Evolution of whales

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Cetaceans include porpoises, whales and dolphins, and they are an order of mammals that originated in the Eocene epoch about 50 million years ago. Their predecessors were mostly terrestrial artiodactyls and they looked like small deer. The early cetaceans were amphibians, while the modern ones are obligate aquatic mammals. Hippos and whales share similar terrestrial ancestors even though they evolved over different periods of history. Protocetidea were among the first set of whales over 50 million years ago, and they were typically land animals. Hippos evolved about 15 million years ago from a group called anthracotheres. However, it was recorded that dolphins are mammals that evolved from land and went to the sea about fifty million years ago. All the Cetaceans are of large size with the largest member being the blue whale weighting between 80,000 to 150,000 kilograms and over 24 meters. The smallest member weighs less than 40kgs and 1.4m in length, this cetacean is known as Vaquita.There are interesting facts about whales, one is that they hold the record for the largest animal with the largest brain. This record is held by the blue whale which weighs 7kgs.Cetaceans also enjoy the largest longitudinal range and they are distributed all over the surface of the earth in all oceans, there is no any other mammal that occupies such a huge space in the world. The sperm whales can also dive more than one kilometer below the sea. Scientists hold the view that Dolphins evolved from Mesonyx which were land living mammals before moving to the sea. Animals that are included in cetaceans include the parvoder of Odontocetes which are toothed whales. In this category we also have beaked whales, sperm whales, porpoises, dolphins and killer whales.

Thewissen & Williams 2002 The Early Radiations of Cetacea (Mammalia): Evolutionary Pattern and DevelopmentalCorrelations pp. 73-90

Lande, Mechanisms of limb loss in tetrapod’s, Harvard University (1977)

Their conical teeth enable them to clasp on squid or fish, there are a few killer whales that feed on mammals which are known as pinnipeds. Cetaceans are further categorized into marine cetaceans which include the porpoises, whales and dolphins, all these three are also known as placental marine mammals1. Modern members of these marine cetaceans live in Open Ocean since they are fully aquatic but there is an exception of few species of dolphins that inhabit estuaries and rivers. There is a school of thought that believe whales originated from wolves. This is surprising to many considering the marine mammals live in the ocean while wolves are land mammals. There are however many similarities between walruses, whales and manatees but the truth of the matter is whales are more related to elephants and cows more than they are to each other.

Known ancestry of Cetaceans.

Development stages of Cetaceans from land to sea were documented in Pakistan from 1970s which revealed various stages of evolution2. This evolution is believed to have occurred over 50 million years ago in the Indian subcontinent. Cetaceans are fully aquatic marine mammals which belong to the order Artiodactyl and they are believed to have evolved from Eocene where they are believed to share an ancestor with hippopotamuses. Despite being fully aquatic cetaceans behave more like land mammals. They nurse their young ones and often surface to the land to breathe and go back to the ocean. In the second cetacean radiation which is believed to have occurred between 28-33 million years ago the two modern parvorders of cetaceans are believed to have separated1. There are two different types of cetaceans that is Odontoceti also known as toothed whales and the Mysticeti the baleen whales. They are believed to have separated from the acheaocetes during this period. The earlier baleen whales had little of baleen and it’s believed that it increased gradually with size. This change in baleen in these whales was attributed to the baleen dependence and the increase in filter feeding.

Thewissen & Williams 2002 The Early Radiations of Cetacea (Mammalia): Evolutionary Pattern and DevelopmentalCorrelations pp. 73-90

There has been simple models of evolution advanced by several authors in understanding the ancestry of cetacean. There are scholars who have indicated that the taxonomic sampling can affect the phylogenetic accuracy especially where the phylogeny examined includes long terminal branches. In order to improve the phylogenetic accuracy it is important to have taxa that bisect long branches.

Different groupings of cetacean’s phylogenetic tree.

