大型身体帮助已灭绝的海洋爬行动物游泳, 研究发现

Ancient sea monsters with long necks had to evolve large bodies to rule Earth’s oceans around 200 一百万年前, 研究发现

  • Scientists used 3D models and computer simulations of extinct marine animals
  • These creatures evolved large bodies allowed them to build up speed in waters
  • This let them compensate for odd appendages that stuck out, like long necks
  • Ancient sea monsters with long necks had to evolve large bodies to rule Earth’s oceans around 200 一百万年前, 僵硬的下颚让霸王龙可以咬碎骨头.

    Large bodies helped extinct creatures swim at speed even if they had bizarre appendages that could otherwise have increased drag and slowed them down.

    Bigger bodies meant more muscle mass, and so allowed them to increase their strength and speed.

    This was particularly the case for the elasmosaurs, known for their giraffe-like neck and a head resembling that of a snake.

    The elasmosaurs, known for a giraffe-like neck and a head resembling that of a snake, is a genus of plesiosaur that lived in North America during the Campanian stage of the Late Cretaceous period, 关于 80.5 一百万年前

    The elasmosaurs, known for a giraffe-like neck and a head resembling that of a snake, is a genus of plesiosaur that lived in North America during the Campanian stage of the Late Cretaceous period, 关于 80.5 一百万年前

    WHAT IS AN ELASMOSAURUS?

    Elasmosaurids are a monophyletic group of plesiosaurs with extremely long necks.

    They had the longest necks of the plesiosaurs and survived from the Late Triassic to the end of the Cretaceous.

    Their diet mainly consisted of crustaceans and molluscs.

    Although abundant elasmosaurid material has been collected from the Upper Crustaceous of Antarctica, skull material is extremely rare.

    The earliest elasmosaurids were mid-sized, 关于 6 米 (20 英尺).

    In the Late Cretaceous, elasmosaurids grew as large as 11.5–12 m (38–39 ft), such as Styxosaurus, Albertonectes, and Thalassomedon. Ť

    heir necks were anywhere between 32 和 76 (Albertonectes) cervical vertebrae. They weighed up to several tons.

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    The study was conducted by researchers at the University of Bristol, who created various 3D models and performed computer flow simulations to study different extinct tetrapods (four-limbed vertebrates), based on evidence from fossils.

    These included ichthyosaurs, a large group of fish-shaped marine reptiles that first appeared around 250 million years ago and disappeared before the end-Cretaceous extinction (65 一百万年前).

    Another extinct tetrapod group looked at was the plesiosaurs, marine reptiles with four flippers and extraordinarily long necks. Plesiosaurs are thought to have appeared in the latest Triassic Period, 关于 203 一百万年前.

    The plesiosaurs included elasmosaurs, which had the longest necks of the plesiosaurs.

    All the extinct creatures were compared with cetaceans – a modern-day order of aquatic mammals comprising the whales, the dolphins and the porpoises.

    ‘We created various 3D models and performed computer flow simulations of plesiosaurs, ichthyosaurs and cetaceans,’ said study author Dr Susana Gutarra Díaz of Bristol’s School of Earth Sciences and the National History Museum of London.

    ‘These experiments are performed on the computer, but they are like water tank experiments.

    研究人员发现 that big bodies help overcome the excess drag produced by extreme morphology, such as long necks.

    Pictured are 3D models of aquatic tetrapods, including the extinct plesiosaurs and the ichthyosaurs

    Pictured are 3D models of aquatic tetrapods, including the extinct plesiosaurs and the ichthyosaurs

    ICHTHYOSAURS AND PLESIOSAURS

    Ichthyosaurs, a large group of fish-shaped marine reptiles that were most abundant during the Jurassic period and disappeared before the end-Cretaceous extinction (65 一百万年前).

    Plesiosaurs were marine reptiles with four flippers and extraordinarily long necks. The plesiosaurs included elasmosaurs, which had the longest necks of the plesiosaurs.

    Dolphins and ichthyosaurs have similar body shapes, adapted for moving fast through water producing low resistance or drag.

    另一方面, plesiosaurs, who lived side by side with the ichthyosaurs in the Mesozoic Era, had entirely different bodies.

    Their enormous four flippers which they used to fly underwater, and variable neck lengths, have no parallel amongst living animals.

    Some elasmosaurs had extreme proportions, with necks up to 20 脚 (6 米) 长, which likely helped them to snap up quick-moving fish.

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    This debunks a long-standing idea that there is an optimal body shape for ensuring low drag through the water.

    Body size is more important than body shape in determining the energy economy of swimming for aquatic animals, 专家说.

    Another key finding was that the large necks of extinct elasmosaurs did add extra drag, but this was compensated by the evolution of large bodies.

    Although plesiosaurs did experience more drag than ichthyosaurs or whales of equal mass, these differences were relatively minor.

    ‘When we examined a large sample of plesiosaurs modelled on really well preserved fossils at their real sizes, it turns out that most plesiosaurs had necks below this high-drag threshold, within which neck can get longer or shorter without increasing drag,’ said study author Dr Benjamin Moon at Bristol.

    ‘But more interestingly, we showed that plesiosaurs with extremely long necks also had evolved very large torsos, and this compensated for the extra drag.

    For elasmosaurs, long necks were advantageous for hunting, but they could not exploit this adaptation and catch their prey until they became large enough to offset the cost of high drag on their bodies.

    ‘We found that in elasmosaurs, neck proportions changed really fast,’ said study co-author Dr Tom Stubbs.

    ‘This study shows that, in contrast with prevailing popular knowledge, very long necked plesiosaurs were not necessarily slower swimmers than ichthyosaurs and whales, and this is in part thanks to their large bodies.

    Computer simulation of flow over the 3D model of an elasmosaur (elasmosaurs had the longest necks of the plesiosaurs)

    Computer simulation of flow over the 3D model of an elasmosaur (elasmosaurs had the longest necks of the plesiosaurs)

    然而, in the evolutionary quest for swimming speed, body sizes cannot get indefinitely large, as there are some constraints to very large sizes as well.

    ‘The maximum neck lengths we observe, seem to balance benefits in hunting versus the costs of growing and maintaining such a long neck,’ said Professor Mike Benton, another co-author.

    '换句话说, the necks of these extraordinary creatures evolved in balance with the overall body size to keep friction to a minimum.

    到现在, it has not been clear how shape and size influenced the energy demands of swimming in these diverse marine animals, 根据团队.

    The new study has been published today in Communications Biology.

    HOW DID PLESIOSAURS SWIM?

    Plesiosaurs used two near-identical pairs of flippers to propel themselves through the water.

    Both sets of flippers provided propulsive power, the new study suggests.

    相比之下, other animals such as turtles and sea lions use their front flippers for thrust and back flippers for steering.

    The team found that swirling movements in the water created by the plesiosaur’s front flippers increased the thrust of the back flippers by 60 per cent and their efficiency by 40 百分.

    Compared to other marine reptiles, they had a short tail that was only used for steering.

    The long extinct paddle saurians easily could have held their own against today’s water animals.

    Plesiosaurs used two near-identical pairs of flippers to propel themselves through the water (艺术家的印象如图)

    Plesiosaurs used two near-identical pairs of flippers to propel themselves through the water (图为艺术家的印象)

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