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The earliest marine reptiles arose in the Permian. During the Mesozoic many groups of reptiles became adapted to life in the seas, including ichthyosaurs, plesiosaurs, mosasaurs, nothosaurs, placodonts, sea turtles, thalattosaurs and thalattosuchians. Marine reptiles were less numerous after mass extinction at the end of the Cretaceous.
Marine reptiles are reptiles which have become secondarily adapted for an aquatic or semiaquatic life in a marine environment. Only about 100 of the 12,000 extant reptile species and subspecies are classed as marine reptiles, including marine iguanas , sea snakes , sea turtles and saltwater crocodiles .
As embryos, vertebrates still have a notochord; as adults, all but the jawless fishes have a vertebral column, made of bone or cartilage, instead. [7] Vertebrate embryos have pharyngeal arches; in adult fish, these support the gills, while in adult tetrapods they develop into other structures. [10] [11]
Because of overhunting, the blue whale population has dropped from what biologists estimate was 200,000 in the 1800s to approximately 20,000 today. They are listed as endangered by the IUCN. Their ...
Scientists consider the blue whale, which grows up to 110 feet (33.5 meters) long, to be the largest known animal ever to exist on the planet. But it’s possible that the 202 million-year-old ...
The blue whale is the largest animal known ever to have existed. [43] [44] [45] Some studies have estimated that certain shastasaurid ichthyosaurs and the ancient whale Perucetus could have rivalled the blue whale in size, with Perucetus also being heavier than the blue whale with a mean weight of 180 t (180 long tons; 200 short tons).
While today's blue whale has long held the title, scientists have dug up fossils from an ancient giant that could tip the scales. ... 13 vertebrae from the whale’s backbone, four ribs and a hip ...
Each is a member of one of three monophylitic clades. All tunicate larvae have the standard chordate features, including long, tadpole-like tails. Their larva also have rudimentary brains, light sensors and tilt sensors. [28] The smallest of the three groups of tunicates is the Appendicularia. They retain tadpole-like shapes and active swimming ...