, Researchers in a 2014 study in Shark Bay found the fatty acid analyses between the West and East Gulf populations to differ, which is due to the two areas having different food sources. As for dorudontines, there are some species within the family that do not have elongated vertebral bodies, which might be the immediate ancestors of Odontoceti and Mysticeti.  According to stable oxygen isotopes analysis, most remingtonocetids did not ingest fresh water, and had hence lost their dependency on fresh water relatively soon after their origin.  This is an intermediate stage in the evolution of cetacean locomotion, as modern cetaceans swim by caudal oscillation (a way of swimming similar to caudal undulation, but is more energy efficient). Generally it is speculated the four modern mysticete families have separate origins among the cetotheres. They too had a fluke and, unlike basilosaurids, they probably swam similarly to modern cetaceans, by using caudal oscillations. Mesonychids also became specialized carnivores, but this was likely a disadvantage because large prey was uncommon. , Basilosaurids and dorudontines lived together in the late Eocene around 41 to 33.9 million years ago, and are the oldest known obligate aquatic cetaceans. Hence pakicetids were most likely aquatic waders. around Shark Bay of Western Australia can be divided into spongers and nonspongers.  Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed a range of water with different degrees of salinity, with some specimens having no evidence of sea water consumption and others that did not ingest fresh water at the time when their teeth were fossilized. The pelvic bones associated with these hind limbs were not connected to the vertebral column as they were in protocetids. The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and littoral mollusks. The first members of both groups appeared during the middle Miocene. It is believed to be a slow process that allows survival of the fittest to occur. Skulls of Squalodon show evidence for the first hypothesized appearance of echolocation. The shape of the ear region in pakicetids is highly unusual and the skull is cetacean-like, although a blowhole is still absent at this stage. In the Eocene, ambulocetids inhabited the bays and estuaries of the Tethys Sea in northern Pakistan. Since dolphins do not chew their food, the mastication of their meal is taken care of in their first or fore stomach. , Remingtonocetids lived in the Middle-Eocene in South Asia, about 49 to 43 million years ago. Their fossils were first discovered in North Pakistan in 1979, located at a river not far from the shores of the former Tethys Sea.  Based on their skeletal remains, remingtonocetids were probably amphibious cetaceans that were well adapted to swimming, and likely to swim by caudal undulation only.  Also, some bottlenose dolphins in Moreton Bay, Australia followed prawn trawlers to feed on their debris, while other dolphins in the same population did not. The argument against this though is that the hippo wasn’t found in the fossil records until about 1 million years after the Pakicetus, which is one of the first known ancestors of the whale. Instead, they move vertically which is the same as animals that live on land. , Ambulocetus, which lived about 49 million years ago, was discovered in Pakistan in 1994. , Indohyus was a small chevrotain-like animal that lived about 48 million years ago in what is now Kashmir. The mandibular foramen of basilosaurids covered the entire depth of the lower jaw as in modern cetaceans. The process of evolution has been identified in dolphins and it is believed they are connected to many terrestrial mammals. Hence, the method of sound transmission that were present in them combines aspects of pakicetids and modern odontocetes (toothed whales).  A single fossil from Baja California indicates the family once inhabited warmer waters. Getting A Leg Up On Whale And Dolphin Evolution New Comprehensive Analysis Sheds Light On The Origin Of Cetaceans main content. , Early archaeocetes such as Pakicetus had the nasal openings at the end of the snout, but in later species such as Rodhocetus, the openings had begun to drift toward the top of the skull. It is easier to think about as a many-branched tree or and interconnected network of rivers and streams.  Contrary to modern sperm whales, most ancient sperm whales were built to hunt whales. They have changes both to the size and the shape of them. Terrestrial locomotion of Rodhocetus was very limited due to their hindlimb structure. The information found indicates that during the early Miocene Period, the use of echolocation formed.  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