Cetacean Phylogenetic Tree

Pakicetidae occupy the basal branch on the cetacean tree and are the earliest form of cetaceans known. Pakicetidae are in three genera, Nalacetus, Ichtholestes and Pakicetus. Ochthyolestes were the first set of pakicetid and were referred to as fish eaters and not as cetacean. This was followed by discoveries of a more genus gene of pakicetus which was named pakicetus. Most of the late to early Eocene deposits of pakicetids are found in northern Pakistan where they have been discovered as deposits in river channels. The second stage is Ambulocetidae and have also been discovered in northern Pakistan and northwestern India and are the most basal amphibious marine cetaceans. It is estimated that they lived in estuaries and bays as most of the deposits are found on the bays with abundant marine plant. Ambulocetids are believed to be as large as a male lion and were amphibious mammals. They could easily move from land to water though their swimming is relatively inefficient.

Remingtonocetids were well adapted to swimming and were amphibious whales with long narrow snout, with long sinuous body. Scientists understanding of remingtonocetids is through genus Kutchicetus which has a muscular and long tail and back, they have no hands or feet but this is compensated by long stocky bones.

Protocetidae were marine cetaceans with a lifestyle close to that of modern seals. They represent a wide range of aquatic adaptations in which some are able to support weight on land while others are unable to support. Protocetids are the most basal cetaceans and among the earliest to leave the South Asian subcontinent and disperse across the globe.

Dorudontidae resembled dolphins in both appearance and body shape and they were mostly found in northern Africa as well as North America. Dorudontids and basilosaurids were the first discovered Eocene cetaceans they had powerful verbal column and short fore limbs.

Basilosauridae are commonly found in association with dorudontids and are the latest Eocene cetaceans to be found. The have snake like bodies which is associated with extreme elongation of their lumbar vertebrae.

Lande 1977 Mechanisms of limb loss in tetrapod’s, Harvard University

Cetacean phylogeny

The body plan of the present day cetaceans is believed to have originated within the Eocene-Oligocene boundary. Before the body plan, the mammals had an estimated 15 million years of which of the body systems could be adapted to the sea life. Eocene cetaceans are made up of six families: pakicetids, ambulocetids, and dorudontids. Protocetids, basilosaurids and remingtonocdtids. The modern baleen whales do not have teeth and in its place they have baleen, the use of baleen differs among different species. There are some that use it for skim-feeding while others use it for bottom plowing. There is also evidence of some genetic whales that were toothless and through multiple mutations have been identified to have genes related to production of enamel. Odontoceti are also referred as toothed whales are believed to have split apart from the baleen whales which is believed to have happened in over 34 million years ago. In order to hunt prey the modern toothed whales do not use sense of sight rather they use their sonar. The toothed whales make series of clicks emitted at various frequencies and it is through these sounds that are reflected off the objects and later retrieved through the lower jaw that assist in hunting prey. Modern cetaceans have rudimentary internal hind limbs such as tenias and fibulas as well as pelvic girdle. Modern cetaceans have a similar composition of teeth which is composed of calcium phosphate which is required by aquatic animals in eating and drinking. Most the skeletons of the modern cetaceans resemble a wolf but the difference is on eye sockets which are much closer to the head than most of the terrestrial mammals.

Subgroup limb usage

During the normal limb development the hind limb muscle is derived from myoblasts that invade the limb buds from adjacent somites. With time there has been major morphological shift in whale evolution. The modern cetaceans have consistently reduced the hind limb skeleton embedded in the ventral abdominal wall. Whale’s vestigial hip bones have always been an area of debate among scientists, they often ask what the use of these traits is while they don’t walk. The argument is that not unless they were remains of their ancestors who were land walking mammals. Recent research by 1has indicated though the whales do not walk they use the hind limbs in a special manner. According to 1hind limbs aid the cetaceans in reproduction and not for limb articulation as earlier thought. A further research by 1 indicated that in order to reproduce, the cetaceans require relatively large pelvic bones to serve as anchors for the males. Scientists are yet to observe their mating period but there is speculation that considering there is a great distance between their reproductive organs and eyes, their males need extra measure of control which is only achieved through bigger pelvic bones.

