Marine life is very essential to the planet Earth. They help in retaining carbon and producing oxygen. Some marine species are also known to create new terrestrial grounds. Many hypotheses state that life began developing in the oceans. About 90% of the living space on the planet is provided by the oceans and seas. The first form of vertebrates appeared as a fish that was able to survive only in the water. Later, some species of fish evolved into amphibians which were able to survive both on land and water. However, simultaneously some species evolved into mammals such as dolphins and whales.
The word “marine†is derived from a Latin word that means ocean or sea. There are quite a few theories that suggest that marine animals evolved from terrestrial animals. However, scientific evidence has been found out about marine having entirely different ancestry from that of land animals. The evolution of classified based on the type of organisms as cetaceans that involved whales, porpoises, and dolphins, pinnipeds that included seals and walruses, and sirenians that involved manatees and dugongs. They evolved convergently and developed similar skeletal and body structures to adapt to the depth and salt conditions of the ocean.
The majority of the marine mammals belong to the group of cetaceans. They have the highest level of diversity based on ecosystem and taxon. They also occupy a wide range of geographic locations. This category of mammals has known to be evolved from land animals or the order of Artiodactyla that existed before 50 million years. The closest living animals to these are the hippopotamuses and ruminants such as sheep, cows, and deer. This is because similar to the ruminants, the cetaceans possessed a chambered stomach and gut. However, they also exhibited carnivore’s behavior by not chewing their prey.Almost all the whales are filter feeders which means they possess teeth in the form of filters to take up only the food that they want and eliminate the rest from the water. This filter-feeding system is named baleen and hence the name ‘baleen whales’. This is very useful in the way as the whales can efficiently gain enough energy resources to satisfy the large body size of the animal. It is known that filter-feeding might have resulted in the environmental and physical changes in the seas and oceans. Whales are also known to regulate the temperature and ocean currents. However, earlier baleen whales had very few baleens and more teeth.
Toothed Whales:
Kentriodonts are the first variety of dolphins that evolved in the Oligocene and mid-Miocene period. However, they later got diversified into numerous other species. The first fossil records were found in the oceans near North Pacific. It belonged to the Semirostrum species. Later, these animals spread across the coast of Europe and happened to possess teeth with no baleens. The presence of teeth adapted them to survive in a hotter condition as well. Hence, they were found to habituate the warmer waters of California.
Sirenians:
Sirenians were a group of marine mammals that were not meat-eaters. They were herbivores. They were known to have evolved in the same period as cetaceans. However, just like the cetaceans, they were also known to have evolved from the hoofed animals but they do not have any ancestors like the ruminants. Instead, they were more related to animals such as hyraxes and elephants. Sirenians have a heart with one pointed tip at the bottom unlike the hearts of other mammals. Elephants were known to have spherical hearts. Like the elephant’s tusk, these fishes have incisors and they also had fingernails like projections at the tip of the limbs.
Pinnipeds:
The youngest group of marine mammals is the pinnipeds. They originated during the Oligocene period. They were considered to have evolved from the primitive bear species and belong to the category of carnivores. They also had features associated with mustelids such as skunks, weasels, and otters. Just like the terrestrial carnivores, they had simple stomach structures with non-retractable claws on their flippers. These three groups shared certain common features as they evolved to diversify into different species. The moving tail, blubber of subcutaneous skin to remain warm, and loss of fur were some of the environmental adaptations that resulted in the gradual evolution of marine species.
