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Summary: This article’s main focus is on the phylogenetic relationship based on mitochondrial genomes. The results are based on an explosive radiation, dividing cetaceans into two different groups. Throughout this article, a series of tests are run to support this hypothesis. Findings suggest that the comparison of the groups are a result of convergence and not common ancestry.

Summary: Cetaceans originated from land animals. The characteristics of hair or fur show this relationship to land mammals, with many cetaceans having whiskers. Cetaceans also come to the surface for air, which shows signs of their mammalian history. Throughout this article, the lineage of cetaceans is described in much detail. In conclusion, the question of why these animals entered the ocean is still observed.

Summary: In this article by Thewissen, the main focus is the morphological evolution of the limbs of the cetaceans. The adaptation of these animals made it no longer possible for them to walk on land thus sending them to the ocean. Fish’s study in this article goes into depth of the different locomotor stages of the skeleton and origin of the cetaceans.

Summary: In this article, Spoor explains the evidence of change in behavior through morphologies of cetaceans swimming. Spoor states that these changes must have corresponded with otter-like swimming with different terrestrial capability. It is unclear whether these were obtained from the ancestor or if they became newly acquired roles as cetaceans evolved. This article’s main focus is the study of the cetaceans canal and how this may have evolved and led to different morphological changes.

Summary: One of the hardest questions for marine mammalogists is why the marine animals went back to sea. This article goes into detail on certain groups of mammals evolving to the sea, one of them being the cetaceans. Cetaceans were believed to have walked around in shallow waters in their time on land. Their limbs eventually evolved to functioning for aquatic living. Different morphological traits led to this evolution.

Wikipedia Assignment II (10/1/14)

Article sentence : Pakicetus had a pelvic bone most similar to that of terrestrial mammals. As the pelvic bone changed throughout species, Basilosaurids had a pelvic bone that was no longer attached to the vertebrae and the ilium was reduced.

https://en.wikipedia.org/wiki/Evolution_of_cetaceans

Suggestions: https://en.wikipedia.org/wiki/Talk:Evolution_of_cetaceans
 * I agree with the thought on adding more detail on the loss of hind limbs, considering it is one important transitions from land to water. Another idea, I have added a few points to the pelvic bone section of the skeletal evolution. I think this section could go way more in depth. One last idea is maybe a section on why some think the transition from land to water occurred

Iwais.1 (talk) 21:17, 1 October 2014

Edits 11/17/14

https://en.wikipedia.org/wiki/Evolution_of_cetaceans Pakicetus had a pelvic bone most similar to that of terrestrial mammals. As the pelvic bone changed throughout species, Basilosaurids had a pelvic bone that was no longer attached to the vertebrae and the ilium was reduced.

Modern cetaceans have rudimentary hind limbs, such as reduced femurs, fibulas, and tibias, and a pelvic girdle, consisting of an ilium, ischium, and pubis bone. Cetacean hind limbs and bones of the pelvic girdle can be compared to terrestrial mammals. The Indohyus have a thickened ectotympanic internal lip of the ear bone. This feature compares directly to that of the cetacean. The most striking similar feature was that of the composition of the teeth of the Indohyus. The composition of the teeth contained mostly calcium phosphate, which is needed for eating and drinking of aquatic animals. The next evolutionary change was to the first actual cetacean known as Pakicetids. Although they somewhat resembled a wolf on the outside, the skeleton showed eye sockets were much closer to the tops of their heads than normal. This compared to the structure of the eyes in cetaceans. The Pakicetids made the most dramatic change, going from land to water. This lead to rebuilding of the skull and food processing equipment because the eating habits were changing. Ultimately, the change in position of the eyes and limb bones is what lead the Pakicetids to become waders. The next evolutionary changed occurred to that of the first marine cetacean known as the Ambulocetidae. The ambulocetidaes also began to develop long snouts, which we see in current cetaceans. Limbs were compared closely to otters because of the swimming that occurred with their hind legs. The evolution of the skeleton continues to change until the modern day cetaceans known as odontocetes and the mysitcetes. Over the course of evolution, the cetacean skeletal structure went through many alterations that now make it very distinguishable from terrestrial mammals.

