The Evolution of Whales
Whales are magnificent creatures of the sea; they are massive gentle beautiful and hold great intelligence. They started evolving over 50 million years ago and only have one living relative, which is the hippopotamus, however they are not ancestors. Early whales do not look or relatively compare to what we picture in our minds of a whale. Early whales such as the Balisosaurus and Pakicetus were typically land animals however had the ability to go into the water. They had a thick bony wall around the middle ear, which is incredibly similar to living whales yet so different to other mammals, and lived in a freshwater semi-aquatic environment. This meant that they could posses fur coats rather than a thick layer of blubber as, it was not yet needed to stay warm, as they weren’t spending majority of their time within water.
Around
50-55 million years ago, early Cenozoic era, India began it course to collide
with Asia. This caused a great collision where the Tibetan plateau and the
Himalayas were created. During this time climate wise, Earth had a relatively consistent
climate no matter where you were situated (i.e. proximal to the equator). The
most significant global warming period took place in this time period around
55.8 million years ago, it is known as the Paleocene-Eocene Thermal Maximum.
This is linked with a 5°C increase in temperature as well as changes within the
carbon cycle, it saw an immense decrease in the 13C/12C
ratio of
marine and terrestrial carbonates as well as organic carbons. It also saw a
large number of foraminifera to become extinct as well as sudden appearance of
mammals on land within Europe and North America.
Photo source: http://www.scotese.com/newpage9.htm
Tectonically,
Earth was very active during this time. There were many land bridges connecting
many parts of the world together, such as Australia and Antarctica, North
America and Europe through Greenland and potentially Asia and North America
though the Bering Strait. These bridges were altered via tectonic movement.
This rearrangement of boundaries saw changes within the dynamics of Earth. They
way things functioned was altered and saw very significant alterations within
the ocean and atmospheric circulation and temperature. The disconnection
between Antarctica and Australia saw an incredibly large canyon between the now
two continents, which created the circum-Antarctic Current. Change was seen in
temperatures within the ocean due to the circulation patterns and global heat
transport was altered. Currently it moves cooler water up from this canyon and
circulates it around the different continents, resulting in a global cooling
event as seen in towards the end of the Eocene. Increases in variability and
seasonality, saw changes within mammals’ body size to increase to cope with the
temperature changes throughout the year.
With
the increase in temperature from the thermal maximum, land mammals such as
Pakicetus and Balisosaurus (early whales) were forced to become more water
orientated with the way they lived due to the selective pressure of increase in
temperature. This increase saw theses mammals become more adaptive with water
and learn to swim, hunt and live predominantly within the water, however still
only semi-aquatic. Oceans became more temperate and species for marine animals started
to diverse, with phytoplankton and zooplankton becoming a main food source of
modern day whales and some earlier whales. With the change of becoming more marine
orientated species, adaptation was necessary to live, limbs became shorter and
bodies became more streamline, this is seen in Rodhocetus. The long hands and
feet of Rodhocetus were probably webbed to enhance ability to swim efficiently.
It also had a sacrum (a bone produced when vertebrae are fused together), which
is positioned between the hipbones of the pelvis, causing to it to be immobile.
The Rodhocetus also uses the power of its feet and uses the tail as a rudder,
it spent majority of it’s time on the surface of the ocean, it was also had fur
as insulation rather than blubber like modern day whales.
Photo source: http://www.wunderground.com/climate/PETM.asp
Modern whales have come along way from living on land to living
completely within the ocean. The Baleen Whales is the first whale that is still living within the ecosystem.With the circumpolar current being active, this ensured and increased the upwell of nutrient rich water full of phytoplankton and zooplankton, which is one of the main food sources for the Baleen whale. The Baleen whale is a toothless whales, therefore
has a palate for phytoplankton and zooplankton, food that is small enough to be filtered
from water. To be able to feed in this way, Baleen whales need an enlarged
cavity that is adapted to suction feeding; this was evolved before specialisation
for bulk filter feeding or raptorial feeding. In the evidence it came be seen
that ancestral forms of mysticeti who had both regular and baleen teeth, which
shows that this has been a change in the evolution of the genes due to the
success of it.
Whales have come along way in their evolution to get to where they are
today. It can be assumed that tectonic processes paly a great role within the
way they have developed from fur to blubber, paws to webbed feet and
carnivorous teeth to filter feeders. It is hard to say that without the
tectonic event that occurred (Australia rifting away from Antarctica) would
have Balisosaurus
and Pakicetus move from a fresh semi-aquatic environment to a salt habitat or
the need to get away from the heat that was being generated from the Paleocene-
Eocene Maximum. It is hard to imagine however, what would we have instead of
whales? What mammal would have evolved to take the whales spot in the food
chain? Would it be possible to go, as far to say that tectonics are the only
reason we have these specific animals on Earth?
Rosie Hebden
References:
Gingerich, Philip D. 2003. "Land-to-Sea
Transition in Early Whales: Evolution of Eocene Archaeoceti (Cetacea) in
Relation to Skeletal Proportions and Locomotion of Living Semiaquatic
Mammals." Paleobiology 29 (3):429-454. doi: 10.2307/4096936.
UCMP (University of California Museum of
Paleontology). (2011). The Miocene Epoch.
Retrieved 27/09/2015 from http://www.ucmp.berkeley.edu/tertiary/eocene.php
PALEOMAP Project. (2003). The Cenozoic Era. Retrieved 28/09/2015 from http://www.scotese.com/newpage9.htm
Live Science. (2013) CEnozoic Era: Facts About Cliamte, Animals &
Plants. Retrieved 29/09/2015 from http://www.livescience.com/40352-cenozoic-era.html
UCMP (University of California Museum of
Paleontology). (2011). The Eocene Epoch.
Retrieved 30/09/2015 from http://www.ucmp.berkeley.edu/tertiary/eoc/eoctect.html
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