Its evident that the evolution to the present day whale has been a long one. A lot of changes have taken place from the postulated land dwelling animal to a fully marine organism. It is a step wise process that first started with a partially marine organism and then to a fully marine organism. The fully marine organism under went both physical and physiological changes to evolve to the present day whales. These changes included alteration of limbs to flippers, fusion of some of the skull bones and modification of the olfactory organs.
These organs have since developed more advanced functions with the increased demand for more metabolic rates. The theories explaining the linage of whales are very important and to some extent have shown significant similarities among the species thought to be related to whales. The ancestors of whale underwent lot of modification for example ability to hear under water, reduction of the limbs, and change of nostrils towards the posterior and greater chances to move under water. Archaeocete Gray Roqual Sperm Pigmy right Right(whales)
Beaked White Dolphins Porpoises Gange river River (Dolphins) Extinct whale ancestors Sinonyx The fossil study of sinonyx shows that it lived about 50 million years ago and fed on other animals (carnivorous). Sinonyx was about five feet long with a small brain, drawn out head, short legs and a large skull. It was found along sea shores and was thought to be related to whales and dolphins because of their long muzzle which almost resembled that of present day dolphins. The long muzzle was associated with fish hunting.
It is suggested that with time these features evolved to the present day whale specialized organ. However, later phylogenetic tests showed that sinonyx is not closely related to cetaceans. It had 44 teeth consisting of molars with many cusps and thin tearing teeth. Sinonyx lived along sea shores, smaller body, and long muzzle compared to the present day whales which live in the ocean, are very large and very huge. They had large sagittal crest with a large mass of muscles for strong grip of their prey.
The present day whales either have teeth or not. Those without teeth have a baleen having on their upper jaw mostly adapted for feeding Archaeocete (ancient whales) This stage is thought to be between sinonyx and whales. Its remnants were first excavated in Pakistan and are thought to have been living around 52 million years ago. They had better developed limbs which allowed them to live in both terrestrial and aquatic environments. As time passed they are thought to have started spending more time in the ocean and giving birth on land.
It was similar to sinonyx apart from its evolved premolars which were more triangular with a single serrated cusp and two rooted molars. With time they spread to many oceans and seas of the world. The Archaeocete lived in water but were not able to acquire deep dives like the present day whales. They later evolved to zygorhiza which was the first fully aquatic Archaeocete. Ambulocetus (the walking whale) It was similar to the present day crocodile with its limbs adapted for swimming more than walking which were flippered.
They swimmed predominantly and the fossils show that they lived about 49 million years ago and survived on both salty and fresh waters. It had teeth similar to those of Archaeocete and an elongated muzzle. Though it lacked a blow nose it was able to swim well under water. The limbs of ambulocetus were changing allowing them to be more adapted for swimming and less adapted for walking. Their cervical vertebrae of the ambulocetus were relatively long than those of the present day whales. Ambulocetes did not have ears and received information by use of vibrations on both land and sea.
They usually lay on the ground to detect the vibrations of their prey. The muzzle of this organism has changed very much in comparison to the present whale in which there is formation of baleen plate. The throat of a whale grooves and expands to hold a lot of water that contain krill which is fed on by the whale. The baleen plate pushes the water out of the mouth remaining with only the food. Baleen therefore acts as a sieve separating the krill from the water. Basilosaurus (king lizard) It is estimated to have lived between 34 and 40 million years ago with a length of around 60 feet.
It looked like a whale snake and budged horizontally or vertically. They had small hind limbs that were not able to effectively propel basilosaurus when moving. It had a single large nostril that was used for breathing effectively especially when in water. With time these organisms were unable to move on terrestrial environment. There is difference of the position of the nostril in which the present whale nostril is more to the posterior position for blowing off air when breathing at the water surface.
These changes have made the present day whale to be more adapted especially in breathing while in aquatic environment. Pakicetids These organisms lived at around 53 million years ago and looked like dogs or wolves with hoofed feet. Their skull and teeth resembled the of the present day whales. The saggital crest was high and narrow with the lambdoidal crest being predominant. Pakicetids resemble the mesonychid hunting both on land and along the shores. They had small ears that were adapted for hearing and are later adapted for hearing under water in the present day whales.
The middle ear of pakicetids was not well developed thus unable to maintain its pressure that limited this organism to dive in deep waters like the present day whales. The preset whale is more adapted to survive in water for instance it is able to dive deeply and swim swiftly since the limbs have been adapted to the aquatic environment. The ears of the ancestor whales have gradually disappeared. The ears are adapted for hearing in the deep seas by receiving vibration signals from other whales like during courtship or tracking their prey.
Courtney from Study Moose
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