FINAL DRAFT SARTS HERE'''

Cetaceans are a group of aquatic mammals that include dolphins, whales and porpoises. Even though these mammals are strictly aquatic and cannot function out of water, they have a line of ancestors that were terrestrial mammals. Cetaceans looked so different from all terrestrial mammals that it was very difficult for their relationship to be traced back to an early ancestor. With the fact that it was very difficult for cetaceans to be given a relationship to that of terrestrial mammals, it was believed that the evolution of the cetaceans was a fast morphological adaptation. After further analyses in the 1950s and 60s, the cetaceans were placed within the order of Artiodactyla, which is an even-toed hoofed mammal (Arnason et. Al 2004). The question of “why” cetaceans went back to water is always a complicated question and is not given a clear answer. Although the “why” aspect cannot be clearly answered, by looking at the evolution of the cetaceans from their early ancestors, the adaptive changes they have made and the most modern cetaceans can give us a helpful hint as to “why” (Uhen, 2007). Earliest ancestors of the cetacean taxa can relate back to one of the families of artiodactyls. The skulls and skeletons of these families go one to be known as Indohyus. Many of the fossils of the Indohyus were found in and around freshwater streams. Considering these findings, the phylogeny among the fossils allowed for placement of cetaceans with similar traits into a cladogram with these taxa. Using this cladogram, strong features of the Indohyus were assessed to similarities of the cetaceans. Unlike any other terrestrial mammal, the Indohyus have a thickened ectotympanic internal lip of the ear bone. (Spoor, 2002). This feature compares directly to that of the cetacean. The most striking similar feature was that of the composition of the teeth of the Indohyus. The composition of the teeth contained mostly calcium phosphate, which is needed for eating and drinking of aquatic animals. With these findings, the Indohyus were suggested to have been aquatic and that the cetaceans evolved from them (Thewiseen et. Al, 2009). From the artiodactyls, the next evolutionary change was to the first actual cetacean known as Pakicetids. Although they somewhat resembled a wolf on the outside, the skeleton showed eye sockets were much closer to the tops of their heads than normal. This compared to the structure of the eyes in cetaceans, which could group these taxa together. Another feature that compared the Pakicetids to the Indohyus was that of the thickened lip on the tympanic bone of the ear. The fact that these two traits compared shows that the Pakicetids were closely related to the aquatic artiodactyls. Consequently, the most significant evolutionary change from artiodactyls to the Pakicetids was that aquatic adaptation of wading and walking in the water. Considering the Pakicetids were moving into the water, this lead to rebuilding of the skull and food processing equipment because the eating habits were changing. Ultimately, the change in position of the eyes and limb bones is what lead the Pakicetids to become waders (Thewiseen et. Al, 2009). From the pakicetids, the next evolutionary changed occurred to that of the first marine cetacean known as the Ambulocetidae. These taxa are much larger than the pakicetid and evolved with short legs with five fingers and four toes. The composition of the skeleton suggested that they were not fast moving predators and did not pursue their prey over large areas. It is shown that the adaptation of aquatic animals is becoming more prevalent in the ambulocetidae. Similar to the pakicetids, the eye sockets are located more towards the top of their heads, but for ambulocetidaes the eye sockets began to move more towards the side. The ambulocetidaes also began to develop long snouts, which we see in current cetaceans. Limbs were compared closely to otters because of the swimming that occurred with their hind legs. The fossils of ambulocetidaes were found near shallow seas and swamps giving them the aquatic characteristics (Thewiseen et. Al, 2009). The taxa following the ambulocetidaes are known as the remingtonocetidae. This taxa is characterized by their long snout that takes up about two thirds of their skull. This feature of the long snout was similar to that of the snout in the ambulocetidaes. Along with the longer snout, evolutionary changes began to take place so the remingtonocetidaes were able to adapt to a more aquatic life. The teeth of the remingtonocetidaes began to change, most likely meaning that their diets have changed from that of the terrestrial mammal diet. One of the most important changes were that the ears of the remingtonocetidaes became larger and more set apart. This in turn caused the jaw and ear to connect through a pad of fat, similar to that of modern cetaceans. The set up of their ears shows that there must be underwater sound transmission happening, which is a clear aquatic trait. A hypothesis made by Frank Fish proposed that cetaceans used only their tails for locomotion and limbs for steering, which is consistent with the skeletal composition of remingtonocetidaes. They have very short