In the late Eocene, 38-45 million years ago, basilosaurids (Basilosauridae Family) appeared and lived, fully aquatic archaeocytes whose limbs were more elongated resembling the flippers. The family of the Archaeoceti (Family Archaeoceti) to which the Pakicetus belonged, evolved from the Mesonickids and lasted until the Eocene-Oligocene, becoming increasingly aquatic members.
They still had paws and depended on the land to raise their offspring, but they were looking for their food in the ocean. Thus, the family pakicetidae was a transition link between the exclusively terrestrial mammals and the aquatic as they started to spend more and more time in the water.Īnother theory suggests that modern cetaceans have their origin in mesonychids (Mesonychid family), a group of carnivorous ungulate mammals with teeth similar to whales and also related to artiodactyls that lived 55 million years ago and began to inhabit shallow water regions due to shortages of food supplies on land and climate changes.Īfter that, the following 15 million years the archaic cetaceans continued developing evolutionary modifications to adapt to life at sea. We know now that the bone structure of the flippers of dolphins is similar to that of the extremities of mammals, with five phalanges and joints.
They had four legs and hooves such as camels and horses. The pakicetidae family, these first cetaceans, appeared about 50 million years ago during the Eocene. The bones were of an elementally terrestrial mammal believed to have been an archaic cetacean because although it could remain on land, it also ventured into shallow waters. In 1983 the fossil of an animal that the scientists named Pakicetus was discovered in Pakistan. Scientists believe that the oldest ancestors of dolphins were land animals and not aquatic. They were hoofed mammals looking for their food walking near bodies of water. The most direct link of the first cetaceans in the line of the artiodactyls is with the extinct taxonomic family Raoellidae, whose members lived during the early and middle Eocene in China, Pakistan, India, and Mongolia. These limbs aren’t on the outside, but they are vestigial legs. The body of actual dolphins does show two small pelvic bones that are rod shaped. Those changes over time helped them survive in the water. The movement of the ears closer to the eyes is also part of the evolution process. The nostrils later evolved into blowholes, so they spend less energy when reaching the surface of the water to breathe. Instead, they continued to be carnivorous in the water as they would have been on land. What is interesting though is that unlike many other mammals, they didn’t change to being herbivores. It extends to the middle ear, and this helps dolphins to be able to hear as well as to use the process of echolocation. It has become larger which is an indication that the development of the fat pad in the lower jaw emerged. The size and the shape of the skull have changed over time too. The process of evolution can be very slow, and it allows living creatures to develop an anatomy that is structured to thrive in their environment. When you look at the movements of fish, their spines move horizontally. Dolphins have spines that move vertically, and this is a characteristic mainly found in animals that can run on land. Their bones strongly resemble the limbs of many animals that live on land.
They need to come to the surface of the water to get air is one factor. Many signs prove that dolphins have terrestrial origins. Artiodactyls have two fused fingers, and the tip of their legs are called hooves. The fossil records indicate that the origin of all members of the modern order Cetacea is the order Artiodactyla, a group whose members are terrestrial mammals being their closest living relatives those belonging to the family Hippopotamidae (Hippopotamuses). They were terrestrial animals that today seem to have little in common with dolphins. It is hard to believe, but scientists think that the earliest ancestors of dolphins were not marine creatures. The initial presence of dolphins on the Earth dates back millions of years.
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