and fossils.
It's not uncommon for instance to find layers of carbonate sediments
in marine shelly fossils like clams,
associated with layers of terrestrial sediments containing plant roots.
The water here is brackish, meaning it's saltier than fresh water,
but not as salty as ocean water.
Some types of animals and plants are adapted to live in this type of water.
So, finding them is also a good indicator of the deltaic or estuarine environment.
Depositional environments at the meeting of sea and
land like estuaries are heavily influenced by actions of the tide.
The rhythmic movement of the rising and falling tides creates a tidal current.
This current produces some characteristic sedimentary structures that make it easier
for geologists and paleontologists to identify ancient, coastal environments in the rock record.
This rock has a pattern of alternating parallel layers,
called herringbone cross stratification.
And what do you think it tells us about the environment it was deposited in?
Does it tell us that A, the sediments were probably deposited in still water.
B, the sediments were probably deposited in a river.
C, the sediments were probably deposited in a tidal environment.
Or D, the sediments were probably deposited in deep water.
The sand was deposited in one direction, when the tides rose,
and then the opposite direction when the tide fell.
Each of these tidal layers was deposited at an angle.
And together, they formed horizontal beds.
This tells us, these rocks probably represent
a tidal environment like an estuary or a delta.
So C is the correct answer.
The horizontal beds in this example are called cross-beds.
And the inclined layers within them are cross-strata.
Together, this type of sedimentary structure is called cross-stratification.
Looking at the inclined layers,
a geologist can tell not only that the sediments were deposited in flowing water,
but also the direction of the paleocurrent or which way the water was flowing.
This herringbone cross-stratification tells us the water
flow was alternating regularly between two different directions.
it was also laid down in an environment where the sediment was being deposited
very fast, enough to preserve cross stratification, like an estuary or delta.
Storms and waves on the surface can also result in sedimentary structures.
When wind blows across the surface of a body of water like a lake,
it makes the water particles move in vertical circles.
We call the depth to which the surface waves affect
a body of water, wave base.
Wave base changes with the weather, being shallower in calm conditions.
This is called fair weather wave base.
The wave base gets deeper during storms, which is the storm wave base.
If rock layers have a lot of ripple marks, they were probably
deposited in shallow water, above fair weather wave base.
If the layers are very fine, and don't have any ripples in them,
they were probably deposited in deep water,
below storm wave base.
This would be where the water was calm enough for fine sediments
to be deposited in flat, undisturbed layers.
And that could mean anything from two hundred to three hundred meters,
to three thousand meters, or even deeper.
Many fossil localities were deposited below storm wave base
in calm environments, and have thin beds of fine sediments, usually shales.
Based on what you know about wave depth, put these rock samples in
order of depositional environment, from shallowest to deepest.
Is the correct order A, B, C; B, A, C; or C, B, A?
The rock with the most ripples is from the shallowest environment.
And the rock with the least ripples is from the deepest environment.
C, is the correct answer.
Some coastal environments, don't have very strong tidal currents.
Without the influence of these strong currents dominating the sedimentary
environment, waves can play more of a role in depositing and reshaping sediments.
On coast lines where wave energy is high, barrier islands
and spits can form parallel to the coast line and behind them are lagoons:
areas of quiet, low energy water.
Sheltered by the barrier island, lagoons are not greatly effected by currents,
tides, or waves and therefore, fine sediments can be deposited here.
If lagoons are very shallow, enough water may able to evaporate
so that the water left behind concentrates minerals and
can become much saltier than ocean water.
If the water in a lagoon continues to evaporate, these minerals will precipitate
out of solution forming a sedimentary rock called an evaporite.
Common evaporite minerals are calcite, gypsum, anhydrite, and halite.
Now, calcite is a calcium carbonate.
Anhydrite is a calcium sulfate.
And halite is sodium chloride, which is what we usually mean, when we say salt.
These can be interbedded with muds, as the water level in the lagoon rises and
falls as new sea water flows in periodically.
If we find layers of these minerals in the geological record,
there's a good chance we're looking at what used to be a shallow lagoon.
Do you think lagoons could be good environments for preserving fossils?
A.
No, because the water is shallow.
B.
Yes, because the water is still.
C.
No, because the water is warm or D, yes, because the water is cold.
Although deep water is a good environment for preserving fossils, shallow lagoons
have produced some excellent, beautifully preserved fossils as well.
This is because the water is very still and quiet.
B is the correct answer.
One effect of lagoons being very still or
becoming very salty, is that not a lot of things can live in them.
Very quiet, still water can easily become stagnant and anoxic,
meaning, there's very little dissolved oxygen in the water.
This means if something dies and sinks into an anoxic lagoon or
the anoxic bottom of a deep lake, or the ocean floor,
there aren't many animals living there to scavenge the carcass.
The small grain size of these lagoonal muds and clays, tells us
that the water was very still, allowing them to slowly settle out.
These very fine sediments are also very good for preserving fine details.
This combination of still water, very fine sediments, and
an anoxic environment means that some of the most
exquisitely preserved fossils ever found, like the famous <i>Archaeopteryx</i>,
have come from lagoons or lagoon-like environments.
If we move a bit further away from the shore,
we find ourselves in progressively deeper, continental shelf environments.