Stem odontoceti and crown odontoceti

Stem Odontoceti, Mysteceti and Stem Cetacean are cetaceans whose evolution has changed the development and role of their limbs. Initially, the hind limb was used to maneuver on the terrestrial environment. However, with respect to the change in the environment, cetaceans also evolved with an objective of surviving in the new environment. As a result, the hind limbs shortened overtime, a process reflected in various fossils from mammals that lived during different times. Eventually, the hind limbs were ‘gone and the mammals developed flippers which aid them in their new environment.

Stem Odontoceti are toothed whales porpoises and dolphins. These animals are more diverse and disparate than the baleen whales. They are generally smaller in size and most of them have erupted teeth. Where teeth is not available they have vestigial teeth, the living stem odontoceti are predators and usually swallow their prey in whole. Their teeth is not for chewing their prey but rather for capturing their prey and sometimes for sexual pleasure. The earliest named stem odontoceti are Simocetus.

Mysticeti.

Mysticeti is in sub order of the Cetacean and are often called the baleen whales or the great whales. The baleen whales and Odontoceti have distinct differences in feeding. The toothed whales tend to eat larger prey while the baleen whales strain plankton out of water .Mysticeti are in Animalia kingdom, class of Mammalia and in the order of Cetacea, so their sub order is what is known as Mysticeti.

Stem Cetacea

Whales have evolved over time with new features witnessed by scientists in baleen whales where they are able to completely jump out of water. The humpback whales for instances are able to jump out of the water with their bodies. No reason has been documented yet why they do this but it is speculated they communicate through jumping. There is similarity in the developmental pattern of snakes and evolutionary pattern of cetaceans. It has been suspected that similar genes control development of these animals as they both develop vertebral column morphology and hind limb at some level.

Vestigial pelvic girdle

The vestigial pelvic girdle in whales has been discovered to help in control of motion in the ocean as well as sexual orientation. The body size and the shape also has influence on their sexual selection.

Jonathan & Sanders 2003 Morphological Evidence for the Phylogeny of Cetacean

Thewissen & Williams 2002 The Early Radiations of Cetacea (Mammalia): Evolutionary Pattern and DevelopmentalCorrelations pp. 73-90

Since whales and dolphins seized to be land dwelling animals over 40 million years ago the hind limbs have slowly evolved into highly reduced pelvic girdle.

The main purpose of this pelvic girdle is to anchor muscles used for maneuvering the penis during mating. It was assumed that the pelvic girdle would disappear with time but this has never been so. Male genitalia are known to evolve rapidly over time and this is a result of sexual orientation. The cetacean penis has been known to have a high degree of mobility and the penis is directly attached to the curved pelvic bones.

Conclusion.

There has been historical correlation between hind limb reduction and homogenization of the vertebral column. Patterns of correlation evolutionary history can be used to identify various development correlations on cetaceans and fossils have shown intriguing patterns. The transition from terrestrial ancestry to a fully pelagic life clearly altered the cetaceans body systems with morphological changes witnessed in their feeding apparatus, skull and hind limbs. The Oligocene Epoch is definitely a crucial time in the evolution of cetaceans where the ancestors of modern dolphins and whales underwent major diversification.

Work Cited

Dines, J.P. et al. Sexual selection targets cetacean pelvic bones. Evolution. Published online before print, September 3, 2014.

Jonathan & Sanders 2003 Morphological Evidence for the Phylogeny of Cetacean

Lande 1977 Mechanisms of limb loss in tetrapod’s, Harvard University

Perkins, R. Whale Sex: It’s All in the Hips. USC News Release.Posted on pressroom.usc.edu September 8, 2014, accessed September 30, 2014. (Perkins, R,2014)

Ridley, M. 2004. Evolution. Malden, MA: Blackwell, 60.

Thewissen & Williams 2002 The Early Radiations of Cetacea (Mammalia): Evolutionary Pattern and DevelopmentalCorrelations pp. 73-90

Thewissen et al 2005 Developmental basis for hind-limb loss in dolphins and origin of the cetacean body plan.

Thomas, B. 2012. Ways That Whales Display Their Creator. Acts & Facts. 41 (8): 18-19.

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