legs with powerful tails, making them that more closely related to modern day cetaceans (Thewiseen, 1997). The first cetaceans that actually conquered the ocean were the taxa protocetids. This taxa allowed the cetaceans to spread out across the globe. This taxa is a diverse group with variety in snout and ear morphology. The eyes are usually very large and face laterally and therefore are set far apart from the midline. The nasal bones become located further away from the top of the snout. With this retraction of the bone, it eventually allows for the blowhole of the whale to evolve. This also affects how the whale is able to breathe while still being under water. The most similar feature of the protocetids to previous taxa was the limbs. The limbs of the protocetids were closely related to the ambulocetidaes, which had five fingers and four toes that allowed for locomotion on land. This taxa show the first cetaceans across the world (Thewiseen et. Al, 2009). The first fully aquatic cetaceans are the modern Basilosaurids. This taxa represents the modern cetaceans the closest of any of the previous taxa. They have nasal openings that are shifted back to create that blowhole, have flippers, a fluke at the tail end and tiny hind limbs. The Basilosaurids are more closely related to the cetaceans that protocetids and are most likely the first cetaceans because they did not come out of the water. All characteristics are similar to that of the most modern day cetaceans, which is why they are known as the first fully aquatic cetaceans (Thewiseen et. Al, 2009). Bringing all of these taxa together to finally have the modern day cetaceans create the odontocetes and the mysitcetes. These two groups are the first representatives of modern cetaceans who are known to have rose from the same ancestor making them monophyletic. Although these two groups have different evolutionary themes, they make up for all cetaceans around the globe. Odontocetes are known for being able to see and hear reflected objects around them to protect themselves. The mysitcetes have an innovative feeding tactic that allows them to feed for bulky prey using the evolutionary adaptive teeth. These two modern cetaceans conquer all type of freshwater forms and show the evolutionary history of the cetaceans (Thewiseen et. Al, 2009). Knowing the evolutionary change of the early ancestors of the cetaceans, the artiodactyls, to the most modern day cetaceans, the Basilosaurids, allows us to study the adaptive changes that occurred throughout. One of the biggest differences through the evolutionary change is the skeletal evolution, which according to a study Shixia Xu may have been highly due to osmoregulation genes bringing the aquatic adaptations. (Xu et. Al 2013). The biggest structural changes in the skeleton of the cetacean were the nasal bone, the teeth, the number of feet and fingers and the hind limbs (Read, 200). The nasal bones were eventually set to the back to create a blowhole, flippers evolved along with a fluke at the tail end and tiny hind limbs. These skeletal evolutionary changes would eventually lead to show how the terrestrial origins came about. These terrestrial origins include the need to breathe air from the surface, bones of their fins and vertical movement of spines. These origins came from the early ancestors of the cetaceans and allow for the modern cetaceans we see today (Culik, 2004). Although cetaceans are strictly aquatic animals, their trace back to terrestrial mammals is clear throughout their evolutionary changes. Tracing back to their earliest ancestor, being the artiodactyls and finding fossils near freshwater streams allows us to believe the earliest ancestors were aquatic mammals. Next, the Pakicetids brought wading through the water to the table, also giving weight that these ancestors were closely related to aquatic mammals. The Ambulocetidae then begin to bring phylogenetic traits that are similar to the modern cetaceans, such as further posterior and lateral position of the eye sockets as well as long snouts. The first cetacean that conquered the sea, known as the Protocetid, evolved a longer and even more prominent snout that we also see in modern cetaceans. The first fully aquatic cetaceans, the Basilosaurids, finally evolved to have the most similar features of that of a modern cetacean by acquiring nasal openings that are shifted back to create that blowhole, have flippers, a fluke at the tail end and tiny hind limbs. These evolutionary changes allowed for many adaptations of the terrestrial origins to create what we know as the modern day cetaceans. This evolutionary tracing ties in with many learning goals during our course, such as the linking of species back to their ancestors. It shows how a genera or taxa can evolve from a common ancestor and then eventually become something we all are familiar with today. It is important to know the lineage of all species so that we can understand how and why species are what they are today. Although the evolutionary track does not give us a clear cut answer as to “why” these once semi aquatic mammals moved back to the water, we now see how their traits over time played a part into pushing them back to being aquatic mammals.