CRETACEOUS PERIOD: 230
million - 65
million years ago
oceans are believed to be more than 4 billion years old. They were
home to the very first known form of life on our planet 3.5 million years
ago, blue-green algae. Invertebrate (without a spine) life forms thrived
in these ancient seas and eventually exploded into dramatic diversity in
the Devonian Period, nearly 400 million years ago but something
else happened around 100 million years earlier than the Devonian that forever changed life
on Earth. It was the emergence of creatures with a backbone -
first vertebrates were fish. Initially,
these were jawless fishes called agnathans and the earliest
examples are known to date back to the Ordovician Period 500
million years ago.
Later, the first
jawed fishes emerged and were similar in appearance with heavily armored
heads and cartilaginous internal skeletons but were equipped with strong
crushing jaws. These were the placoderms and some species like the Dunkleosteus
grew to frighteningly large proportions.
It was also
at this time, over 400 million years ago, that sharks and rays (chondrichthyans)
appeared with the placoderms. Like the agnathans and placoderms, chondrichthyans
had internal skeletons made up entirely of cartilage. The
first fish to have complete skeletons made of bone arrived much later at
200 million years ago. These are known as the bony fishes or osteichthyans.
placoderms, all major groups of fish still survive today. More
primitive groups are such as the lampreys and hagfish are much more
limited in variety than they were millions of years ago. The bony
fishes are the most prevalent still in existence.
the most abundant of all living vertebrates
and make up slightly more than half of the approximately 48,500
described species with us today! With the longest
period of existence known of all vertebrates on our planet, it is no
wonder that fish are so diverse. It
is impossible to accurately quantify the number of species of fish that
have lived since their beginning but based on known rates of evolution and
emergence of new species over the entire fossil record, it is most likely
in the MILLIONS!
their proliferation, fish are relatively rare as fossils. Aside from
shark's teeth with their near indestructible durability, the fragile
nature of a fish body makes it a poor candidate for fossilization.
there are some unique environments and circumstances that existed in
prehistory where some deposits in the world produce the most impressive
and perfectly preserved fossils known to science. With the most
minute details retained in the host rock, these spectacular fish fossils
offer a rare glimpse into life that existed hundreds of millions of years
before the first human walked out planet. Most fish fossils are
found in layered deposits whereby the fossil-bearing rock can be split
to reveal the preserved remains. There are some more unusual
deposits such as the Santana Formation of Brazil, where the fish were
preserved in large concretions. These specimens retain incredible
three-dimensional preserved form as they were when alive. Other
fossil deposits in the world yield isolated remains such as scales,
teeth and fragmentary bones. Analysis of the type of fish found in
fossil deposits can tell scientist much about the type of water
environment they once thrived in. Characteristics of the fish can
reveal if there underwater environment was turbid or placid, deep or
shallow or salt or freshwater. The abundance of fish also usually
leads to other fossils being discovered in the same deposit - those of
the creatures that fed off them.
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