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We know today that when we look out into the waters of the world Cetaceans come in all shapes and sizes and inhabit many different environments from the fresh waters of the Amazon River to the deep submarine canyons throughout the worlds largest oceans.
As you have already learnt whales are mammals grouped in the Order Cetacea (se-tay-sha) along with Dolphins and Porpoises. As we know the first Mammals all lived on the land. How or why the ancestors of whales returned to the sea is still unclear however as research continues we are gaining a greater understanding of this process. The fossil record of whales is increasing and recent discoveries have shed much light on the origin of the Cetacea.
About 55 million years ago the world looked very different from what we see today. The landmass of Gondwana had been breaking up for the past 80 million years and separate continents were moving towards where they sit today. It was about this time that the landmass we now know as India collided with Asia creating the great Himalayas. It was also about this time that the oceans became more shallow and saline fuelling an explosion of sea life.
Most scientists agree that whales have the same ancestors as even toed ungulates (hoofed animals) such as cows and deer. This ancestor is thought to be a mammal from a group called the Creodonts. Some sub groups of the Creodonts were herbivores and some carnivores. One of these subgroups the Mesonychids began hunting close to the waters edge feeding on the dead and dying fish washing up in the shallows. Eventually these creatures started entering the shallow waters and surf to catch their prey. Within a few million years they had developed into semi aquatic creatures and the first primitive whales.
Over the next 10 million years whales where to evolve from these medium sized semi-aquatic land mammals to the giants of the sea we see today. This process involved the evolution of various new species each evolving slight adaptations to suit their new watery environment.
It is known through fossils that early whales such as Rodhocetus and Pakicetus who inhabited the shallow seas around 50 million years ago still had well developed limbs.
Illustration by Carl Buell, and taken from
Ambulocetus evolved large hind feet which and a very strong tail, both would have aided them in swimming.
Illustration by Carl Buell and taken from
Remingtonocetids are fossil whales that lived in South Asia, approximately from 49-43 million years ago. Their heads were very different from other whales. Their eyes were small, their snout long.
Protecetids inhabited the waters from approximately 48-25 million years ago and were the first whales to explore the world's oceans and move long distances. It is also thought that this group of animals had evolved tail flukes to help them with propulsion.
Basilosaurids and Dorudontids lived approximately 41-31 million years ago. These creatures some what resembled the whales we see today. They were massive in size and had a streamlined body. Whilst these species had maintained a strong and effective forelimb, their hind limbs had almost disappeared.
IN JUST OVER 10 MY THE EARLY WHALES HAD LEFT THE LAND FOREVER
These primitive whales had teeth designed to grasp and shear, eventually these modified to become long rows of large uniform teeth as we see in today's toothed whales (Odontocetes). Around 25 million years ago the baleen whales (Mysticetes) evolved from a toothed ancestor, even today the foetuses of some baleen whales have some remnants of teeth.
For a land mammal to have evolved into the creatures we see today the evolution had to be chemical as well as physical, livers became specialised to deal with excess salt, fluids covered the eyes to protect them from the saline environment and body hair was lost, no other mammal has a body covering so sleek.
These small adjustments put together over 10 million years transformed medium sized land mammals into the huge sea creatures we see today.