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About 50 million years ago
- Cetaceans (whales, dolphins, and porpoises) are an order of mammals that originated about 50 million years ago in the Eocene epoch.
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- Bob Strauss
- 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.
May 21, 2007 · Evolution of marine mammals: Back to the sea after 300 million years. Mark D. Uhen. First published: 21 May 2007. https://doi.org/10.1002/ar.20545. Citations: 144. Sections. PDF. Tools. Share. Abstract. The fossil record demonstrates that mammals re-entered the marine realm on at least seven separate occasions.
- Mark D. Uhen
- 144
- 2007
- 21 May 2007
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys.
Between 390 and 360 million years ago, the descendents of these organisms began to live in shallower waters, and eventually moved to land. As they did, they experienced natural selection that shaped many adaptations for a terrestrial way of life. Like other terrestrial sarcopterygians, modern humans still carry the evidence of our aquatic past ...
Evolution of Marine Mammals: Back to the Sea After 300 Million Years. MARK D. UHEN* Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC. ABSTRACT. The fossil record demonstrates that mammals re-entered the marine realm on at least seven separate occasions.
The first fully marine turtles emerged during the Cretaceous Period, a span of time lasting between 145 and 66 million years ago. By 120 million years ago, they resembled the sea turtles we are familiar with today.
Marine mammals are mammals that rely on marine (saltwater) ecosystems for their existence. They include animals such as cetaceans ( whales, dolphins and porpoises ), pinnipeds ( seals, sea lions and walruses ), sirenians ( manatees and dugongs ), sea otters and polar bears. They are an informal group, unified only by their reliance on marine ...
