The order Cetacea is comprised of aquatic mammals ranging from the playful dolphins to the massive blue whale. They have stream-like bodies with compacted neck vertebrae, tail with flukes set horizontally and dorsal fins. Their nostrils are placed at the top of the head forming their distinct blowhole. The forelimbs formed the flippers and the hind limbs, including the pelvis, are so small and do not extend outside the body. The order is separated into two subgroups, namely, the Odontoceti, or the toothed whales, and the Mysticeti, or the baleen whales.
Odontocetes have teeth and uses sound waves for echolocation. Examples are dolphins, sperm whales and killer whales. On the other hand, baleen whales lack teeth and use a baleen, or a hornlike substance that creates filaments, to filter plankton from the water. Examples are blue whales and humpback whales (Waggoner, 2002). The strangeness of these creatures from other mammals fueled the drive for researchers, particularly paleontologists, to explore their ancestry and understand how they evolved in nature.
Links with other mammals were studied using DNA and it was found that they are closest to the ungulates (hoofed mammals), particularly the hippopotamus. Morphological similarities of excavated fossils and present cetaceans were also studied to determine the link between and finally determine the ancestor of cetaceans (BBC, 2001). The first cetacean was the Pakicetus, a wolf-like creature that is assumed to have lived near the water 52 million years ago. It has a skeletal similarity with the present whales and showed early development of hearing underwater.
It was followed by the Ambulocetus, a crocodile-looking creature with hind legs adapted to swimming. It lived 50 million years ago and showed the early form of the specialised ears of whales. Next was the Rhodocetus, an aquatic creature with developed flukes used for faster swimming. However, it still has hind legs depicting that it lived in shallow waters 45 million years ago. The last is the Basilosaurus, an 18-meter long creature that looked nearly like the present whale and lived 38 million years ago.
However, it lacked the melon organ, the organ used for echolocation, and has small brains depicting it is not a social creature unlike the present whales (BBC, 2001). The morphological changes that occurred and are of significance to the evolution of whales include the development of the ear, the disappearance of the hind legs and the positioning of the blowhole. The hearing mechanism of the cetaceans developed to allow underwater hearing. It was found out that they have smaller semicircular canals enabling them to be acrobatic and fast swimmers.
This change happened in a short period of time making them quickly independent of terrestrial life (Braun, 2001). The hind legs’ disappearance was an adaptation to allow faster swimming. Through fossils, it was found out that the hind legs eventually became underdeveloped since they are not anymore used. The nostril of the cetaceans developed into a blowhole by repositioning it at the back of the head. This change is not yet scientifically understood and explained.
However, by observing the embryonic development of a cetacean, it could be seen that the nostril is initially formed at the front and eventually moves to the top of the head. This could illustrate how evolution developed the blowhole (PvM, 2008). Through these changes and evidences, we can clearly observe and understand the development of the cetaceans; from being terrestrial creatures to becoming totally independent sea creatures. This is a good illustration of how evolution shaped the creatures of today and made them successful creatures.
Courtney from Study Moose
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