ORIGIN OF VERTEBRATES
Where we are today is we‟re going to be talking about the origin of vertebrates, as well as the
origin of jaws. We‟ll be talking about the Ordovician and the Silurian periods and a little bit of
the Devonian. Remember, we‟re talking about the periods of the Palaeozoic. Following the
Cambrian explosion, you have the replacement of the Cambrian faunas (dominated by trilobites)
by the brachiopod faunas. These are little hinged vertebrates related to molluscs. The Ordovician
goes from 490 to 443 million years ago. It is followed by the Silurian – 443-417, and the
Devonian 417 to 354. These are significant because we have radiation of plants and animals from
marine environments onto land. There are brachiopod dominated faunas. In the Ordovician, you
have a large Gondwana, sea-levels are very high, and at the end of the Ordovician, there are
glaciations. Hence, you go from a greenhouse environment to an icehouse environment. You
have polar ice. Sea levels fall. Ocean water is locked up as ice. You see the big Gondwana. It is
mainly a southern landmass. You also see laurentia. Sitting almost at the equator at this time is
the GTA. Lollll. Northern Ontario was equatorial at this time. Most of the continental North
American landmass is submerged. We have very high sea levels. We have lots and lots and lots
of shallow green environments with lots of marine animals. How many people have gone to the
escarpment (the green belt to the north and east of Toronto)? That is Ordovician limestone,
shallow marine rocks. When you go to the escarpment, you‟re hiking on the Ordovician. When
sea levels rise, it is called transgression, and when they fall it is called regression. Sea levels
rise and fall all the time. What is so problematic about the current situation is the rate at which
sea levels are rising. What is happening during the Ordovician? Trilobites, the animals
dominating the Cambrian go extinct to a large degree. They go from very having very diverse
populations to having very scarce populations in the Ordovician. They get replaced by
brachiapods and Cephalopods. When you hear the word pod, you should know that it is Latin for
foot. When we talk about molluscs, including clams, oysters, snails, and Cephalopods, we use
the term pod because these animals have evolved the soft tissue structure called the foot. It
doesn‟t have toes though. It secretes minerals and is used for a variety of functions. Molluscs all
have a shell that is made from the same stuff that pearls are made of. Their bodies are very
different. We call different creatures different things. For example, Gastropod literally means
stomach-foot. Essentially, any name which has „pod‟ in it does so because it describes the
arrangement of the foot relative to the rest of the body.
We also have the evolution of true fish. They‟re pretty crazy looking primitive fish. Graptolites
are also around. They‟re related to us and are colonial organisms. They lived in colonies or tubes
and would come out with an oral disk, gather nutrients out of a water column and go back in.
They hung out like big floating reefs. They were floating. Hence, they‟re called planktonic.
There are the brachiopods. These are the most diverse animals. They are filter feeders. They have
a special structure called a lophophore. It‟s a soft tissue structure which moves through the water
column and grabs food and brings it to the digestive cavity of the brachiopods.
We also have corals. These are colonial animals. When you see a coral reef, you‟re basically
looking at a giant condominium. It has little holes, which has tiny organisms. They pop out, pull
food out of the water, and go back in. Corals are important indicators of what we‟re doing to the oceans. They‟re just a tool in this regard. Here‟s a question: are coral reefs empty or do they
contain lots of organisms? They have lots of organisms. At a coral reef, you find a diversity of
life. Now, corals form reefs. Corals are not the only things that form reefs. Reefs are centers of
evolution. They‟re important for this reason. They‟re engines of speciation. New species evolve
in coral reefs. When we get reefs, what we can expect is a complicated ecosystem. We get the
first corals. There are little corals called rugose corals and bigger ones called tabulate corals.
When they originally form, they form small reefs. In the Ordovician, they‟re not very large.
There‟s another thing in the Ordovician. These are called conodonts, which literally means „cone
tooth‟. They have very complex teeth structures. We find them in shallow marine rocks between
the late Cambrian and the beginning of the age of dinosaurs. They‟re economically important.
They‟ve metamorphosed into rocks. We can look at marine aged rocks, and by the degree of the
alternation of rocks, we can tell if the oil is located nearby. More importantly, we don‟t know
what these animals are though. They only serve an economic function.
What happened about 10 years ago was that this specimen was found (picture in the power-
point). As you can see, it‟s a slab of rock with a gray streak running up the side (see power-
point). The top appears to be what look like dumbbells. That‟s the first soft tissue preservation of
conodonts. Conodonts look like freaky looking snakes. They have great big eyes and an open
oral cavity. They‟re related to us (vertebrates). Their teeth are complicated structures and allow
them to orally process food. They have four or five different sets of teeth. These were amazing
oral processors. As a result, they could eat a fair variety of food sources. It‟s useful to have
layers of teeth because these creatures don‟t have jaws. Now, another group that evolves and
diversifies during the Ordovician are vertebrates, us.
The oldest evidence of known vertebrates are fish scales from the late Cambrian from Wyoming.
These fish scale, made of calcium phosphate (the same material that our bones are made of) are
the oldest evidence of the oldest groups of vertebrates. These very oldest groups are jawless
fishes. They‟re technically called Ostracoderms. These become numerous in the Ordovician but
are pretty much gone in the Devonian. When vertebrates first evolved, the ecology they evolved
in comprised shallow oceans. They swam in these oceans. They had no jaws, but an open oral
cavity. Think about this. When you go have lunch, what I‟d like you to do is take your food, sit
at the table, put your hands under your leg, open your mouth, and try to get the food in your
mouth without using your hands or lower jaws. The evolution of jaws is a big innovation in
vertebrates. This is fairly late in our evolutionary history. The bony lower jaws shows up after
vertebrates appear. The first vertebrates have no lower jaws. These primitive jawless fishes have
something on the underside of their head. Remember, they don‟t swim well (since they do not
yet have fins) and also suction feed off the bottom. Now, if you look at the picture at the top,
there‟s a spike on the nose. At the bottom, the fish has fins. The fins! What are they use for?
They help in swimming. What they were primitively used them for was not to generate thrust,
but to manoeuvre. Thrust is generated by the tail. The most primitive jawless fishes don‟t have
fins. They‟re not good at swimming in straight lines. They‟re like drunk fishes. lollll Having fins
allows the fish to have control over where it‟s swimming. The animal at the bottom (in the
picture) is a better swimmer, the top one is worse.
The first evidence of bones is found in these fishes. This (in the power-point) is a cross-section
of these bones. There are many species when they first evolve. As we look at more and more derived jawless fishes, they become efficient at swimming.
Anaspids have paired fins.
The thelodonts are below them. Their armour is removed, allowing them to swim faster. They‟re
probably still feeding on the bottom though.
The big derived group are the osteostraci. This group does a lot of weird stuff. There‟s a fossil at
the bottom (i