Aug 17, 2019 · The First Canids: Hesperocyon and the "Bone-Crushing Dogs". Paleontologists agree that the late Eocene (about 40 to 35 million years ago) Hesperocyon was directly ancestral to all later canids — and thus to the genus Canis, which branched off from a subfamily of canids about six million years ago.
Case Studies in Ecology and Evolution DRAFT D. Stratton 2011 1 1 Phylogenetic History: The Evolution of Marine Mammals Think for a moment about marine mammals: seals, walruses, dugongs and whales. Seals and walruses are primarily cold-water species that eat mostly fish and can spend part of their time on land (or ice).
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In the 1980s, Karen Pryor (who specialized in training marine mammals and dogs) published Don’t Shoot The Dog, a guide to clicker training and positive reinforcement in dog training. Many other ‘purely positive’ trainers followed, alongside a variety of other dog training styles and schools of thought which has evolved into what dog ...
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May 21, 2007 · Also, the two recently evolved marine mammals, the polar bear and sea otter, spend most of their time in the water feeding. This is no great surprise, because a great deal of mammalian evolution has been linked to changes in feeding ecology.
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Mar 21, 2012 · In addition, the Pinnipedia (seals, sea lions, and walruses) evolved from a group of dog-like Carnivora in the late Oligocene. Pinnipeds are all semi-aquatic, coming ashore to breed and have their...
Marine mammals have evolved a wide variety of features for feeding, which are mainly seen in their dentition. For example, the cheek teeth of pinnipeds and odontocetes are specifically adapted to capture fish and squid. In contrast, baleen whales have evolved baleen plates to filter feed plankton and small fish from the water.
The evolution of cetaceans is thought to have begun in the Indian subcontinent from even-toed ungulates 50 million years ago and to have proceeded over a period of at least 15 million years. Cetaceans are fully aquatic marine mammals belonging to the order Artiodactyla and branched off from other artiodactyls around 50 mya (million
- Fish and Sharks. Between 500 and 400 million years ago, vertebrate life on earth was dominated by prehistoric fish. With their bilaterally symmetric body plans, V-shaped muscles, and notochords (protected nerve chords) running down the lengths of their bodies, ocean dwellers like Pikaia and Myllokunmingia established the template for later vertebrate evolution It also didn't hurt that the heads of these fish were distinct from their tails, another surprisingly basic innovation that arose during the Cambrian period.
- Tetrapods. The proverbial "fish out of water," tetrapods were the first vertebrate animals to climb out of the sea and colonize dry (or at least swampy) land, a key evolutionary transition that occurred somewhere between 400 and 350 million years ago, during the Devonian period.
- Amphibians. During the Carboniferous period, dating from about 360 to 300 million years ago, terrestrial vertebrate life on earth was dominated by prehistoric amphibians.
- Terrestrial Reptiles. About 320 million years ago, give or take a few million years, the first true reptiles evolved from amphibians. With their scaly skin and semi-permeable eggs, these ancestral reptiles were free to leave rivers, lakes, and oceans behind and venture deep into dry land.
- Other animals
These first whales, such as Pakicetus, were typical land animals. They had long skulls and large carnivorous teeth. From the outside, they don't look much like whales at all. However, their skulls particularly in the ear region, which is surrounded by a bony wall strongly resemble those of living whales and are unlike those of any other mammal. Often, seemingly minor features provide critical evidence to link animals that are highly specialized for their lifestyles (such as whales) with their less extreme-looking relatives.
Compared to other early whales, like Indohyus and Pakicetus, Ambulocetus looks like it lived a more aquatic lifestyle. Its legs are shorter, and its hands and feet are enlarged like paddles. Its tail is longer and more muscular, too. The hypothesis that Ambulocetus lived an aquatic life is also supported by evidence from stratigraphy Ambulocetus's fossils were recovered from sediments that probably comprised an ancient estuary and from the isotopes of oxygen in its bones. Animals are what they eat and drink, and saltwater and freshwater have different ratios of oxygen isotopes. This means that we can learn about what sort of water an animal drank by studying the isotopes that were incorporated into its bones and teeth as it grew. The isotopes show that Ambulocetus likely drank both saltwater and freshwater, which fits perfectly with the idea that these animals lived in estuaries or bays between freshwater and the open ocean.
Whales that evolved after Ambulocetus (Kutchicetus, etc.) show even higher levels of saltwater oxygen isotopes, indicating that they lived in nearshore marine habitats and were able to drink saltwater as today's whales can. These animals evolved nostrils positioned further and further back along the snout. This trend has continued into living whales, which have a \\"blowhole\\" (nostrils) located on top of the head above the eyes.
These more aquatic whales showed other changes that also suggest they are closely related to today's whales. For example, the pelvis had evolved to be much reduced in size and separate from the backbone. This may reflect the increased use of the whole vertebral column, including the back and tail, in locomotion. If you watch films of dolphins and other whales swimming, you'll notice that their tailfins aren't vertical like those of fishes, but horizontal. To swim, they move their tails up and down, rather than back and forth as fishes do. This is because whales evolved from walking land mammals whose backbones did not naturally bend side to side, but up and down. You can easily see this if you watch a dog running. Its vertebral column undulates up and down in waves as it moves forward. Whales do the same thing as they swim, showing their ancient terrestrial heritage. As whales began to swim by undulating the whole body, other changes in the skeleton allowed their limbs to be used more for steering than for paddling. Because the sequence of these whales' tail vertebrae matches those of living dolphins and whales, it suggests that early whales, like Dorudon and Basilosaurus, did have tailfins. Such skeletal changes that accommodate an aquatic lifestyle are especially pronounced in basilosaurids, such as Dorudon. These ancient whales evolved over 40 million years ago. Their elbow joints were able to lock, allowing the forelimb to serve as a better control surface and resist the oncoming flow of water as the animal propelled itself forward. The hindlimbs of these animals were almost nonexistent. They were so tiny that many scientists think they served no effective function and may have even been internal to the body wall. Occasionally, we discover a living whale with the vestiges of tiny hindlimbs inside its body wall. This vestigial hindlimb is evidence of basilosaurids' terrestrial heritage. The picture below on the left shows the central ankle bones (called astragali) of three artiodactyls, and you can see they have double pulley joints and hooked processes pointing up toward the leg-bones. Below on the right is a photo of the hind foot of a basilosaurid. You can see that it has a complete ankle and several toe bones, even though it can't walk. The basilosaurid astragalus still has a pulley and a hooked knob pointing up towards the leg bones as in artiodactyls, while other bones in the ankle and foot are fused. From the ear bones to the ankle bones, whales belong with the hippos and other artiodactyls.