If we assume that the Brachiosaurus and the modern Giraffe with their long necks both came to be by the need to reach higher-situated vegetable nutrition (which is a somewhat controversial assumption to make, I understand Evolution of long necks in giraffes), what biological reason is there we don't see any kind of modern day equivalent to the Elasmosauri?

Put differently, what environmental factor promoted the evolution of water reptiles with extremely long necks that ceased to exist in a post-cretaceous world? What benefits did Elasmosauri have that later water creatures would not?

Assuming that evolution is shaped by environmental factors, of course.

(And assuming the "water giraffes" weren't simply called "hydras" and extirpated by the ancient greeks. :P )

  • $\begingroup$ I think you already visited this Wiki page, but just in case you didn't I'll post it here: Neck movement in Elasmosaurus. $\endgroup$ Jun 21 '18 at 15:43
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    $\begingroup$ Do we not? en.wikipedia.org/wiki/Heron en.wikipedia.org/wiki/Hydrophiinae $\endgroup$
    – Bryan Krause
    Jun 21 '18 at 16:22
  • $\begingroup$ Are there any existing environments where such long necks would be advantageous, and which are large enough to support a population of Elasmosaurus-sized creatures? Such necks are certainly common in aquatic birds: not just herons, but geese, swans, cranes, flamingos... $\endgroup$
    – jamesqf
    Jun 22 '18 at 4:49
  • $\begingroup$ Is your question simply: Given that elasmosaurs went extinct, why did an analogue of elasmosaurs not evolve after the Cretaceous? $\endgroup$
    – kmm
    Jun 23 '18 at 12:58

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