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  1. 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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  2. 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.

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    • Mammals vs. Reptiles
    • Lifestyles of First Mammals
    • Split in Family Tree
    • Mammals Survive Extinction Event

    Before discussing how the first mammals evolved, it's helpful to define what distinguishes mammals from other animals, especially reptiles. Female mammals possess milk-producing mammary glands with which they suckle their young. All mammals have hair or fur during at least some stage of their life cycles, and all are endowed with warm-blooded (endothermic) metabolisms. Regarding the fossil record, paleontologists can distinguish ancestral mammals from ancestral reptiles by the shape of their skull and neck bones, as well as the presence, in mammals, of two small bones in the inner ear (in reptiles, these bones constitute part of the jaw).

    The most distinctive thing about the mammals of the Mesozoic Era is how small they were. Although some of their therapsidancestors attained respectable sizes. For example, the late Permian Biarmosuchus was about the size of a large dog. Very few early mammals were larger than mice, for a simple reason: dinosaurs had already become the dominant terrestrial animals on earth. The only ecological niches open to the first mammals entailed a) feeding on plants, insects and small lizards, b) hunting at night (when predatory dinosaurs were less active), and c) living high up in trees or underground, in burrows. Eomaia, from the early Cretaceous period, and Cimolestes, from the late Cretaceous period, were fairly typical in this regard.

    Recently, paleontologists discovered conclusive fossil evidence for the first important split in the mammal family tree, the one between placental and marsupial mammals. Technically, the first, marsupial-like mammals of the late Triassic period are known as metatherians. From these evolved the eutherians, which later branched off into placental mammals. The type specimen of Juramaia, the "Jurassic mother," dates to about 160 million years ago, and demonstrates that the metatherian/eutherian split occurred at least 35 million years before scientists had previously estimated.

    Ironically, the same characteristics that helped mammals maintain a low profile during the Mesozoic Era also allowed them to survive the K/T Extinction Event that doomed the dinosaurs. As we now know, that giant meteor impact 65 million years ago produced a kind of "nuclear winter," destroying most of the vegetation that sustained the herbivorous dinosaurs, which themselves sustained the carnivorous dinosaurs that preyed on them. Because of their tiny size, early mammals could survive on much less food, and their fur coats (and warm-bloodedmetabolisms) helped keep them warm in an age of plunging global temperatures.

    • Bob Strauss
    • Science Writer
    • 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.
  4. 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...

  5. 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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