High in the MacKenzie Mountains of the Northwest Territories of Canada,
the locality called Man On The Hill or MOTH,
preserves 415 million year old vertebrate fossils
that accumulated in deep water on a continental shelf.
Now rocks from MOTH
are a mix of finely laminated carbonates and clays, meaning,
the environment was close enough to land for fine sediments to be deposited, but
far enough off shore that most of the sedimentation was carbonate.
As water gets deeper, there's less influence by wave action and
ripples become less common.
Man On The Hill was also probably anoxic,
which in addition to the water being relatively deep and still,
helped preserve the remains of the animals there, similar to a lagoonal environment.
Relatively shallow continental shelf environments like these are where we
commonly find areas of carbonate deposition, forming carbonate platforms.
Marine fossils, especially invertebrates, are very common here,
where the water is shallow enough for light
to penetrate with enough intensity to support photosynthesis, and
this range of water depth is called the photic zone.
And it can be anywhere from a few centimeters deep to 200 meters deep,
depending on the clarity of the water.
Photosynthetic plants and algae, and
the microscopic organisms that feed on them, flourish in the photic zone.
These organisms are collectively called plankton,
meaning that they float, and they form the basis of the marine food chain.
Many of these organisms extract calcium and
other ions from the water to form calcareous skeletons.
And these skeletons form carbonate sediments when the animals die.
It is in these shallow carbonate platform environments
that we also find reefs, which are often made up of corals.
The types of corals that build reefs thrive
in high energy environments with nutrient rich water.
The landward side of reefs may protect lagoons while the seaward side
of a reef will form a steeper slope leading down into quieter water.
And the steeper the slope, the less stable the slope.
So, these steeper slopes are prone to underwater mudslides and slumps.
The type of coral we find can let us know where in the reef it was from.
Sturdier forms are generally located in the shallowest parts of the reef,
where the energy of the water is high.
And more delicate branching forms of coral
are found progressively deeper on the seaward side.
Plate corals, which are able to make the most use of the least light,
are found at the deepest parts of the reef.
Reefs also provides homes for many other animals, and
most of these are specially adapted for life in shallow water.
As we move further away from shore,
below storm wave base, into progressively deeper marine segments,
our chances of finding vertebrate fossils becomes relatively small.
The slope between the continental shelf and
deep ocean floor is steep and unstable.
And most of the sediment becomes disturbed and
redistributed by slumps, underwater landslides, and debris flows.
Turbidites are deposits that can form in many environments, but
are particularly known from deep water.
These are deposited by a special type of current called a turbidity current.
The Burgess Shale in Yoho National Park in the Rocky Mountains of British Columbia
is probably the most famous Cambrian fossil locality in the world.
The Burgess Shale was likely deposited by a turbidity current.
The fossil locality is known worldwide for its beautifully preserved
soft bodied animals, including one of our earliest chordate relatives.
Deeper still, on the ocean floor, we don't find very many vertebrates.
The sediments here are mostly made up of the skeletons of plankton.
These animals are pelagic,
meaning they spend their lives floating in the water column.
When they die their skeletons slowly sink to form muds and
oozes on the sea floor, up until a certain depth.
These muds are mostly calcareous and
the rocks they form is a pelagic limestone.
The solubility of calcium carbonate increases with pressure,
so below a certain depth the calcium dissolves in seawater.
Today, this pressure is found at a depth of 4,000 to 5,000 meters below sea level.
The sediments deposited below this depth are made up entirely
of the remains of microscopic planktonic animals with skeletons made up of silica.
The siliceous ooze formed by these skeletons becomes finely laminated chert.
Fossils from deep marine deposits,
other than these planktonic invertebrates, are extremely rare.
What do you think is the age of the oldest oceanic crust
at the bottom of the ocean today?
Is it A, 4 billion years old.
B, 500 million years old.
C, 150 million years old.
Or D, 1 million years old.
Most of the seafloor throughout Earth's history has been melted and
recycled into new crust, taking its fossils with it.
The oldest ocean crust still in today's ocean is about 150 million years old.
So, C is the correct answer.
This is still pretty old, but not when you compare it to the oldest
continental rocks, which are over 4 billion years old.
This seafloor recycling means that even though the ocean
is a huge area of sedimentation, ocean basin deposits
make up only a small fraction of a fossil record we have today.
Oceanic crust is denser than continental crust.
So, when an oceanic plate meets a continental plate,
the oceanic plate sinks.
The oceanic crust is pushed underneath, or subducted beneath,
the continental plate, destroying whatever fossils were part of it.
The continental crust is therefore much older than the oceanic crust and
holds more fossils.
But wait, now you're thinking if the oldest crust in the ocean is only 150
million years old, how do we have any marine fossils from the Paleozoic?
Good question.
There are two main reasons.
First, most marine fossils come from continental
shelf deposits, which are deposits on continental crust.
Secondly, when an oceanic plate gets pushed underneath a continental plate,
the pile of sediments on the oceanic crust may be scraped off the downgoing plate and
become stuck to the leading edge of the overriding plate.
In some cases oceanic crust is obducted, or pushed up,
on top of continental crust when multiple plates are converging at a boundary.
In this case, oceanic plate will again be preserved.
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